Multiple stress degradation analysis of the active layer in organic photovoltaics

Under general working conditions, organic solar cells are exposed to a multitude of degradative stresses, including light, oxygen, temperature and humidity. We selectively exposed the layer stacks of three different material systems, P3HT, PCDTBT and AnE-PVcc in bulk heterojunction with fullerene, to different combinations of stresses, starting from one single stress and then iteratively adding a new stress to the combination. This allowed us to determine how harshly different polymers get affected by individual stress factors. P3HT based devices show an overall higher stability compared to PCDTBT or AnE-PVcc. All of the devices are affected the most by combined influences of light, oxygen, humidity and temperature, and they cease functioning within a couple of hours of exposure. A thermodynamically driven formation of a thin PCBM layer at the anode is observed with the prolonged storage, which causes an increase in the current densities for the large reverse voltages. We explain this phenomenon by tunneling from the fullerene HOMO to the LUMO, as a result of a large gradient in the electric potential over this layer. The increase of open circuit voltage of the pre-annealed P3HT devices upon UV photolysis was investigated, and attributed to the UV irradiation facilitated destruction of the face-on P3HT layer grown on the surface of the active layer. The effect of the sequence in which the annealing step is performed with respect to evaporation and degradation on the performance upon degradation is discussed.