Interface Structure and Band Alignment of CZTS/CdS Heterojunction: An Experimental and First-Principles DFT Investigation.

Sachin Rondiya,Nelson Y Dzade,Yogesh Jadhav,Sandesh Jadkar,Mamta Nasane

doi:10.3390/ma12244040

Abstract

We report a phase-pure kesterite Cu2ZnSnS4 (CZTS) thin films, synthesized using radio frequency (RF) sputtering followed by low-temperature H2S annealing and confirmed by XRD, Raman spectroscopy and XPS measurements. Subsequently, the band offsets at the interface of the CZTS/CdS heterojunction were systematically investigated by combining experiments and first-principles density functional theory (DFT) calculations, which provide atomic-level insights into the nature of atomic ordering and stability of the CZTS/CdS interface. A staggered type II band alignment between the valence and conduction bands at the CZTS/CdS interface was determined from Cyclic Voltammetry (CV) measurements and the DFT calculations. The conduction and valence band offsets were estimated at 0.10 and 1.21 eV, respectively, from CV measurements and 0.28 and 1.15 from DFT prediction. Based on the small conduction band offset and the predicted higher positions of the VBmax and CBmin for CZTS than CdS, it is suggested photogenerated charge carriers will be efficient separated across the interface, where electrons will flow from CZTS to the CdS and and vice versa for photo-generated valence holes. Our results help to explain the separation of photo-excited charge carriers across the CZTS/CdS interface and it should open new avenues for developing more efficient CZTS-based solar cells.

Highlights

The development of scalable, sustainable and economical solar cells demands stable, earth-abundant, non-toxic, and highly efficient absorber materials
To provide an atomic-level insight into the structure and composition of the CZTS/CdS interface and to better understand the energy band alignment derived from the Cyclic Voltammetry (CV) characterizations, we carried out electronic structure density functional theory (DFT) calculations as implemented in the Vienna ab initio simulation package (VASP) [23,24]
We have synthesized a single-phase kesterite-CZTS films with highly uniform surface and optimum band gap of 1.39 eV using radio frequency (RF) sputtering followed by low-temperature H2 S annealing

Summary

Introduction

The development of scalable, sustainable and economical solar cells demands stable, earth-abundant, non-toxic, and highly efficient absorber materials. Owing to efficient charge separation, heterostructures of copper-zinc oxide (CZO) have shown better activity towards hydrogen evolution reaction under sunlight than individual CuO and ZnO materials [5] Notwithstanding their superior optical and electronic properties, to date, the highest reported power conversion efficiencies (PCEs) of Cu2 ZnSnSe4 (CZTS), Cu2 ZnSnSe4 (CZTSe) and Cu2 ZnSn(S,Se). The band alignment is arguably one of the most important properties of the semiconductor–semiconductor heterostructure as it dictates the barrier height, and, controls charge carrier generation and transport phenomena at interfaces and characteristics of the devices employing these interfaces Due to their small extent in one dimension and typical location buried within bulk materials, interfaces are difficult to resolve or access by purely experimental means, but a synergistic experimental–computational approach promises to offer an in-depth understanding structure–property relationship of the CZTS/CdS heterojunction interface. The atomic-level insights derived in the present study should be relevant to other related scientific disciplines such as photocatalysis

Experimental Details

Computational Details

Results and Discussion

Energy

Conclusions