Nanomaterials and nanotechnology for advanced photovoltaics
€ 798 979
Tove Lillian Hønstad (Ms.)
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STICHTING ENERGIEONDERZOEK CENTRUM NEDERLAND
€ 598 512,49
HELMHOLTZ-ZENTRUM BERLIN FUR MATERIALIEN UND ENERGIE GMBH
€ 376 898
UNIVERSITAT DE VALENCIA
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TECHNISCHE UNIVERSITAET MUENCHEN
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LEIBNIZ-INSTITUT FUER PHOTONISCHE TECHNOLOGIEN E.V.
€ 297 147
CONSIGLIO NAZIONALE DELLE RICERCHE
€ 277 612
RUDER BOSKOVIC INSTITUTE
€ 225 000
CENTRAL LABORATORY OF SOLAR ENERGY& NEW ENERGY SOURCES OF THE BULGARIAN ACADEMY OF SCIENCES
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Oxford Instruments Plasma Technology Ltd
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SCHOTT SOLAR AG
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INNOVATIVE MATERIALS PROCESSING TECHNOLOGIES Ltd
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Grant agreement ID: 246331
1 March 2011
28 February 2014
€ 5 153 458,02
€ 3 853 315
Cost-effective, high-efficiency solar cells
Silicon (Si)-based photovoltaic (PV) devices can now be seen on rooftops and solar farms across Europe, yet widespread implementation is hindered by costs and limits in efficiency. The EU-funded project 'Nanomaterials and nanotechnology for advanced photovoltaics' (NANOPV) addressed current bottlenecks in production with large-scale processes and equipment for production of PVs from nanomaterials. The consortium studied a large variety of such materials from zero-dimensional quantum dots, nanocrystals and nanoparticles, to 1D nanowires and nanorods and even 2D ultra-thin nanolayers. In addition to materials' development to enhance efficiency, scientists developed cost-effective large-scale processes and equipment for integration into existing pilot and industrial production lines. Nanomaterials can significantly enhance efficiencies by making better use of the electromagnetic spectrum, taking advantage of a broader range of available wavelengths. More than 20 % energy conversion efficiency was targeted for wafer-based Si devices and more than 15 % for epitaxially grown thin-film Si solar cells. Scientists also assessed low-cost solar cells produced entirely from nanomaterials. Of the more than 50 materials tested, 9 were integrated in wafer-based Si devices and 11 in thin-film Si solar cells. Enhanced conversion efficiency was seen in some cases. A database of the most promising candidates has been created and several were chosen for optimisation within the project. For instance, the team optimised fabrication of solar cells with Si nanorod/nanowire structures. This resulted in thin film solar cells of high efficiency (10 %) that can be produced cost effectively at industrial scale. In addition, incorporation of Si-based nanolayers into Si-based solar cell structures resulted in high conversion efficiency for both wafer-based (> 20 %) and epitaxially grown thin-film (~ 15 %) architectures. NANOPV demonstrated the feasibility of high-efficiency, low-cost PV devices using nanomaterials. The project has contributed to development of a safe, cost-effective and sustainable energy supply with technologies that will enhance the competitive position of the EU in a market poised for a major breakthrough.
Grant agreement ID: 246331
1 March 2011
28 February 2014
€ 5 153 458,02
€ 3 853 315
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Final Report Summary - NANOPV (Nanomaterials and nanotechnology for advanced photovoltaics)
The nanoPV project aims at making a breakthrough step-change in photovoltaics by the removal of a set of bottlenecks to the application of nanostructures for high-efficiency low-cost solar cells. These include the present lack of relevant equipment and up-scalable processes that can meet the needs for nanomaterials in PV applications. To achieve this, a consortium of 9 complementary research partners and 2 industries have been assembled and nanoPV project objectives were:
(i) to develop technologies that can increase the efficiency and reduce the processing cost of existing silicon solar cell technologies using nano-scale effects provided by nanomaterials to above 20% for wafer based and above 15 % for thin film silicon based solar cells at a processing cost for modules well below 1 €/watt;
(ii) to design and to fabricate low cost solar cells entirely from nanomaterials by using nanostructures. An efficiency of above 10 % at processing costs well below 1 €/watt is targeted with potential of further significant improvements in the future;
(iii) to develop up-scalable cost effective processes and equipment in order to implement both enhanced standard solar cells and solar cell based on nanomaterials as well as related modules to existing pilot and industrial lines;
(iv) to create new market opportunities for the industrial partners.
Nanotechnology has been applied for both existing conventional Si solar cells (wafer and thin-film based), and for advanced solar cells entirely based on nanostructures. Scientific efforts were focused on exploitation of nanomaterials such as: (i) 0D quantum dots, nanocrystals and nanoparticles, (ii) 1D nanowires and nanorods, and (iii) 2D nanomaterials such as ultrathin layers. A large number of specialised vacuum (magnetron sputtering, ALD, PECVD, e-beam) and non-vacuum (sol-gel, spray, electrostatic spray, spin coating, tape casting) technologies have been applied. Manufacturing procedures for the processing of nanoparticles and inks compatible with industrial requirements have been developed. Several methods have been compared for cost-effective production of such nanoparticles (e.g. sol-gel, laser assisted, thermal spraying, and magnetron sputtering). More than 50 different types of nanomaterials have been tested and after thorough selections 9 nanomaterials were implemented in Si wafer based Si cells and 11 in thin-film Si cells. Proof of principle tests of integration of nanoparticles and related layers into processing of conventional Si based solar cells has been carried out and resulted in enhanced conversion efficiency in some cases. As an example, efficiency gains for Si based solar cells (compared to relevant references) were up to 1.5% absolute (e.g. with Black silicon nanotexture). A database of most promising nanomaterials candidates to improve properties of Si based solar cells has been created. Further optimization is, however, required to achieve record high efficiencies. Detailed investigations and control of nanostructures and relevant solar cell structures for selected cases has been performed. As an example, optimization of fabrication of Si based solar cells based on Si nanorod/wire structures resulted in high efficiency solar cells (10%), which can be industrially produced at a low cost. It has also been demonstrated that incorporation of a-Si:H nano-layers into Si based solar cell structures resulted in high (>20%) conversion efficiency for Si wafer based solar cells and ~15% for thin epi-Si (in this case a stack of ultra-thin a-Si:H and Al2O3 layers was implemented) based solar cell structures. It is expected that the developed methods can be adjusted to an industrial level to process high efficiency solar cells at a lower cost. NanoPV has, therefore, demonstrated that it is possible to develop a process chain based on cost-effective manufacturing of Si based solar cells and modules using implementation of nanomaterials. A technological platform, which includes innovative equipment and cost effective up-scalable processes has been developed. The platform covers the complete fabrication process chain and enables cost-effective and large scale production. In this way the full potential of the technology can be exploited in combination with the throughput and cost per unit necessary for commercialization.
Project Context and Objectives:
The main vision of NanoPV is to reach a breakthrough towards cost-effective manufacturing of Si based solar cells partially or fully based on nanomaterials. The use of nanomaterials can enhance the solar cell efficiency (e.g. by making better use of the solar spectrum) at reduced cost and make PV technology more competitive with conventional energy technologies. Main efforts have been focused on understanding and exploitation of nanomaterials such as: (i) zero-dimensional (0D) quantum dots (QDs) and nanocrystals (NCs) and nanoparticles (NPs); (ii) one-dimensional (1D) nanowires (NWs) and nanorods (NRs), and (iii) two-dimensional (2D) nanomaterials such as ultrathin layers of semiconductors and dielectrics. Nanotechnology has been applied for enhancing the conversion efficiency of: (i) already existing conventional Si-based solar cells (1D and 2D structures applied to wafer as well as thin film based) and (ii) advanced solar cells based on 1D nanostructures. Thus, the first route can be seen as an add-on technology to existing cell and module concepts. The second route is a new cell concept.
A set of innovative technological processes have been developed to realise the cell concepts described above using nanostructures. The aim is to lower the cost to well below than 1€/W for industrially relevant module technology by higher efficiencies using nanostructures. NanoPV has thoroughly investigated phenomena in nanomaterials at the molecular level, up to a scale comparable with the wavelength of the solar spectrum, with a focus on the underlying nanometric processes and nano-scale effects. A PV “revolution” has been expected through the accelerated evolution and merging of two known but so far almost independent fields: conventional PV and nanotechnology.
Increasing the efficiency and decreasing the production cost of terrestrial photovoltaic devices is important for their widespread acceptance as a major power generation technology for the future. One of the main factors limiting the efficiency of solar cells is the spectral mismatch between the band gap energy (absorption spectrum) of the absorber material and the incident solar light. The solar spectrum is very broad and contains significant power density both below and above the band gap of silicon. Therefore the conversion efficiency limit for a single-junction solar cell, according to the detailed balance analysis, yields a maximum of ~31% under standard AM 1.5 solar spectrum irradiation. Several approaches have recently been proposed to increase the efficiency of single–band gap solar cells above the theoretical limit. These approaches are based on a “photon management” concept, which can employ such phenomena as: (i) spectral modification of the incident light (up-/down-conversion, down shifting), (ii) multi-exciton generation (MEG) and (iii) surface plasmon resonance effect. In addition tandem and multi-junction approaches are considered as a possible solution to the problem.
Another main factor, which limits large-scale use of solar energy, is the cost, which is still too high. The most advanced solar cell processes make extensive use of high temperature processes, vacuum processes, pattern definition steps, and/or expensive materials (including the silicon wafer). Therefore, not only the efficiency enhancement but also the cost reduction of solar cells has to be a target of innovative developments.
An analysis of current trends in the PV field shows that both improved efficiency and lowered cost can be obtained by implementing nanomaterials and nanotechnology to existing solar cells. Solar cells entirely based on nanostructured materials, which are currently a subject of intensive investigation, can ultimately be an alternative to the dominant bulk crystalline Si technology (1st generation) and thin-film based PV technology (2nd generation). However, it is also clear that 1st and 2nd generation cells will dominate for many years to come. Thus, incorporation of nanomaterials in 1st and 2nd generation solar cells, to enhance conversion efficiency and lower cost per unit watt, is required and can provide an evolution process towards creation of a new generation of advanced solar cells. For this to come true, progress in advanced materials as well as advanced solar cell structure concepts is, which are the topics of this proposal, is needed.
In NanoPV a set of cost-effective methods for synthesis of nanomaterials and related solar cell structures has been developed and incorporated into an up-scalable solar cell processing flow. Two ambitious complementary routes have been followed in NanoPV to achieve the aim of enhanced efficiency and reduced cost:
Route 1: Implementation of nanomaterials into already existing Si solar cell technologies to improve their cost/Wp (Watt-peak) ratio by improved photon management (add-on technology). The concepts have been ranged from lower-risk low-cost uses of nanomaterials, to higher risk concepts for radical breakthroughs.
Route 2: Combination of developments from route 1) to fabricate a new solar cell concept entirely based on nanomaterials.
Route 1 uses wafer-based and thin-film poly- and a-Si:H/µc-Si silicon based solar cells, which are currently dominating (>90%) the PV market and can be reliably fabricated within the consortium. NanoPV has developed advanced solutions for Si based solar cells, by implementation of nanomaterials and nanotechnology. This implementation has been done in a complementary and a smooth manner, using as a background conventional Si solar cell based technologies, which are already well developed by several NanoPV participants. NanoPV route 1 is expected to improve the performance/cost ratio of conventional Si based solar cells and modules made on the basis of such cells significantly above current values, targeting well below 1 €/W. Therefore, route 1 can be considered as a nanotechnology based breakthrough step for the further lowering of the performance/cost ratio for the conventional Si based solar cell processing. It is important to emphasize that results from route 1, which is considered as an extension of current PV technologies (an add-on technology), can be implemented in industrial manufacturing lines relatively fast. It is important to note that the proposed concept of the solution is fully in line with other recent world-wide efforts in this field to merge nanotechnology and PV in order to solve a global problem of development cost effective energy sources. Route 2 has been realised using step-by-step implementation of nanomaterials and nanotechnologies to substitute or drastically modify currently developed and widely used Si based solar cells and conventional technologies. This route has a target to establish an advanced nanotechnology based process flows and utilise the photon management effects for the cost effective fabrication of advanced Si solar cells entirely based on nanomaterials. The NanoPV route 2 can be considered as a complimentary and a wider approach compared to that of the recent EU project ROD-SOL . NanoPV considers not only: (i) growing Si NRs/NWs by several low-cost methods, different from those of ROD-SOL but also (ii) implementing NRs/NWs made from conductive oxides and metals, which can be used as supporting substrates/back side electrodes for a-Si:H/µc-Si based solar cells.
