Expelled Water Diffusion from Buried ITO-Polymer Interfaces for Durable Soft Photocathodes During H₂ Evolution

Electrolyte diffusion through conjugated polymers provides novel opportunities for photo-fuel conversion at polymer-electrolyte interphase in soft photoelectrodes. However, their solar-to-hydrogen (STH) conversion efficiencies and lifespans (STH < 4% for a few hours) lag far behind industrial requirements (STH > 10% for 10 years). In addition to many factors already studied in other organic (opto)electronic devices, electrolyte ingress into the polymers provides novel degradation pathways in soft photoelectrodes. At the interphase, electrodes or charge transport layers under polymers are exposed to the diffused electrolytes and induce unwanted (photo)electrochemical reactions. In this work, we demonstrate surface treatments on a transparent electrode (ITO) control electrochemical reactions at buried ITO-polymer to enhance lifespans of soft photocathodes (ITO/Polymer blend/Pt) during H₂ evolution. By four different experimental sets, we reveal hydroxylated ITO surface electrochemically reacts with diffused water molecules in an acidic electrolyte (0.1 M H₂SO₄ in DI water). This reaction immediately reduces current density at zero potential less than 2% of initial value within 500 s. Transient current and open circuit potential measurements and electrochemical impedance spectroscopy show the electrochemical reaction forms hole traps at the buried interface. Phosphonic acid self-assembled monolayer (PA-SAM) treatment is a key strategy to suppress the side reaction. Hydrophobized ITO surface expels the electrolyte from the buried interface and suppress the side reaction. The photocathodes with PA-SAM-treated ITO preserve high current density at zero potential for at least 1 h. This work has amplified attention on the chemical treatments at buried interfaces to control electroactivities in soft (photo)electrochemical platforms.