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 (C ₆₀, C₇₀) as acceptors. The higher absorption of C ₇₀ 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 (T_c, 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.