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International cooperation for the development of cost-efficient kesterite/c-Si thin film next generation tandem solar cells

Periodic Reporting for period 1 - INFINITE-CELL (International cooperation for the development of cost-efficient kesterite/c-Si thin film next generation tandem solar cells)

Reporting period: 2017-11-01 to 2019-10-31

INFINITE-CELL aims to stablish and consolidate an International and Intersectoral Cooperation Project between 6 Academic European and Associated Countries Institutions, 2 European Companies, and 4 Third Country Academic Institutions, for the sustainable development of cost-efficient advanced photovoltaic tandem devices, based on the combination of wide band-gap kesterite (Cu2Zn(Si,Ge,Sn)(S,Se)4) technologies as top cell, and low cost c-Si thin film ones as bottom cell, thanks to the collaborative combination of the know-how and partnership generated in two previous and successful FP7 projects: PVICOKEST (269167) and EUROSUNMED (608593). The availability of a very high-efficiency PV technology using exclusively earth-abundant and cheap elements is of crucial importance for our society, in order to consolidate a sustainable and competitive European PV Industry. In order to contribute to this challenge, INFINITE-CELL has implemented a very ambitious plan of secondments with special emphasis in promoting women, Young researchers and intersectoral staff Exchange. In this sense, 32% of the PMs has being performed by women, 63.5% by Young researchers and 34% with the involvement of the Industry. The consortium has demonstrated a thin film c-Si device with over 15%, a wide bandgap kesterite device with over 8% efficiency, and a combined stacked 4T tandem solar cell with almost 1.5 V.
In WP1 conventional high-temperature, as well as low-temperature heterojunction approaches were applied for the fabrication of thin Si based solar cells and mini-modules. Conversion efficiencies above 15% for both types of Si based solar cells have been achieved. It is decided to continue WP1 focusing on development and optimization of Hydrogen implanted and exfoliated as well as e-beam/CVD deposited and exfoliated Si structures (and relevant solar cells) bonded on low-cost substrates, which can used as bottom solar cells for CZTS/Si tandems.
In WP2 different methodologies have been explored to increase bandgap energy of kesterite material. One of them was the partial or total substitution of Se with S, Cu2ZnSn(S,Se)4 (CZTSSe). By this way, efficiencies higher than 6 %, Eg = 1.63 eV and transmittance about 30 % in the Vis-NIR region was obtained for ITO/i-ZnO/CdS/CZTS/Mo/FTO/glass solar cell structure. Another option was the partial or total substitution of Sn by Ge. The formation of Cu2ZnGeSe4 (CZGSe) with Eg = 1.45 eV has been investigated using PVD techniques and efficiencies of 6.5 % and 4.8 % (only 0.9 µm of thickness in this latter case) have been obtained for CZGSe-based devices. Additionally, semi-transparent kesterite-based devices have been fabricated with T > 25 % in the NIR region. Efficiencies of 7.9 % and 5.6 % have been achieved for ITO/i-ZnO/CdS/CZTSSe/V2O5/FTO/glass and for ITO/i-ZnO/CdS/CZTGSe/Mo/ V2O5/FTO/glass solar cell devices respectively.
In WP3 the design principle to consider when making tandem solar cells is choosing the right band gaps in order to optimize harvesting of the solar spectrum. Among the potential top cell absorbers, Cu2Zn(Sn,Ge)(S,Se)4 has an appropriate crystal structure with a direct band gap that can be tuned. The purpose of this workpackage 3 is to fabricate appropriate bottom (N+P and N+P+N junctions) silicon cells, optimized Cu2Zn(Sn,Ge)(S,Se)4 top cells and finally stacking them to form 2T or 4T tandem cells.
In WP4 HZB in close cooperation with IAP-ASM has performed detailed investigations on structural, optoelectronic and transport properties of wide-gap quaternary compound semiconductors. Applying neutron diffraction as well as multi-energy anomalous X-ray diffraction we have determined the crystal structure of Cu2ZnGeS4 and Cu2ZnSiSe4 and revealed the presence of Cu/Zn disorder in these quaternary semiconductors. Also we have started to investigate the phase relations and changes in the crystal structure of anion and cation mutation series. In the Cu2Zn(GexSi1-x)Se4 series the crystal structure changes from the tetragonal kesterite type (Cu2ZnGeSe4) to the monoclinic wurtz-kesterite type structure (Cu2ZnSiSe4). Within 0.45≤x≤0.55 a monoclinic and a tetragonal phase coexist. IREC in close collaboration with UM5, MASCIR and UWC, has carried out a deep analysis of Cu2ZnGeSe4 (CZGeSe) photovoltaic absorber synthesized by a sequential process. Introducing a combinatorial sample which includes all the interesting compositions, cationic ratios were correlated with secondary phases and crystalline quality of CZGeSe, as well as solar cell devices performance. This combinatorial study allows to determine the optimal composition of CZGeSe that seems to be slightly Cu- and Zn-poorer than in the case of the pure Sn compound. First transfer towards transparent substrates shows some deterioration of the absorber quality, which further optimization is ongoing.
In WP5 during this period, the consortium focuses in the awareness of public and scientific dissemination of the project using the different media available including the webpage, social media, outreach activities, publication of scientific papers and presentation of the results at international conferences. For the training of the researchers, the project has implemented several seminars and the first workshop has been organized. For the exploitation of the project results, the EIB has been formed, and the activities have been
INFINTE-CELL is demonstrating key progresses in the two involved technologies. In particular in the thin film c-Si solar cells, conventional high-temperature, as well as low-temperature heterojunction approaches were applied for the fabrication of PV devices. Conversion efficiencies above 15% for both types of Si based solar cells have been achieved. In the case of kesterite, Semi-transparent kesterite-based devices have been fabricated with T > 25 % in the NIR region. Efficiencies of 7.9 % and 5.6 % have been achieved for ITO/i-ZnO/CdS/CZTSSe/ V2O5/FTO/glass and for ITO/i-ZnO/CdS/CZTGSe/Mo/V2O5/FTO/glass solar cell devices respectively. Very relevant progresses are also being obtained in the combination of both technologies in a tandem concept, and first essays related to the implementation of stacked 4T devices demonstrates devices with 1.5V Voc, and 3% efficiency. In terms of characterization of materials and devices, by applying neutron diffraction as well as multi-energy anomalous X-ray diffraction we have determined the crystal structure of Cu2ZnGeS4 and Cu2ZnSiSe4 and revealed the presence of Cu/Zn disorder in these quaternary semiconductors. These systems are very relevant for the development of wide bandgap kesterite.
These progresses can have a strong impact in the near future, that is currently demonstrated by the strong involvement of women, young researchers and specially industry in the plan of secondments of INFINITE-CELL.
c-Si/kesterite stacked 4T tandem prototype
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