Effect of heat treatment in sulfur on structural, optical and electrical properties of thermally evaporated In2S3 thin films

M.S Tivanov,I.A Svito,S Rasool,K Saritha,K.T Ramakrishna Reddy,V.F Gremenok

doi:10.1016/j.solener.2021.04.057

Abstract

Crystallinity, optical band gap, resistivity and photoresponse of thermally evaporated In2S3 thin films deposited at a temperature of 350 °C and further annealed in sulfur vapour at different temperature range of 200–300 °C is investigated. It is observed that with an increase of annealing temperature, predominantly β-In2S3 phase is formed and the optical band gap for indirect allowed transitions increases from 1.6 eV to 2.0 eV and for direct allowed transitions from 2.3 eV to 2.7 eV. The electrophysical properties indicate that the activation mechanism of conductivity with an activation energy in the range of 0.5–0.73 eV, which is typical for the presence of indium vacancies in the β-In2S3 crystal structure and for the replacement of sulfur by oxygen atoms. It is also noted that sulfur annealing at temperatures of 250–300 °C leads to an increase in the conductivity and photosensitivity of films, which is suitable for photovoltaic applications.

Highlights

IntroductionIndium sulfide (In2S3) is one of the potential materials used in advanced solar cells, as buffer/window layer in Cu(In,Ga)Se2, Cu2ZnSnS4 and Sb2S3 based thin film solar cells (Spiering et al, 2016; Lee and Yang, 2021; Lugo-Loredo et al, 2014; Shi et al, 2021), as an electron transport layer (ETL) in perovskite solar cells (Yang et al, 2019; Hou et al, 2017) and as a photosensitizer in sensitized solar cells (Yang et al, 2015; Zhang et al, 2014) due to its high stability at room tem perature, transparency in a wide wavelength range, less toxicity of its constituent elements and photosensitive nature
As shown by scanning electron microscope (SEM) images taken in the mode of elastically reflected electrons detection (Fig. 1), the surface of both as-deposited and after sulfur annealing at various temperatures films is developed with a granular structure
0.87 structure, the size of granules enhanced with increase in annealing temperature

Summary

Introduction

Indium sulfide (In2S3) is one of the potential materials used in advanced solar cells, as buffer/window layer in Cu(In,Ga)Se2, Cu2ZnSnS4 and Sb2S3 based thin film solar cells (Spiering et al, 2016; Lee and Yang, 2021; Lugo-Loredo et al, 2014; Shi et al, 2021), as an electron transport layer (ETL) in perovskite solar cells (Yang et al, 2019; Hou et al, 2017) and as a photosensitizer in sensitized solar cells (Yang et al, 2015; Zhang et al, 2014) due to its high stability at room tem perature, transparency in a wide wavelength range, less toxicity of its constituent elements and photosensitive nature. In2S3 is used as a photo-anode in photoelectrochemical (PEC) cells for hydrogen produc tion (Wang et al, 2020). Crystalline β-In2S3 is an n-type semiconductor with direct band gap energy varies from 1.8 eV to 2.4 eV depending on its composition and deposition conditions (Sanz et al, 2013; Bouabid et al, 2004; Ji et al, 2015; Nehra et al, 2015). It is the most stable compound at room temperature (Pistor et al, 2016). We observed the effect of subsequent heat treat ment in sulfur vapor on the structure, phase composition, optical and electrophysical parameters of In2S3 films obtained by the vacuum thermal evaporation

Experimental

Crystal structure

Optical properties

Conclusion