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 focused in stablishing the background and foreground of the different partners to define the first PEDR.
Finally, in WP6 all the financial aspects of the project are being closely monitored including the distribution of the budget, the plan of secondment is frequently updated, all the deliverables and reports have been submitted, and project meetings are regularly organized (every 6 months).
