Ruthenium Chloride as a Versatile and Stable Hole Transport Material for Organic Solar Cells and Photocathodes

Organic solar cells (OSCs) demand hole transport layers (HTLs) that simultaneously ensure efficient hole extraction, favorable energy-level alignment, and long-term stability for real application. Here, we introduce ruthenium chloride (RuCl₃) as a robust HTL and investigate the effect of annealing temperature on its electrochemical properties and the resulting device performance. The RuCl₃ film processed at 100 °C exhibited a suitably deep work function for organic photoactive materials and effective charge extraction dynamics. Additionally, a low leakage current and reduced interfacial charge transfer resistance enabled the device to achieve a power conversion efficiency of 18.02%. Crucially, RuCl₃ demonstrated exceptional operational durability compared to the conventional HTLs, retaining 80.3% of its initial efficiency after 1724 h under 1-sun illumination and 73.7% after 692 h of thermal stress at 85 °C. Beyond photovoltaics, the implementation of RuCl₃ in photocathodes yields a photocurrent of 14.9 mA cm^-2 and a <i>T</i>₈₀ of 5 h during chronoamperometric operation, underscoring its resilience to electrochemical stress. Together, these results highlight RuCl₃ as a promising alternative HTL for efficient and stable organic optoelectronic devices.