As organic photovoltaics (OPVs) based on organic semiconductor active layer materials continue to improve in device efficiency, the need to efficiently and quantitatively understand the degradation processes that lead to a decline in their performance metrics is greater than ever. While traditional analyses of spectroscopic data collected before and after photodegradation of OPV active layer materials rely on qualitative comparisons between spectra from the pristine and degraded materials, spectral changes associated with early-stage degradation are often too subtle to be readily noticed. Development of a diagnostic for defining degradation state of these materials, especially in early-stage device efficiency loss, would be tremendously useful. Toward that end, we employ <i>operando</i> FTIR spectroscopy in a custom-built ATR-FTIR spectroelectrochemical cell coupled with advanced data analysis tools to more quantitatively study degradation processes in hole-only OPV devices based on poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) and 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 layer materials. The multivariate analysis tools of principal component analysis (PCA) and/or linear discriminant analysis (LDA) are coupled with 2D generalized correlation spectroscopy to demonstrate the ability of data-driven, multivariate analyses to distinguish visually identical spectra based on degradation extent and to elucidate the finer details and order of independent degradation processes. Ultimately, this works lays a solid foundation for realization of a diagnostic tool for assessing OPV active layer material degradation.

Novel approaches for the analysis and classification of photodegraded solid-state organic photovoltaic active layer materials utilizing a custom spectroelectrochemical cell

ACS Spring 2022

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