Experimental and Theoretical Study into Interface Structure and Band Alignment of the Cu₂Zn_1-x Cd _x SnS₄...

To improve the constraints of kesterite Cu₂ZnSnS₄ (CZTS) solar cell, such as undesirable band alignment at p-n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu₂Zn_1-x Cd _x SnS₄ through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental-theoretical approach was employed to characterize and assess the optoelectronic properties of Cu₂Zn_1-x Cd _x SnS₄ materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu₂Zn_1-x Cd _x SnS₄ nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu₂Zn_1-x Cd _x SnS₄ helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu₂CdSnS₄ (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p-n junction in the ultrafast time scale and highlight a route to improve device performances.