Efficient bilayer polymer solar cells possessing planar mixed-heterojunction structures

We have investigated the influence of thermal annealing on the performance of polymer bilayer solar cell devices incorporating poly{2,6-(4,4-bis[2-ethylhexyl]-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)} (PCPDTTBT) as the donor and two kinds of fullerenes (C60, C70) as acceptors. The higher absorption of C70 increased the external quantum efficiency in the spectral range 400–600 nm. We observed morphological changes of the polymer films when the pre-annealing temperature was near the crystalline temperature (Tc, 207 °C). These nanostructural transformations resulted in a modified interfacial morphology of the donor phases and, therefore, greatly influenced the device performance. Post-annealing treatment reorganized the interface between the donor and acceptor phases, leading to better contact. The highest power conversion efficiency (2.85%) was obtained when we performed device pre- and post-annealing both at 200 °C for 30 min; the open-circuit voltage was 0.69 V and the short-circuit current was 8.42 mA cm−2.