The degradation of polymer organic solar cells and materials is one of the most pressing problems facing scientific and commercial development of organic electronics. Degradation can occur in the presence of light exposure together with external oxygen and moisture exposure. We light soak organic solar cell films and then utilize infrared (IR) spectroscopy to identify IR active vibrational modes and the atomistic changes occurring before and after degradation. We find significant measurable IR changes when light exposure is performed in the presence of oxygen or in an ambient environment.
As a prototype, we investigate the low-band-gap polymer PTB7 which has led to higher efficiency organic solar cells. After light exposure, the PTB7-PCBM blend films display significant increases of increased absorption at 1727 cm-1 attributable to increased C=O modes. In conjunction there is a broad increase at 3240 cm-1 attributed to increased hydroxyl (OH) groups within polymer . Light soaking performed in the absence of oxygen/moisture do not lead to large changes in the IR active modes. Our ab-initio electronic structure simulations interpret these by oxidation at the α-C site of the alkyl chains in PTB7, with an irreversible rupture of the alkyl chain and formation of new C=O and C-O-H conformations at the α-C. P3HT-PCBM blends do demonstrate small changes around 2500 cm-1 after light soaking, that may be connected to local H-motion induced rearrangements. Films exposed to the ambient atmosphere in the dark do not show IR active changes, identifying photo-excited singlet oxygen to be the detrimental factor. We will discuss the threshold of photon energies needed to observe photo-structural changes. Understanding nanoscale light-induced structural changes will open pathways to designing more stable organic materials for organic electronics.
 S. Shah, R. Biswas, T. Koschny, V. Dalal, Unusual Infrared Absorption Increases in Photo-degraded Organic Films, Nanoscale 9, 8665-8673 (2017).