Precise construction of PCBM aggregates for polymer solar cells via multi-step controlled solvent vapor annealing

Polymer solar cells based on poly(3-hexylthiophene)/[6,6]-phenyl-C-61-butyric-acid methyl ester (P3HT/PCBM) composite are one of state-of-the-art polymer photovoltaic devices in terms of performance. In this work, we applied two-step controlled solvent vapor annealing (C-SVA) to achieve an optimized morphology for the photoactive layer with both an appropriate size of PCBM aggregates and an improved crystallinity of P3HT. As revealed by bright-field transmission electron microscopy (TEM), and atomic force microscopy (AFM), X-ray diffraction (XRD) and UV-Vis spectroscopy, PCBM forms aggregates with sizes of ca. 30 nm during the first step C-SVA in tetrahydrofuran vapor. The second step treatment using carbon disulfide vapor on one hand reduces the large size of these PCBM aggregates to ca. 20 nm, and on the other hand substantially increases the crystallinity of P3HT. The polymer solar cells employing a thus-treated composite film gave a power conversion efficiency as high as 3.9%, in contrast to 3.2% for the thermally annealed device under the same characterization conditions. This result shows the importance of a precisely controlled morphology of the photoactive layer in device performance.