Discrepancy of optimum ratio in bulk heterojunction photovoltaic devices: Initial cell efficiency vs long-term stability

Organic photovoltaic devices are difficult to commercialize because of their vulnerability to chemical degradation related with oxygen and water and to physical degradation with aging at high temperatures. We investigated the photophysical degradation behaviors of a series of poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PC₆₀BM) bulk heterojunctions (BHJs) as a model system according to the donor-acceptor ratio. We found that the optimum P3HT:PC₆₀BM ratio in terms of long-term stability differs from that in terms of initial cell efficiency. On the basis of cell performance decays and time-resolved photoluminescence measurements, we investigated the effects of oxygen and material self-aggregation on the stability of an organic photovoltaic device. We also observed the changes in morphological geometry and analyzed the surface elements to verify the mechanisms of degradation.