By incorporating the two described routes in one project, a smooth process of merging of nanotechnology and nanomaterials with PV has been provided. The advantage of such strategy based on the proposed methodology: nanomaterials, being tested on well-known Si based solar cells can be used for the construction of innovative solar cell structures. Moreover a step-by-step testing and substitution of conventional materials and technologies by their “nano” based alternatives will provide a smooth transition from already existing to advanced solar cells structures on the basis of short, medium and long term developments. It is expected that such developments will offer a set of benefits for the PV industry.
The solutions from NanoPV can be applied in current state of the art processing lines, lowering the cost/Wp on a short term. In addition, NanoPV has contributed to the vision of cost-effective 3rd generation of ultrahigh-efficiency solar cells in the medium to long term. In spite of intensive current investigations world-wide, several bottlenecks exist and many new developments are needed before low-cost processing of nanotechnology-based solar cells can be realised. Therefore the following whole process flow has been addressed:
Up-scalable processing of 0D, 1D and 2D nanomaterials for PV needs, as add-on components in existing Si based solar cells, which provides performance enhancement by nano-scale effects (bottleneck 1)
Development of 0D, 1D and 2D nanomaterials and their up-scalable processing for the fabrication of solar cells entirely based on nanomaterials (bottleneck 2)
Implementation of nanomaterials and nanotechnology into already existing Si solar cell technologies, pilot and industrial lines (bottleneck 3)
Development of advanced equipment and processes for the implementation of nanomaterials into solar cell processing (bottleneck 4)
The complexity of nanomaterials-based solar cells requires high-level experience in a number of different scientific fields, which are not directly linked. Hence, a strong interdisciplinary activity has been realized in NanoPV. Therefore, the consortium partners have been chosen such that all the required and necessary experience has been available: (i) nanomaterials processing; (ii) development of an advanced up-scalable equipment for processing of nanomaterials and related solar cell structures; (iii) analysis of nanomaterials and interfaces; (iv) analysis of solar cell structures and solar cells; (v) processing and testing of solar cells and modules in research pilot lines. World leading research groups as well as leading European industrial partners formed a very strong consortium for NanoPV. Important to mention, that NanoPV was using a wide competence of all partners, which was built in previous numerous EU projects with participation of the nanoPV consortium members.
The main objectives of NanoPV aimed at the removal of a set of bottlenecks which have been identified to block the application of nanostructures for high-efficiency, low-cost solar cells. The objectives were:
1. To develop technologies that can increase the efficiency and reduce the processing cost of existing silicon solar cell technologies using nano-scale effects provided by nanomaterials to above 20% for wafer based and above 15 % for thin film (poly- and a-Si:H/µc-Si) silicon based solar cells at a processing cost for modules well below 1 €/watt. All available dimensions of nanomaterials have to be used to achieve this objective (0D, 1D and 2D) as well as advanced designs for new solar cell structures (elimination of bottleneck 1)
2. To design and to fabricate low cost solar cells entirely from nanomaterials by using 0D, 1D and 2D nanostructures. An efficiency of above 10 % at processing costs well below 1 €/watt is targeted with potential of further significant improvements in the future (elimination of bottleneck 2)
3. To develop up-scalable cost effective processes and equipment in order to implement both enhanced standard solar cells and solar cell based on nanomaterials as well as related modules to existing pilot and industrial lines (elimination of bottlenecks 3 and 4)
4. To create new market opportunities for the industrial partners
To be able to reach the main objectives, a set of sub-objectives were defined in addition:
5. To develop a thorough understanding of the electronic and optical properties of nanomaterials and interfaces, their dependence on processing parameters and their relationship with the final properties of solar cells. Ab initio simulation of electronic structure of selected nanomaterials and interfaces in some selected solar cell structures have to be performed.
6. Selected processes developed in NanoPV have to be adapted to up-scaled process flows for fabrication of solar cells containing nanomaterials.
To reach these objectives the following technical developments have been performed in NanoPV:
Development of cost effective non-vacuum as well as vacuum based and combined methods for the processing of semiconducting, metallic and insulating nanoparticles (0D). This includes:
o Fabrication of individual nanoparticles with desired optical and electrical properties, composition, purity, crystallinity, particle size etc.
o Development of non-vacuum and vacuum based processes and related equipment for the formation of nanoparticles embedded in different solid matrices.
o Development of cost effective non-vacuum chemical methods for the formation of thin layers containing nanoparticles with desired passivating and optical properties suitable for up- and down-conversion processes (0D + 2D nanomaterials).
o Exploitation of size quantization effects in Si based superlattices for band gap engineering in high-efficiency tandem cells. (2D nanomaterials).
Development of cost effective non-vacuum as well as vacuum based methods for the formation of ultra-thin nanostructured passivating layers, which can be used as thin emitters/back surface field (BSF) for the advanced hetero-junction solar cells. (2D nanomaterials).
Development of cost effective non-vacuum as well as vacuum based methods for the processing of NRs/NWs (1D) with desired properties. This includes:
o Development of cost effective bottom-up and top-down processes for the formation of nanorods/nanowires from Si based materials using vapour-liquid-solid, electrodeposition and low-cost template assisted methods.
o Development of low-cost processes for the fabrication of oxide based 1D structures (ZnO, TiO2, SnO, FTO, ITO) and implementation of them as anti-reflective coatings (ARC) for the fabrication of conventional Si based (thin film and wafer based) solar cells.
o Development of low-cost processes for the fabrication of highly conductive (TCO and metallic based) NRs/NWs, which can be used as substrates for a-Si:H/µc-Si based solar cells
Optimization of selected processes developed in NanoPV for up-scaling in solar cell fabrication (solar cell level up-scaling)
It was aimed for the transfer of developed innovative up-scaling technologies and equipment prototypes to industrial type of equipment and pilot lines by the end of the project.
Main S&T results/foregrounds
Main results and foregrounds presented below follow the R&D workplan which has been devided into 6 major R&D topics: i) Nanoparticle based materials (WP1), ii) Nanowires (WP2), iii) Ultrathin nanolayers (WP3), iv) Electrical, optical and structural properties of individual nanomaterials (WP4), v) Design, processing, charaterisation and testing of solar cells and modules (WP5), and vi) Assessement of transferability to industrial production (WP6).
NANOPARTICLE BASED MATERIALS AND RELATED SOLAR CELLS
Work on this topic was part of WP1, which had the following objectives:
To fabricate individual nanoparticles with desired optical and electrical properties, composition, purity, crystallinity, particle size etc and inks to disperse such nanoparticles in a desirable manner
To optimise processes for incorporation/assembling of semiconductor nanoparticles in thin films, which can be used as a part of solar cell structures, providing: (i) nano-particle based emitter (ii) MEG process, (iii) up- and down conversion (shifting) effects
To optimise processes for incorporation of metallic nanoparticles, which can provide: (i) formation of local contacts or (ii) surface plasmon based improvement of the solar cell conversion efficiency
To develop cost-effective non-vacuum and vacuum based equipment and processes for the formation of nanoparticles embedded in different solid matrices and thin layers or assembled in nanolayers
During the project period, cost-effective processing of nanoparticles and related inks adjusted to PV needs has been developed in WP1. The developed processes have been used as a background for innovative technologies based on an implementation of nanoparticles in PV. Manufacturing procedures for the processing of nanoparticles and inks compatible with industrial requirements have been developed in WP1. Several alternative methods have been compared for cost-effective production of such nanoparticles: sol-gel, laser assisted, thermal spraying, magnetron sputtering, etc.
Synthesis of nanoparticles and inks
Several vacuum (magnetron sputtering, e-beam deposition, PECVD, H+ plasma) and non-vacuum (sol-gel, laser ablation, cryo-milling, reduction of oxides by annealing, reduction of oxides by hydrogen radicals) technologies have been developed to sinter semiconductor and metallic based nanoparticles, suitable for PV applications. Several types of inks have been successfully tested to disperse NPs and to form layers from NPs containing solutions.
Different methods of deposition of colloidal Ag NPs of 100-nm and 200-nm diameters bearing different stabilization ligands (PVP and Citrate) on different surfaces (Si, SiO2, SiNx, ZnO) were elaborated, such as deposition by physical adsorption and adsorption due to chemical functionalization of a solar cell interface.
Developments based on implementation of vacuum methods (PECVD, e-beam) for forming arrays of Si nanoparticles in SiOx were performed. Some indications, that such Si NPs can be formed in this way were obtained. Formation of Si NPs on top of Si has been realised in frame of room temperature process. Implementation of the developed NPs and related layers for the processing of advanced solar cell structures has been done during second period of the nanoPV project activity and results can be seen in the section on Design, processing, charaterisation and testing of solar cells and modules.
Nanoparticles for fabrication of heterojunction solar cells
Results of this type of developments can be summarized as follows:
• Heterojunction Si based solar cells have an emitter, which consists of highly conductive local regions. These conductive local regions provide current flow through the ITO/a-Si:H interface. It can be assumed that such channels can be constructed from highly conductive Si nano-particles.
• It has been shown that microcrystalline or rather nanocrystalline emitter layers processed by PECVD are compatible with high efficiency heterojunction solar cells if a nanometer-thin intrinsic amorphous buffer layer is used for interface passivation.
• Si NPs solutions based on ethanol with different concentration of Si NPs have been prepared. Thin mono-layers of such Si NPs based solutions have been successfully deposited on a-Si:H/Si solar cell structures, which shows reasonable values for the minority carriers lifetime and will be used for further processing of solar cells.
• It is shown that a-Si:H layers deposited at low temperatures on Si substrates can be converted into Si NPs containing layers after annealing at 700-800 ⁰C.
Silicon heterojunction solar cells with aSi:H emitter layers and indium nanoparticles have been prepared by H-plasma induced reduction of an ultrathin ITO layer with a thickness of 5 to 10 nm. As substrate n-type wafers with a planar (100) surface and with an alkaline etched, random pyramids texture have been used. A conventional a-Si:H emitter stack based on ultrathin layers and additionally a 5 to 10 nm thin ITO layer has been deposited.
Fig. 1 shows SEM images of ITO/Si structures after H plasma treatments at 100 ͦ C for 10 min.These NPs can be used for some applications: (i) as local conductive channels in some advanced solar cell structures; (ii) as seeds for the growth of Si NWs by VLS methos. Indium based NPs were intensively tested for the processing of HJ solar cells in WP5.
Nanoparticles for processing of local contacts/conductive channels
The following methods, which can be used for the formation of nano-contacts/conductive channels, have been developed in frame of the nanoPV project:
• Synthesis of metal contacts using hydrogen initiated reduction of ITO layers.
• Synthesis of conductive channels in nanolaminate dielectric structures.
• Synthesis of nano contacts using sol-gel and laser ablation methods.
As a result, the following nano/micro contacts can be used for an conversion efficiency enhancement of Si based solar cells: (i) In/Sn obtained by the hydrogen initiated reduction of ITO layers; (ii) conductive channels formed in nanolaminate dielectric structures and (iii) Ag nano-particles obtained using sol-gel and laser ablation methods.
All mentioned above methods for fabrication of nano/micro contacts have been tested for the processing of relevant Si based solar cell structures during the second period of the project.
An innovative approach has been developed by CLS. The idea is to use an electronic conductivity in granular materials. The granules are arrays of metallic particles of sizes ranging usually from a few to hundreds of nanometers embedded into an insulating matrix. Granular arrays can be treated as artificial solids with programmable electronic properties. The ease of adjusting electronic properties of granular metals assures them an important role for applications.
Technological approach: the multilayer structure is formed using one or two different dielectrics. The electrical conductivity is modified with planar located metal granules. Sheet resistance has been measured using the four-point probe method, Transmission Line Method (TLM), van der Pauw method and I-V measurements of the resistors. The studied samples are nanolaminate dielectric structures on the thermally oxidized silicon wafers. Four- point probe method and transmisssion line method gave the following number for the sheet resistance Rsh = 5-7 Ohm/sq. Results from van der Pauw metod: the electrical resistivity of the conductive area at room temperature is 9. 9 10-5 Ohmcm. Conductive areas with different forms and sizes can be formed on nanolaminate dielectric structure with the help of template mask or photolithography after additional treatment.
