Identification of Degradation Pathways of Organic Solar Cells Using Infrared Spectroscopy

Abstract

The degradation of organic solar cells is one of the most urgent problems facing further scientific and commercial development of organic electronics. Degradation can occur in the presence of light exposure together with external oxygen and moisture. We utilize infrared (IR) spectroscopy to identify IR active vibrational modes and the atomistic changes occurring during degradation of organic solar cell films, before and after degradation. We find measurable changes when light exposure is performed in the presence of oxygen or an ambient environment. The low band gap PTB7-PCBM blend and PTB7 films display significant increases of increased absorption at 1727 cm−1attributable to increased C=O modes in conjunction with a broad increase at 3240 cnr attributed to hydroxyl (OH) groups within polymer. Ab-initio modeling indicates that this can be explained by an oxidation of the PTB7 polymer at the a-C site and a irreversible cleaving of the polymer. Light induced degradation performed in the absence of oxygen/moisture do not lead to large changes in the IR active modes. 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 detrimental. The identification of light induced changes in atomic bonding configurations can open up pathways to stabilizing organic solar cells.