Reactive Ion Etching Activating TiO₂ Substrate for Planar Heterojunction Sb₂S₃ Solar Cells with 6.06% Efficiency

Antimony sulfide is a promising photovoltaic material for the top subcell of Si-based tandem solar cells due to its suitable bandgap, high absorption coefficient, low cost, and environmentally friendly properties. However, the electron transport layer (ETL) of antimony sulfide solar cells is generally based on CdS, while narrow-bandgap CdS (E_g = 2.4 eV) absorbs part of the short-wavelength light causing spectral loss. Unfortunately, Sb₂S₃ films could not be deposited uniformly on TiO₂ substrate by the hydrothermal method. For the first time, reactive ion etching (RIE) treatment to TiO₂ surface is developed to activate it for later Sb₂S₃ deposition. Based on this strategy, the obtained Sb₂S₃ film is almost the same as that deposited on CdS, smooth, dense, and uniform. The optimal device efficiency can reach 6.06%, a top value among TiO₂/Sb₂S₃ devices with a new record of short-circuit current density (≈19.4 mA cm⁻²). The high efficiency on RIE-treated TiO₂ ETL is attributed to the high transparency of TiO₂ and the high-quality Sb₂S₃ thin film with suppressed recombination in the Sb₂S₃ bulk film and TiO₂/Sb₂S₃ interface. This work proposes a simple and efficient strategy for deposition of high-quality Sb₂S₃ thin films on inert substrates.