Probing Ultrafast Charge Separation in CZTS/CdS Heterojunctions through Femtosecond Transient Absorption Spectroscopy

Abstract

Chalcogenide-based type II heterojunctions are considered to be a promising candidate for optoelectronic device fabrication such as in solar cells, photodetectors, and light-emitting diodes. Type II heterosystems effectively help in the delocalization of charge carriers owing to facile band arrangements, preventing the recombination of photogenerated carriers resulting in improved photovoltaic efficiency. Herein, we report the synthesis of CZTS nanoparticles and CdS quantum dots using a facile and low-cost hot-injection method followed by fabrication of CZTS, CdS, and CZTS/CdS heterojunction thin films with the help of a spin-coating technique at room temperature. We demonstrated through a combination of steady-state photoluminescence and femtosecond pump–probe spectroscopy experiments that a type-II, staggered band alignment of a CZTS/CdS junction is encouraging for charge carrier transport. We monitored the ultrafast charge carrier dynamics in the junction and confirmed the efficient separation of photoexcited charge carriers in the CZTS/CdS heterojunction. In the CZTS/CdS heterojunction, the photoexcited electrons are transferred from CZTS to CdS, which resulted in a drastic increment of the bleach signal intensity compared to that of bare CdS. Similarly, the photoexcited holes are transferred from CdS to CZTS, monitored by steady-state and time-resolved spectroscopy. A slower bleach recovery confirms the spatial charge separation at the interface of the CZTS/CdS heterojunction, placing electrons and holes at CdS and CZTS, respectively. The controlled introduction of charge carriers and charge separation dynamics in the heterointerface reported here provides a promising approach toward designing CZTS-based solar cells and will open up new avenues for developing more efficient Cu chalcogenide-based photovoltaic and photocatalytic devices.

Topics & Concepts