Impact of excess and disordered Sn sites on Cu2ZnSnS4 absorber material and device performance: A 119Sn Mossbauer Study

TL;DR

This study uses Mossbauer spectroscopy to analyze how excess and disordered Sn sites in Cu2ZnSnS4 affect material properties and solar cell efficiency, showing high-temperature processing reduces Sn disorder and improves device performance.

Contribution

It provides new insights into the impact of Sn site disorder on CZTS absorber quality and solar cell efficiency through detailed Mossbauer analysis and comparative processing conditions.

Findings

High-temperature annealing reduces Sn disorder and dislocations.

Device efficiency improves from ~0.145% to ~1% after high-temperature treatment.

Sn-rich samples show higher recombination and lower acceptor concentration.

Abstract

Mossbauer analysis is carried out on CZTS samples, subjected to a low temperature processing at 3000C (S1) and high temperature processing at 5500C under sulfur environment (S2). Loss of Sn is observed in sample S2 due to high temperature thermal treatment.The isomer shifts obtained in the Mossbauer spectra confirms the existence of Sn at its 4+ valance state in both the samples. Relatively high quadriple splitting is observed in S1 with respect to S2, suggesting dislocations and crystal distortion present in S1, which are reduced drastically by high temperature annealed S2 sample. The fabricated solar cell with S1 and S2 absorbers showed significant improvement in efficiency from ~0.145% to ~1%. The presence of excess Sn in S1 allows enhanced recombination and the diode ideality factor shows larger value of 4.23 compared to 2.17 in case of S2. The experiments also validate the fact that S1 with Sn rich configuration shows lower acceptor carrier concentration as compared to S2 because of enhanced compensating defects in S1.