Reversible and irreversible degradation of organic solar cell performance by oxygen

The effect of oxygen on the degradation of inverted bulk heterojunction solar cells based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blends has been investigated by monitoring current–voltage (jV)-curves, impedance spectra and charge extraction by linearly increasing voltage (CELIV) traces during the degradation process. The use of gas permeable top electrodes allows monitoring the kinetics of degradation without being limited by the diffusion through a compact metal electrode. A computational model is used to provide a phenomenological explanation of the experimental results. The charge distribution inside the device is modeled by solving the fully coupled set of nonlinear differential equations for the quasi-one-dimensional transport of electrons and holes. Degradation of the cells in the presence of oxygen results mainly in the reduction of short circuit current (jsc), while the concomitant loss in light absorption is negligible. The rate of degradation is enhanced significantly by illumination. A significant part of the loss in jsc is reversible upon annealing under nitrogen or in vacuum. The irreversible part of the degradation is assigned to the photochemical formation of carbonyl and carboxylic groups, which act as traps for electrons. The reversible component of degradation is due to p-doping of the photoactive layer by oxygen, which results in the formation of mobile holes and immobile superoxide anions. This leads to the formation of a space charge region in front of the electron extracting electrode whose width depends on the doping level as well as on the applied bias. The space charge region shields the electric field inside the photoactive layer and hence hampers charge carrier extraction, which leads to the observed loss in short circuit current.