Low-temperature APPJ printing of Kesterite on flexible substrates for heterojunction solar cells

Abstract

Kesterite Cu₂ZnSnS₄ (CZTS), composed of earth-abundant, non-toxic elements, is considered a promising chalcogenide photoabsorber. One of the major roadblocks in its large-scale implementation is limited scalability for roll-to-roll fabrication at low temparatures. Herein, we reported an ambient-air deposition method for CZTS films onto rigid as well as flexible surfaces at a low temperature (∼170 °C) and tested their applicability in heterojunction solar cells. In this process, the precursor solution was directly coated onto conductive substrates using an atmospheric pressure plasma jet (APPJ). The rich chemistry of APPJ was used to engineer the surface energy of the substrates, while the charged particles, neutral metastable species, and radicals were used to activate reactions that were thermodynamically unfavorable under ambient conditions. Here, we carefully controlled a high temperature in the plasma while maintaining a low temperature at the substrate. This open-air processing overcame the complex and high-temperature (∼500 °C) requirements for controlled sulfurization or selenisation. By employing an environmentally benign molecular precursor made with a green solvent, our method aligned well with the increasing demand for industry-relevant, eco-friendly manufacturing processes. The plasma-printed CZTS showed p-type characteristics and a band gap in the visible region. As a proof of concept, we measured the power conversion efficiencies (PCEs) of functional CZTS/CdS heterojunction solar cells on both rigid ITO substrates (~3.1%) and flexible ITO substrates (~1.1%). The measured PCEs set new benchmarks for low-temperature-processed CZTS devices. Through favourable engineering of interfacial layers and the integration of passivation layers, higher efficiency and stability could be achieved.