Successes and Challenges Associated with Solution Processing of Kesterite Cu₂ZnSnS₄ Solar Cells on Titanium Substrates

Roll-to-roll (R2R) processing of solution-based Cu₂ZnSn­(S,Se)₄ (CZT­(S,Se)) solar cells on the flexible metal foil is an attractive way to achieve cost-effective manufacturing of photovoltaics. In this work, we report the first successful fabrication of solution-processed CZTS devices on a variety of titanium substrates with up to 2.88% power conversion efficiency (PCE) collected on flexible 75 μm Ti foil. A comparative study of device performance and properties is presented aiming to address the key processing challenges. First, we show that a rapid transfer of heat through the titanium substrates is responsible for the accelerated crystallization of kesterite films characterized with small grain size, a high density of grain boundaries, and numerous pore sites near the Mo/CZTS interface, which affect charge transport and enhance recombination in devices. Following this, we demonstrate the occurrence of metal ion diffusion induced by the high-temperature treatment required for the sulfurization of the CZTS stack: Ti⁴⁺ ions are observed to migrate upward to the Mo/CZTS interface while Cu¹⁺ and Zn²⁺ ions diffuse through the Mo layer into the Ti substrate. Finally, residual stress data confirm the good adhesion of stacked materials throughout the sequential solution process. These findings are evidenced by combining electron imaging observations, elemental depth profiles generated by secondary ion mass spectrometry, and X-ray residual stress analysis of the Ti substrate.