Although organic photovoltaic devices (OPVs) continue to advance in terms of efficiency, they still suffer from inferior lifetimes compared to their traditional silicon counterparts due to degradation of the active layer material from reactive oxygen species (ROS), which can be generated in situ through electric fields and/or photochemistry. Characterizing the presence and reactivity of ROS under different operational conditions is a critical first step to mitigating device degradation. Operando molecular spectroscopy is one attractive approach for such studies. Toward this end, a novel device-like architecture was constructed on an internal reflection element (IRE) to support operando ATR-FTIR spectroscopy on the 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) active layers. Devices based on this material has been reported to have high power conversion efficiencies and good solution processability, but little consensus has been reached about the degradation pathways for PTB7. In the operando ATR-FTIR studies reported here, degradation of thin solid-state films was induced by exposure to light, potential, or a combination of both, enabling a focus on in situ conversion of atmospheric constituents into ROS. Although all spectra of degraded films exhibit changes regardless of degradation conditions, the spectral signatures for each degradative condition are unique. Advanced spectral analysis using generalized 2D correlation spectroscopy (2D-COS) supports the assertion that degradation mechanisms and products depend on the dominant ROS present and hence, operational conditions. These results emphasize the importance of systematic investigation of the different ROS generated under different device conditions and their role in loss of device efficiency.

Sarah Tyler

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