While organic photovoltaics (OPVs) continue to advance in terms of device efficiencies, they still suffer from lower device lifetimes when compared to their traditional silicon counterparts due to degradation of the active layer material from reactive oxygen species (ROS). These ROS can be generated from<i> in situ</i> and external sources, including electric fields, photosensitization, or ozone generation. Correlating the degradation mechanisms to specific ROS within the active layer is crucial to identify the sources of device performance degradation in OPVs. By differentiating the degradation pathways of individual ROS, we can address device vulnerabilities and provide a framework for future material or encapsulation designs. A new donor-acceptor polymer, 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) has high reported power conversion efficiencies and solution processability, but there is little consensus about its degradation pathways. Infrared reflection absorption spectroscopy (IRRAS) has been used to study the degradation of PTB7 exposed to various ROS. Degradation conditions include exposure of thin, solid state films to light, potential, or reactive gaseous species focusing on the <i>in situ</i> conversion of atmospheric constituents into ROS or other reactive species. Although the spectra of each degraded film exhibit changes, the spectral signatures obtained under each degradative condition are unique. Analysis via generalized 2D correlation spectroscopy (2D-COS) and computational modeling support the assertion that degradation mechanism and products are dependent on dominant ROS present. These results emphasize the importance of systematic study of different ROS and their associated degradation products to determine which ROS are primarily responsible for losses in device efficiency.

Degradation of PTB7 by different reactive oxygen species

ACS Spring 2022

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