Investigation on limiting factors affecting Cu2ZnGeSe4 efficiency: Effect of annealing conditions and surface treatment

This work aims to unveil the optimal annealing conditions and surface treatments of CZGeSe absorbers, syn- thesized using vacuum-based deposition technique, with an eye to optimizing the main parameters allowing better control of secondary phases formation and improving crystalline quality of this absorber. Firstly, a comparative study is given of one and two-step annealing profiles, where, for each thermal treatment, the optimal temperature is probed. The second section of this study underlines the evaluation of the surface treat- ment effect on the as-annealed absorber using different etching agents. Finally, the effect of different post- annealing treatment temperatures on the overall performance of the fabricated devices is evaluated. For the studied optimizations, a deep understanding of the cell behavior is provided through structural, morphological and electrical characterizations. Preliminary results have given an efficiency up to 5.6% with higher Voc = 572 mV and FF = 65% compared to the reported record cell using similar absorber (Voc = 558 mV, FF = 59%). This performance is linked to the implementation of a two-step annealing process with lower temperatures (330 ◦C/ 480 ◦C) as it showed the best crystallinity-efficiency trade-off along with the smallest amount of ZnSe secondary phase among all the thermal routines studied. In addition, after the evaluation of several etching agents, the implementation of a KCN etching has shown to be the most effective leading to a remarkable improvement of the PN junction through a surface passivation.