Strain modulation for enhancing Cu–Zn ordering in CZTS absorber layer using seed layer assisted growth for efficient carrier transport

Abstract

Cu2ZnSnS4 (CZTS) solar cells suffer from lower power conversion efficiency relative to its fellow copper indium gallium selenide thin-film technology, which have been asserted on the existence of non-stoichiometry and high degree of Cu–Zn disorder. Huge disparity among the lattice constants of Mo and CZTS is one of the causes of inducing strain in the film, which often creates defects in the CZTS structure. This work focused on investigating the effect of strain modulation using seed layer (SL) assisted growth on the structural and optoelectronic properties of CZTS films. The results indicate that SL growth of CZTS reduces strain in the film and improves the crystallinity and overall quality of the CZTS absorber, as indicated by SEM and x-ray diffraction studies. Raman shifts to higher wavenumber and photoluminescence (PL) energy shift corresponding to dominant band-to-band transition in SL CZTS correlate perfectly with the high value of order parameter. Bandgap enhancement and reduction in the Urbach energy of SL CZTS implicate higher ordering (reduction in Cu–Zn disorder) due to strain modulation. Consequently, substantial improvement from 2.13 to 13.5 cm2/V s in hole mobility is achieved. Finally, the faster response of the photodetector based on SL CZTS compared to without SL growth supports all the findings. Our results imply that SL assisted growth of CZTS could be critical to obtain a high-quality CZTS absorber layer.