Long time thermal annealing effects on the film morphology and performance of polymer solar cells with calcium electrode

Conclusions  We found that the performance of P3HT:PCBM solar cells with a thin Ca electrode was improved by thermal annealing, but its trend was not monotonic with the annealing time. This result could not be explained with the surface morphology measurement, but it was reasonably correlated with the composition changes in the surface region of blend films by the photoelectron spectra measurement. In particular, it is noteworthy that the device performance was not much degraded by the longest time annealing (5 h here), in spite of the formation of PCBM microcrystals, which disclosed the durability of the thin (30 nm) Ca electrode on top of the P3HT:PCBM blend film upon thermal annealing for such long time at 140 °C.