Characterization of the degradation process of Si-PCPDTBT:PC 70 BM(1:2) blend layers deposited on ITO/glass substrate

In organic photovoltaic (OPV), the photoactive layer is the main part, where the energy conversion process takes place. In our work we focus on Si-PCPDTBT:PC70BM (1:2) photoactive blend layers, where Si-PCPDTBT and PC70BM are the poly[2,6-(4,4-bis(2-ethylhexyl)dithieno[3,2-b:2,3-d]silole)-alt-4,7-(2,1,3 benzothiadiazole)] and [6,6]-phenyl-C71-butyric acid methyl ester, respectively. The mixture of these materials was deposited on the ITO/glass substrate and investigated using different analytical methods such as secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM) and scanning electron microscopy (SEM) before and after the degradation. The different degree degradation of layers measured as % of UV absorbance loss was performed under the illumination at ambient air using a solar simulator (AM 1.5, Xenon arc lamp). SIMS depth profiles reveal a diffusion of atmospheric oxygen through the photoactive layer leading to the photooxidation of organic materials as unveiled from the distribution of selected ions fragments (SO−, NO−, SO2−, NO2− and CO−). AFM and SEM images of the non-degraded sample show a relatively smooth surface. SEM investigations of the degraded layers reveal a homogenous formation of dispersed fibril-like nano-objects at the surface. The formation of such nano-objects may explain the increase of the layer surface oxidation and volatilization during the long-term degradation.