Final Report Summary - GO-NEXTS (Graphene doping and texturing in efficient electrodes for organic solar cells)
Executive Summary:
In the last the organic semiconductor solar cells (OSCs) and Perovskite solar cells (PSCs) have been proposed as a promising route to scalable, economically viable, energy conversion technologies due to the potential for development of low-cost, flexible, large-area cells and modules, reaching efficiencies of 11% (11.6%) for OSCs and of 21% for PSCs respectively.
In this context, the replacement of the transparent conductive oxide (TCO), for the semitransparent conductive electrode, which presents serious issues related to their stiffness, poor transparency in blue region, release of oxygen in the active layer, scarcity and high cost of materials, represents a critical issue. Recently graphene electrodes have been proposed as a promising candidate for the realization of the semitransparent contact. Research on graphene electrodes is however still at its infancy, so that if this material will manage to accomplish this task has still to be proved. In particular, many questions remains open concerning the degree of interaction of graphene with the polymeric layer, which could degrade the outstanding graphene electron conductivity, or the graphene/polymer electron affinity, which plays an important role in the overall solar cell efficiency.
The GO-NEXTs project has proposed to study a new kind of electrodes based on doped, textured (i.e. 3D) graphene electrodes, in order to increase the overall efficiency and performance of organic solar cells. To our knowledge, this represented the first attempt to enhance light trapping in a solar cell by structuring one or more graphene contact electrode(s) in such a way that it will act as photonic crystal(s).
The project benefitted of a theoretical-experimental approach. During the project it has been developed a multi-scale method based on optical and electrical simulations for the study of the theoretical aspects related to the use of doped and textured graphene as a contact on OSCs and PSCs. In particular, it has effectively assessed OSC performance, while investigating the sensitivity of the main figures of merits (i.e. Power-conversion efficiency (PCE), Fill-factor, open-circuit voltage etc.) spanning over the device parameter space (i.e. different materials, gate workfunctions, electrode gratings etc.), as well as outlining the most critical aspects to address in order to obtain large PCE.
From an experimental point of view the graphene-related issues that may affect the device performance have been identified.It was demonstrated that the sheet resistance and transparency of graphene grown at sub-900 °C temperatures were not adequate for the project requirements. Specific protocols based on temperature and solvent treatments have been identified to clean the final layer allowing a substantial reduction of the contaminant on the graphene contact. Centrimetre-scale graphene electrodes doped with inorganic (MoO3) and organic materials (TFSA and PEIE) has been demonstrated although a variability in performance for the organic adsorbate-doped devices has been found; this could be related to changes in interface stability for the ageing of the materials. Furthermore that nanotexturing of graphene electrodes was not possible as both the transferring of the planar graphene sheet onto a nanotextured surface and the direct growth on nanopatterned catalyst were leading to low quality electrodes. In this case, the price for an increase in light absorption of about 20% as expected from simulations would be a drastic increase in sheet resistance, leading to an overall much poorer performance of the fabricated cell. The performances of the graphene electrodes in terms of sheet resistance, have been improved introducing a grid underneath which helped the collection of charges in the devices. Thanks to this improvement it was possible to realize both OSCs with a maximum efficiency of 6.3% in a tandem configuration and PSCs with a maximum efficiency of 7.1%.
Project Context and Objectives:
Motivation.
The primary energy supply in the EU is currently strongly dependent on fossil fuels. Reinventing the energy system based on a low carbon model is thus one of the critical challenges of the 21st Century. Solar energy is by far the largest of all carbon-neutral energy sources. More energy from sunlight strikes the Earth in one hour (4.3 × 1020 J) than all the energy consumed on the planet in a year (4.1 × 1020 J). Development of low-cost photovoltaic energy conversion technologies is thus of key importance.
In the last decade, the organic semiconductor solar cells (OSCs) have been proposed as a promising route to scalable, economically viable, energy conversion technologies due to the potential for development of low-cost, flexible, large-area cells and modules. OSCs have recently shown an impressive improvement in cell efficiency starting from few percent in 2000, they have achieved in the last months values of above 11% (11.6%) for a triple junction device [C. Tao et al., “17.6% stabilized efficiency in low-temperature processed planar perovskite solar cells,” Energy Environ. Sci., vol. 8, no. 8, pp. 2365–2370, 2015]. In the last three years, however, a new technology based on the use of perovskite materials (PSCs), which can be processed by solution and have many realization aspects in common with organic solar cells, has gained the attention of the scientific community reaching a certified value of 21% (http://www.nrel.gov/ ) .
Current state-of-the-art BHJ-SCs are typically fabricated on substrates with sputtered indium tin oxide (ITO) transparent electrodes, which unfortunately presents serious issues related to i) the release of oxygen and indium into the organic layer, ii) the poor transparency in the blue region, iii) its stiffness, which prevents its use in flexible solar cells, and iv) the large cost due to the limited supply of indium. All these aspects make the development and commercialization of a replacement for ITO a major focus in the BHJ-SCs Research and
In this context graphene electrodes have been recently proposed as a promising candidate for the realization of the semitransparent contact. Research on graphene electrodes is however still at the beginning, so that if this material will manage to accomplish this task has still to be proved. In particular, many questions remains open concerning the degree of interaction of graphene with the polymeric layer, which could degrade the outstanding graphene electron conductivity, or the graphene/polymer electron affinity, which plays an important role in the overall solar cell efficiency.
Up to now, the highest efficiency achieved with graphene electrodes are 8% with tandem polymer solar cell, and in a recent paper published in December 2015 the 17.1% with Perovskite solar cell.
The GO-NEXTs project has proposed to study a new kind of electrodes based on doped, textured (i.e. 3D) graphene electrodes, in order to increase the overall efficiency and performance of organic solar cells. To our knowledge, this represents the first attempt to enhance light trapping in a solar cell by structuring one or more graphene contact electrode(s) in such a way that it will act as photonic crystal(s).
The project leverages the combination of two different fabrication processes, and in particular the doping of the graphene, to obtain semi-transparent electrodes as well as the texturing of the electrodes/substrates. This approach, which has never been proposed before, represents a high-risk, high-impact approach. If successful, it should lead to improvements in solar cell efficiency by up to 14%. Furthermore, all the technologies proposed are suitable for large area realization paving the way for scalable fabrication technologies on low-cost flexible substrates.
During the GO-NEXTS project the following concepts have been studied, which all represent significant innovations with respect to current OSC and PSC technology.
Concept: Doped graphene as a (flexible) transparent electrode replacement for indium tin oxide (ITO).
Films of tin-doped indium oxide (ITO) sputter-deposited on glass substrates are widely used as a transparent electrode contact in organic photovoltaic cells and modules. However, this technology has a number of key drawbacks including:
• Shortage of indium supply (from China) and corresponding increase in material cost.
• Material-inefficient deposition process (sputtering).
• Incompatibility with flexible substrates due to the fragility of the material.
Graphene, a monolayer sheet of sp2-hybridized carbon atoms arranged in a honeycomb lattice, has emerged as a promising candidate for (flexible) large-area transparent electrodes.
Concept: Reduced sheet-resistance of doped mono- and multi-layer graphene.
Since intrinsic exfoliated graphene has a sheet-resistance ~ 6 kΩ/square, significant levels of charge-transfer adsorbate doping are required to achieve acceptable sheet resistances for transparent electrode applications, e.g. for displays or photovoltaics. P-doping (HNO3 treatment) was employed to achieve sheet-resistances ~ 130 Ω/square for graphene monolayers (with an induced carrier concentrations > 9x1012 cm-2). A graphene monolayer has a transparency > 97% across the visible range, thus a stack of 4 individually p-doped graphene monolayer films was fabricated by the SKKU/Samsung team to meet the figure of merit required, i.e. sheet resistance of only ~ 30 Ω/square at a transparency of 90%.
