1119 Full Paper Macromolecular Chemistry and Physics wileyonlinelibrary.com © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/macp.201500440 Aqueous Photoelectrochemical Reduction of Anthraquinone Disulfonate at Organic Polymer Films Pankaj Chowdhury, Patrick Fortin, Graeme Suppes, Steven Holdcroft* The photoelectrochemistry (PEC) of poly(3-hexylthiophene) (P3HT) is investigated by studying the photocathodic reduction of anthraquinone-2,7-disulfonate (AQ27DS). A photo- current of ≈ 100 μA cm -2 is obtained using ≈ 5 × 10 -3 M AQ27DS and 100 mW cm -2 visible light. The photon-to current-efficiency approaches ≈ 1% when irradiated with low intensity monochromatic light. The photoreduction proceeds by either a 2-electron, 2-proton, or 2-electron, 1-proton process depending on the solution pH. The band energy levels of P3HT shift by ≈ 150 mV in contact with aqueous solutions over a 1–12 pH range, and considerable more in the presence of AQ27DS during PEC, to the extent that Fermi level pinning occurs. This work reveals that the efficiency of electron–hole separation is required to improve photoefficiency and the interaction of redox couples with the P3HT semiconductor interface is war- ranted in order to understand the photovoltages developed across the semiconductor/electrolyte interfaces. P. Chowdhury, P. Fortin, G. Suppes, Prof. S. Holdcroft Department of Chemistry Simon Fraser University Burnaby Greater Vancouver BC V5A 1S6, Canada E-mail: holdcrof@sfu.ca In addition to hydrogen-evolving, water splitting reactions, other interesting PEC redox reactions may be induced at illuminated semiconductors. For example, photoelectrochemical reduction of carbon dioxide and its conversion to value-added products have been investigated using p-type GaP, Si, GaAs, and Co 3 O 4 . [10–13] Other studied PEC reactions include the reduction of anthraquinone as an intermediary for H 2 O 2 synthesis, [14] and the reduction of nitrate ion to nitrite, ammonium, and nitrogen. [15] A class of semiconductor materials that have received significantly less attention with respect to PEC activity are the π-conjugated polymers. In the field of organic elec- tronic devices, the use of π-conjugated polymers has been demonstrated to offer ease of processing and the poten- tial for manufacturing large area, mass produced devices using established polymer processing routes. [16] A ubiqui- tous organic p-type π-conjugated polymer that has been studied in several application areas of molecular elec- tronics, including organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photo- voltaics (OPVs), is poly(3-hexylthiophene) (P3HT). [17–19] 1. Introduction Photoelectrochemistry (PEC) as a means for energy conver- sion and storage has been investigated for several decades, prompted by the oil crisis period of 1970s. [1–4] The seminal research in photoelectrochemical energy conversion was based on a n-type TiO 2 semiconductor anode and a noble metal cathode to achieve water splitting into hydrogen and oxygen. [1] This has inspired PEC studies that employ many other semiconductor electrodes, both n-type and p-type. [5–7] In spite of extensive research work, the effi- ciency of photoelectrochemical solar hydrogen production from water remains low, or the materials are far too costly for large-scale commercialization. [8,9] Macromol. Chem. Phys. 2016, 217, 1119−1127