Distinguishing photodegradation pathways of organic semiconductors on ITO and Ag electrode contacts using IR reflectance-absorbance spectroscopy with multivariate analysis

Although many efforts to study photodegradation of organic semiconductor (OSC) thin films have been reported, few consider the effect of substrate material on degradation pathways. Given the many electrode contacts available, elucidation of the contributions of substrate material to OSC degradation is essential to facilitate the design of more durable devices. Here, we investigate the effect of substrate material on photodegradation of a model organic semiconductor, 4,7-bis(9,9-dimethyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazole (FBTF), an oligomer of the widely used OSC polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), by monitoring spectral changes in FTIR spectra coupled with multivariate analysis. While the identified degradation products are similar on both substrates, the differences suggest substrate-dependent degradation pathways. Along with the expected polyfluorene ketonic degradation product, strong spectral evidence of anhydride formation implies that a previously unreported interchain coupling mechanism is also present. Additionally, new bands in the 2100-2200 cm⁻¹ region observed in spectra from films on Ag are suggestive of ring opening and rearrangement in the benzothiadiazole unit only on the metallic substrate. Although spectra on different substrates appear visually similar during early-stage degradation, principal component analysis (PCA) and linear discriminant analysis (LDA) reveal differences based on substrate type and degradation extent. This work demonstrates the utility of data-driven, vibrational spectroscopy coupled with multivariate methods for the characterization of OSC degradation and the potential for such approaches as diagnostic tools in operando devices.