Temperature-guided solidification of copper coordination complexes as hole transport materials

We report a rapid and controllable solid-state formation process of copper coordination complex hole-transport materials (HTMs) in dye-sensitized solar cells (DSCs), reducing processing times from over 48 h to 20 min. By thermally-induced phase transition of Cu^(I/II)(tmby)₂-based liquid electrolytes from 50 °C to 110 °C, we demonstrated that a 70 °C post-treatment for 20 min is ideal for creating an amorphous HTM with minimal crystallization. Time-dependent Raman spectra confirmed near-complete solvent removal within 20 min, while scanning electron microscopy highlighted a compact, defect-minimized HTM morphology when 4-tert-butylpyridine was employed versus N-methylbenzimidazole. Transient absorption spectroscopy revealed ultrafast dye regeneration (t_1/2,reg = 487 ns) and near-unity regeneration efficiency (99.2%) for short heat treatments, whereas extended treatments (e.g., 60 min) led to μs-scale recombination (26.8 μs) and lower performance. Electrochemical impedance spectroscopy indicated stable charge-transfer resistances at the TiO₂/HTM interface (from 25.6 Ω in liquid state to 27.5 Ω in solid state), confirming efficient hole transport pathways. Under 1 sun illumination, devices retained up to ≈10% power conversion efficiency, while indoor (1000 lux) conditions yielded photocurrents up to 79 μA cm^-2 and peak efficiencies of 16%. These findings establish a robust, reproducible route to form Cu-based HTMs in solid-state DSCs with enhanced low-light performance and highlight key design parameters controlling morphology, interfacial charge transfer, and photovoltaic yield.