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Development of thin film Solar cells based on WIde band Gap kesterite absorbers

Periodic Reporting for period 1 - SWInG (Development of thin film Solar cells based on WIde band Gap kesterite absorbers)

Reporting period: 2015-06-01 to 2016-11-30

The aim of this project is to develop wide band gap thin film solar cells based on kesterite absorbers for future application in high efficiency and low cost tandem PV devices. The SWInG working group focuses both on the development of the processes for the synthesis of such solar cells based on the Cu2ZnXY4 (with X = Sn, Si, Ge and Y = S, Se) compounds, and on the understanding of the physical and electrical properties of the high band gap absorber in order to reach high conversion efficiency. The key research challenges are: developing up-scalable processes for the synthesis of the absorbers; defining the specifications for high quality wide band gap absorbers as well as suitable back contact and buffer/window layers; and assessing the potential of this technology for PV applications. The wide band gap thin film solar cells developed in this project are expected to reach state-of-the-art efficiencies. Publication of specifications for the synthesis of a high quality Cu2ZnXY4 absorber as well as suitable back and front contacts are expected. The lead users will be PV modules manufacturers and companies that design and produce the machines for the synthesis of such devices. The results will be disseminated and communicated to the European PV industries and the scientific community. The intensive exchange of researchers between the partners during the project will also lead to an enhanced European collaboration in the research field of thin film solar cells.
A summary of the key technical results achieved so far:
- Screening of different ternary, quaternary and pentanary absorber materials
- Fabrication of an evaporation-based Cu2ZnGeSe4 absorber with an efficiency of 7.0 %
- Fabrication of an solution-based Cu2ZnGe(S,Se)4 absorber with an efficiency of 6.0 %
- Development of selective etching for ZnSe vs CZGSe, resulting in a 120 mV VOC improvement
- Development of alternative buffer layer (CdIn2S4) for kesterite material, with 7.4 % efficiency
- In-depth study of new materials using XPS, HAXPES and XES
- Scientific modelling of CZTS cells with (non-metallic) transparent back electrode
- Benchmarking of transparent back electrode stack configurations
- Development of functioning semi-transparent CZGeS solar cells
- Successful radio-frequency sputtering of CZGSe quaternary target
- Uniform CZGSe sputtering deposition over 6-inch stainless steel substrates
- Successful transfer of the CZGSe sputtering process to 10x10 cm2 glass substrates
This project is challenging a new, out-of-the-box idea for high efficiency and low cost tandem PV devices. Therefore, the project is advancing the Cu2ZnGe(S,Se)4 wide band gap technology to higher TRL, i.e. from TRL 2 to TRL 4. Small area solar cell efficiencies above 7.0 % have been achieved, which is well beyond the state of the art. In addition, the project already brings better scientific understanding concerning the development of these wide band-gap absorbers, the absorber’s front interface (i.e. etching, cleaning and buffer formation), and the transparent back contact. Now, focus is on improving the absorber layer and its front and rear interfaces, but also the upscaling of these technologies. Developing a new technology based on these materials, the European industries could be brought to consider a new opportunity for the thin film (TF) PV market. This project is a decisive first step to significantly improve the performance (> 30 % efficiency) of the current PV technologies for a reasonable increase of the fabrication costs and to sustain the development of the TF-PV market in Europe. The wide band gap kesterite technology developed in this project will lead to a breakthrough for sustainable and non-toxic TF-PV, applied on its own in, e.g. BIPV, and as a new semi-finished product for the c-Si industry, the enabled high efficiency of the tandem cells helping to accelerate PV use in the energy-mix.