Experimentaland Theoretical Study intoInterface Structure and BandAlignment of the Cu2Zn1–xCdxSnS4 Heterointerface forPhotovoltaic Applications
2020
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 Cu2Zn1–xCdxSnS4 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 Cu2Zn1–xCdxSnS4 materials. Tunable
direct band gap energy ranging from 1.51 to 1.03 eV with high absorption
coefficient was demonstrated for the Cu2Zn1–xCdxSnS4 nanocrystals
with changing Zn/Cd ratio. Such bandgap engineering in Cu2Zn1–xCdxSnS4 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.
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