| Summary: | Poly(3‐alkylthiophene) films play a central role in various organic devices due to their solvent processability and their remarkable electrical and optical properties. The (photo)electrocatalytic abilities of unsubstituted and solvent‐insoluble polythiophenes in the reduction of O2 to H2O2 in a basic aqueous electrolyte have recently emerged as an advanced function. Herein, the electrocatalytic and photoelectrocatalytic abilities of solvent‐processable poly(3‐alkylthiophene) films at pH 12 are demonstrated, as well as their characteristics are re‐examined from the viewpoints of the polymer structure, electrochemistry, photochemistry, and film nanostructure. A comparison of the above characteristics reveals the requirements for effective (photo)electrocatalytic O2 reduction to H2O2 production. In addition, the addition of an organic salt to the polymer solution changes the formed film characteristics. The thin film of the regioregular poly(3‐hexylthiophene‐2,5‐diyl) containing a small amount of tetramethylammonium bis(trifluoromethanesulfonyl) imide is easily formed by a solvent‐based process and features lower crystallinity, a porous film nanostructure, and high conductivity. This polymer acts as a robust photoelectrocatalyst for the reduction of O2 to H2O2 with a conversion rate of 3.9 × 103 mg (H2O2) gphotocat−1 h−1 or ≈0.040 mg (H2O2) cm−2 h−1 and a high Coulombic efficiency of >95% at 0.1 V bias from the theoretical potential.
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