The electrical conductivity is studied depending on the type of the dielectric used and on the thickness of the dielectric layers in this nanolaminate structure. Good conductivity can be obtained for nanolaminate structures with dielectrics such as SiO2, TiO2, ZrO2, Al2O3 and (Al2O3)x(B2O3)1-x, deposited as a first layer on substrate ( ”bottom” layer of the nanolaminate structures). It can be concluded that innovative types of nanolaminate structures discussed above can be considered as a promising approach for processing of low-cost Si based solar cells.
Metal nanoparticles and related layers for plasmonic effects
Though plasmonics is a wide area of study, its application in the field of solar cells has recently attracted
a great attention. Metals in general support surface plasmons that are collective oscillation of excited free electron and characterized by a resonant frequency. The resonances of noble metals are mostly in the visible or infrared part of the spectrum, which is of interest for photovoltaic applications. It is demonstrated that metallic nanoparticles (Au, Ag, Cu) and related layers, which exhibit plasmon resonance, can be formed using magnetron sputtering (or co-sputtering) of thin layers followed by an annealing step at temperatures in the range 300-600 ⁰C. In situ synthesized Au and Ag NPs in thin layers of SiO2 and TiO2 on Si wafers can be formed using spin coating. ESAVD (electrostatic spray) can be used for the processing of SiOx layers containing Ag NPs. Metal NPs have been intensively tested in WP5 related developments.
Pre-assessment application of nanotechnologies for processing of related solar cells
The key conclusions from the pre-assessment application of nanotechnologies for processing of related solar cells that could result in cell efficiency gain are listed below:
• Microcrystalline or rather nanocrystalline emitter layers are compatible with high efficiency heterojunction solar cells. For this approach it is crucial to introduce a nanometer-thin intrinsic amorphous buffer layer at the emitter as well as at the BSF side of the solar cell. Further increase of efficiency is to be expected when building a µc-Si(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n+) solar cell with optimized TCO and antireflective/scattering layers/textures.
• Highly crystalline silicon nanodots embedded in a SiO2-matrix were successfully grown onto oxidized c-Si wafers and effective doping of those nanodots has been realized. It has been found that SHJ solar cells with silicon nanodot/silicon oxide emitter exhite rather low efficiencies and these development have been put on hold.
• SiO2:Tb films have demonstrated absorption in the UV range (224 nm) of the solar spectrum and photoluminescence in the visible range (~ 550 nm). First tests of application as a down-convertor in poly-Si thin films solar cells have been reported. Poly-Si thin film solar cells with this type of SiO2:Tb films directly deposited on top of the TCO layer have been processed. Initial tests carried out without any optimisation of the deposited ZnO/NRs stack with respect to ARC show an increased EQE in the UV region, which is the desired effect of down conversion in those cells. However the overall effect is a slight decrease in short circuit current since the stack needs to be further optimized not only for down conversion but also for ACR in future experiments.
• Al2O3 with Ag + Cu nanoparticles after annealing exhibit better properties for plasmonic application to solar cell – more pronounced plasmonic peak of absorption for samples annealed at 5000C, the transparency is higher than 95% and the values of reflection are lower in the wave range 400-1000 nm.
• Multilayer structures such as Al2O3(70nm)/Ag(12nm)/Al2O3(50nm) and ZnO:Al(70nm)/Ag(12nm)/ZnO:Al(30nm) exhibited potential for solar cell application.
• CdSe/CdS Core/Shell nanorods (NRs) possess very high PL quantum yield (~60%) and very large quasi-Stokes shift between absorption and PL band maxima (~100 nm), which made these NRs promising for spectrum down-conversion.
• ESAVD produced Ag nanoparticles in SiO2 have shown to exhibit plasmonic effects. The cell parameters can be optimised further by tuning the content, size and shape of the nanoparticles further in such a way that the system is optimized for the rear side of the cell.
• Nanolaminate structures have been explored and tested as nano-thin transparent back side electrodes with local contacts providing at the same time good passivation for mc-Si or c-Si solar cells. Further optimizations are required to implement such structures for the processing of Si based solar cells.
Assessment application of nanoparticles to solar cells
The main conclusions with respect to the assessment of the application of nanoparticles to solar cells are:
• Plasmonic layers, applied on the front side of solar cells may not be the optimal choice. In the case of Au nanoparticles in TiO2, deposited on Si wafer based cells, the absorption at the localized plasmon resonance decreases the photocurrent of the solar cell. Besides that, the layer reduces the passivation quality of the interface, which can be improved by introduction of an intermediate passivation layer.
• Nanoparticle emitter formation by nano-/microcrystalline emitter (µc-Si/a-Si:H/c-Si solar cell) with an a-Si(i) buffer layer, has led to a Voc of 689 mV, which is a very promising result. On SHJ cells with a phosphorus doped silicon nanocrystal emitter and a-Si(i) buffer layer, a high implied Voc of 700 mV is achieved.
• Spectral conversion layers of CdSe/CdS nanorods show an enhanced response in the blue spectrum, which proves the principle. However, absorption in the longer wavelength spectrum reduces the effect. Directly applied on a different cell sample, the same particles resulted in only 0.5% current loss, which indicates that it is necessary to tune properties of such layer further to achieve current gain.
It is shown that in most cases implementation of NPs for Si solar cell processing requires essential modification of the solar cell processing steps and not always properties of individual nanoparticles and relevant layers can improve properties of Si based solar cells. Nevertheless, nanoPV consortium selected most promising NPs based structures, which potentially can result in an enhancement of Si based solar cell efficiencies.
NANOWIRES AND RELATED SOLAR CELLS
Work on this topic was part of WP2. In this WP, Cost effective methods for fabrication of nanorods and related solar cell structures are developed: All silicon, TCO, and metallic based nanorods on highly conductive substrates (metal, highly doped low-cost Si substrates) with the diameter of NRs below 200 nm and methods for their incorporation into the solar cell structures are developed. Several processes and approaches are used: (i) bottom-up using vapor-liquid-solid, electrodeposition assisted by low-cost template based methods, (ii) top-down alternative route based on electroless metal-assisted etching of low-cost Si thin layers deposited on low-cost conductive substrates, (iii) vacuum and non-vacuum low-cost methods for fabrication of bottom-up and top-down 1D ZnO and TiOx based structures for implementation as ARCs in conventional Si (thin film and wafer based) solar cells, (iv) cost effective bottom-up processes for formation of metallic 1 D based structures, which can be used as substrates for advanced thin film based solar cells. Methods for nanotexturing of Si substrates are developed and evaluated.
This WP has the following objectives:
• To fabricate individual Si and TCO based nanorods with desired optical and electrical properties, composition etc.
• To develop methods for effective passivation of Si nanorods
• To optimize processes for incorporation of nanorods into solar cell structures (route 1)
• To develop cost-effective non-vacuum and vacuum based processes and equipment compatible with industrial requirements for the implementation of Si (route 2) and TCO nanorods (routes 1,2) for solar cells
• To develop optimal process for nanotexturing of wafer and thin film Si substrates to obtain "black, light-trapping silicon" with high electronic quality and technological compatibility with the standard processes of solar cell manufacturing (route 1)
• To develop processes for merging of 0-D and 1-D nanostructures for fabrication of advanced solar cell structures in frame of routes 1 & 2
Silicon nanorods/nanowires synthesis
Silicon nanorods have been prepared in a bottom up approach by VLS growth and in a top down approach by etching. In both cases, axial as well as radial pn-junctions were fabricated to get Si nanowire solar cells (route 2).
For the VLS growth as substrates silicon wafers, laser crystallized multicrystalline silicon thin films as well as TCO layers (aluminum doped zinc oxide) have been used. As a nanotemplate for the VLS growth a 2 nm thick gold films was evaporated on the substrate layer from which nanodroplets were formed by annealing. VLS growth was performed at about 630 °C from silane to result in several µm long nanowires with a diameter in the range of 50 to 100 nm. For doping the nanowires, doping gases were added to silane (diborane B2H6 for boron doping or phosphine PH3 for phosphorus doping). The growth rate was about 300 nm/min. NW solar cells were prepared based on these nanowires, both following the axial and radial heterojunction concepts. In the reporting period, regarding the VLS-grown NWs we mainly worked on the embedding and contacting of axial heterostructure NW solar cells. However, we did not reach a satisfactory level now, especially envisaging the removal of the topmost layer of the capping material to successfully contact the NWs. Combination of axial and radial junction solar cells grown by a combination of thermal CVD and PECVD has been investigated. In this concept, we try to reach a combination of axial and radial junction by first growing a NW by thermal CVD and in a second step growing an intrinsic as well as an oppositely doped top part onto this NW by PECVD. PECVD results in the deposition of a-Si on the NW’s surfaces, so that a radial p-i-n junction will also evolve.
Nanowire solar cells prepared by etching were investigated on wafers and on polycrystalline (pc-Si) silicon on glass substrates. The best solar cell with nanowires, etched in to Si wafer, showed a Voc of 517 mV and an efficiency of 10.0%. For nanowire solar cells on polycrystalline thin films, the pc-Si was prepared from electron beam evaporation (EBE) deposited a-Si followed by CW diode laser crystallization. Etching was done by AgNO3/HF solutions. Based on etched nanowires, radial pn-junction nanowire solar cells were prepared on the pc-Si thin film. It was observed, that the solar cells showed better performance in superstrate configuration than in substrate configuration. The best cell so far on a 8 µm thin pc-Si thin film reached, in superstrate configuration, Voc of 520 mV, jsc of 28.1 mA/cm², fill factor of 68.4%, and efficiency of 10%. With this result, the main aim of this task has been achieved.
Regular silicon nanowire arrays were prepared by nanosphere lithography on silicon wafers and on crystalline silicon thin films on glass substrates as sketched in Fig. 2. The c-Si thin films were prepared by diode laser crystallization of electron beam evaporated amorphous silicon films, deposited on borosilicate glass substrates. For the nanosphere lithography first a monolayer of polystyrene spheres was prepared on the silicon substrate by a Langmuir-Blodgett technique.
Fig. 2: Steps for preparing regular nanowire arrays
To this end, 0.6 and 1.03 µm diameter spheres suspended in water were used (microParticles GmbH). To 100 µl of the suspension 100 µl ethanol and 5 ml hexylamine (5%) were added and sonicated for 30 min. The samples were placed on a pedestal in a glass bowl filled with DI water. A drop of the nanosphere suspension was added. To get a monolayer of nanospheres on the water surface, several drops of sodium dodecyl sulfate solution (10%) were added. The surfactant pushes the nanospheres together. Finally the water level in the dish was lowered by a hose. In this way the nanosphere monolayer on the water surface settles on the substrate. Fig. 3 shows an SEM image of a nanosphere array prepared in this way.
Fig. 3: 1.03 µm diameter nanosphere array on a silicon substrate
In a next step the diameter of the nanospheres is reduced by etching in an oxygen plasma (Fig. 4). The final size of the nanospheres can easily be controlled by the etching time.Onto the samples a 50 nm thick silver layer was deposited by evaporation. The silver covered nano¬spheres were removed by sonicating the sample in a mixture (10:1) of methanediol and acetone. As a result a silver mask remained on the silicon surface (Fig. 5).
Nanowires were prepared by metal assisted etching using a mixture of HF:H2O2 (20:1). Silicon is etched below the silver mask to result in a regular array of nanowires as shown in Fig. 6.
Fig. 4: Nanosphere array after 120 s plasma etching
The regular nanowire arrays on silicon thin films were optically characterized. The results are described in WP4. Moreover, solar cells with radial pn-hetero-junction were prepared in way similar as in case of irregular nanowires. Fig. 7 shows a SEM top and side view of the final solar cells based on regular nanowire arrays.
