Regulating charge carrier recombination in Cu2ZnSn(S,Se)4 solar cells via cesium treatment: bulk and interface effects

Abstract

Alkali post deposition treatment (PDT) of the absorber material is an efficient strategy to boost the desired photovoltaic performance of kesterite-based Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells. Here, a simple and innovative solution-processed cesium PDT method was employed as an effective strategy for the CZTSSe absorber bulk and interface modification toward efficient CZTSSe-based photovoltaic devices. The material properties of the CZTSSe thin film and the photoelectric properties of the corresponding photovoltaic devices are investigated subsequently. After Cs treatment, the Cs element is mainly detected on the surface of the CZTSSe-based sample, while the phase structure itself is not affected. Systematic studies reveal that the presence of small amounts of cesium in the CZTSSe absorber bulk results in a low defect concentration, prevents the bandgap tail states, enlarges the depletion region width, and reduces the carrier recombination. Meanwhile, the majority of the Cs element detected on the surface inhibits the carrier recombination, and leads to improved carrier transport and extraction. Benefiting from the synergistic optimization effects brought by Cs treatment, a champion device with 12.83% efficiency is achieved using the 1.25 wt% CsCl hot solution with a soaking time of 30 s, which is ≈26% higher than that of its pristine counterpart (10.21%). Our work offers judicious direction to expedite design strategies for solving bottleneck problems regarding the bulk and interface recombination, showing a promising future in developing efficient kesterite-based CZTSSe photovoltaic devices.