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All-inorganic nano-rod based thin-film solarcells on glass

Final Report Summary - ROD-SOL (All-inorganic nano-rod based thin-film solarcells on glass)

Executive Summary:
Title: ROD_SOL

Grant agreement no: FP7-NMP-227497
Start and end dates: 01.01.2009 - 31.12.2011

Institute of Photonic Technology, Germany, Dr. Silke H. Christiansen, e-mail: , phone: +49 1796894182, fax: +49 3641 206499
-Simulation of cell concepts, VLS-CVD growth of Si nanorods, p-n junction development, chemical etching of NRs, TCO contacts using ALD, electrical and optical characterization

Friedrich-Alexander Universität Erlangen-Nürnberg, Max-Planck Research Group, Germany
- VLS-CVD growth of GaN and Si NRs,
EMPA - Swiss Federal Laboratories for Materials Testing and Research, Switzerland
-Chemical etching of NRs, electrical electrical, optical, and mechanical characterization of NR ensembles and individual NRs, electrochemical deposition of TCO
Hungarian Academy of Science, Research Institute for Technical Physics and Materials Science, Hungary
-Determination of structural properties of NRs (TEM)
AIT – Austrian Institute of Technology GmbH, Nano-System-Technologies, Austria
- Metal templates for VLS NR growth, optimization of TCO layers by magnetron sputtering
VTT Micro- and Nanoelectronics, Finland
-Metal templates for VLS NR growth, fabrication of NRs using RIE, optimization of TCO layers by ALD, doping by diffusion following ALD deposition of dopants
PICOSUN oy, Finland
-Optimization of TCO layers by ALD, doping by diffusion following ALD deposition
Aixtron AG, Germany
-VLS-CVD growth of GaN and Si NRs, optimization of CVD equipment
BiSOL d.o.o. Slovenia
-Determination of solar cell parameters, demonstration of a device based on NR material
iSupply Deutschland GmbH, Germany
-Technology watch of thin film photovoltaic technologies and benchmarking the ROD_SOL results

Objectives: The project has explored new innovative concepts for thin film solar cells based on nanorods (NR). The nanorods were fabricated either by the vapour-liquid-solid (VLS)-CVD growth method on various substrates such as Si-wafer and glass or by wet chemical etching or reactive ion etching of thin multi-crystalline Si layers on glass or Si wafers. The VLS growth has been reached by statistical or patterned metal templates and yields dense ensembles of randomly oriented single-crystalline NR. The NR diameter of choice and dopant concentration for optimum solar cell efficiencies was derived based on numerical simulations and on experiments. The most promising synthesis method for high efficiencies and best large area manufacturability has been identified. The materials optimization is based on structural, optical, electrical, and mechanical characterization of the NRs.
The targeted thin film solar cell processing of NRs includes many materials optimization steps like the realization of axial or preferably radial p-n junctions and the deposition of transparent conductive oxides as front contacts.
Technology watch of thin film photovoltaic technologies and benchmarking the ROD_SOL concept with respect to other thin film technologies as well as validation of the NR based solar cell material has been carried out.
First thin film devices were realized on small area substrates within the duration of the project. Further research is needed to upscale the processes and to improve materials properties to approach the efficiency of planar wafer based materials.

Project Context and Objectives:
II. Summary description of project context and objectives

II.a Concept and project objectives
Semiconductor NRs in solar cells are suggested more recent with the convincing offer of applicability in solar cells as antireflective films and as active elements in organic dye-sensitized and inorganic solid-state devices. In ROD_SOL we have made use of NRs as the active absorber material in an all-inorganic thin film solar cell and thus created a novel, 3D device architecture that needed strong research to develop all the components needed to fully exploit the potential of that cell.

The ROD_SOL project has aimed at making available to the markets of novel materials and nanomaterials for energy applications a thin film material based on silicon NRs. Materials developments and optimization based on NRs have been one of the major tasks in ROD_SOL next to the integration of these NRs in working thin-film solar cell devices, their testing and benchmarking of solar cell parameters. The leading European materials oriented groups have contributed to this task together in one project. Innovation has taken place in areas of materials processing of NRs such as adequate preferably radial doping, NR contacting based on conformally wrapped around transparent conducting oxide (TCO) layers. These materials developments were guided by device simulations.

