Effects of active layer thickness and thermal annealing on polythiophene: Fullerene bulk heterojunction photovoltaic devices

The effect of thermal annealing on photovoltaic devices comprising poly(3-hexylthiophene):[6,6]-phenyl ${\text{C}}_{61}$ butyric acid methyl ester (P3HT:PCBM) with thicknesses up to 1200 nm was investigated. Without thermal annealing, the efficiency of the as-prepared devices decreased with increasing active layer thickness, reflecting largely a reduction in the short-circuit current density and an inverse photocurrent spectral response. Thermal annealing of the full devices was found to substantially recover thick-film device efficiencies while reducing the thin-film device efficiencies. The profound variations in photovoltaic characteristics were interpreted in terms of vertical phase separation in the P3HT:PCBM blend film and ${\text{Li}}^{+}$ diffusion from the LiF/Al contact.