Impact of selenium content on the growth kinetics and optoelectronic properties of Ag-substituted Cu2ZnSnSe4 thin films

Abstract

Silver-substituted Cu₂ZnSnSe₄ has been studied experimentally and theoretically as one of the most promising intermediate band photoabsorbers for thin-film solar cells. Selenization of stacked precursors is an established procedure for synthesizing (Cu,Ag)₂ZnSnSe₄ (CAZTSe) thin films. Various process parameters during selenization have been widely researched; however, the availability of selenium quantity during selenization has limited reports and plays a vital role in controlling the film's composition and properties. This study aims to investigate the effect of selenium quantity (100 – 500 mg) during the pre-annealing at 275 °C for 30 min and selenization at 450 °C for 2 min on the structural, morphological, optical, and electrical properties of CAZTSe films. X-ray diffraction (XRD) analysis confirmed a single-phase kesterite structure for films with lower selenium content (≤ 300 mg), while higher selenium content (> 300 mg) led to the formation of secondary SnSe₂ phases. Raman spectroscopy further confirmed the formation of CAZTSe films. Field emission scanning electron microscopy (FESEM) indicated that grain size increased with selenium content, though excessive selenium availability led to surface deterioration. Optical analysis revealed a bandgap increase from 0.99 to 1.12 eV, attributed to the changes in band edges. Electrical measurements showed consistent p-type conductivity, and significant changes in carrier concentration, mobility, and resistivity with increasing selenium content. Therefore, managing appropriate selenium quantity during selenization is crucial for optimizing the process and producing high-quality absorber layers suitable for efficient solar cells.