Fig.5: Silver mask Fig. 6: Regular nanowire array on a silicon thin film on glass (left)
and on a wafer (right)
Fig. 7: SEM top view and cross section of a solar cell based on regular nanowire arrays
Fig. 8 shows the I-V curve of a solar cell prepared on a regular silicon nanowire array. An efficiency of 5% was reached, which is somewhat lower that reached in irregular arrays. Particularly the open circuit voltage is a bit low. However, in case of regular arrays much less optimization efforts were done.
Fig. 8: I-V curve of a solar cell based on a regular silicon nanowire array
Growth of Si nanowires by thermal CVD and PECVD processes has been done to develop f novel structures of silicon nanowire based solar cell. Al, Au and ITO reduced by H plasma treatments were used as catalysts for the preparation of Si nanowires and junctions.
Fig. 9 shows the p-n junction obtained from ITO catalyst on Al substrate and Si substrate. The red arrow in Fig. 9 points to the p-n junction area, which results from different shape of Si nanowires for p-type and n-type. The purple arrow indicates a 3D structure of Si NWs. Similar structures have been observed in Au/Si and ITO/stainless steel.
ITO/Al ITO/p-type Si
Fig. 9: p-n junction and 3D structure obtained using reduced ITO as a catalyst
Electrochemical deposition of ZnO NW as ARC on Si solar cell structure and (Si solar cell structure)/ITO
Development of low-cost processes for the fabrication oxide based nanostructured (ZnO) by electrochemical deposition to be implemented as anti-reflective coatings (ARC) for the fabrication of conventional Si based (thin film and wafer based) solar cells. Experiments of deposition of ZnO NW as ARC were performed on c-Si solar cell structures with ITO ARCs. Two different types of solar cells have been studied - with and without textured surface before the deposition of ITO. ZnO nanostructured layers are deposited by electrochemical method on the front side of the solar cell structures. Optical and structural properties of the deposited structures were studied.
SEM images (with different magnification) of the surface of electrochemically deposited ZnO on the top of the solar cell with textured front side surface are presented in Fig.10. ZnO nanowires with hexagonal shape have been grown on different side of the pyramidal etched surface of the solar cell.
Fig. 10: SEM images (with different magnification) of the surface of electrochemically deposited ZnO on the top (textured surface) of the solar cell (ITO/n+-Si (~0.4µm, emitter)/p-Si – base, ~200µm/p+ Si – BSF) are presented The ZnO was deposited for 20 min.
Fig. 11 shows the spectra of reflectance and diffuse reflection for c-Si solar cell structure)/ITO. Solar cell structures are: ITO/n+-Si (~0.4 µm, emitter)/p-Si – base, ~200 µm/p+ Si – BSF, ~2 µm without metal contacts. The front side of the solar cells is textured. ZnO layers are deposited for different times. The spectra of solar cells structures before deposition of ZnO are given for comparison as well.
Electrochemical deposition of the ZnO results in decreasing of the intensity of the band of reflectance at about 400 nm by about 10% and slightly increase (about 0.2%) of the diffuse reflection in the range 550-100 nm compared to the value of the based structure: ITO/Si solar cells. Applying deposited ZnO on the surface of the solar cell can increase generation of the carriers and the value of the photocurrent in the spectral range 400-480 nm, but a decrease in the visible range occurs. Optimization of deposition process and ZnO NWs structures are still required.
Fig. 11: Spectra of reflectance and diffuse reflectance for structures ITO/n+-Si (~0.4 µm, emitter)/p-Si – base, ~200 µm/p+ Si – BSF (untextured surface) with ZnO deposited for different time.
Results, concerning TCO NWs obtained in naoPV project can be summarized as follows: ZnO nanowires useful as antireflection layers on solar cells were successfully prepared by electrochemical deposition, by spray pyrolysis, by CVD, by low temperature non-vacuum deposition, and by etching.
Nano-texturing of silicon substrates
Modified nano-textures, with life-time of the photo-generated carriers up to 500 µs, have been obtained by applying SC1 post-treatment to the nano-texture. This value is approaching the threshold level, required for high quality back contact solar cells. Further improvement of the electronic quality of c-Si nano-textures has been achieved by a-Si:H passivation.
During the implementation of black nano-texture to actual solar cell structures the nano-texturing process has been continiously improved. Important particular achievements are: (i) a modified procedure for black etching of poly-Si thin film absorbers (for HZB) has been developed. This procedure results in strong reduction of reflectivity, as well as in ultimate light-trapping within the thin absorber due to enhanced, difractive scattering in size-optimised nano-texture; (ii) the standard black etching process has been developed to work also with highly doped Si substrates with donor (acceptor) concentrations up to 1018 cm-3. This allows integration of black nano-textured surfaces in highly doped regions of a large variety of prefabricated solar cell structures in route 1; (iii) a new, buffered, black etching solution has been developed, which works even with more heavy doped Si. Substantial advantage of this solution is that nano-texture with desired morphology and optical properties is obtained without additional post-treatments, which are otherwise necessary with the “standard solution” that was used earlier (Fig. 12). The new buffered solution also do not destroy photoresist protective layers in contrast with the “standard solution”. This expands significantly the technological flexability of the nano-texturing process.
Fig. 12: Comparison of the nano-textures as prepared by a)”standard” black etching and b) “buffered” black etching.
The scalability of the metal-catalized black etching to wafers of industrial scale in batch processing has been demonstrated for all developed versions of the nano-texturing process. Obtained results can be summarised as follows: black etching of silicon was successfully extended to highly doped wafers and to polycrystalline silicon thin films. The material shows very low reflectance and enhanced absorption as expected. On wafers the carrier lifetime could be extended to 0.5 ms and with highly efficient a-Si:H passivation above 1 ms. Thus, excelent optical and electronic quality of the nano-textured sssilicon surfaces have been achieved. These surfaces fulfill all the requirements for successful application in photovoltaic solar cells.
Development of the advanced equipment for the processing of Si based solar cells using nanotechnology approaches
The hardware tests and process verification has been done as specified for the task:
(i) It is tested that Si NW pn-junction is possible to be done in a single chamber when the hardware performance is correct. As an example for the easy identification, the thermal CVD intrinsic layer and PECVD for p layer of Si NWs is prepared. Fig.13 shows that Si NWs grown by pure thermal CVD have the same diameter along the length of nanowires with the length of about 0.9-1.4 µm, and the PECVD grown Si NWs show tapered shape and the length is 1-1.3 µm.
(ii) Development of a single chamber/cluster system process based on deposition of thin metal catalyst layer followed by an in situ anneal and VLS growth of Si NWs. This part of verification was performed in two separate cases:
Case 1: The catalyst layer (Al) has been deposited at different temperature, or annealed after the deposition, as shown in the following SEM images (Fig. 14). It has been demonstrated that when catalyst layer is deposited at room temperature, it needs to be annealed at high temperature for the formation of nanoparticles, whereas the temperature of the table can be lower when the table is heated while the deposition is performed;
Fig. 14: 10 nm Al layer on Si substrate deposited at room temperature (upper left) and after anneal at 600 °C (upper right) showing the formation of Al nanoparticles. 10 nm Al layer deposited at 400 °C (bottom, left).
Case 2: the catalyst can be annealed in the chamber that is used for the growth of Si NW and then Si NWs can be grown. Au nanoparticles can be formed in the process chamber, which then can be used for the growth of Si NWs by CVD or PECVD, as shown in above SEM images.
(iii) Combination of Si NRs/NWs growth and a-Si PECVD
(iv) Si/SiOx multilayers plus annealing for nanodots
IMPT contributed to this task with the unique non-vacuum equipment for the deposition of TCO based nanostructures layers. IMPT hardwares have been optimized on the basis of feedbacks obtained from NanoPV partners (especially HZB and ECN) with the characterized nanomaterials and related solar cell structures processed with this equipment. Process and related equipment developments that underpin reliable process delivery have been performed within the following strategy by IMPT: (i) verification and development of heater and module for the deposition of ZnO seed layer for the grown of ZnO nanorods; (ii) module for the deposition of nanowires based TCO plus low temperature in-situ annealing.
The cost-effective non-vacuum based processes and equipment with the relevant modules compatible with industrial requirements for the implementation of TCO nanorods (routes 1 & 2) for solar cells have been developed by adapting the in-house dedicated Electrostatic Spray Assisted Vapour Deposition (ESAVD) for the deposition of nanomaterials in NanoPV project. Fig. 15 shows the advanced prototype equipment developed for the processing of Si based solar cells using nanotechnology approaches.
Both non-vacuum low-cost methods for the processing of Si-based solar cells using: a) bottom-up; and b) top-down have been established.
The ESAVD tool can be clustered with other tools or modules for continuous or in-line production processes using top down and bottom up approaches.
Fig. 15: The layout of ESAVD equipment for the processing of Si based solar cells using nanotechnology approaches.
Proof of principle of application of nanorods and nanotexturing to solar cells
The overall progress in this activity can be summarized as follows:
Silicon nanowire solar cells were prepared with an efficiency of up to 10% on polycrystalline silicon thin films on glass. Black etching of silicon wafers reached a quality concerning low reflectance and surface passivation that it can be used in solar cells to reach 15% efficiency on n-type Si wafers. With the HIT concept, even an efficiency of 17.2% was reached using black etching. Black etching of poly¬crystalline silicon layers was shown to be promising. The efficiency of HIT solar cells could be improved by ZnO nanostructures on top by up to 0.4% absolute, which was due to reduced reflection and increased short circuit current. TCOs with inserted Ag nanowires, deposited at low temperature, were tested.
Significant results obtained in WP2:
• Silicon nanowires were etched into low cost substrates such as crystalline silicon thin films on glass.
• Silicon nanowire solar cells etched into wafers which are based on a-Si heterojunction reached 10.0% efficiency.
• Silicon nanowire solar cells etched into polycrystalline silicon thin films on glass, with a radial pn-heterojunction, reached 10.0% efficiency (see project objectives).
• Regular silicon nanowire array was prepared by nanosphere lithography and their optical properties were determined. First solar cells on these structures reached an efficiency of 5%.
• TCO nanorods were prepared as antireflection layers by electrochemical deposition and by RIE. By adding these structures to HIT solar cells, their efficiency could be improved by 0.4%.
• On black etched wafers the carrier lifetime could be extended to 0.5 ms and with highly efficient a-Si:H passivation above 1 ms. On n-type wafers an efficiency of 15% was reached, with HIT concept even 17.2%.
• Metallic nanorods were prepared by electrochemical deposition into AAO porous membranes
ULTRATHIN NANOLAYERS AND RELATED SOLAR CELL STRUCTURES
Work on this topic was part of WP3. In this WP, cost effective vacuum and non-vacuum based methods for fabrication of ultrathin nanolayers and related solar cell structures have been developed. The developed processes have been used as a background for innovative technologies based on an implementation of nanolayers in PV. Manufacturing procedures and optimized equipment for the processing of nanolayers compatible with industrial requirements have been developed in WP3. Several alternative methods have been developed for the cost-effective production of such nanolayers: (i) electro-deposition, (ii) ECR-PECVD, (iii) low-temperature oxidation, magnetron sputtering, (iv) ESAVD. The results have been evaluated based on comprehensive innovative and elaborated analytical techniques.
Si nanodots in SiO2 and Si3N4 matrixes as nano emitters
Si-nanostructures have been fabricated by decomposition of amorphous SiOx layers. The layers have been optimized with respect to nanodot formation for different O/Si-ratios between 0 and 2 have and annealing temperatures of up to 1000°C. In agreement with previous studies the following details with respect to nanodot formation and properties could be confirmed: For completing the decomposition temperatures of around 900°C are needed. The size and density of the nanodots decreases with decreasing O-content and nanodot diameters between 5 and 10 nm can be reached. For higher O-content a blue-shifted PL signal can be observed, indicating quantum size effects within the sample. Furthermore an effective doping of the nanodot layers could be obtained as evidenced by SPV and current-voltage measurements. Both for n-type and p-type doped nanodot layers on p-type/n-type substrates a band bending of up to 700mV could be proven by SPV. This opens the possibility for applying those layers as an emitter in solar cells. For the first time such a nanodot layers have been used as an emitter for a solar cell based on a poly-Si thin film absorber. Open circuit voltages of ~400mV could be reached which are comparable to poly-Si thinfilm solar cells with conventional emitter. Possible advantages of such an emitter include a high temperature stability which is needed during poly-Si thin film formation and a high optical transparency. For application in wafer based solar cells a higher interface passivation would be needed and thus a temperature stable passivation layer. Possibly those layers could be combined with an ultrathin, tunneling, passivation plasma oxide layer which have already been tested as passivation layer which can survive subsequent high temperature steps.
