Controlling the Polarity of Fullerene Derivatives to Optimize Nanomorphology in Blend Films

Developing a design strategy to establish the compatibility of acceptor materials with donor materials is important for the rational development of organic solar cells. We synthesized 2,6-dimethoxyphenyl methanofullerene derivatives to realize an enhanced open-circuit voltage, and we investigated polarities and their effects on the film morphology of the active layer. The polarities of the synthesized fullerene derivatives were affected significantly by the presence of functional groups, such as methoxy, ether, and ester groups. Macro/nanoscopic morphological investigation and spectroscopic analysis of the blend films of the poly­(3-hexylthiophene)­(P3HT)/fullerene derivatives showed that a balanced polarity between materials results in the formation of optimized nanomorphology without grains and robust phase separation. Measurements of the device performance of the photovoltaic cells composed of P3HT and the fullerene derivatives confirmed the same tendency as that shown in the morphological analysis. This finding enables us to obtain an improved power conversion efficiency because of the enhanced open circuit voltage derived from the fullerene derivatives

Design of Fullerene Derivatives for Stabilizing LUMO Energy using Donor Groups Placed in Spatial Proximity to the C₆₀ Cage

A series of arylated dihydrofullerene derivatives were synthesized to elucidate the effective design of fullerene derivatives for enhancing the performance of organic photovoltaics. The LUMO energy of the fullerenes was estimated by the first reduction potential and theoretical calculations. The results showed that the methoxy groups substituted at spatial proximity to the fullerene core offered significant stabilization of the LUMO level. The stabilizing effect of the directly arylated fullerenes is more significant than that of conventional methanofullerenes. The theoretical investigation was performed with regard to the electronic interaction between the methoxy and fullerene moieties