The goal of ROD_SOL is to realize optimized thin film material systems based on semiconductor NRs that show photovoltaic behaviour and properties optimized for that application. The NRs can be grown or realized bottom-up or top-down.
In ROD_SOL we will essentially try to optimize and understand the bottom-up so called vapour-liquid-solid (VLS) growth process first described by Ellis and Wagner in 1963. The VLS growth process is based upon a droplet of a metal/semiconductor alloy that catalyzes the local growth of a NR which is a shaft of the semiconductor material. This NR crystallizes with the diameter of the catalyzing droplet and the NR pushes the metal alloy droplet away from the substrate when, upon supersaturation with semiconductor species from the gas phase (e.g. silane, DTBS) the semiconductor shaft crystallizes on the substrate.
The catalyzing metal nano-droplet template determines the VLS process a lot and metal template optimization using for comparison Au, Pd, Al and Cu will be a task in ROD_SOL. The preference is for metals that do not contribute deep traps in semiconductors, therefore Al or Pd are in favour, but reports on successful NR growth are quite rare. However, processing is difficult with these metals (oxidation and silicide formation problems). Au and Cu induce deep traps in silicon, but processing of NRs is well established. Gold (Au) works easiest for silicon NR growth at comparably low growth temperatures (eutectic temperature of Au-Si alloy system is 363°C) and the largest body of literature of almost 10 years is available. A strategy of which metals to be in favour for the processing of solar cells (minority carrier device, cost efficiency an issue) will be derived in ROD_SOL. Thick and thin metal layers can be used determining the size of the droplets that will form upon heating and liquefying and thus determining the NR size. As a last option we also look into the integration of NRs of a direct bandgap material from the group-III nitride systems in a thin-film solar cell. This approach is highly novel and opens up the option to satisfy absorption in the direct bandgap material with possibly even thinner films that in the Si NR case. The commercial RTD as well as production type reactors available in the consortium (high temperature CVD tools, MPRG, AIX) allow the growth of these NRs at elevated temperatures. GaN-based NRs will be grown on directly on silicon wafers or on Si NRs that themselves reside on Si wafers. These GaN-based NRs grow by VLS on Si wafers so that for integration onto glass or foil, the depilation strategy described before needs to be followed.

As VLS NRs are not the easiest achievable semiconducting nanostructure it is also an option and will be tested in ROD_SOL to etch (top down) NRs into silicon wafers and depilate them from the wafer while being able to re-use the wafer several times. In any case we will be using the etched NRs that can easily be produced at larger volumes to provide these NRs essentially in the beginning of ROD_SOL to the partners to establish the different characterization techniques (electrical, optical, mechanical- after embedding).

Processing of NRs for integration in thin film solar cells
For the integration of NR in thin film solar cell device concepts additional processing steps and properties need to be developed:
(i) Metal nanotemplates for the VLS CVD growth of NR ensembles
(ii) p- and n-doping of NRs (axial and preferably radial)
(iii) realization of back and front contacts with low contact resistances and good transmission properties: optimizing transparent conductive oxide (TCO) films covering the 3D NRs surface
(iv) passivation of large surface area using CVD (e.g. atomic layer deposition) (additional passivation layer between TCO and semiconductor NR may be needed)
care for sufficient absorption in the thin indirect semiconductor film of indirect silicon (or direct InGaN): optimize NR ensembles (random or patterned, growth directions, etc. to account for multiple scattering in NR ensembles).

Materials optimization based on characterization and simulation
For comparably complex highly challenging materials optimization at the nano-scale overall sophisticated characterization is key for the success of materials developments. Techniques to be used are structural, electrical, optical and mechanical characterization of individual and NR ensembles. The 3D NR materials optimization for the envisaged application can only be successful when supported by numerical simulations

Strategic objectives
- To enable seven world class research institutions (including a leading group from the USA) to realize based on basic materials science a novel nano-material based on semiconductor NRs for the application of a thin-film solar cell concept that has the potential for high efficiencies (>15%) at competitive production costs.
- To transfer results from basic materials developments in research directly to two equipment manufacturing companies that transfer materials developments/processes into technological relevance at a scale needed in production lines.
- To validate and benchmark the novel NR-based thin film material at an end users test site and compare it to other bulk and thin film concepts that are continuously monitored at a consulting company specialized in PV technology watch.

Scientific objectives
- semiconductor (mostly Si) NR deposition or integration on glass or foil substrates need to be developed and among different options of deposition/synthesis such as bottom-up and top-down methods the most promising one with the potential for best materials/device
properties, best up-scalability at lowest production costs needs to be identified;
- Identification of the most suitable material based on numerical and physical device simulations and experimental solar cell parameter extraction based on electrical measurements (ensembles and single processed NRs) is needed;
- establishment of physical device simulations for the novel 3D NR-based solar cell using the COMSOL and r-soft or ATLAS simulation software;
- establish parameter extraction from electrical measurements that need to be established for single processed NRs and NR ensembles (preferably also at an end user test site-BISOL);
- materials optimization based on combined structural, electrical, optical and mechanical characterization by TEM and SEM techniques, I-V measurements based on 4-point-probe measurements or AFM-tip based contacting of single NRs in an SEM and contacting processed NR ensembles, as well as optical characterization in an integrating sphere of wavelength dependent light absorption or optical standard characterization for solar cells at the end users site as well as mechanical tests of stability of single processed NRs and reliability tests of embedded NR-based thin films with respect to delamination, bending, cracking and disintegration of components.

