The Role of Morphology Control in Determining the Performance of P3HT/C-70 Bulk Heterojunction Polymer Solar Cells

Morphology is one of the critical elements in determining the performance of polymer solar cells. As an important member in fullerene family, unsubstituted C-70 is scarcely employed to prepare polymer solar cells as compared with its derivatives, which is ascribed to its poor solution processability from a single organic solvent. Hereby, we show a method by using mixed solvent to prepare poly(3-hexylthiophene) (P3HT)/C-70 photoactive layer for high-performance polymer solar cells. Composite films spin coated from the solution with different mixed solvents are characterized by optical microscopy, transmission electron microscopy, UV-vis spectroscopy, and photovoltaic (PV) device characterization. As a result, heptane/o-dichlorobenzene (ODCB) mixture is found to be a good combination to prepare homogenous P3HT/C-70 film, and a high-performance device is thus obtained. By further optimizing the ratio between the donor P3HT and acceptor C-70 in the composite film, the PV cells with a power conversion efficiency of 2.24% have been achieved, which is much higher than that of the device prepared from ODCB-only solution. Dynamic light scattering measurement implies that C-70 molecules form aggregates with size of ~250 nm in ODCB solution, while in heptane/ODCB mixture, the aggregates are much smaller, which is responsible for the homogenous P3HT/C-70 film and also high device performance. This paper demonstrates a vivid approach to control the morphology of polymer solar cells, and thus significantly improved performance via deliberately designed solvent system for thin-film deposition.