Several altternative nano-emiters have been developed in frame of WP3: (i) SiOx and Si NDs in SiNx and SiCx structures nano-emitters and processed by magnetron sputtering; (ii) Si nanodots on Si surface or in SiO2 and Si3N4 matrices by e-beam evaporation. In all cases, further optimization of nano-emitters is required.
SiO2 layers for tunneling and passivation
Several technologies for processing of ultrathin SiO2 layers have been developed: (i) plasma oxidation; (ii) Wet-chemical oxide as ultrathin tunneling, passivation layer in SHJ solar cells; (iii) sol-gel deposition of thin SiO2 films as passivating layers; (iv) ultra-thin SiO2 passivation layers by ALD; (v) ultrathin SiOx layer formed at the interface of ITO/Si. All mentioned above SiO2 layers have been tested for the processing of solar cell structures and can be considered as promising options for the cost effective Si based PV.
Passivation by alternative oxides
TCO based layers deposited by ALD based methods: for the silicon nanowire solar cells according to the radial pn-junction concept as sketched in Fig. 16, a TCO layer is required as a contact to the a-Si shell. In order that the TCO layer fills the space between the nanowires, deposition has to be conformal, even in the deep and narrow grooves between the nanowires. To achieve this, atomic layer deposition is perfectly suited, whereas the more conventional sputtering technique is not useful. For the concept of n-type nanowires and p-type a-Si:H emitter as a shell, a good contact between TCO and p-type silicon is needed. Aluminum doped zinc oxide (AZO) is well suited for that. For ALD deposition of ZnO, consecutive gas pulses of diethyl zinc (DEZ) and water are introduced into the device, separated by neutral gas flushing. For doping, every 15th to 30th (DEZ) pulse is replaced by a trimethyl aluminum (TMA) pulse. Optimization was done with respect to the Al:Zn ratio and the deposition temperature.
Nanolayers with nanoparticles
Several types of nanolayers with nanoparticle have been processed and tested: (i) nanolayers with nanoparticles fabricated by chemical liquid deposition; (ii) Ge, Au nanoparticles by means of e-beam deposition on top of oxide layers (SiO2, HfO2); (iii) ITO layers with In NPs. N-type epi-Si(30 µm)/n+ Si substrate structure has been used to process HJ solar cells. Ultrathin Al2O3 passivation tunnel layer between AZO and a-Si:H emitter have been employed. Nanotexturing of the AZO layer through etch back has been done. Jsc of 30.3 mA/cm2 determined by small cell cut from large piece with edge passivation. The following results have been obtained: Results: Improved VOC: 618 mV, JSC: 30.3 mA/cm2 FFP: 81.1%, ηP: 15.2%
Proof of principle of application of nanolayers to solar cells
Solar cells involving nanolayers with nanoparticles. Within WP3 several concepts for solar cells with nanolayers containing nanoparticles have been investigated. All concepts are based on silicon heterojunction solar cells and on the application of nanoparticles inside the stack of a-Si:H nanolayers that form the emitter (Fig. 17).
Fig. 17 Heterojunction solar cells with Si NPs.
In summary a slight increase of conversion efficiency of silicon heterojunction solar cells could be obtained by introducing nanoparticles to ultrathin emitter layers, albeit that the conversion efficiency of the reference solar cells where not on a top level (=15.2%). The efficiency increase up to =16.4%, obtained for solar cells involving In-NPs processed from ultrathin ITO layers and sandwiched in between the ultrathin a-Si:H emitter and the ITO front window layer, stems from a slight increase in Voc and an increase in short circuit current. While the Voc could be increased due to the additional H-plasma treatment which was performed to create the In-NPs, the increase in current must be attributed to the In-NPs themselves, which cause increased internal quantum efficiency in the short wavelength range. Furthermore, silicon heterojunction solar cells with Si-NCs embedded in ultrathin a-Si:H emitter layers have been developed and conversion efficiencies of =15.2%, on the same level as the reference solar cell, have been obtained. Such high efficiencies can only be reached if an ultrathin intrinsic a-Si:H buffer layer is present between the absorber and the Si-NCs to passivate the interface defects and suppress recombination such that high open circuit voltages >700 mV can be reached. The short circuit current of such solar cells are slightly increased due to slightly reduced reflection on planar wafers and slightly increased internal quantum efficiency on textured wafers. The high internal quantum efficiency indicates that the light absorbed in the Si-NCs generates charge carriers that can be extracted from the solar cells. This opens up new perspectives for applying those nanoparticles embedded in ultrathin emitter layers. At the current stage of development the solar cells with Si-NCs suffer from a low fill factor, due to a high serial resistance which is presumably due to an oxide shell surrounding the Si-NCs.
Finally it has to be noted that the overall low level of conversion efficiency can largely be attributed to sample contamination during transport between laboratories and sample shipment. Reference solar cells processed at HZB that undergo the routine process flow reach conversion efficiencies of up to 20%. For the final solar run with solar cells involving In-NPs special attention has been paid to fast sample processing and special protection of solar cells during shipment and solar cell efficiencies of 18.4 % have been reached (see description of WP 5 activity). Several other solar cell structures with ultra-thin oxides have been tested: (i) ALD-Al2O3 integration in n-Pasha solar cells; (ii) (Al2O3)x(B2O3)1-x passivation layers for rear side of c-Si solar cells; (iii) Solar cells involving a nanodot emitter (Si NPs in SiOx). In all cases rather promising results have been obtained, but high efficiencies for solar cells were not reached, which indicated that further technological optizations are required.
Significant results obtained in frame of WP3 can be summarised as follows:
• Indium based NPs, obtained by the reduction process of ITO initiated by H+ plasma have been fabricated and it has been concluded that they could be combined with a-Si:H emitters to form high conductive channels at ITO/a-Si:H interface. Solar cells based on this concept have been processed and conversion efficiencies are higher than the reference and reach 18.4 %.
• Heterojunction solar cells with a-Si:H emitters incorporating Si-NCs have been processed by HZB and conversion efficiencies of 15.2 % have been reached, which is on the same level as the reference solar cell. The high internal quantum efficiency indicates that the light absorbed in the Si-NCs generates charge carriers that can be extracted from the solar cells.
• Ultrathin a-Si:H layers for the passivation of nanotextured silicon surfaces have been developed. It was shown that it is possible to passivate nanotextured silicon surfaces of different pre-etching crystallographic orientations with a-Si:H and to reach very high effective charge carrier lifetimes of 1.3 ms. Heterojunction solar cells (wafer and thin film) with ultrathin layers for the passivation of nanostructures have been processed by and conversion efficiencies of 17.2 % have been reached (WP5).
• AL2O3 passivation layers have been optimized and applied for passivation of black-Si textures in IBC solar cells and for the passivation of the backside in in n-Pasha solar cells by ECN in WP5. Furthermore Al2O3: B2O3 layers (CLS) have been optimized and applied at the rear side of c-Si solar cells been by ECN in WP5.
• Different methods to fabricate ultrathin SiO2 layers have been investigated. Based on the wet-chemical deposition approach ultrathin SiO2 (0.8nm) passivation, tunneling layers have been developed. It has been shown, using cell precursor structures as well as finalized solar cells, that high effective lifetimes of 1ms can be reached and that it can be beneficial to substitute the (i)a-Si:H buffer layer in a solar cell by a SiOx tunnel layer at the front side of the cell. An unexpected additional benefit of this layer is the improved band bending and thus current collection efficiency and fill factor. This benefit is only present if the (i)a-Si:H layer is substituted in the p/n junction on (n)c-Si substrates. An overall improvement of the cell efficiency by ~0.3% abs. could be shown, albeit on a generally very low level of cell performance.
• A nanolayer with nanodots based on Si-NDs in SiO2 has been developed and high transmittance and effective doping as well as electrical conductivity have been realized. Therefore this layer could be applied as an emitter in thin film poly-Si solar cells. Although important progress has been made and fundamental scientific questions have been addressed this approach seems not suitable for application to solar cells with increased efficiency within the short time period left in the project.
• It has been shown that Si and/or Ge nanoparticles embedded in Si oxide, nitride and carbide matrix in the form of superstructure can be fabricated by magnetron sputtering. It is also shown that a very good control of the average diameter can be obtained by preselecting the desired thickness of the corresponding layer. Although important progress has been made and fundamental scientific questions have been addressed this approach seems not suitable for application to solar cells with increased efficiency within the short time period left in the project.
• SiO2/SiO multilayer were deposited by e-beam and subsequently annealed at high temperature in order to form Si nanocrystals embedded in a thin SiO2 matrix. The resulting nanostructures were characterized using electro-optical techniques in order to text the electrical response of the system to monochromatic light.
• Atomic Layer Deposition (ALD) Al2O3 layers were developed with excellent passivating properties when processed at low temperatures. The integration of the ALD-Al2O3 films in silicon based solar cells revealed that, under higher temperature processing, their performance deteriorates. Extensive investigation of the chemical-structural evolution of the Al2O3 film was performed to identify the origin of lifetime deterioration upon high temperature annealing.
• SiO2 layers were grown on Si substrates by ALD at low temperature and their passivating properties were investigated. Lifetime measurements revealed these films provide a limited passivation effect. The results were supported by electrical characterization of the resulting SiO2/Si interface in terms of density of fixed charges and of interface states.
• Comprehensive morphological and structural characterization of Au deposition on SiO2 and HfO2 was performed and the final characteristics of the Au nanoparticle population were investigated as a function of the deposition conditions and subsequent thermal treatment.
• ~15% conversion efficiency by implementation of ultra-thin Al2O3 passivation tunnel layer between AZO and a-Si:H emitter for n-type epi-Si(30 µm)/n+ Si substrate solar cell structure has been achieved.
ELECTRICAL, OPTICAL AND STRUCTURAL PROPERTIES OF INDIVIDUAL NANOMATERIALS AND RELATED SOLAR CELL STRUCTURES
Work on this topic was part of WP4. In this WP, analysis of composition, electrical, optical and structural properties of the nanostructures, layers and solar cell structures supported by ab initio simulations in some cases have been done.
Analysis and characterization of composition and electrical properties of individual nanolayers and related solar cell structures
Characterization of Si/TiO2:AuNPs, Si/SiO2 and Al2O3 interfaces was implemented by HF-CV experiments. Formation of defects at the interface between Si and different crystalline SiO2 polymorphs upon thermal treatment was explored in the force field approach. For quantitative assessment of an influence of nanostructured layers deposited on top of silicon wafer, the photoconductivity method is developed and used to evaluate the EQE and IQE characteristics of Si substrates covered by these nanostructured layers. SPV method was used to determine band bending of various nanolayers and nanolayers with nanoparticles. Voltage-dependent SPV method was used to determine interface defect densities of SiO2 passivation layers. The electrical properties of chemically deposited thin films of (Al2O3)x(B2O3)1-x with dispersed Si nanoparticles (20 nm) have been studied. The transverse conductivity and the sheet resistance have been evaluated, when different types of substrates are used.
Characterisation techniques used for developing an amorphous silicon emitter on nanotextured black silicon surfaces
The development of a silicon heterojunction solar cell with a hydrogenated amorphous silicon emitter layer on top of a nanotextured “black” silicon surface poses high challenges and thus during optimization different characterisation techniques, such as scanning electron microscopy (SEM), field-dependent surface photovoltage (SPV), lateral conductivity measurements, spectroscopy and quantum efficiency (QE) measurements in the UV and visible range as well as simulation based analysis of QE measurements. Simulations have been performed with using the AFORS-HET solar cell simulator and a simple optical model.
Optical analysis and characterization of individual nanomaterials and related solar cell structures
PL method was applied to investigate quantum size effects and to determine the photogenerated minority charge carrier lifetime in Si nanodots.
Optical transmission and reflectance of various nanomaterials such as ITO nanorods, plasmonic particles were measured.
The optical characteristics (transmittance and reflectance) have been studied for nanolaminate structures with TiO2 depending on film thickness of the individual layers and of the density of the metal granules. The ratio between the transmittance and reflectance is analyzed in order to detect the absorbance. The aim is to find out optimal conditions for decreasing the reflectance in the infrared spectral region.