Concept: Stability of adsorbate-doped graphene.
Although these initial adsorbate doping results are encouraging, it must be emphasized that, to date, no systematic study of the ambient temporal or temperature stability of the doping process has been reported. In fact, very recent results by the SKKU group on use of graphene anodes in flexible organic light emitting diodes, showed that the sheet resistance of the HNO3-doped graphene electrodes increased by ~30% over the first 30 hours following fabrication. The GO-NEXTS project aimed at addressing these issues in order to achieve the goal of obtaining a sheet-resistance smaller than 100 Ω/square (for a single layer of doped graphene) and smaller than 30 Ω/square for stacked multi-layer graphene. GO-NEXTS will also focus on enhanced ambient stability for molecule-doped transparent graphene electrodes, targeting <10% change in sheet resistance over a period of 14 days. The stability of the electrodes within the encapsulated solar cells is foreseen exceed one year.
Concept: Structured graphene electrodes as photonic crystals for improved light trapping.
The texturing of graphene will be achieved using large area nanopatterning techniques combined with the growth techniques.
Two different approaches have been considered for the nanostructuring of the surfaces: Nanoimprint lithography and electrochemical growth of anodic alumina .
The growth of the graphene on textured substrates will be performed by means of chemical vapour deposition for which a low temperature process will be developed targeting ultimate compatibility with roll-to-roll processing. The use of the structured graphene electrodes as photonic crystals will induce an increase of the light absorption in the active layer of the solar cell.
Concept: Graphene-based organic photovoltaics.
The concept of graphene doping and graphene structuring was used for the realization of single active layer or tandem OSCs or PSCs. Many different architectures can be foreseen for such devices. The p- or n-type doped graphene electrode was used as bottom directly in contact with the active layer or in combination with a proper hole and electron transporting layers at the interface with the active film. Light harvesting of the active layer has been enhanced through the inclusion of an optimized grating in the architecture. The grating will be realized via a texturing of the graphene electrode and or of the active layer.
Concept: Multi-scale simulations of graphene-based solar cell
Due to the novelty of graphene research field, design principles of graphene based solar cell have not yet been extensively elaborated, and different design options must be explored, evaluated, and optimized. From this perspective, numerical simulations can greatly help in order to overcome fabrication issues. To this purpose, a multi-scale method based on atomistic simulations (both ab-initio and tight-binding) for the calculation of the electrical properties of the material and a drift-diffusion model for the study at the macroscopic level of the complete device performance have been extensively exploited.
The developed approach allows to consider different architectures of graphene based organic solar cells, while assessing their performance and providing to the fabrication side relevant information concerned with i) the suitable material parameters ii) electrodes geometries and iii) configuration to be chosen to obtain large efficiencies.
Light management also plays an important role in the overall efficiency of a planar solar cell. The use of nanopatterned graphene electrodes or of a combination of nanopatterned gratings and graphene electrodes induced a light trapping within the solar cell enabling effective photon management while having a minimal impact on the design of current solar cells and their processing techniques. Even in this case, simulations will represent an important tool in order to drive fabrication process towards the realization of high efficient BHJ-SC.
The expected final results from GO-NEXTs consortium were the delivering of significant innovations in transparent electrode materials, fabrication processes and device architectures in order to reach the ultimate goal of low-cost organic photovoltaic technologies. The following objectives are targeted:
• Development of fabrication and doping processes to realize stable monolayer graphene p- and n-oped electrodes with sheet resistances < 100 Ω/square and <10 % change in sheet resistance over a period of 14 days. The stability of the electrodes within the encapsulated solar cells is foreseen to exceed one year.
• Development of organic photovoltaic cells with one or both electrodes fabricated from doped graphene (ie single or tandem cell architecture).
• Development of processes for fabrication of nanostructured graphene-based photonic crystal electrodes targeting improved light trapping in the organic photovoltaic cells in the visible and/or near-infrared spectral ranges.
• Development and realization of graphene-based high-efficiency tandem solar cells (efficiency≥14%)
• Development of experimentally validated models for the simulation of solar cells with graphene contacts, and distribution of the simulation tools implemented under BSD open-source license.
To achieve these objectives, the GO-NEXTS project benefited from the tight collaboration between the experimental and theoretical framework. In particular, multi-scale simulations are providing both performance evaluations of the entire solar cell, as well as the guidelines to the fabrication side. From the experimental point of view, technological processes for the realization of the textured doped contacts are under evaluation and the crucial elements of the device fabrication process are identified (e.g. films thickness, morphology, operational areas, texturing dimensions and doping) and optimized.
Project Results:
The project proposed a theoretical/experimental approach for the analysis and development of organic solar cells (OSCs) and after the first reviewing period perovskite solar cells PSCs using graphene textured electrodes deposited with chemical vapour deposition (CVD). The aim wss to obtain, through the use of graphene contacts, high-performance ITO-free solar cells on rigid and flexible substrates.
The scientific approach that was employed to attain our objectives wss based on the use of the state-of-the-art technology for the growth and doping of graphene contact on large area substrates, for the nanopatterning (NIL apparatus, Obducat Eitre ) and for the realization of OSCs and PSCs.
The activity is divided into five work packages (WPs); three work packages dedicated to the RTD.
In particular, WP2 aimed at developing a multi-scale method for the study of the theoretical aspects related to the use of doped and textured graphene as a contact on OSCs and PSCs.
The activity carried out within the this WP has resulted to be really useful in providing guidelines to the fabrication side. In particular, it has effectively assessed OSC performance, while investigating the sensitivity of the main figures of merits (i.e. Power-conversion efficiency (PCE), Fill-factor, open-circuit voltage etc.) spanning over the device parameter space (i.e. different materials, gate workfunctions, electrode gratings etc.), as well as outlining the most critical aspects to address in order to obtain large PCE.
This has been possible through the development of a multi-scale simulation approach (ranging from atomic to the device level), which, for the first time, has led to a comprehensive analysis of OSCs with graphene exploited as transparent electrode. In particular, simulations have shown that, graphene OSCs can exhibit better performance as compared to ITO counterpart, if series resistance of the contact is minimized (e.g. if high-quality graphene is used), and if the workfunction is optimized in order to reduce series resistance and induce a higher built-in electric field in the cell. Further improvement can be obtain with graphene electrode grating. The same techniques can also be used with the new and promising Perovskite materials for achieving record efficiency.
WP3 was devoted to the development of doped and textured graphene electrodes. Low temperature CVD graphene growth techniques and molecular doping processes were considered targeting stable, low-resistance transparent graphene electrodes. The texturing will be achieved with two different strategies: the nanoimprint lithography, which allows different kind of shaping of the patterning and the use of anodic alumina as template for the growth of 3D graphene electrodes.
During this activity many graphene-related issues emerges i.e. the quality of the graphene, the process for transferring graphene from the Cu foil it is grown on onto the target substrate, the fabrication of metal grids, the doping of graphene, the nanopatterning and the fabrication of the solar cell itself. In the following the resulting considerations related to each topic.
Assessment of low-temperature CVD growth of graphene: For the first half of the project, graphene electrodes were fabricated by graphene grown at AMO (using CVD on Ni substrates) and Tyndall (using CVD on Cu substrates). During the project it emerged that for chemical vapour deposition of planar graphene at sub-700 °C process temperatures, the sheet resistance and transparency of graphene grown at sub-900 °C temperatures were not adequate for the project requirements. It has been demonstrated that growth temperatures in excess of 900 °C are required for production of high quality graphene (sheet resistance ~1 kΩ/sq and transparency > 95%). Lower growth temperatures resulted in graphene films with sheet resistance values >> 1 kΩ/sq.
