Abstract
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.
Highlights
The advancement of safe, renewable, and low-cost clean energy technologies to substitute the environmentally unfriendly provider of fossil fuels have attracted significant scientific interest in recent years
In earlier investigations,[18−22] we have reported a direct correlation between efficiency and carrier dynamics in solar cells
Interface are 0.21 and 1.15 eV, respectively, compared to 0.11 and 1.38 eV at the CCTS/CdS interface. These results are consistent with the experimental cyclic voltammetric (CV) measurements and point to efficient charge separation with photoexcited electrons migrating to the buffer CdS layer and holes to the absorber
Summary
The advancement of safe, renewable, and low-cost clean energy technologies to substitute the environmentally unfriendly provider of fossil fuels have attracted significant scientific interest in recent years. Cation and anion substitution have been employed to tune the bandgap of CZTS materials in progression toward CZTS thinfilm solar cells.[12,13,24] Low efficiencies of CZTS-based PV cells have been attributed to several factors including defects, poor interface chemistry, inefficient charge separation due to poor band offsets, and carrier trapping. Until now there has been no systematic study dedicated to providing detailed insight into the carrier dynamics and interface chemistry in Cu2Zn1−xCdxSnS4 thin films, which makes this investigation timely. In this communication, we present a synergetic experimental and computational evidence of the beneficial role of cation exchange (Cd for Zn) in Cu2Zn1−xCdxSnS4. Measurements for determining the band structure parameters were carried out using Metrohm potentiostat/galvanostat, Autolab PGSTAT 100 as per our recent studies.[3,25] The femtosecond transient absorption spectroscopy was carried out using HELIOS ultrafast setup.[26]
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