The optical properties of the chemically deposited (Al2O3)x(B2O3)1-x with dispersed Si nanoparticles (20 nm) films are studied as a function of the thermal treatments.
Structural and chemical characterization of nanomaterials and interfaces
NPs and NWs containing structures produced by sevreal nanoPV partners have been characterized by SEM, AFM, XPS, Raman, FTIR. In some cases, ab-initio simulations for advanced electronic structures using DFT approach have been done. Fig. 18 shows that thin Si NW (labeled as "mini", 1.2 nm in diameter) privide formation of a direct band gap structure. The cross-over for a transition between in-direct and direct Si Nws structures is in the range of ~5 nm ("full") for the NW diameter.
Characterization and testing of solar cells containing nanomaterials
A special workstation is designed for measuring I-V characteristics for small solar cells with micromanipulator for small contact areas. The illumination intensity is calibrated of 100mW/cm2. The measuring conditions are programmed and the results are recorded in digital form. I-V characteristics are measured sequentially in the dark and lighting. Measurement of I-V characteristics of HIT and SHJ solar cells before and after deposition of ZnO NWs by electrochemical method and of the structure n-type c-Si/p+ NP Si (deposited by magnetron sputtering and annealing); were made. IV, Suns-Voc, and EQE characterization of nanowire-based solar cells was performed. This applies to nanowires prepared on silicon wafers and on polycrystalline silicon thin films deposited on glass, with axial as well as with radial pn-junction. Spatially resolved EQE measurements were performed by light beam induced current (LBIC) to give local information on nanowire based solar cells prepared on polycrystalline silicon deposited on glass.
Significant results of WP4 activities can be summarised as follows:
• It has been shown (PL) that a band gap narrowing is possible by adjusting the nanodot size and thus the absorbance can in principle be adjusted by band gap engineering. However an efficiency increase of solar cells by exploiting this effect is still far from realization, in particular because of low passivation quality of Si nanodot surfaces.
• Optical experiments show that ZnO NW layers can be deposited on the surface of different solar cells - on poly-Si wafers, HIT structure by electrochemical method. The films demonstrate antireflection properties. The optimisation of the conditions of growing has to be optimised to avoid creation of damages around the grid contacts of HIT solar cells. ZnO NW films are deposited on the surface of steak structures glass/Ag/ZnO:Al/Ag/ZnO:Al with plasmonic properties. These structures can be applied as a back reflector with higher diffused reflection properties and increased effective area for this films solar cells deposition.
• Interface between silicon and its oxide has been investigated theoretically using molecular dynamics based on relax force field. It is shown shown that low efficiency of Si nanocrystal solar cells may be a result of the difference in the structural relaxation in Si and SiO2 prior and during annealing that leads to stress formation in Si lattice due to presence of the interface.
• Spatially resolved EQE measurements were performed by light beam induced current (LBIC) to give local information on nanowire based solar cells prepared on polycrystalline silicon deposited on glass. It has been demonstrated, that the EQE of these cells is rather homogeneous varying by about 10% on the cell area that is acceptable for further scaling the method of nanowire based solar cells preparation.
• It is shown that at the ITO/Si interface in Si based heterojunction solar cell structures, formation of Indium based local conductive channels as well as thin SiOx layer occurs.
DESIGN, PROCESSING, CHARACTERIZATION AND TESTING OF SOLAR CELLS AND MODULES
Work on this topic was part of WP5. This WP has the following objectives:
• Selection of 3-5 most promising nano-materials (particles and layers) for proof of concept demonstration on wafer-based silicon solar cells and/or modules (all partners); This selection will be made from the broad range of different activities, i.e. advanced nanotechnology based emitters, BSF & local contacts, advanced passivation and antireflection layers, implementation of MEG as well as up- and down conversion effects.
• Selection of 3-5 most promising nano-materials (particles and layers) for proof of concept demonstration on thin-film silicon solar cells and/or modules (all partners). This selection will be made from the same range of activities as indicated above for wafer-based silicon solar cells.
• Selection of 2-3 most promising NRs/NWs based technologies for advanced cell structures.
• Development of cost effective application of nanomaterials to solar cells and modules. The proof of concept with respect to industrial processes and efficiency gain will be demonstrated in this WP (proof of concept means: proof of the technology on tens of cells).
Based on the results of the material investigations, 9 materials were implemented in wafer-based Si cells, and 11 materials were implemented in thin-film Si cells. Based on these results the most promising concepts have been selected for further optimization, leading to efficiency enhancement in 5 cases within the wafer-based Si concepts, and 3 cases within the thin-film Si concepts.
Solar cell results (selected cases)
The results for the implementation of nanomaterials in solar cell structures, leading to efficiency enhancement (selected cases), are shown below.
ZnO nanowires for SHJ solar cells
The solar cells have a structures: Ag grid/ITO/(p)a-Si:H/(i)a-Si:H/(n)c-Si/(i)a-Si:H/(n+)a-Si:H/Al (see Fig. 19a). The current-voltage characteristics of the samples before and after deposition of ZnO NS array films on the top side of the SHJ solar cells were evaluated. It is seen that after implementation of the ZnO NS array films, a few percent relative increase in short circuit current, Jsc, and efficiency, η (up to 2.7% relative), is achieved in some of the samples, as shown in Fig. 19b.
Fig. 19 Implementation of TCO nanorods in SHJ solar cells: a) schematic drawing of TCO nanorods integration (note that in final design, the ZnO front contact was replaced by ITO, b) I-V parameters of SHJ cells before and after ZnO nanorods application.
Black silicon nanotexture for bifacial solar cells
Bifacial solar cells on n-type material were prepared with standard random pyramid texture for the reference cell, and with black silicon nanotexture on the front side to enhance the performance, as described in Table 1. The cell results are summarized in Table 2.
Table 1 Description of sample configuration with black silicon etching (2.5x2.5 cm2)
Description Sample Comments
redistributed diffused profiles, annealing in N2, 1000°C, 1h 30' reference Normal pyramid textured front p-side & Normal pyramid textured rear n-side, reference cell
p+nn+/ p-side black,
redistributed diffused profiles Black Si Black front p-side & Normal pyramid textured rear n-side
Table 2 Solar cell results with black Si nanotexture on the front, compared to reference
reference Black Si
pseudo-FF [%] 67.9 75.6
Jsc [mA/cm2] 30.7 33.0
Voc [mV] 572 576
FF [%] 65.3 68.6
Efficiency [%] 11.5 13.0
SiO2 tunnelling passivating layer
In-NPs for TCO/a-Si:H/Si
In silicon heterojunction solar cells, emitters with In-NP integrated are compared to reference cells (Fig. 21).For both planar and textured cells, the integration of In-NP at the front leads to an increase of efficiency, due to increase in both Jsc and Voc.
a) b) c)
Figure 21: Silicon heterojunction solar cell with In-NP at the emitter/contact interface: a) schematic cross-section, b) efficiency plots of run 1, c) IV curves and IV parameters after optimization of the H-plasma treatment
For this approach In-NPs were processed by a H-plasma induced reduction of an ultrathin ITO layer with a thickness of 5 to 10 nm at OIPT. As substrate n-type wafers with a planar (100) surface and with an alkaline etched, random pyramid texture have been used. A conventional a-Si:H emitter stack based on ultrathin layers and additionally a 5 to 10 nm thin ITO layer has been deposited. Afterwards, plasma processing was done at 100°C for 2 min. To finalize the solar cell an a-Si:H back surface field layer stack as well as ITO layers at front and back side to optimize optics and contacting, have been deposited. An improved efficiency as compared to the reference of 0.6% absolute in efficiency can be observed for the solar cell with In-NPs processed at 100°C. Maximum efficiencies of 18.4% have been reached and thus we can recommend this approach for improved silicon heterojunction solar cell design.
a-Si:H emitter nanolayer for SHJ cells
In silicon heterojunction solar cells, the a-Si:H emitter was optimized for enhanced performance. Background knowledge from ECN was brought in to present the optimization of deposition parameters and thickness of the emitter nanolayer. The implementation of this optimized nanolayer led to an efficiency increase of 0.6% absolute, at the level of 19.8% (20.2% best) cell efficiency. In Table 3 the cell parameters are listed for the reference emitter (1), after deposition parameter optimization (2) and thickness optimization (3).
Table 3 Median (best) I-V parameters of the SHJ cells of 156x156 mm2 size with emitter nanolayer.
Emitter 1 Emitter 2 Emitter 3
Jsc [mAcm-2] 35.8 36.0 35.9
Voc [V] 0.721 0.724 0.726
FF [-] 0.744 0.757 0.760
efficiency 19.2% (19.5%) 19.7% (19.9%) 19.8% (20.2%)
Ag NP plasmonic nanostructures for thin-film Si tandem cell
Plasmonic silver nanoparticles (Ag NP) have been introduced at the back side of thin film solar cells. To implement the Ag NP layer on larger area and for solar cells processed with industrial equipment, a-Si:H/µc-Si thin film tandem solar cells have been prepared, as sketched in Fig. 22.
a) b) (c)
Fig. 22 a) a-Si:H/ µc-Si tandem solar cell, a-Si:H/µc-Si absorber on glass b) schematic layout of the cells on a wafer, c) after deposition of Ag nanostructures and AZO deposition, and after contact structuring.
Table 4 Photovoltaic parameters of the best thin film Si solar cells with plasmonic layer, as compared with the reference (without back reflector).
sample Jsc Voc FF eta
Ag NP by ESAVD 13.81 937 75.5 9.8%
Reference 13.98 917 74.0 9.5%
The presence of Ag nanoparticulate (NPs) layer by ESAVD has increased the thin film Si solar cell efficiency by 0.3% absolute to 9.8% as compared with the reference solar cell without the Ag nanoparticulate layer (9.5%), as listed in Table 4.
Black nanotexture for thin-film FrontERA cell
Cells have been prepared on 5x5 cm2 substrates using the advanced FrontERA contacting scheme (see Fig. 23). Because shallow texture is of advantage in terms of passivation while a deep texture is of advantage in terms of anti-reflection properties, samples with two different b-Si etching times (2 min and 3 min) have been prepared. From the results, presented in Fig. 24, it can be concluded that black Si is well suited as a texture for thin film solar cells. The average efficiency increase is >5% relative for the nanotexture compared to the reference.
Fig. 23 FrontERA contact system for contacting poly-Si thin film solar cells with b-Si texture
Fig. 24 Solar cell parameters (a) and external QE (b) for 10x6 mm2 solar cells on a 5x5 cm2 substrate with a poly-Si thin film absorber and a black-Si texture with different texture depth. In (a) the black dots indicate the average value and the pink dots the solar cell with the maximum efficiency. In (b) the external QE measurement two cells for each MCE time are shown to provide prove of the reproducibility.
ZnO nanowires for thin-film nip cells
ZnO nanowires were integrated at the rear side of the cells, for rear side scattering purposes. For ensuring good contact to the cell, it was necessary to deposit the back contact on top of the ZnO nanowires (Fig. 30, top). This leads to an efficiency gain of 0.7% absolute compared to the reference cell, as shown in Fig. 25 (bottom).
Significant results of WP5 activities can be summarised as follows:
Wafer-based silicon solar cells: (i) efficiency gains for ZnO nanowires, black silicon nanotexture, SiO2 tunneling passivating layer, In-NP contacting layer (18.4%) and a-Si:H emitter layer (20.2%); (ii) highlights for Al2O3 (Voc gain); successful integration of black silicon in SHJ (17.2%) and Al2O3/B2O3 in p-Pasha (17.0%).
Thin-film silicon solar cells: (i)efficiency gains for Ag NP plasmonic nanostructures, black silicon nanotexture and ZnO nanowires; (ii) highlights for Ag/AO/Ag/AZO plasmonics (high Isc gain), nanotextured Al (7.7%), epi-Si cells with a-Si:H emitter (12.6%), epi-Si cells with Al2O3+ a-Si:H emitter (15.2%), epi-Si cells with ITO (14.0%).
Nanomaterial (Si NWs) based solar cells: (i) Si NWs based solar cells with radial a-Si hetero-emitter etched into wafer: 10.0% efficiency; (ii) fabrication of Si NW solar cells on larger (50x50 mm2) substrates.
