Rear interface engineering of kesterite Cu2ZnSnSe4 solar cells by adding CuGaSe2 thin layers

Kesterite Cu2ZnSn(S,Se)4 thin film technology has been thoroughly investigated dur- ing the last decade as a promising solution in the field of low-cost, sustainable, and environmental-friendly photovoltaic technologies. However, despite efforts to boost kesterite solar cells performance by numerous strategies, the efficiencies remain stagnant around 13%. Some commonly observed issues in this technology refer to recombination events due to the likely presence of defects and, largely in line with the latest, the presence of voids and poor morphologies at the rear interface. This work, partly inspired by the copper indium gallium selenide (CIGS) technology and the use of wide-bandgap Ga-rich region as back surface field (BSF), focuses on an innovative approach using ultrathin CuGa layers at the rear interface to promote the formation of wide-bandgap CuGaSe2, acting as an efficient electron reflector or BSF, and to function as an effective interlayer improving the kesterite crystallinity at the back interface. Kesterite Cu2ZnSnSe4 devices fabricated with added CuGa layers show a general increase in photovoltaic parameters and a significantly enhanced col- lection efficiency compared with reference devices without CuGa. This strategy proves to be successful, for not only passivating but also for improving the Mo/kesterite interface morphology, preventing to a large extent the presence of voids at the back region of the absorber.