Bandgap engineering of Cu2CdxZn1−xSnS4 alloy for photovoltaic applications: A complementary experimental and first-principles study

We report on bandgap engineering of an emerging photovoltaic material of Cu2CdxZn1-xSnS4 (CCZTS) alloy. CCZTS alloy thin films with different Cd contents and single kesterite phase were fabricated using the sol-gel method. The optical absorption measurements indicate that the bandgap of the kesterite CCZTS alloy can be continuously tuned in a range of 1.55-1.09 eV as Cd content varied from x = 0 to 1. Hall effect measurements suggest that the hole concentration of CCZTS films decreases with increasing Cd content. The CCZTS-based solar cell with x = 0.47 demonstrates a power conversion efficiency of 1.2%. Our first-principles calculations based on the hybrid functional method demonstrate that the bandgap of the kesterite CCZTS alloy decreases monotonically with increasing Cd content, supporting the experimental results. Furthermore, Cu2ZnSnS4/Cu 2CdSnS4 interface has a type-I band-alignment with a small valence-band offset, explaining the narrowing of the bandgap of CCZTS as the Cd content increases. Our results suggest that CCZTS alloy is a potentially suitable material to fabricate high-efficiency multi-junction tandem solar cells with different bandgap-tailored absorption layers. © 2013 AIP Publishing LLC