Small- and Wide-Angle X-ray Scattering Characterization of Bulk Heterojunction Polymer Solar Cells with Different Fullerene Derivatives

The aim of this study is to quantitatively investigate the effect of different fullerene type (PC₆₀BM and PC₇₀BM) on various morphological structures and power conversion efficiency (PCE) in the bulk heterojunction (BHJ) P3HT/PC_xBM solar cells. The solar cells are fabricated by spin coating without thermal annealing. The quantitative investigations of three-dimensional self-organized nanostructures are performed by using combined grazing-incidence small- and wide-angle X-ray scattering technique (GISAXS/GIWAXS). Two types of nanostructures are observed due to the phase separation in the BHJ films during the processing. They include (1) intercalated PC_xBM molecules around boundary of P3HT crystalline domain and within amorphous domain and (2) aggregated PC_xBM clusters in PC_xBM domains. The lamellar spacing of P3HT crystalline domains in P3HT/PC₇₀BM is larger than that in P3HT/PC₆₀BM. This result indicates more interfacial areas are generated between PC₇₀BM and P3HT at the molecular scale for more efficient charge separation. On the other hand, the size, volume fraction, partial attachment, and spatial distribution of PC₆₀BM clusters are larger than that of PC₇₀BM clusters, which reveals more efficient electron transport in P3HT/PC₆₀BM. We deduce the correlation between nanostructures and PCE (3.25% and 2.64%, respectively, for P3HT/PC₇₀BM and P3HT/PC₆₀BM). The structure of fullerene intercalated with P3HT rather than the size of fullerene cluster plays a major role in the PCE performance of BHJ solar cell without thermal annealing.