Graphene nanopatterning: According to simulations performed in GO-NEXTS, a nanopatterned 3D electrode could increase light absorption by up to 20 %. The fabrication of nanotextured graphene electrodes was explored in GO-NEXTS. Simply transferring a planar graphene sheet onto a nanotextured surface will not work. Due to the increased area of the nanotextured surface compared to a planar surface, the graphene placed on top of e.g. an array of pillars will stretch to the point of being torn apart. The other alternative was growing graphene directly onto a nanotextured metal surface. For this to work, the graphene growth temperature must be low enough to not damage the texturing of the metal growth substrate. The temperature for which the metal catalyst layers deposited onto 3D substrates underwent significant roughening was below 500 °C, but for this value CVD growth of continuous graphene electrodes onto nanopatterned substrates was not possible.
The maximum growth temperature that does not destroy the texturing was determined to be around 400 °C, which is too low to grow high-quality graphene. In fact, the grown films, although graphitic in nature, more closely resemble amorphous carbon than graphene. In other words, the price for an increase in light absorption would be a drastic increase in sheet resistance, leading to an overall much poorer performance of the fabricated cell. For this reason, nanotextured graphene electrodes are presently not a viable option to increase cell performance. This could change in future if a process for low temperature growth of high quality graphene ever becomes available, although it is unlikely that low temperature growth will ever be able to compete with high temperature graphene growth in terms of quality.
Graphene Transfer: The graphene used in GO-NEXTS wss grown on copper foil and needed to be transferred to a glass substrate for the fabrication of the solar cell. The transfer process involves coating on side of the graphene/Cu foil with Poly(methyl methacrylate) (PMMA) as a protective layer, reactive-ion etching on the other (uncoated) side of the Cu foil to remove the graphene on that side and then wet etching to dissolve the Cu foil. The PMMA/graphene is subsequently transferred to water for rinsing and then transferred to the target substrate, where the film is dried and baked on a hot plate before the PMMA is finally removed with solvents. Two potential problems arise from the transfer process. First, complete removal of the PMMA with solvents is not possible. Although the optimization of the transfer process during the course of the project has significantly reduced the amount of PMMA residue found on the graphene, some residue still remains. Because this residue is on top of the graphene, it will be between the graphene and any material placed on top such as molecular dopants or the hole/electron transporting layers of the solar cell possibly leading to diminished device performance. And second, the etching of the copper foil also causes residue, large particles from the etching solution that adhere to the bottom of the graphene during etching and are transferred to the target substrate along with the graphene. Because these particles are underneath the graphene, they locally raise the graphene electrode into the solar cell. Depending on the thickness of the polymer layers deposited onto the electrode, this can lead to a decrease of the shunt resistance or even to short circuits within the cell. In the project Specific cleaning procedure based on temperature treatment, proper solvent choice and rapid termal annealing have been developed allowing to achieve a better quality of the film in terms of surface cleaning and reduced roughness without affecting the graphene quality evaluated through raman spectroscopy.
Metal grids: The metal grids were introduced as part of the graphene electrode to reduce the sheet resistance without having to stack multiple graphene layers that would need to be transferred individually on top of each other, which is not only time consuming but also causes additional particle contamination with each transfer step. The grid geometry was chosen to achieve an overall transmittance of around 90% for the graphene/grid electrode. For this given geometry, the sheet resistance then depends on the thickness of the metal lines. As mentioned in the last section, much better cell performance is achieved when using a graphene/grid hybrid electrode compared to a graphene electrode without a grid, but the actual sheet resistance of the grid doesn’t appear to have a noticeable effect on the PCE of the cell. To increase the PCE, the grid spacing could be reduced to decrease the distance charge carriers need to travel through the graphene. If the linewidth is reduced proportionately, transmittance and overall sheet resistance are not affected.
Molecular adsorbate doping: Reducing the sheet resistance is also possible by functionalizing graphene with n- or p-type adsorbate dopants. During the project a number of candidate molecules for both p- and n-type doping were assessed. p-type doping is of interest to create graphene layers as a direct alternative to ITO. n-type doping was of interest for the proposed tandem solar cells containing both p- type graphene electrodes and also n-type graphene. s-triazine and PEIE (polyethylenimine 80% ethoxylated) were investigated as n-type dopants. PEIE-functionalised graphene micron-scale field-effect devices showed a shift in the Dirac point consistent with (net) n-type doping, however the sheet resistance (at zero gate voltage) remained high (several kΩ/sq ), P-doping with bis(trifluoromethanesulfonyl)amide (TFSA) showed more promising results with a reduction in sheet resistance from 2400 Ω/sq to 400 Ω/sq. Nevertheless, the sheet resistance was still considerably larger than the target value, therefore further improvements in functionalization protocols are needed and the stability of molecule-functionalised graphene is still a concern.
In WP4 the realization of the OSCs and PSCs with graphene textured electrodes has been perfomed.
The biggest issue with graphene electrodes during fabrication of the devices was the compatibility of graphene with the process steps and the materials used. For example, annealing at high temperatures (> 300 °C) in air or with oxygen containing materials such as TiOx, which is used in the mesoporous perovskite cells, will damage the graphene. Evaporation of materials, as is the case for MoO3, which is used as hole transporting layer, does not damage the graphene. However, most materials are spin coated onto the graphene and in this case, wettability can be an issue depending in the material or solvent involved. Due to the hydrophobic nature of the graphene surface, aqueous solutions are especially prone to dewetting when deposited onto graphene. In the worst case, complete dewetting takes place, but partial dewetting can also occur locally around the particles transferred along with the graphene, which act as nucleation sites for dewetting. This limits the materials available for the fabrication of solar cells on graphene electrodes although the addition of surfactants into the solution could improve the wettability.
In this workpackage many different architectures have been tested to realize OSCs and PSCs both single layer and tandem using substrates with gridded graphene on top of grids with variable thicknesses. In particular for the single active layer OSCs both direct and inverted architecture have been tested using the high efficiency blend PTB7:PCBM, while for PSCs the planar inverted was the only structure which could be realized without damaging the graphene contact.
Single layer OSCs reached a maximum efficiency of 4.8% using an inverted configuration, while for tandem devices the maximum value was 6.3%, with an overall improvement of 1.8% respect to the single layer device.
In the case of perovskite solar cells a maximum efficiency of 7.1% has been reached which is very close to the value obtained on the reference device on ITO (8.5%).
An additional activity has been performed to nanopattern the active layer of the solar cell using nanoimprint lithography on ITO substrates. The solar cell with the patterned active layer showed an efficiency 14% higher than the flat cell, in accordance with the assumption of increasing performances with the introduction of a nanostructured, light-trapping layer.
The same imprinting technique has been tested on graphene on grid substrates, but it was not possible to reproduce the same improvement. This was probably due to the fact that the graphene contacts, when treated in the high pressure environment of the NL, tend to degrade more easily, as much as the blend P3HT:PCBM tends to preferentially form big cluster of PCBM.
In conclusion, the GO-NEXTs consortium have made an in-depth investigation of the key process challenges involved in development of graphene-based transparent electrodes for organic photovoltaic devices. This work is also relevant to other organic electronic devices. Through careful process development and comparison with “like-for-like” reference devices, the consortium has identified key challenges to be overcome for graphene-based electrodes to compete with incumbent metal-oxide based transparent electrode technologies.
Potential Impact:
Impact
The research performed in the project will have a specific impact on solving the outstanding technical problems of achieving high efficiency OSCs and PSCs using new semitransparent graphene electrodes and introducing efficient light management in the device. Such research will have a fundamental impact not only in the field of OSCs and PSCs but for all photovoltaic technologies.
The semitransparent electrodes in fact are a fundamental component of all solar cells being used as contact layers, either as cathodes or anodes or both. Many transparent conductive oxides (TCO’s) (indium oxide, molybdenum-doped indium oxide, zinc oxide, tin-doped zinc oxide, tin oxide, and fluorine-doped tin oxide) have been used to make solar cells. Among all these TCO’s, tin doped indium oxide (ITO) represented the preferred choice due to performance and film characteristics.
By considering the technological development of graphene, the hype following graphene discovery in 2004 is coupled with the question whether graphene can live up to its expectations in terms of industrial applications, such as those investigated in the GO-NEXTS project. Some of the most likely applications to emerge in the next few years are those that use transparent, flexible graphene electrodes such as solar cells, light emitting diodes (LEDs), organic light emitting diodes (OLEDs), touchscreens and LCD displays to replace ITO. The application as consumer products will be highly visible, potentially changing everyday life, and thus subsequently creating a profound impact on future graphene research and marketability.
Europe right now has the leading edge on graphene research, thanks also to the Graphene flagship action, especially from the experimental point of view as demonstrated by the recent awarded Nobel Prize in Physics to the Manchester group. The results achieved in the GO-NEXTs project could contribute in the advance in this field, ensuring Europe’s competitiveness with USA and Asia in the market of graphene-based technology.
Dissemination
The major activity on dissemination was directed to the scientific community by publications in journals and by contributions to specific conferences. So far 11 papers have been published and 32 talks as well as 8 posters have been presented at different conferences and workshops. The rather low number of publications so far is due to difficulties during the first half of the project in achieving the high risk goals of developing low-temperature chemical vapour deposition routes for graphene and fabricating textured graphene electrodes. However, after a subsequent refocusing of our tasks, very promising results have been achieved in the last months so that there are several papers in preparation on the fabrication and characterization of organic solar cells with graphene electrodes using different device architectures. Lists of the publications, conference contributions and papers in preparation can be found at the end of this report.
Additionally, different specific dissemination activities have been carried out to inform the general public:
• AMO hosted a visit of a German Army Delegation (Blauer Bund) to inform about the material Graphene (February 12 2014).
• Visit of school students (Inda Gymnasium, Aachen) was hosted at AMO (July 5 2013).
• There was a public information event on Graphene organized by RWTH Aachen in the town Hall of Aachen (July 11 2013). Daniel Neumaier from AMO contributed to that event by participating at the panel discussion.
• At the researcher night in Lucca activities of GO-NEXTs were presented: 20-09-2013 - Real Collegio, Lucca, Researchers Night – "SHINE"
• At the public event NanoDay organized at the Deutsches Museum in Munich on Nov. 22, 2014
For training PhD students and young researchers a summer school on organic photovoltaics (ISOPHOS 2013) was organized by Center for Hybrid and Organic Solar Energy (CHOSE), University of Rome Tor Vergata with support from the project GO-NEXTs.
At AMO THz-based non-contact conductivity mapping was applied to graphene based electrodes. Therefore GO-NEXTS opened up a new market for this technology at AMO, which has so far attracted several new customers. The activities on THz-based conductivity have been running since April 2014 at the AMO spin-off Protemics (http://www.protemics.com).
Exploitation
The exploitation of GO-NEXTS results focuses primarily on the continued use of know-how generated by each partner within the project in future research activities.
At UTV, the competencies gained within the GO-NEXTS project led to an increase of know how both in theoretical and in experimental field. A simulation system based on integration of the in-house developed software tiberCAD and on the commercial software COMSOL, allowed to model 3-dimesional optoelectronic properties of nanostructured organic/perovskite solar cells extracting the main solar cell characteristic parameters such as absorption of the nanopatterned layers, power conversion efficiencies, fill-factor, open circuit voltage, short circuit current. Thanks to the versatility of the simulation procedure implemented, the knowledge developed will be exploited in solar cell architectures as well as in other fields of research related to the simulation of other organic/perovskite optoelectronic devices such as nanostructured photodetectors, LEDs, LASERs.
From an experimental point of view, UTV has developed several technological processes to realize solar cells on graphene contacts. State of the art devices for single layer and tandem polymer bulk heterojunction solar cells have been realized both on ITO and on graphene. A low temperature process to realize planar perovskite solar cells has been implemented and the compatibility of this process with graphene contact has been demonstrated. UTV started a specific research line on graphene based solar cells which resulted in many collaborations around Europe with leading groups active in the field. In this context, UTV has been invited to participate to the COST action “StableNextSol” on the stability of new generation solar cells as working group leader, the task is the use of perovskites and new materials including graphene to improve the stability of the devices. Furthermore, UTV entered in the Graphene Flagship in the work-package on energy with a specific activity on perovskites and graphene based materials.
The knowledge gained within the GO-NEXTs project related to graphene semitransparent electrodes will be also exploited by UTV to realize other TCO-free optoelectronic devices such as LEDs, LASERs and photodetectors.
At Quantavis, the activity within the GO-NEXTs projects has eventually led to the integration within the NanoTCAD-ViDES simulation framework of a module able to simulate bulk-heterojunction solar cells with graphene electrode, so to extract all the main figures of merit like Voc, Fill-factor and power conversion efficiency. This makes Quantavis ready to enter the growing market of organic and flexible solar-cells, through providing consulting services to industries involved in solar cell fabrication. The experience gained in the investigation of graphene/metal contacts as well as the developed simulation framework, will allow Quantavis also to provide guidelines not only in the solar cell field, but also in the field of two-dimensional-based transistors, where the contact resistance is largely degrading performances of current fabricated devices.
At AMO, improvements in the graphene transfer process achieved during the project have enabled AMO to transfer graphene to target substrates with significantly reduced residue. Because the transfer of clean graphene is still a critical issue within the graphene research community, the extensive graphene research at AMO will greatly benefit from the results achieved during Go-NEXTs. Additionally, the development of a process for the fabrication of embedded metal grids to decrease the sheet resistance of the graphene electrode can also be exploited in ongoing and future research at AMO. The process can be easily adapted for the fabrication of local embedded back gates for high frequency graphene transistors. When using an embedded gate, the substrate surface remains flat and problems associated with transferring graphene onto non-planar surfaces are avoided.
At Tyndall-UCC, key know-how generated during the project included development of adsorbate doping (p-type) recipes for graphene. Significant reductions in sheet resistance were achieved, however further work is needed to enhance stability of functionalised graphene electrodes (a common problem in the community). Tyndall-UCC has also developed cleaning protocols and associated metrology protocols for minimisation of post-transfer residue on graphene surfaces to improve functionalisation efficiency. Finally, in developing 3-D anodised aluminium oxide honeycomb templates and ultra-thin conformal coatings of Pt catalyst layers (initial concept for development of 3-D graphene electrodes through subsequent CVD), Tyndall has significantly enhanced its capabilities in development of high surface area electrodes and devices for energy storage applications (electrolytic and electrostatic supercapacitors).
At TUM, the activities focused mainly on the realization of nanopatterned contacts / active layers for polymeric solar cells architectures. The nanofabrication was performed by the mean of nanoimprint lithography: this technique introduced several criticalities over the final realization of bulk heterojunction solar cells. The overcoming of issues like the nano-confinement of polymeric blends and the clusterization caused by temperature and pressure during the nanoimprint, lead to the development of a deep know-how about nanofabrication of patterned organic electronics devices that will be extensively used for photonic crystal effects on both OSC and OLED devices. In addition, TUM investigated the use of other nanomaterials for the realization of transparent electrodes. In particular, the applicability of carbon nanotubes and of silver nanowires electrodes has been demonstrated not only for solar cells but also for other organic devices, e.g. photodetectors.
However, because of the scientific nature of the project, it did not result in the development of a product or prototype device. On a similar note, the results generated within the project are not capable of industrial or commercial application so no patents were filed or other forms of intellectual property protection pursued.
List of Websites:
http://www.go-nexts.eu/
COORDINATOR
Dr.-Ing. Francesca Brunetti
Dept. of Electronics Engineering
via del Politecnico 1
00133 Rome, Italy
Phone +39-06-72597366
Fax +39-06-72597939
francesca.brunetti@uniroma2.it
In the last the organic semiconductor solar cells (OSCs) and Perovskite solar cells (PSCs) have been proposed as a promising route to scalable, economically viable, energy conversion technologies due to the potential for development of low-cost, flexible, large-area cells and modules, reaching efficiencies of 11% (11.6%) for OSCs and of 21% for PSCs respectively.
In this context, the replacement of the transparent conductive oxide (TCO), for the semitransparent conductive electrode, which presents serious issues related to their stiffness, poor transparency in blue region, release of oxygen in the active layer, scarcity and high cost of materials, represents a critical issue. Recently graphene electrodes have been proposed as a promising candidate for the realization of the semitransparent contact. Research on graphene electrodes is however still at its infancy, so that if this material will manage to accomplish this task has still to be proved. In particular, many questions remains open concerning the degree of interaction of graphene with the polymeric layer, which could degrade the outstanding graphene electron conductivity, or the graphene/polymer electron affinity, which plays an important role in the overall solar cell efficiency.
The GO-NEXTs project has proposed to study a new kind of electrodes based on doped, textured (i.e. 3D) graphene electrodes, in order to increase the overall efficiency and performance of organic solar cells. To our knowledge, this represented the first attempt to enhance light trapping in a solar cell by structuring one or more graphene contact electrode(s) in such a way that it will act as photonic crystal(s).
The project benefitted of a theoretical-experimental approach. During the project it has been developed a multi-scale method based on optical and electrical simulations for the study of the theoretical aspects related to the use of doped and textured graphene as a contact on OSCs and PSCs. In particular, it has effectively assessed OSC performance, while investigating the sensitivity of the main figures of merits (i.e. Power-conversion efficiency (PCE), Fill-factor, open-circuit voltage etc.) spanning over the device parameter space (i.e. different materials, gate workfunctions, electrode gratings etc.), as well as outlining the most critical aspects to address in order to obtain large PCE.
From an experimental point of view the graphene-related issues that may affect the device performance have been identified.It was demonstrated that the sheet resistance and transparency of graphene grown at sub-900 °C temperatures were not adequate for the project requirements. Specific protocols based on temperature and solvent treatments have been identified to clean the final layer allowing a substantial reduction of the contaminant on the graphene contact. Centrimetre-scale graphene electrodes doped with inorganic (MoO3) and organic materials (TFSA and PEIE) has been demonstrated although a variability in performance for the organic adsorbate-doped devices has been found; this could be related to changes in interface stability for the ageing of the materials. Furthermore that nanotexturing of graphene electrodes was not possible as both the transferring of the planar graphene sheet onto a nanotextured surface and the direct growth on nanopatterned catalyst were leading to low quality electrodes. In this case, the price for an increase in light absorption of about 20% as expected from simulations would be a drastic increase in sheet resistance, leading to an overall much poorer performance of the fabricated cell. The performances of the graphene electrodes in terms of sheet resistance, have been improved introducing a grid underneath which helped the collection of charges in the devices. Thanks to this improvement it was possible to realize both OSCs with a maximum efficiency of 6.3% in a tandem configuration and PSCs with a maximum efficiency of 7.1%.
Project Context and Objectives:
Motivation.
The primary energy supply in the EU is currently strongly dependent on fossil fuels. Reinventing the energy system based on a low carbon model is thus one of the critical challenges of the 21st Century. Solar energy is by far the largest of all carbon-neutral energy sources. More energy from sunlight strikes the Earth in one hour (4.3 × 1020 J) than all the energy consumed on the planet in a year (4.1 × 1020 J). Development of low-cost photovoltaic energy conversion technologies is thus of key importance.
In the last decade, the organic semiconductor solar cells (OSCs) have been proposed as a promising route to scalable, economically viable, energy conversion technologies due to the potential for development of low-cost, flexible, large-area cells and modules. OSCs have recently shown an impressive improvement in cell efficiency starting from few percent in 2000, they have achieved in the last months values of above 11% (11.6%) for a triple junction device [C. Tao et al., “17.6% stabilized efficiency in low-temperature processed planar perovskite solar cells,” Energy Environ. Sci., vol. 8, no. 8, pp. 2365–2370, 2015]. In the last three years, however, a new technology based on the use of perovskite materials (PSCs), which can be processed by solution and have many realization aspects in common with organic solar cells, has gained the attention of the scientific community reaching a certified value of 21% (http://www.nrel.gov/ ) .
Current state-of-the-art BHJ-SCs are typically fabricated on substrates with sputtered indium tin oxide (ITO) transparent electrodes, which unfortunately presents serious issues related to i) the release of oxygen and indium into the organic layer, ii) the poor transparency in the blue region, iii) its stiffness, which prevents its use in flexible solar cells, and iv) the large cost due to the limited supply of indium. All these aspects make the development and commercialization of a replacement for ITO a major focus in the BHJ-SCs Research and
In this context graphene electrodes have been recently proposed as a promising candidate for the realization of the semitransparent contact. Research on graphene electrodes is however still at the beginning, so that if this material will manage to accomplish this task has still to be proved. In particular, many questions remains open concerning the degree of interaction of graphene with the polymeric layer, which could degrade the outstanding graphene electron conductivity, or the graphene/polymer electron affinity, which plays an important role in the overall solar cell efficiency.
Up to now, the highest efficiency achieved with graphene electrodes are 8% with tandem polymer solar cell, and in a recent paper published in December 2015 the 17.1% with Perovskite solar cell.
The GO-NEXTs project has proposed to study a new kind of electrodes based on doped, textured (i.e. 3D) graphene electrodes, in order to increase the overall efficiency and performance of organic solar cells. To our knowledge, this represents the first attempt to enhance light trapping in a solar cell by structuring one or more graphene contact electrode(s) in such a way that it will act as photonic crystal(s).
The project leverages the combination of two different fabrication processes, and in particular the doping of the graphene, to obtain semi-transparent electrodes as well as the texturing of the electrodes/substrates. This approach, which has never been proposed before, represents a high-risk, high-impact approach. If successful, it should lead to improvements in solar cell efficiency by up to 14%. Furthermore, all the technologies proposed are suitable for large area realization paving the way for scalable fabrication technologies on low-cost flexible substrates.
During the GO-NEXTS project the following concepts have been studied, which all represent significant innovations with respect to current OSC and PSC technology.
Concept: Doped graphene as a (flexible) transparent electrode replacement for indium tin oxide (ITO).
Films of tin-doped indium oxide (ITO) sputter-deposited on glass substrates are widely used as a transparent electrode contact in organic photovoltaic cells and modules. However, this technology has a number of key drawbacks including:
• Shortage of indium supply (from China) and corresponding increase in material cost.
• Material-inefficient deposition process (sputtering).
• Incompatibility with flexible substrates due to the fragility of the material.
Graphene, a monolayer sheet of sp2-hybridized carbon atoms arranged in a honeycomb lattice, has emerged as a promising candidate for (flexible) large-area transparent electrodes.
Concept: Reduced sheet-resistance of doped mono- and multi-layer graphene.
Since intrinsic exfoliated graphene has a sheet-resistance ~ 6 kΩ/square, significant levels of charge-transfer adsorbate doping are required to achieve acceptable sheet resistances for transparent electrode applications, e.g. for displays or photovoltaics. P-doping (HNO3 treatment) was employed to achieve sheet-resistances ~ 130 Ω/square for graphene monolayers (with an induced carrier concentrations > 9x1012 cm-2). A graphene monolayer has a transparency > 97% across the visible range, thus a stack of 4 individually p-doped graphene monolayer films was fabricated by the SKKU/Samsung team to meet the figure of merit required, i.e. sheet resistance of only ~ 30 Ω/square at a transparency of 90%.
Concept: Stability of adsorbate-doped graphene.
Although these initial adsorbate doping results are encouraging, it must be emphasized that, to date, no systematic study of the ambient temporal or temperature stability of the doping process has been reported. In fact, very recent results by the SKKU group on use of graphene anodes in flexible organic light emitting diodes, showed that the sheet resistance of the HNO3-doped graphene electrodes increased by ~30% over the first 30 hours following fabrication. The GO-NEXTS project aimed at addressing these issues in order to achieve the goal of obtaining a sheet-resistance smaller than 100 Ω/square (for a single layer of doped graphene) and smaller than 30 Ω/square for stacked multi-layer graphene. GO-NEXTS will also focus on enhanced ambient stability for molecule-doped transparent graphene electrodes, targeting <10% change in sheet resistance over a period of 14 days. The stability of the electrodes within the encapsulated solar cells is foreseen exceed one year.
Concept: Structured graphene electrodes as photonic crystals for improved light trapping.
The texturing of graphene will be achieved using large area nanopatterning techniques combined with the growth techniques.
Two different approaches have been considered for the nanostructuring of the surfaces: Nanoimprint lithography and electrochemical growth of anodic alumina .
The growth of the graphene on textured substrates will be performed by means of chemical vapour deposition for which a low temperature process will be developed targeting ultimate compatibility with roll-to-roll processing. The use of the structured graphene electrodes as photonic crystals will induce an increase of the light absorption in the active layer of the solar cell.
Concept: Graphene-based organic photovoltaics.
The concept of graphene doping and graphene structuring was used for the realization of single active layer or tandem OSCs or PSCs. Many different architectures can be foreseen for such devices. The p- or n-type doped graphene electrode was used as bottom directly in contact with the active layer or in combination with a proper hole and electron transporting layers at the interface with the active film. Light harvesting of the active layer has been enhanced through the inclusion of an optimized grating in the architecture. The grating will be realized via a texturing of the graphene electrode and or of the active layer.
Concept: Multi-scale simulations of graphene-based solar cell
Due to the novelty of graphene research field, design principles of graphene based solar cell have not yet been extensively elaborated, and different design options must be explored, evaluated, and optimized. From this perspective, numerical simulations can greatly help in order to overcome fabrication issues. To this purpose, a multi-scale method based on atomistic simulations (both ab-initio and tight-binding) for the calculation of the electrical properties of the material and a drift-diffusion model for the study at the macroscopic level of the complete device performance have been extensively exploited.
The developed approach allows to consider different architectures of graphene based organic solar cells, while assessing their performance and providing to the fabrication side relevant information concerned with i) the suitable material parameters ii) electrodes geometries and iii) configuration to be chosen to obtain large efficiencies.
Light management also plays an important role in the overall efficiency of a planar solar cell. The use of nanopatterned graphene electrodes or of a combination of nanopatterned gratings and graphene electrodes induced a light trapping within the solar cell enabling effective photon management while having a minimal impact on the design of current solar cells and their processing techniques. Even in this case, simulations will represent an important tool in order to drive fabrication process towards the realization of high efficient BHJ-SC.
The expected final results from GO-NEXTs consortium were the delivering of significant innovations in transparent electrode materials, fabrication processes and device architectures in order to reach the ultimate goal of low-cost organic photovoltaic technologies. The following objectives are targeted:
• Development of fabrication and doping processes to realize stable monolayer graphene p- and n-oped electrodes with sheet resistances < 100 Ω/square and <10 % change in sheet resistance over a period of 14 days. The stability of the electrodes within the encapsulated solar cells is foreseen to exceed one year.
• Development of organic photovoltaic cells with one or both electrodes fabricated from doped graphene (ie single or tandem cell architecture).
• Development of processes for fabrication of nanostructured graphene-based photonic crystal electrodes targeting improved light trapping in the organic photovoltaic cells in the visible and/or near-infrared spectral ranges.
• Development and realization of graphene-based high-efficiency tandem solar cells (efficiency≥14%)
• Development of experimentally validated models for the simulation of solar cells with graphene contacts, and distribution of the simulation tools implemented under BSD open-source license.
To achieve these objectives, the GO-NEXTS project benefited from the tight collaboration between the experimental and theoretical framework. In particular, multi-scale simulations are providing both performance evaluations of the entire solar cell, as well as the guidelines to the fabrication side. From the experimental point of view, technological processes for the realization of the textured doped contacts are under evaluation and the crucial elements of the device fabrication process are identified (e.g. films thickness, morphology, operational areas, texturing dimensions and doping) and optimized.
Project Results:
The project proposed a theoretical/experimental approach for the analysis and development of organic solar cells (OSCs) and after the first reviewing period perovskite solar cells PSCs using graphene textured electrodes deposited with chemical vapour deposition (CVD). The aim wss to obtain, through the use of graphene contacts, high-performance ITO-free solar cells on rigid and flexible substrates.
The scientific approach that was employed to attain our objectives wss based on the use of the state-of-the-art technology for the growth and doping of graphene contact on large area substrates, for the nanopatterning (NIL apparatus, Obducat Eitre ) and for the realization of OSCs and PSCs.
The activity is divided into five work packages (WPs); three work packages dedicated to the RTD.
In particular, WP2 aimed at developing a multi-scale method for the study of the theoretical aspects related to the use of doped and textured graphene as a contact on OSCs and PSCs.
The activity carried out within the this WP has resulted to be really useful in providing guidelines to the fabrication side. In particular, it has effectively assessed OSC performance, while investigating the sensitivity of the main figures of merits (i.e. Power-conversion efficiency (PCE), Fill-factor, open-circuit voltage etc.) spanning over the device parameter space (i.e. different materials, gate workfunctions, electrode gratings etc.), as well as outlining the most critical aspects to address in order to obtain large PCE.
This has been possible through the development of a multi-scale simulation approach (ranging from atomic to the device level), which, for the first time, has led to a comprehensive analysis of OSCs with graphene exploited as transparent electrode. In particular, simulations have shown that, graphene OSCs can exhibit better performance as compared to ITO counterpart, if series resistance of the contact is minimized (e.g. if high-quality graphene is used), and if the workfunction is optimized in order to reduce series resistance and induce a higher built-in electric field in the cell. Further improvement can be obtain with graphene electrode grating. The same techniques can also be used with the new and promising Perovskite materials for achieving record efficiency.
WP3 was devoted to the development of doped and textured graphene electrodes. Low temperature CVD graphene growth techniques and molecular doping processes were considered targeting stable, low-resistance transparent graphene electrodes. The texturing will be achieved with two different strategies: the nanoimprint lithography, which allows different kind of shaping of the patterning and the use of anodic alumina as template for the growth of 3D graphene electrodes.
During this activity many graphene-related issues emerges i.e. the quality of the graphene, the process for transferring graphene from the Cu foil it is grown on onto the target substrate, the fabrication of metal grids, the doping of graphene, the nanopatterning and the fabrication of the solar cell itself. In the following the resulting considerations related to each topic.
Assessment of low-temperature CVD growth of graphene: For the first half of the project, graphene electrodes were fabricated by graphene grown at AMO (using CVD on Ni substrates) and Tyndall (using CVD on Cu substrates). During the project it emerged that for chemical vapour deposition of planar graphene at sub-700 °C process temperatures, the sheet resistance and transparency of graphene grown at sub-900 °C temperatures were not adequate for the project requirements. It has been demonstrated that growth temperatures in excess of 900 °C are required for production of high quality graphene (sheet resistance ~1 kΩ/sq and transparency > 95%). Lower growth temperatures resulted in graphene films with sheet resistance values >> 1 kΩ/sq.
Graphene nanopatterning: According to simulations performed in GO-NEXTS, a nanopatterned 3D electrode could increase light absorption by up to 20 %. The fabrication of nanotextured graphene electrodes was explored in GO-NEXTS. Simply transferring a planar graphene sheet onto a nanotextured surface will not work. Due to the increased area of the nanotextured surface compared to a planar surface, the graphene placed on top of e.g. an array of pillars will stretch to the point of being torn apart. The other alternative was growing graphene directly onto a nanotextured metal surface. For this to work, the graphene growth temperature must be low enough to not damage the texturing of the metal growth substrate. The temperature for which the metal catalyst layers deposited onto 3D substrates underwent significant roughening was below 500 °C, but for this value CVD growth of continuous graphene electrodes onto nanopatterned substrates was not possible.
The maximum growth temperature that does not destroy the texturing was determined to be around 400 °C, which is too low to grow high-quality graphene. In fact, the grown films, although graphitic in nature, more closely resemble amorphous carbon than graphene. In other words, the price for an increase in light absorption would be a drastic increase in sheet resistance, leading to an overall much poorer performance of the fabricated cell. For this reason, nanotextured graphene electrodes are presently not a viable option to increase cell performance. This could change in future if a process for low temperature growth of high quality graphene ever becomes available, although it is unlikely that low temperature growth will ever be able to compete with high temperature graphene growth in terms of quality.
Graphene Transfer: The graphene used in GO-NEXTS wss grown on copper foil and needed to be transferred to a glass substrate for the fabrication of the solar cell. The transfer process involves coating on side of the graphene/Cu foil with Poly(methyl methacrylate) (PMMA) as a protective layer, reactive-ion etching on the other (uncoated) side of the Cu foil to remove the graphene on that side and then wet etching to dissolve the Cu foil. The PMMA/graphene is subsequently transferred to water for rinsing and then transferred to the target substrate, where the film is dried and baked on a hot plate before the PMMA is finally removed with solvents. Two potential problems arise from the transfer process. First, complete removal of the PMMA with solvents is not possible. Although the optimization of the transfer process during the course of the project has significantly reduced the amount of PMMA residue found on the graphene, some residue still remains. Because this residue is on top of the graphene, it will be between the graphene and any material placed on top such as molecular dopants or the hole/electron transporting layers of the solar cell possibly leading to diminished device performance. And second, the etching of the copper foil also causes residue, large particles from the etching solution that adhere to the bottom of the graphene during etching and are transferred to the target substrate along with the graphene. Because these particles are underneath the graphene, they locally raise the graphene electrode into the solar cell. Depending on the thickness of the polymer layers deposited onto the electrode, this can lead to a decrease of the shunt resistance or even to short circuits within the cell. In the project Specific cleaning procedure based on temperature treatment, proper solvent choice and rapid termal annealing have been developed allowing to achieve a better quality of the film in terms of surface cleaning and reduced roughness without affecting the graphene quality evaluated through raman spectroscopy.
Metal grids: The metal grids were introduced as part of the graphene electrode to reduce the sheet resistance without having to stack multiple graphene layers that would need to be transferred individually on top of each other, which is not only time consuming but also causes additional particle contamination with each transfer step. The grid geometry was chosen to achieve an overall transmittance of around 90% for the graphene/grid electrode. For this given geometry, the sheet resistance then depends on the thickness of the metal lines. As mentioned in the last section, much better cell performance is achieved when using a graphene/grid hybrid electrode compared to a graphene electrode without a grid, but the actual sheet resistance of the grid doesn’t appear to have a noticeable effect on the PCE of the cell. To increase the PCE, the grid spacing could be reduced to decrease the distance charge carriers need to travel through the graphene. If the linewidth is reduced proportionately, transmittance and overall sheet resistance are not affected.
Molecular adsorbate doping: Reducing the sheet resistance is also possible by functionalizing graphene with n- or p-type adsorbate dopants. During the project a number of candidate molecules for both p- and n-type doping were assessed. p-type doping is of interest to create graphene layers as a direct alternative to ITO. n-type doping was of interest for the proposed tandem solar cells containing both p- type graphene electrodes and also n-type graphene. s-triazine and PEIE (polyethylenimine 80% ethoxylated) were investigated as n-type dopants. PEIE-functionalised graphene micron-scale field-effect devices showed a shift in the Dirac point consistent with (net) n-type doping, however the sheet resistance (at zero gate voltage) remained high (several kΩ/sq ), P-doping with bis(trifluoromethanesulfonyl)amide (TFSA) showed more promising results with a reduction in sheet resistance from 2400 Ω/sq to 400 Ω/sq. Nevertheless, the sheet resistance was still considerably larger than the target value, therefore further improvements in functionalization protocols are needed and the stability of molecule-functionalised graphene is still a concern.
In WP4 the realization of the OSCs and PSCs with graphene textured electrodes has been perfomed.
The biggest issue with graphene electrodes during fabrication of the devices was the compatibility of graphene with the process steps and the materials used. For example, annealing at high temperatures (> 300 °C) in air or with oxygen containing materials such as TiOx, which is used in the mesoporous perovskite cells, will damage the graphene. Evaporation of materials, as is the case for MoO3, which is used as hole transporting layer, does not damage the graphene. However, most materials are spin coated onto the graphene and in this case, wettability can be an issue depending in the material or solvent involved. Due to the hydrophobic nature of the graphene surface, aqueous solutions are especially prone to dewetting when deposited onto graphene. In the worst case, complete dewetting takes place, but partial dewetting can also occur locally around the particles transferred along with the graphene, which act as nucleation sites for dewetting. This limits the materials available for the fabrication of solar cells on graphene electrodes although the addition of surfactants into the solution could improve the wettability.
In this workpackage many different architectures have been tested to realize OSCs and PSCs both single layer and tandem using substrates with gridded graphene on top of grids with variable thicknesses. In particular for the single active layer OSCs both direct and inverted architecture have been tested using the high efficiency blend PTB7:PCBM, while for PSCs the planar inverted was the only structure which could be realized without damaging the graphene contact.
Single layer OSCs reached a maximum efficiency of 4.8% using an inverted configuration, while for tandem devices the maximum value was 6.3%, with an overall improvement of 1.8% respect to the single layer device.
In the case of perovskite solar cells a maximum efficiency of 7.1% has been reached which is very close to the value obtained on the reference device on ITO (8.5%).
An additional activity has been performed to nanopattern the active layer of the solar cell using nanoimprint lithography on ITO substrates. The solar cell with the patterned active layer showed an efficiency 14% higher than the flat cell, in accordance with the assumption of increasing performances with the introduction of a nanostructured, light-trapping layer.
The same imprinting technique has been tested on graphene on grid substrates, but it was not possible to reproduce the same improvement. This was probably due to the fact that the graphene contacts, when treated in the high pressure environment of the NL, tend to degrade more easily, as much as the blend P3HT:PCBM tends to preferentially form big cluster of PCBM.
In conclusion, the GO-NEXTs consortium have made an in-depth investigation of the key process challenges involved in development of graphene-based transparent electrodes for organic photovoltaic devices. This work is also relevant to other organic electronic devices. Through careful process development and comparison with “like-for-like” reference devices, the consortium has identified key challenges to be overcome for graphene-based electrodes to compete with incumbent metal-oxide based transparent electrode technologies.
Potential Impact:
Impact
The research performed in the project will have a specific impact on solving the outstanding technical problems of achieving high efficiency OSCs and PSCs using new semitransparent graphene electrodes and introducing efficient light management in the device. Such research will have a fundamental impact not only in the field of OSCs and PSCs but for all photovoltaic technologies.
The semitransparent electrodes in fact are a fundamental component of all solar cells being used as contact layers, either as cathodes or anodes or both. Many transparent conductive oxides (TCO’s) (indium oxide, molybdenum-doped indium oxide, zinc oxide, tin-doped zinc oxide, tin oxide, and fluorine-doped tin oxide) have been used to make solar cells. Among all these TCO’s, tin doped indium oxide (ITO) represented the preferred choice due to performance and film characteristics.
By considering the technological development of graphene, the hype following graphene discovery in 2004 is coupled with the question whether graphene can live up to its expectations in terms of industrial applications, such as those investigated in the GO-NEXTS project. Some of the most likely applications to emerge in the next few years are those that use transparent, flexible graphene electrodes such as solar cells, light emitting diodes (LEDs), organic light emitting diodes (OLEDs), touchscreens and LCD displays to replace ITO. The application as consumer products will be highly visible, potentially changing everyday life, and thus subsequently creating a profound impact on future graphene research and marketability.
Europe right now has the leading edge on graphene research, thanks also to the Graphene flagship action, especially from the experimental point of view as demonstrated by the recent awarded Nobel Prize in Physics to the Manchester group. The results achieved in the GO-NEXTs project could contribute in the advance in this field, ensuring Europe’s competitiveness with USA and Asia in the market of graphene-based technology.
Dissemination
The major activity on dissemination was directed to the scientific community by publications in journals and by contributions to specific conferences. So far 11 papers have been published and 32 talks as well as 8 posters have been presented at different conferences and workshops. The rather low number of publications so far is due to difficulties during the first half of the project in achieving the high risk goals of developing low-temperature chemical vapour deposition routes for graphene and fabricating textured graphene electrodes. However, after a subsequent refocusing of our tasks, very promising results have been achieved in the last months so that there are several papers in preparation on the fabrication and characterization of organic solar cells with graphene electrodes using different device architectures. Lists of the publications, conference contributions and papers in preparation can be found at the end of this report.
Additionally, different specific dissemination activities have been carried out to inform the general public:
• AMO hosted a visit of a German Army Delegation (Blauer Bund) to inform about the material Graphene (February 12 2014).
• Visit of school students (Inda Gymnasium, Aachen) was hosted at AMO (July 5 2013).
• There was a public information event on Graphene organized by RWTH Aachen in the town Hall of Aachen (July 11 2013). Daniel Neumaier from AMO contributed to that event by participating at the panel discussion.
• At the researcher night in Lucca activities of GO-NEXTs were presented: 20-09-2013 - Real Collegio, Lucca, Researchers Night – "SHINE"
• At the public event NanoDay organized at the Deutsches Museum in Munich on Nov. 22, 2014
For training PhD students and young researchers a summer school on organic photovoltaics (ISOPHOS 2013) was organized by Center for Hybrid and Organic Solar Energy (CHOSE), University of Rome Tor Vergata with support from the project GO-NEXTs.
At AMO THz-based non-contact conductivity mapping was applied to graphene based electrodes. Therefore GO-NEXTS opened up a new market for this technology at AMO, which has so far attracted several new customers. The activities on THz-based conductivity have been running since April 2014 at the AMO spin-off Protemics (http://www.protemics.com).
Exploitation
The exploitation of GO-NEXTS results focuses primarily on the continued use of know-how generated by each partner within the project in future research activities.
At UTV, the competencies gained within the GO-NEXTS project led to an increase of know how both in theoretical and in experimental field. A simulation system based on integration of the in-house developed software tiberCAD and on the commercial software COMSOL, allowed to model 3-dimesional optoelectronic properties of nanostructured organic/perovskite solar cells extracting the main solar cell characteristic parameters such as absorption of the nanopatterned layers, power conversion efficiencies, fill-factor, open circuit voltage, short circuit current. Thanks to the versatility of the simulation procedure implemented, the knowledge developed will be exploited in solar cell architectures as well as in other fields of research related to the simulation of other organic/perovskite optoelectronic devices such as nanostructured photodetectors, LEDs, LASERs.
From an experimental point of view, UTV has developed several technological processes to realize solar cells on graphene contacts. State of the art devices for single layer and tandem polymer bulk heterojunction solar cells have been realized both on ITO and on graphene. A low temperature process to realize planar perovskite solar cells has been implemented and the compatibility of this process with graphene contact has been demonstrated. UTV started a specific research line on graphene based solar cells which resulted in many collaborations around Europe with leading groups active in the field. In this context, UTV has been invited to participate to the COST action “StableNextSol” on the stability of new generation solar cells as working group leader, the task is the use of perovskites and new materials including graphene to improve the stability of the devices. Furthermore, UTV entered in the Graphene Flagship in the work-package on energy with a specific activity on perovskites and graphene based materials.
The knowledge gained within the GO-NEXTs project related to graphene semitransparent electrodes will be also exploited by UTV to realize other TCO-free optoelectronic devices such as LEDs, LASERs and photodetectors.
At Quantavis, the activity within the GO-NEXTs projects has eventually led to the integration within the NanoTCAD-ViDES simulation framework of a module able to simulate bulk-heterojunction solar cells with graphene electrode, so to extract all the main figures of merit like Voc, Fill-factor and power conversion efficiency. This makes Quantavis ready to enter the growing market of organic and flexible solar-cells, through providing consulting services to industries involved in solar cell fabrication. The experience gained in the investigation of graphene/metal contacts as well as the developed simulation framework, will allow Quantavis also to provide guidelines not only in the solar cell field, but also in the field of two-dimensional-based transistors, where the contact resistance is largely degrading performances of current fabricated devices.
At AMO, improvements in the graphene transfer process achieved during the project have enabled AMO to transfer graphene to target substrates with significantly reduced residue. Because the transfer of clean graphene is still a critical issue within the graphene research community, the extensive graphene research at AMO will greatly benefit from the results achieved during Go-NEXTs. Additionally, the development of a process for the fabrication of embedded metal grids to decrease the sheet resistance of the graphene electrode can also be exploited in ongoing and future research at AMO. The process can be easily adapted for the fabrication of local embedded back gates for high frequency graphene transistors. When using an embedded gate, the substrate surface remains flat and problems associated with transferring graphene onto non-planar surfaces are avoided.
At Tyndall-UCC, key know-how generated during the project included development of adsorbate doping (p-type) recipes for graphene. Significant reductions in sheet resistance were achieved, however further work is needed to enhance stability of functionalised graphene electrodes (a common problem in the community). Tyndall-UCC has also developed cleaning protocols and associated metrology protocols for minimisation of post-transfer residue on graphene surfaces to improve functionalisation efficiency. Finally, in developing 3-D anodised aluminium oxide honeycomb templates and ultra-thin conformal coatings of Pt catalyst layers (initial concept for development of 3-D graphene electrodes through subsequent CVD), Tyndall has significantly enhanced its capabilities in development of high surface area electrodes and devices for energy storage applications (electrolytic and electrostatic supercapacitors).
At TUM, the activities focused mainly on the realization of nanopatterned contacts / active layers for polymeric solar cells architectures. The nanofabrication was performed by the mean of nanoimprint lithography: this technique introduced several criticalities over the final realization of bulk heterojunction solar cells. The overcoming of issues like the nano-confinement of polymeric blends and the clusterization caused by temperature and pressure during the nanoimprint, lead to the development of a deep know-how about nanofabrication of patterned organic electronics devices that will be extensively used for photonic crystal effects on both OSC and OLED devices. In addition, TUM investigated the use of other nanomaterials for the realization of transparent electrodes. In particular, the applicability of carbon nanotubes and of silver nanowires electrodes has been demonstrated not only for solar cells but also for other organic devices, e.g. photodetectors.
However, because of the scientific nature of the project, it did not result in the development of a product or prototype device. On a similar note, the results generated within the project are not capable of industrial or commercial application so no patents were filed or other forms of intellectual property protection pursued.
List of Websites:
http://www.go-nexts.eu/
COORDINATOR
Dr.-Ing. Francesca Brunetti
Dept. of Electronics Engineering
via del Politecnico 1
00133 Rome, Italy
Phone +39-06-72597366
Fax +39-06-72597939
francesca.brunetti@uniroma2.it