To improve the constraints of kesterite Cu 2 ZnSnS 4 (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 Cu 2 Zn 1−x Cd x SnS 4 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 Cu 2 Zn 1−x Cd x SnS 4

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... als. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu 2 Zn 1−x Cd x SnS 4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu 2 Zn 1−x Cd x SnS 4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu 2 CdSnS 4 (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.