SnS thin films for photovoltaic solar cell applications

AlZaidy, Ghadah (2018) SnS thin films for photovoltaic solar cell applications. University of Southampton, Doctoral Thesis, 153pp.

Record type: Thesis (Doctoral)

Abstract

Currently, a wide range of semiconductors are explored for their potential use as absorbers in photovoltaic (PV) applications. General criteria determining the choice of a particular semiconductor are efficiency, cost, availability, scale production and toxicity of the raw material. Among these materials tin (II) sulphide (SnS) has recently received great attention due to its desirable properties for thin film photovoltaic (PV) applications.
This thesis research has focused on two important goals: the synthesis of efficient thin film absorbers using cost-effective techniques, and incorporating it into a PV device showing its capability of working as a PV absorber. SnS thin films were deposited using two different scalable fabrication methods: chemical vapour deposition (CVD) and radio frequency (RF) sputtering.
First a novel deposition method for SnS films was achieved by room temperature CVD using tin (IV) tetrachloride and H₂S as precursors, providing a facile route to high-quality thin films. As-deposited amorphous sulphur rich tin sulphide thin films were transformed to crystalline p-type single phase SnS thin films by an optimised post annealing treatment. The phase transition was confirmed by x-ray diffraction (XRD), Raman spectroscopy and energy dispersive X-ray spectroscopy (EDX). Single phase SnS thin films with a high carrier concentration around 10¹⁵ cm^-3 and mobility of 2.35 cm²/V.s.
Second, as a comparable study SnS thin films were grown by RF sputtering. The as-sputtered thin films were subjected to post-annealing treatment to improve thin film quality. Specifically, the phase change and improvements in optical and electrical properties were studied with various annealing conditions to find the optimal annealing procedure. Experimental observations shows that annealing at 400 ºC under low pressure for annealing time of 4 hours is optimal to obtain single-phase SnS with a high carrier concentration around 10¹⁶ cm^-3 and mobility of 4.74 cm²/V.s.
SnS thin films by both deposition methods were directly grown on n-type Si wafers. The photovoltaic operation was observed in both heterojunctions from their clear rectifying behaviour signifying the formation of p–n junctions. The RF sputtered films exhibit better photovoltaic performance which can be explained by its better crystallinity which improves the electrical properties allowing lower carrier losses and long diffusion length.
Both growth processes were found to give good quality thin films, showing tin monosulphide as the main phase, large grains and suitable properties for PV application, but higher homogeneity and stoichiometry control were achieved using the RF-sputtering route.
The results from these devices show that both methods for preparing single-phase SnS thin films used here produces high quality material that works excellent as an absorber material for PV applications. This study shows a new opportunity to discover new physics and properties in two-dimensional heterojunctions.