Defects passivation by solution-processed titanium doping strategy towards high efficiency kesterite solar cells

The performance of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin films solar cells is limited mainly because of the two technical bottlenecks, including multilayer crystallization in non-hydrazine solvents, and the presence of severe non-radiative recombination within the absorber layer and/or heterojunction region, resulting in large open-circuit voltage (Voc) deficit. Herein, we show that by adding an environment-friendly 2-methoxyethanol solvent and adjusting the solution processing conditions without inserting any insulating layer between the Mo substrate and CZTSSe absorber, the multilayer crystals can be removed via a hybrid layered deposition (HLD) approach. In addition, we found that the introduction of Ti4+ in CZTSSe can substantially improve the film morphology, increase the grain size and suppress CuSn defects. Consequently, solar cells based on Ti4+ doped CZTSSe film manifest a conspicuous enhancement in the Voc from 0.455 to 0.505 V with the lowest Voc deficit of 0.278 V, JSC from 36.12 to 39.43 mA/cm−2 along with a remarkable increment in the power conversion efficiency (PCE) from 9.48 to 12.07 %. These results unveil the encouraging prospects of judiciously developed Ti-doped CZTSSe thin-films to alleviate the Voc deficit and boost the performance of kesterite solar cells.