Comparing the loss of charge carrier generation with the loss of VOC at low temperatures in organic bulk-heterojunction blends

We estimate the temperature dependence of charge carrier generation in P3HT:ICBA and PTB7:PCBM using continuous-wave photoinduced absorption. For P3HT:ICBA, the generation rate increases fivefold from 79 K to ca 250 K, above which it saturates. We compare this to the temperature dependence of open-circuit voltage (VOC) which at low temperatures shows a decrease with decreasing temperatures, in disagreement with the typically expected linear rise. This voltage loss is too large to be explained by the decrease of generation with decreasing temperature. For PTB7:PCBM the dominant species has a generation rate increasing from 90 to 150 K, after which it decreases. For PTB7:PCBM there is no decrease in VOC at low temperatures. Our findings support the lack of correlation between the decrease in VOC and a decrease in generation rate. We conclude that the loss of VOC does not originate in charge carrier dynamics in the bulk material, but is due to contact effects.