Photovoltage Generation at p‑Type Semiconducting\nPolymer/Electrolyte Interfaces

p-Type organic semiconductors are\ngaining interest as both phototransducer\nmaterials for optically controlled bioelectronic devices and photocathode\nmaterials for oxygen reduction reactions. Understanding the different\ncompeting optoelectronic phenomena arising at the direct interface\nbetween the organic semiconductor and an aqueous electrolyte is, therefore,\nof central importance in the development of future devices. Here,\nwe perform systematic transient and spectroscopic photovoltage measurements\non two prototypical conjugated polymers, namely, poly(3-hexylthiophene-2,5-diyl)\n(P3HT) and poly[[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4<i>H</i>,8<i>H</i>-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]]\n(PBDB-T), thin films in contact with phosphate buffered saline electrolyte.\nTo identify the role of the electrolyte and the impact of electrochemical\nreactions, we compare the measurements to intrinsic photovoltage generation,\nas observed in capacitively coupled photoelectrodes in the absence\nof an electrolyte. We explain the large photovoltages observed in\nwater and their dependence on light intensity with an analytical model\nbased on photoactivated forward and backward charge transfer reactions\nacross the organic semiconductor/water interface. The model quantitatively\ncaptures all experimental observations, and fitting parameters match\nwith values obtained by chronoamperometric measurements and impedance\nspectroscopy. The findings are of particular relevance to understand\nwireless, optically triggered bioelectronic transduction as achieved\nwith p-type organic semiconductors in the form of transducer patches\nor micro- and nanoparticles in contact with biological cells.