Technological objectives
The integration of selected processes in a technological process or processing environment (up-scaling needed) with the potential to yield a highly efficient and cost effectively to be produced thin film NR based solar cell on cheap substrates need to be carried out at three industry/SME sites. These industry partners carry on the tool and process optimization/modification and the following technological objectives can be derived from industry partners’ tasks:
- AIX, PICO and BISOL are requested to support optimized NR synthesis and NR processing (doping, contacting, integration on cheap substrates) based on the evaluation of materials developed by the research partners;
- AIX, PICO and BISOL are requested to support the transfer of processes into a technologically viable environment;
- AIX, PICO and BISOL are requested to support materials characterization of optimized materials from institute partners using optimized standard characterization techniques available at their sites for their internal materials/process developments;
- BISOL and iSD are requested to support the benchmarking of the novel ROD_SOL thin film material compared to current bulk and thin film concepts that are partly already in production;
- tool and process developments for the (MO)CVD deposition of NRs by the bottom-up VLS mechanism at different temperatures is needed;
- tool and process developments for the TCO deposition by either ALD, spray pyrolysis, magnetron sputtering is needed;

Policy objectives
Today, European companies can compete in the PV market (bulk and thin-film materials). The technology leadership in thin-film photovoltaic could potentially be gained by the accumulated knowledge and expertise in Europe once research and development is financially supported. ROD_SOL will enable a European solar module producer (BISOL) to possibly advance the novel NR based thin film PV concept and prepare it with the help of the strong, multi-disciplinary consortium for a future generation of thin-film PV and make it attractive for the thin film PV market. ROD_SOL also supports European equipment manufacturers (AIX, PICO) that deliver equipment for materials processing most suited for all sorts of novel thin film and composite materials and in particular also for the NR-based thin-film processing needed for the proposed concepts. For these equipment manufacturres it will be extremely attractive to commercialise their tools in a tremendously growing field of green energy.

Project Results:
The final report_Pdf document contains the the main S & T results/foregrounds of the report. (including some figures and images)
Potential Impact:
V. Potential impact (including the socio-economic impact and the wider societal implications of the project so far) and the main dissemination activities and exploitation of results

Among various sources of energy, sunlight is the most abundant and cleanest natural energy resource. In principle, photovoltaics (PVs) hold a great promise to exploit the sunlight to generate clean energy to accommodate the ever-increasing energy demands. Photovoltaic (PV) is the field of technology and research related to the application of solar cells for energy generation by converting solar energy directly into electricity. Solar cells are particularly useful as power generators in distant and terrestrial places like weather stations and satellites. Solar cells are used in small devices like watches and calculators, but mostly they are used to produce electricity either in large-scale power plants or by being incorporated into walls and rooftops. The function of solar cells essentially involves the presence of a p-n junction close to its surface in order to create potential difference in the bulk. About 90% of solar photovoltaic modules are silicon-based, but in recent years increased demand for silicon solar cells has inflated the price of solar-grade silicon. The price of Si accounts for about half of the price of solar cells. The grid parity of electricity produced by solar cells is close to 1 USD/Watt assuming a 20-year lifetime of the cells, but the price is currently four times higher.
One obvious way to lower the cost is to reduce the amount of silicon in the cells. By using thin-film technology, the thickness of the silicon can be reduced from 200 ?m to 0.2–5 ?m. Another way to cut down production cost is to use low-grade Si instead of ultra-pure Si currently being used. However, this lower-grade material is less efficient. It is presumed that the grid parity of solar cells will be reached by using thin-film technology or a new design that is most-likely based on nanotechnology.
Rod-Sol project is unique as it involves partnerships/cooperation across disciplines from fabrication of nanostructures (chemist/material scientist), thin-films technology (device engineer) to a large range of advanced analytical material science methods (physicist/engineer/material scientist). The silicon based approaches are certainly favored because of material abundance and non-toxicity at a high level of materials control and understanding together with a huge industrial infrastructure to account for low production/processing costs and high production yields. For all device concepts based on nanostructures, the crystal structure, geometry, interfacial properties between the SiNW and the substrate as well as the Si core and the shell of the SiNW, dopant concentrations and impurity levels are of key importance for functioning of the devices. VLS approach gives high-quality NWs but requires the use of hazardous silane gases at high temperature and metal catalyst.
Alternatively, catalyst free SiNWs can be realized by electroless wet chemical etching or electrochemical etching into bulk Si wafers or even thin silicon layers (they can be single-, multi-, nano-crystalline or even amorphous) on substrates such as glass as was developed and investigated during Rod-Sol project. Engineering flexibility in doping characters of silicon nanowires is highly desirable to widen the range of their potential applications. Metal-assisted wet chemical etching (MAWCE) is a simple and low-cost approach to fabricate SiNWs with designable doping nature. In MAWCE, SiNWs are fabricated through (non)uniform etching on silicon substrates in aqueous acid solutions, which is catalyzed by electroless deposition of metal nanoparticles on the substrate surface.
The knowledge acquired from this “Rod-Sol” project regarding the synthesis, processing, optimization and characterization of silicon nanostructures which can be used to form the fundamental basis for the design and construction of efficient, cost-effective energy-harvesting device production lines.
This project has also involved exploratory work on surface treatment of SiNWs to address the limiting issue of high energy conversation degrees and testing of different surfaces for the low cost production. During Rod-Sol project the new type of photovoltaic architecture based on non-typical p-n junction was developed. The novelty of depositing transparent conductive oxide and barrier layers lies in the fact that unlike the ALD approach they can be used to simultaneously, in one step, passivate and embed in a matrix the SiNWs which is essential for “real life” solar cell devices.
The anticipated benefit from this project is firstly the new knowledge gained from these innovative nanostructures which will be invaluable both from a fundamental point of view and for the near future development of new advanced energy technologies. The highest open-circuit voltage under AM 1.5 illumination was 470 mV. Under these conditions a short-circuit current density of 35 mA/cm2 was measured. The highest power conversion efficiency of SiNWs based SIS solar cell was over 9 % which shows potential for low cost (<1.0 $/Wp) solar cells with such a design.
The most promising and important factor is the improvement of the Voc which can be tuned by the SiNW dominated surface structure. We see a real potential for further improvement of solar cell parameters such as Voc to 600-700 mV and a power conversation efficiency of >15% as a main objective of this project – “from laboratory to industrial scale”. The transfer of SIS concept on R2R production line can give the following global exceptional advantages like: (i) reducing of silicon consumption; (ii) reducing of total PV module weight; (iii) reducing of building static costs; (iv) reducing of CO2 emissions during the production process.

