Solution processed manganese-based thin films for photovoltaic applications.

Cu2MnSnS4 (CMTS) is regarded as a promising material for thin film solar cells thanks to the abundance, low cost and low toxicity of its constituents and its good optical properties. The record efficiency (1.13%) is still low due to the presence of manganese oxides in the absorber [1]. In this work, CMTS was synthesized by a cheap sol-gel methodology. The thin film was made by blade coating of a solution containing metal precursors and thiourea, acting as the unique source of sulfur, followed by annealing at 550°C. The commonly used CdS was added by chemical bath deposition to act as buffer layer. Solution composition was fine-tuned and effect of deposition under controlled atmosphere and of post-deposition treatments (HCl etching of the absorber and thermal treatment of complete device at 260°) were studied. XRD and Raman spectroscopy revealed the formation of CMTS with stannite structure while optical bandgap was found to be comprised between 1.5 and 1.6 eV. X-ray photoelectron spectroscopy (XPS) was used to determine manganese oxidation state finding only Mn (II), which is the correct state in stannite structure. No oxygen was detected in the material bulk, but deposition under a controlled atmosphere was found to be important to avoid the formation of oxidized species on the samples’ surface. Besides, post-deposition treatments were found to be beneficial for photovoltaic performance, as well as device ageing. The champion device displayed 0.92% efficiency, which is the current record for wet-synthesized CMTS in substrate configuration. The still poor efficiency could be linked to CdS used as a buffer layer. XPS and energy dispersive X-rays (EDX) revealed a manganese loss from the samples’ surface due to ammonia presence in CdS chemical bath deposition; moreover, ultraviolet photoelectron spectroscopy (UPS) found a detrimental cliff-like band alignment between CMTS and CdS. [1] V. Trifiletti et al., Solar Energy Materials & Solar Cells 254 (2023) 112247