Voltage-Induced Degradation of PTB7 with Atmospheric Exposure and the Contributions of Simultaneous Exposure to Light

Previous studies of organic semiconductor (OSC) degradation have focused primarily on photochemical processes, which are thought to be the main cause of shortened device lifetimes. However, OSC optoelectronic devices operate through charge generation, charge mobility through the active layer material, and/or charge recombination that require the presence of an electric field across the device for charge harvesting or injection. The presence of moving charges in these devices implies the presence of polaron forms in the OSC that may have vastly different chemical reactivity than the neutral OSC. Despite the critical role of the electric field in generating, collecting, and mobilizing charge carriers in OSC active layers, the contributions that these charges make to OSC degradation is not well understood. The role of device voltage as a contributor to OSC degradation is studied here for films of poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) using infrared reflectance–absorbance spectroscopy and X-ray photoelectron spectroscopy. It is proposed that the application of voltage across thin film OSCs under dark ambient atmosphere conditions, in combination with doping by atmospheric O2, leads to the generation of superoxide anions that degrade the film through pathways different from those of purely photodegradative processes in which singlet oxygen is proposed to be the predominant player. PBT7 films exposed simultaneously to (light + voltage) host both reactive oxygen species with initial degradation pathways apparently directed to the more reactive polaron sites generated by the applied voltage. These results demonstrate that the effects of applied voltage must be considered when understanding degradation processes in OPVs.