Summary of efficiency gains obtained by implementation of nanomaterials
Si wafer-based cells: (i) ZnO nanowires for antireflection coating: 2.7% relative; (ii) Black silicon nanotexture: 1.5% absolute; (iii) SiO2 tunneling passivating layer: 0.5% absolute; (iv) In-NPs contacting layer: 0.6% absolute; (v) a-Si:H emitter layer: 0.6% absolute (enhanced to >20% efficiency).
Thin film silicon cells: (i) Ag NP plasmonic nanostructures: 0.3% absolute; (ii) Black silicon nanotexture: >5% relative; (iii) ZnO nanowires for antireflection coating: 0.7% absolute.
ASSESSMENT OF TRASFERABILITY TO INDUSTRIAL PRODUCTION
Work on this topic was part of WP6. The main goal of this work package is to prove the viability of some selected new technologies developed by the nanoPV Consortium for the processing of Si based solar cells on industrial equipment (lead by ECN). The selected process chains for cost-effective processing of Si based solar cells and related solar cell modules have been tested regarding a feasibility to transfer such chains to production equipment. Based on the results from the fabrication of solar cells and modules on laboratory scale (WP 5) the advantage of the proposed advanced technologies has been evaluated and demonstrated for some selected cases. This WP has the following objective: Transfer of the selected solar cell and solar cell modules processing chain to production equipment.
Transferability to industrial equipment
Concepts for processing equipment chains have been defined for three cell concepts with integrated nanomaterials that were assessed for transferability to industrial equipment. The n-Pasha concept with Al2O3 passivating nanolayer at the rear and the silicon heterojunction cell with a-Si:H (emitter) nanolayers can be completely processed on industrial equipment. The p-Pasha concept with Al2O3/B2O3 passivating nanolayer at the rear can be largely processed on industrial equipment; the transfer to industrial equipment of the Al2O3/B2O3 application is not proposed here, but not seen as a large risk for the transfer of the process to industrial equipment. Processing equipment on industrial level for manufacturing of completely nano-based solar cells (route 2) is proposed.
Wafer-based silicon solar cells have been manufactured on industrial equipment, achieving high efficiencies of over 20%. Three solar cell types have been manufactured in several cell runs. These three concepts are:
• n-Pasha solar cell with thin Al2O3 passivating nanolayer: in this project, the efficiency was on same level as the reference without Al2O3 nanolayer; potential improvement was identified.
• Silicon heterojunction solar cell with a-Si:H nanolayers: 20.2% efficiency achieved; efficiency increase of 0.6% absolute by optimized emitter nanolayer.
• p-Pasha solar cell with an Al2O3/B2O3 passivating nanolayer: successful integration of Al2O3/B2O3 passivating layer; more research is needed to implement this material successfully.
The impact of nanoPV conforms to the very ambitious targets that have been adopted for renewable energy in Europe, for example:
• In March 2007, the Heads of States and Governments of the 27 EU Member States adopted a binding target of 20% renewable energy from final energy consumption by 2020 .
• In September 2008, the European Photovoltaic Industry Association declared that photovoltaic energy could provide 12% of European electricity by 2020 .
Although reliable PV systems are already commercially available and widely deployed, the cost of PV generated electricity is still too high to compete with electricity from non-renewable sources. Therefore, further development of PV technology with the aim to drastically reduce the turn-key system prices and cost/m2 cell area is crucial. Indeed, it was emphasised already in 2005 by the Photovoltaic Technology Research Advisory Council (PV TRAC) that such a development is possible.
Innovative scientific/technical research going well beyond the state of the art
Completely new solar cell concepts have been generated in frame of nanoPV project by combining experience in nano-science and solar cell technology. The main idea is to modify and adapt the nanomaterials and nanostructures to solar cells in order to increase the performance and reduce the cost. The knowledge generated by this fundamental integration of sciences is well beyond the state-of-the-art.
NanoPV has combined research experience in a number of different scientific fields. The partners have been chosen so that a range of competences has been available: nanostructures such as particles, rods, whiskers and films, cost effective preparation and deposition, solar cell science, as well as analysis and testing.
Improved efficiency and more favourable cost/m2 up-scalable solutions
For conventional silicon PV technology experience has shown that the cost is reduced by 20% each time there is a doubling of installed capacity. As a consequence of this rule, a huge and heavily subsidised capacity would have to be installed if the required cost reduction should be realised by the economy of scale alone. Therefore a step-change in the PV performance is required.
NanoPV has realised the required step-change in PV performance by implementing innovative research in nano-science. The project results have demonstrated the feasibility to achieve solar cells with >20% efficiency for Si wafer based, ~15% efficiency for epi-Si based and > 10% efficiency for Si NWs based solar cell structures by implementation of nanomaterials .
The project has been using competence of leading research groups and industry in Europe working along the complete value chain. In this way the full potential of the developed technologies can be efficiently scaled up to commercial production after the project. A full process flow for cost effective solar cell processing on the basis of nanotechnologies has been made for the first time in Europe.
Development of new knowledge with a high prospect for potential applications exceeding current price/performance ratios
nanoPV has developed nanomaterials and nanostructures that can substantially enhance the efficiency of conventional solar cells by utilising physical effects related to dimensions comparable with the wavelength of the solar spectrum. Building on this experience completely new solar cell structures have been developed. These structures are based entirely on nanomaterials and have about 10% efficiency.
The new solar cell concepts are based on low cost processing methods and using of inexpensive materials. For example, by using wet chemical methods the need for expensive clean room conditions will be reduced. The project has been studied both, vacuum-based and low-cost non-vacuum techniques for deposition of layers. The target of less than 1 €/W has been reached by choosing cost-effective conventional industrial equipment.
Contribution to substantial innovations in the European industry and industrial products
The partners of nanoPV hold strong positions in various aspects of nanosciences and nanotechnologies, such as nanosized particles, rods, whiskers and films. These sciences and technologies are of generic in nature and have wide application areas in ‘nano-enabled’ products, components and devices. A range of such applications has been already addressed by the partners in collaboration with industrial companies. Examples of ‘nano-enabled’ products that are currently under development by the partners are; coatings (for reduced wear, icing and fouling), paints (for reduced VOC content), photocatalysts (for water purification), modification of polymers (for improved mechanical properties) and separation membranes (for improved flux and selectivity).
The nanoPV partners will promote industrial innovations across Europe by actively pursuing new commercial applications of the knowledge gained in the project. In this way, the project results will lead to substantial industrial innovations. Both the existing network of industries and new companies will be addressed. More specific impacts for the industries involved are described below.
Impact towards principal objectives of the NMP-theme
A key element of Theme 4 (NMP) is to improve the competitiveness of European industry and to generate knowledge to ensure its transformation from a resource-intensive to a knowledge-intensive base, by creating step changes through research and implementing decisive knowledge for new applications at the crossroads between different technologies and disciplines. The Nanosciences and Nanotechnologies activity will provide support to research and innovation in industries active in the energy market. nanoPV has contributed to these objectives as it was aimed to develop complete innovative solar cell processing chains based on a merger of nano-science and photovoltaics technology. The SME and industry partners in nanoPV has got an opportunity to transfer the new knowledge into their own products, as they see a market potential both as solar cells, technology and equipment providers.
Why a European approach rather than national
It is important to emphasize that the vision and ultimate goal of nanoPV is to develop innovative solar cells and process chains for cost effective manufacturing of nanotechnology based solar cells and modules. In order to achieve this, low-cost but at the same time enough high quality materials, tailored to the application, and efficient fabrication methods are needed, which have been the core issues of nanoPV. It has to be noted that any PV related activity is a multidisciplinary one, since any development in this field requires involvement of scientists from different areas: materials science, technology, simulation and design, equipment fabrication etc.
The proposed consortium was the best possible solution to address the above mentioned needs as the different partners complement each other perfectly, covering all the required scientific fields and the whole value-chain ideally. None of the individual partners would be able to succeed in developing the proposed technology alone and in sufficient time. We are now observing that the interest in PV is growing worldwide, since developments in this field provide society with the sustainable clean energy already today and especially in the near future. In order for Europe to be competitive in this area, it is necessary to join forces, competence and facilities.
Building upon achievements from previous and current research activities (Table 2.3-1) (academic, FP6, FP7 and others), nanoPV ensured a high chance of payback as enhanced competitiveness of European PV industry. In order to create sustainable impacts the consortium is also engaged in an assertive strategy building external partnerships to gain market contacts, improve competitiveness through external research know-how, broaden exploitation potential of our industrial partners, and create greater value for the European marketplace. This process is being shaped by the skills, capabilities and dedication of all the nanoPV consortium members. None of the individual partners or participating countries would be able to succeed in the development of the proposed technologies.
Impact for the industrial partners and the SME
Impacts for the production equipment providers:
OIPT makes thin film processing equipment, specialising in the market of early adopters of new technology. It supplies more than 100 processing tools per year, and could reasonably expect a successful project to impact this by at least 10% within 2 years, with the potential for significant growth if the technique enters manufacturing production at multiple PV suppliers. The whole market for PV production tools is on the order of 150 M€ in Europe alone, and has lifted some much smaller companies from relative obscurity. OIPT is already well known, and could expect to be credible as an equipment supplier for this technology.
The sweep oscillation wafer motion feature was proven to be of great benefit for thin film deposition during nanoPV. Since this work was performed this feature has been incorporated into four tools sold into production environments. Both the sweep oscillation and the in-situ heating are now offered as additional features on OIPT sputtering system 400.
The IMPT is a company specialising in development of non-vacuum deposition equipment for nanomaterials and nanostructures. For this company, the successful completion of NanoPV means that a whole new business area will be open for exploitation. The company is therefore expecting to benefit from and contribute significantly to the commercialisation of the project’s results based on implementation of non-vacuum electrostatic spray based process for fabrication of Si solar cells.
After a successful implementation of NanoPV with promising results, it is envisaged that this form the basis for the development of non vacuum innovative tooling suitable for pilot scale production based on innovative process chain. This has opened up market opportunity for SME like IMPT to exploit ESAVD and non vacuum chemical based processes of nanomaterials, for examples, Ag nanoparticles based plasmonic films, Ag nanowires based transparent conducting oxide films, and arrays of ZnO nanowires as antireflection layers for solar industry. It is envisaged that the commercialisation through the sales of specialised equipment and/or licensing of the ESAVD and non vacuum processes will follow.
Dissemination and/or exploitation of project results, and management of intellectual property
Management of knowledge
Dissemination and use of knowledge generated in nanoPV is governed by the Grant Agreement and the Consortium Agreement. The project’s handling of Intellectual Property (IP) as described in the Consortium Agreement is closely aligned and consistent with Annex II of the Standard (model) Contract in FP7. The Intellectual Property regulations regarding background and foreground knowledge, transfer of knowledge and exploitation of knowledge are closely aligned to the DESCA simplified FP7 model consortium agreement.
To assure successful exploitation of the nanoPV scientific and technical results, dissemination and exploitation activities were running in parallel with the coordination of the project so that all activities were linked to successive knowledge transfer. Disseminating and exploiting nanoPV achievements have been an integral part of all work packages and a responsibility for all partners. The efforts allocated to these activities have been focused along the following main lines:
1. Dissemination of results obtained: All partners collaborated in the dissemination of the results among the research community, the photovoltaics community and their respective industries and markets in a broad sense, including other FP7 programme participants.
2. An open seminar with invitation of participants, involved in other EU PV related projects coordinated by SINTEF (ThinSi and HyperSol), has been organised. Relevant industrial and scientific partners, who are not involved into nanoPV, ThinSi and HyperSol projects, have been invited to this seminar after internal consultations, especially with the industrial partners from these consortiums.
3. Exploitation: Industrial take-up of the technology developed in nanoPV plays a central part in the project. To maximise the likelihood of success, the project has prepared an “exploitation assistance package”. The package contains manufacturing procedures, infrastructure and IPR.
