Abstract
Impedance spectroscopy measurement is employed to study the impact of thermal annealing on charge carrier lifetimes in P3HT: PCBM bulk heterojunction solar cells. Upon thermal annealing at 150 °C, a correlation between charge carrier lifetime and device performance is identified. The best power conversion efficiency reported here corresponds to the devices annealed at 150 °C, yielding the longest charge carrier life time. In addition to lateral segregation, thermal annealing promotes accumulation of PCBM molecular aggregates towards the cathode inducing vertical segregation as is evident from the analysis of results obtained from capacitance – voltage measurements. Based on Mott-Schottky relation, the measured values of the flat band potential as a function of thermal annealing is correlated to the concentration of PCBM at cathode interface and a compositional gradient profile induced by thermal annealing is proposed.
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