Optimization of co-evaporation process for Cu2ZnSnSe4 thin film and Fabrication of kesterite thin film solar cell devices

Abstract

Indium-free kesterite-related materials such as Cu2ZnSnSe4 (CZTSe), Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) have attracted significant research interest due to their similar properties to Cu(In,Ga)Se2 (CIGS). In these materials, indium is replaced with earth-abundant zinc and tin metals. Con-trolled substitution of S with Se allows band gap tuning from approximately 1.5 to 1.0 eV, practically cover-ing the optimal range for photovoltaic absorber similarly to indium-based chalcopyrites. Almost successful kesterite solar cell devices are made by two-step processes, which employ a low-temperature deposition step and a high temperature post-annealing step at atmosphere. Because of the challenges of controlling volatile species, especially under high vacuum deposition condition, the kesterite films have been deposited at low temperature and annealed at atmospheric pressure to suppress the loss of Sn-related phases. Although the vacuum deposition processes for kesterite absorber materials have the drawback related with re-evaporation of volatile elements, high temperature co-evaporation with single-step is still attractive for high efficient solar cells, as it affords the real-time manipulation of elemental deposition rates and their simultaneous reaction to form the final copper zinc tin chalcogenide phase during film formation. In this thesis, CZTSe films have been deposited using single-step four-source thermal co-evaporation and we have concentrated on growth and properties of CZTSe absorber layers prepared by co-evaporation of the constituent elements. Firstly, various deposition strategies were investigated to fabricate high efficient CZTSe solar cells, and then studied on the effect of Cu composition on the CZTSe thin films. When the ab-sorber film was deposited with sufficient time for simultaneous reaction to form copper zinc tin chalcogenide phase, the CZTSe layer showed the large grain size and the good crystallinity for desirable absorber layer of t...