Please find the details on the dissemination activities and the exploitation of results in the attached PDF file.
List of Websites:
European project ROD_SOL: List of participants, Co: Coordinator, Cr: Contractor

1, Co Institute of Photonic Technology IPHT D RTD
Dr. Silke Christiansen
IPHT, Albert Einsteinstr. 9, 07745 Jena
Phone: +491796894182

2, Cr Friedrich-Alexander Universität
Max-Planck Research Group MPRG D RTD
Dr. Christian Tessarek
Friedrich-Alexander-Universität Nürnberg
Max-Planck-Research Group “Institute of Optics, Information and Photonics”
Günther-Scharovsky-Str. 1, Bau 24,
91058 Erlangen
Tel.: 09131 761341

3, Cr Swiss Federal Laboratories for
Materials Testing and Research EMPA CH RTD
Dr. Johann Michler
Mechanics of Materials and Nanostructures Laboratory
Empa - Materials Science & Technology
Feuerwerkerstr. 39
CH-3602 Thun, Switzerland
Phone: +41 33 228 4605 or 4626 (Secr.)

4, Cr Hungarian Academy of Science
Research Institute for Technical Physics and Materials Science MFA Hu RTD
Dr. Bela Pecz
Research Institute for Technical Physics and Matl. Sci., MFA
H-1525 Budapest, POBox 49
Phone: 36-1-392-2587

5, Cr ARC – Austrian Research Center GmbH, Nano-System-Technologies ARC Au RTD
Dr. Hubert Brückl
Austrian Research Centers GmbH - ARC
Donau-City-Straße 1
1220 Vienna
Phone: +43-50550-4301

6, Cr VTT Micro- and Nanoelectronics VTT Fi RTD
Jouni Ahopelto, Dr.Tech.
Research Professor
VTT Micro and Nanoelectronics
P.O. Box 1000, FI-02044 VTT, Finland
Phone: +358 20 722 6644

Dr. Juhana Kostamo
Managing Director
Picosun Oy
Tietotie 3
FI-02150 Espoo
Phone: +358-(0)50 369 9565

8, Cr Aixtron AG AIT D IND
Dr. Christoph Giesen
Kaiserstr. 98
52134 Herzogenrath
Phone: +49 241 8909 507

9, Cr BiSOL d.o.o. BISOL Si SME
Dr. Uros Merc
CEO/General Manager
Bisol d.o.o.
Latkova vas 59a
SI-3312 Prebold
Phone: +386 (0)3 703 22 50

10, Cr iSuppli Deutschland GmbH WTC D SME
Henning Wicht
WTC - Wicht Technologie Consulting
Dr. Henning Wicht
Frauenplatz 5
D- 80331 München
Tel +49 89 207026010

Address of the project public website: