hipoCIGS - Publishable Report - 241384 PROJECT FINAL REPORT Final publishable summary report Grant Agreement number: 241384 Project acronym: HIPOCIGS Project title: New concepts for high efficiency and low cost in-line manufactured flexible CIGS solar cells Funding Scheme: FP7-ENERGY-2009-1 Period covered: from 01/01/2010 to 31/12/2012 Dr. Friedrich Kessler, ZSW Tel: +49 711 7870 201 Fax: +49 711 7870 230 E-mail: [email protected] Project website address: http://hipoCIGS.zsw-bw.de - page 1 of 34 - hipoCIGS - Publishable Report - 241384 TABLE OF CONTENTS 1 Executive summary ..............................................................................................................3 2 Summary of project context and objectives.........................................................................4 Objectives..............................................................................................................................5 3 Main S&T results/foregrounds............................................................................................7 3.1 Alternative substrates, back contact and Na supply................................................7 3.2 High quality CIGS at low and high substrate temperature ..................................11 3.2.1 Low temperature CIGS on polyimide, steel, Al, and enamelled Al foils ...........12 3.2.2 High temperature CIGS on steel and enamelled steel foils.................................19 3.2.3 Conclusion...........................................................................................................21 3.3 Evaporated indium sulphide buffer layers .............................................................21 3.4 Series interconnection of cells on polyimide, steel, and enamels ..........................26 3.5 Atmospheric pressure PECVD ZnO as conductive window layer .......................29 4 Potential impact, main dissemination activities and exploitation of results ....................31 4.1 Potential impact.........................................................................................................31 4.2 Main dissemination activities ...................................................................................32 4.3 Exploitation of results ...............................................................................................32 - page 2 of 34 - hipoCIGS - Publishable Report - 241384 1 Executive summary The present report presents results and highlights of the FP7 EU project hipoCIGS, insofar they are not confidential. Except for module encapsulation all (main) deposition and process steps have been addressed which might be suitable for a future cost effective and highly efficient roll-to-roll processing of Cu(In,Ga)Se2 (CIGS) based solar cells and modules. The CIGS absorber was deposited by the co-evaporation method only. Various CIGS coating plants were available from multistage small area batch to multistage medium area. Although on polyimide foil also roll-to-roll processing was possible, it was not a key focus of this project. Since CIGS is grown at substrate temperatures between about 450°C and 650°C, desired and undesired diffusion of elements plays a major role during the absorber formation and thus the choice of substrate material strongly affects the film growths and process parameters. Most interesting substrate materials with regard to the fabrication of monolithically integrated modules were thin polyimide films (25 µm) for the low temperature processing at about 450°C and enamelled steel for the high temperature processing at 600 - 650°C. However, also aluminium and mild steel foils functioned, whereas enamelled aluminium has shown to be not well suited due to cost arguments and physical properties. Best Na doping method at low substrate temperatures was the deposition of a thin NaF layer onto the finished CIGS layer followed by an annealing process. For the high temperature processing on enamelled steel the composition of the enamel layer itself was optimised in a way that it could serve as precursor layer. An In2S3 buffer layer was evaporated from In2S3 powder on an area of up to 30cm x 30cm. On glass reference substrates a cell efficiency of up to 17.5 % was reached. Film growth is characterised by sulphur depletion, even if flash evaporation of the powder is applied. An atmospheric pressure plasma-enhanced CVD method was successfully developed for the non-vacuum deposition of ZnO window layers. Results achieved so far are very promising and work will be continued. Remarkable progress has been achieved in the low temperature co-evaporation of the absorber: So a small-area cell efficiency of 18.7 % was demonstrated both on polyimide and stainless steel foil, whereas 17.1 % and 15.4 % were reached on aluminium and mild steel foil, respectively. This means that – at least in the laboratory – the efficiency gap between low and high temperature CIGS processing could be closed. With an inline and high temperature CIGS processing on enamelled steel a maximum efficiency of 18.6 % could be achieved without additional external doping. To reduce material costs and deposition time the absorber thickness was reduced. Even with thin CIGS layers of only 0.9 µm and 1.24 µm on polyimide efficiencies above 15 % and 16 %, respectively, were obtained. Monolithically integrated mini-modules with 8 – 52 interconnected cells have been fabricated on polyimide foil and enamelled steel substrates by using complete laser scribing on polymer and mixed laser/mechanical scribing on enamelled steel. On polyimide an efficiency of 14.8 % (8 cells in series) was achieved, on enamelled steel 15.4 % (16 cells) and 12.9 % (52 cells) was reached. - page 3 of 34 - hipoCIGS - Publishable Report - 241384 2 Summary of project context and objectives Project context At the stage of project application (2009) the photovoltaic (PV) market was maintaining an impressive annual growth rate of 30 – 50 % during the last few years. Continuously increasing production capacities, enhanced performance of solar modules and decreasing costs of solar modules were impressive testimonials of the technological success of conventional photovoltaic technology based on Si wafer technology. However, thin film PV technologies, comparatively less developed but having high potential for low production costs were drawing considerable attention. In 2008, the thin-film PV module production increased to 0.89 GW, a growth of 123% compared to the previous year. All thin film solar cells can be grown on flexible foils (metal or polymer), provided they fulfil the minimum requirements for chemical, optical, structural and thermal characteristics. Flexible solar cells based on CdTe, CIGS, a-Si based, TiO2-dye based, organic materials have been developed on different types of substrates. The highest CIGS solar cell efficiencies on foils are significantly higher than 6 - 13% achieved with other materials. Flexible a-Si devices are available since more than fifteen years, however, at a module efficiency level of only 4 – 8 %. Thus, the CIGS technique was and still is the only thin-film technology which allows the fabrication of flexible devices with mean efficiencies clearly above 10 %. Flexible CIGS solar cells and modules could be made even more efficient and they offer a high potential for low manufacturing cost. The transfer of know-how and efficiencies from CIGS-on-glass to CIGS-on-foils, however, is challenging due to: - high temperatures required for the formation of optimum absorbers (i.e. some foils cannot withstand the required high temperatures, or foils have intolerably large thermal expansion coefficient) - contamination of the absorber by diffusion of undesired elements out of the flexible substrate in case of metal foils - need of additional Na doping for the growing absorber (on glass substrates Na diffuses out of the soda lime glass into the CIGS) - lack of a common, low-cost substrate with suitable mechanical, physical, and chemical characteristaics. The highest efficiency of CIGS solar cells on various substrates are achieved by growing a sequence of layers such as a sputtered Mo back electrode, vacuum evaporated CIGS absorber, chemical bath deposited (CBD) CdS buffer layer and sputtered ZnO:Al/ZnO front contact. Different methods are used for incorporation of Na into CIGS to achieve the desired electronic properties for high photovoltaic conversion efficiency. High deposition temperatures of the CIGS layer normally are advantageous to achieve high semiconductor quality. On polyimide and metal foils, respectively, substrate temperatures of ~450°C and 550°C can be used for deposition of “high quality” CIGS layers. Enamelled steel substrates would allow the application of even higher temperatures up to 650°C. However, at such high process temperature thermal expansion mismatch of layers and substrates becomes a critical problem to achieve crack-free and defect-free structures. Optimum control of the composition in CIGS layers, microstructure (grain size), morphology and interface properties of adjacent layers including inter-diffusion and structural defects are the key issues for obtaining high efficiency solar cells. Most of the R&D work on flexible solar cells including achievements of record efficiency solar cells, has been performed on small sized substrates and/or using static deposition methods (substrate remains at a fixed position). For industrial production, however, large area substrates (films or foils) are mandatory. They will be in motion during deposition