The main progresses beyond the state of the art obtained so far for each sub-objective of STARCELL are the following:
SO-1 (Improved modelling of CRM free PV absorbers and devices by identifying core problems and possible solutions): a complete model for kesterite was developed including all type of defects. It was demonstrated that Sn-related defects are most probably the main efficiency limitation, reducing the carriers life-time, increasing the non-radiative recombination, and increasing the Voc deficit.
SO-2 (Optimization of Kesterite bulk properties by modified thermal treatments and advanced absorber engineering): different absorber engineering strategies were investigated including doping, alloying and customized thermal annealing processes. The importance of Li and Ge doping and the interaction of alkalis with Sn were discovered, graded bandgap concepts using Sn/Ge and S/Se were developed, and the main kesterite formation mechanisms elucidated.
SO-3 (Re-design and optimization of the device structure): (Zn,Sn)O buffer layer was developed by CEA, and additionally IREC has developed a Cd-reduced buffer layer, where (Zn,Cd)S is the most promising candidate for the moment, achieving conversion efficiencies over 10%.
SO-4 (Study of kesterite surface modification for improved solar cell devices): EMPA has proposed Li as the most promising alkali dopant, observing a strong dependence of the optimal alkaline content on the Sn concentration. Additionally, IREC has studied surface passivation, optimizing (NH4)2S as very effective passivating agent. IREC has started a very innovative selective surface doping, based in the incorporation of alkali elements during the CdS deposition, demonstrating the effectiveness for incorporating the alkalis at the surface.
SO-5 (Back contact design to reduce recombination and efficiency loss due to non-fully ohmic rear contact): Different transition metal oxides were investigated and compared, and particularly, the use of 20 nm of TiO2 or Nb2O5 allows to improve the long wavelength response of the solar cells. Additionally, some selenides (CuGaSe2), nitrides and other metals were investigated as possible back contacts. Mo still remains as the best option.
SO-6 (Study of non-homogeneity problems at micro and macro scale): the homogeneity of state-of-the-art kesterite based devices were investigated by several techniques including synchrotron based techniques. Slight compositional fluctuations at macro-scale and some secondary phases has been identified. Nevertheless, at micro-scale, relevant compositional fluctuations have been observed.
SO-7 (Sustainability of materials (life-cycle analysis, recycling/reusing). Demonstration of the technology at TRL5): CEA has developed a complete LCA analysis for kesterite demonstrating their sustainability. Additionally, WIREC has developed a route for the complete recycling of mini-modules onto glass and Steel substrates, demonstrating their scalability and cost analysis.
SO-8 (Cost-effectiveness and commercial exploitation potential): A final plan for exploitation and dissemination was presented opening interesting perspectives for the future commercialization of kesterite.