The planning of dissemination activities, which is a horizontal activity along the overall project lifecycle, was started immediately in the first project meeting. Dissemination policies have been based on three major dissemination channels. Each dissemination activity has been designed as a blend of dissemination activities from one or more channels, with respect to the target group(s). The three channels and their components (in bold) are:
a. Online dissemination: A nanoPV website provides a first access point for interested scientific and business parties into the nanoPV project. The objective of the website is to create a community of interested parties around the project, to accelerate their involvement and to create awareness of the results.
b. Non-electronic dissemination: Classical vehicles of knowledge transfer such as open project seminars intended for industrial end-users, articles in technical journals, peer-reviewed publications in scientific journals and presentations in conferences have been focused on the dissemination of project results, mainly to experts and professionals. The foremost scientific journals have been selected for publishing of the scientific results.
c. Personal dissemination: Each partner represents a network of contacts, technology and market overview that is much greater than the consortium itself. The scientists and engineers working for nanoPV were actively seeking to disseminate the results through contacts in formal networks (e.g. participation in other projects and involvement in organisations) and in their personal informal networks.
NanoPV partners that are technology providers were motivated to approach relevant industry sectors and potential markets and customers. The research partners have been focused on researchers and professionals both from industry and academia. To achieve maximum impact for nanoPV, it is imperative that the next users are identified and that the project results are disseminated through the appropriate channels. An overview is given in the following table:
Main next users Main dissemination channels
Process chain for low cost solar cells PV companies Online, conferences, publications, patents, personal networks
Production equipment PV companies Online, conferences, publications, patents, personal networks
Analysis and characterisation tools PV companies Online, conferences, publications, patents, personal networks
Basic knowledge on electronic properties of individual layers and interfaces in solar cell structures Universities, research institutes, industry Conferences, publications, personal networks
Possibility for an industrial take-up of the technology developed within nanoPV was playing a central part in the project. To maximise the likelihood of success, the project was open to provide user-friendly access to the developed technology. At the same time the interests and investments of the partners have been actively pursued by protecting any intellectual property developed within the project through the application for patents. The patenting of technology applies both to solar cell concepts and to developed process technology and will become the shared property of the partners involved after the project period because of some delays caused by technical problems.
Together with project partners the coordinator was monitoring the market situation for solar cells during the whole lifetime of the project. The information has been collected from all relevant sources, including the partners’ existing contact networks, possible clients and relevant investors.
Exploitation of the project results beyond the duration of the project is based on an “exploitation assistance package”. The package contains three components that are essential for industrial take-up, both by the industrial project partners and by other companies outside the project. The exploitation assistance package forms an excellent basis for industrial implementation and scale-up:
1. Manufacturing procedures that present in details the manufacturing approach and parameters to be used. They have been worked out through integration of the new processes developed in WP1 and WP2. The procedures have been checked for industrial viability.
2. Infrastructure: prototypes for production and analysis.
3. IPR: access to protected intellectual property developed within the project.
With the protection of the developed technology in place it is in the interest of the partners to actively pursue dissemination of the technology as to gain wider acceptance and larger product volumes, which will help to reduce cost.
The NanoPV partners have been exploiting the results in many directions. The research partners (SINT, ECN, UVEG, HZB, RBI, CLS, TUM and CNR) were utilizing the academic potential in scientific publications and as a basis for new research. They will also offer product and process development to industries. Two industrial partners, OIPT and IMPT, have offered highly sophisticated production equipment (vacuum and non-vacuum techniques, respectively), which is attractive for the global PV fabrication market.
The nanoPV partners regard exploitation of the project results as an integrated part of their project activities and have a strong motivation to exploit and disseminate the results to the best of any interested party in the European Community. Started with dissemination activities after the first indications of successful results, the exploitation activities have been a topic for discussion in all meetings of the General Assembly. The project coordinator was monitoring the exploitation activities and reported to GA. Each partner was following a specific exploitation/business plan according to the partner’s own strategies.
Exploitation for each partner
With the results from nanoPV, SINT is able to offer a comprehensive package of knowledge about low-cost Si substrates for solar cells to industrial companies on a world wide scale. Through industrial development projects, SINTEF offer assistance to the companies in setting up their own production facilities. We will also be able to produce small series of substrates in our own laboratories.
To develop our capabilities further, we intend to continue the cooperation with the NanoPV partners in new research projects inside and outside the European framework programmes. As a marketing tool for our services and research capabilities SINT intends to publish the project’s results – after securing IPR – in high quality peer-reviewed journals, at relevant conferences, and in application-focused technical journals.
ECN has well established, and successful exploitation strategy for scientific and technological results from EU projects. In agreement with ECN's mission "to develop advanced technologies for the transition to a sustainable energy supply and to bring these technologies to the market", ECN will both, strive for the implementation of industrially relevant results of NanoPV in close collaboration with PV companies (after securing IPR), and publish scientifically relevant results of this project in the form of peer-reviewed articles in high profile scientific journals & presentations/contributions at science and technology conferences in collaboration with academic and industrial partners. Existing as well as new collaboration links within the consortium of NanoPV will be of relevance in the context of our exploitation strategy as well.
Within the NanoPV project mainly precompetitive research was carried out and therefore it is hard to predict future exploitation of the results. However, potentially and after performing more R&D there are technologies that could be implemented in our future solar cell and module processing, which we could license to cell and module manufacturers. ECN has observed that: (i) a successful Al2O3 passivation on n-Pasha cells contributes to the (commercial) development activities for n-type cell technology, (ii) the Al2O3/B2O3 passivation integration trial on p-Pasha cells could contribute to further development activities for this cell type, and (iii) the SHJ results provide a good commercial prospect due to the high efficiencies obtained. For the other nanomaterials researched in the project it is too early to say anything about potential exploitation in future solar cells and modules. But, results could be used as starting point to define new R&D projects partly subsidized by the national agencies or the EU or for joint R&D projects with and paid by private companies.
As a national research institution, HZB has published the scientific achievements obtained within nanoPV in high-ranking peer-reviewed journals (cf. the publication list of the project). Further dissemination has been achieved through presentations at relevant international conferences. Moreover HZB expects to extend the collaboration with the European project partners beyond the duration of the project.
The technological achievements will be made available to European small and medium photovoltaic enterprises. At HZB, the infrastructure for this process is purposefully established in form of the Competence Centre Thin-Film- and Nanotechnology for Photovoltaics Berlin.
Of the results obtained by HZB within the course of the project, the improved light trapping obtained by nanotextured (“black”) silicon and subsequent preparation of silicon heterojunction solar cells is the most promising, especially for thin film crystalline silicon. Further, the wet-chemical processing of a nm-thin SiO2 tunnel oxide as a replacement for amorphous silicon buffer layers appears promising, but the reported results show only a small improvement over the reference, and on a generally low cell efficiency level. On the technology readiness scale, these results are TRL 3/4. (Note, that HZB’s standard silicon heterojunction and polycrystalline silicon on glass technologies are at a higher TRL, but are sideground for the nanoPV project). Therefore, further R&D is required and will be pursued by HZB before a commercialization (in collaboration with industry partners – HZB does not commercialize their work, except by licensing IP) can be envisaged.
University of Valencia
The scientific results acquired by the research group of UVEG within NanoPV have been/will be – after securing IPR – published in high quality peer-review journals and widely presented at national and international conferences. UVEG group expects to use the acquired knowledge for development of new technologies of production of nanostructures useful for the photovoltaics field and collaborate with small and medium enterprises for possible scalable production of the nanostructures. The experience acquired in NanoPV has increased UVEG competences for future research efforts in this area.
Technical University of Munchen (Walter Schottky Institute)
The scientific results concerning the benefits of black nano-texture in photovoltaic devices have been/will be published in international peer-review journals after securing the related IPR. During the project, TUM has accumulated a comprehensive knowledge about the black-etching technique itself and about its integration in the overall manufacturing process of various kinds of solar cells. Thus, TUM is able to offer a detailed know-how to the industrial companies regarding the opportunity to achieve substantial improvement of the efficiency of wafer and thin film Si solar cell by applying black nano-texture with minimal modification of the established standard production technologies.
Institute of Photonic Technology
As a R&D institute, IPHT has published / will – after securing IPR – publish the results gained within NanoPV in high-ranking journals and will present the findings on international conferences. The results will help the European industry to make available production facilities for low cost high efficient thin film solar cells. Moreover, European solar cell companies have been/will be provided with processing details for manufacturing these cells. This is the more important as presently US R&D institutions and companies are very active in the field covered by NanoPV.
As a non-profit research organization, IPHT may not work for commercial return. Nevertheless, IPHT uses the NanoPV results for further R&D: Based on our final results achieved within NanoPV concerning nanowire solar cells etched into laser crystallized silicon thin films on glass, in April 2014 a German thin film solar cell company granted IPHT a R&D contract to further develop this cell type. However, in the meantime the company decided to stop operations completely by the end of this year and stopped the running contract. Due to the present status of the PV industry in Europe, it will be hard to find another company to resume this topic. However, we are presently active, together with industry, to start projects on using silicon nanowires for Lithium-ion batteries, in which the knowledge on nanowire preparation, achieved within NanoPV, will be used.
CNR – Laboratorio MDM
As a Research laboratory, CNR will – after securing IPR – has been published/will publish the main scientific results in international peer-review journals and will assure the widest dissemination of these results by presenting them at national and international conferences. CNR expects to be able to extend the cooperation with the NanoPV partners to new research activities within the European framework programs. The know-how acquired during the NanoPV project could be helpful to start R&D collaboration with small and medium enterprises in the photovoltaic field.
Rudjer Boskovic Institute
The results acquired in this collaboration will help EU and national PV industry to further develop the existing production lines and acquire the new concepts for further upgrade of production of more efficient solar cells at lower cost. Being a R&D institute, RBI has published / will – after securing IPR – publish the results obtained within this collaboration in high ranking international journals and/or present them on international conferences. The new knowledge on the fundamental processes developed in this collaboration will broaden the general fund of knowledge and increase RBI competences for future research efforts this area.
Central Laboratory of Solar Energy and New Energy Sources, Bulgaria
CLS has proposed methods of deposition of different nanostructured materials applied to solar cells and modules for increasing their efficiency and made them available to the other partners in NanoPV and also to European companies and SMEs according to the IPR. The results of the analyses and studies have been/will be presented at international conferences after securing IPR and published in the international scientific journals.
Oxford Instruments Plasma Technology Ltd.
OIPT offers well qualified process tools for the processes developed in this project. Some early sales are expected to the worldwide R&D community in photovoltaics: these customers will be seeking to replicate and extend results achieved in the NanoPV project.
The greatest impact will come from the adoption of any of these techniques by the growing number of PV module manufacturers.
Within a year of the project end, we have been already implemented the enhanced thin film sputtering developed in nanoPV in four customer tools, at least one of which is a production customer. It was very attractive to have highly uniform deposition from the smallest possible target, especially for high value materials such as gold. Enhanced in situ heating is being taken further, to meet the needs of the growing interest in two-dimensional materials.
We have disseminated the project results in several user group meetings and sales promotions: the ability to deposit controlled, very thin layers (few nm) is proving very timely in the current device development landscape.
Innovative Materials Processing Technology Ltd.
After a successful implementation of nanoPV it is envisaged that the commercialisation through the sales of specialised equipment and/or licensing of the ESAVD process for the non-vacuum deposition of nanostructured thin films to solar cell industry will follow. These include TCOs, antireflection coatings layers and ultra-thin films for interface passivation, and low band gap dielectrics for improving conductivity, as well as deposition of nanoparticles incorporated on surfaces of solar cell structures for improved light trapping and down/up conversion. This will open up significant opportunities and generate enormous benefits, and revenues to IMPT. The success of the project would also enable IMPT to partner with a larger equipment manufacturer and developer such as OIPT to create a large volume production of equipment or develop equipment for even larger area deposition (i.e. beyond 8” wafer) for solar cells, as well as other large-area thin film markets.
List of Websites:
Dr.rer.nat.habil. Alexander Ulyashin
Senior Research Scientist
SINTEF Materials and Chemistry
Tel.: +47 93002224
Grant agreement ID: 246331
1 March 2011
28 February 2014
€ 5 153 458,02
€ 3 853 315
Deliverables not available
Grant agreement ID: 246331
1 March 2011
28 February 2014
€ 5 153 458,02
€ 3 853 315
Grant agreement ID: 246331
1 March 2011
28 February 2014
€ 5 153 458,02
€ 3 853 315