Photovoltage Generation at p-Type Semiconducting Polymer/Electrolyte Interfaces

p-Type organic semiconductors are gaining interest as both phototransducer materials for optically controlled bioelectronic devices and photocathode materials for oxygen reduction reactions. Understanding the different competing optoelectronic phenomena arising at the direct interface between the organic semiconductor and an aqueous electrolyte is, therefore, of central importance in the development of future devices. Here, we perform systematic transient and spectroscopic photovoltage measurements on two prototypical conjugated polymers, namely, poly(3-hexylthiophene-2,5-diyl) (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 H,8 H -benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]] (PBDB-T), thin films in contact with phosphate buffered saline electrolyte. To identify the role of the electrolyte and the impact of electrochemical reactions, we compare the measurements to intrinsic photovoltage generation, as observed in capacitively coupled photoelectrodes in the absence of an electrolyte. We explain the large photovoltages observed in water and their dependence on light intensity with an analytical model based on photoactivated forward and backward charge transfer reactions across the organic semiconductor/water interface. The model quantitatively captures all experimental observations, and fitting parameters match with values obtained by chronoamperometric measurements and impedance spectroscopy. The findings are of particular relevance to understand wireless, optically triggered bioelectronic transduction as achieved with p-type organic semiconductors in the form of transducer patches or micro- and nanoparticles in contact with biological cells.