Thermal stabilization of organic photovoltaic cells using [6,6]-phenyl C61-butyric acid methyl ester analogs: Effects of alkyl substituents on the nanostructures of bulk heterojunction films and their stabilities

•Thermally stable OPV cells are prepared using PCBM analogs as an n-type semiconductor.•Nanostructures of P3HT:PCBM and P3HT:PCBE films are compared using solid-state NMR.•The P3HT:PCBE film maintains its dispersed state after aging at 130 °C for 24 h.

Improvement of the thermal stability of organic photovoltaic (OPV) cells using [6,6]-phenyl C61-butyric acid methyl ester (PCBM) analogs that have ethyl (PCBE), butyl (PCBB), and isobutyl (PCBiB) substituents as electron acceptor materials is reported. The power conversion efficiency of an OPV cell consisting of poly(3-hexylthiophene) (P3HT) and PCBE remained at 3.1% upon aging at 130 °C for 24 h, while that of an OPV cell based on PCBM decreased to 1.2%. Structural analyses of the P3HT:PCBM and P3HT:PCBE films were performed using solid-state nuclear magnetic resonance and two-dimensional grazing incidence wide angle X-ray scattering. These analyses revealed that PCBE retained its nanoscale dispersed state in the P3HT:PCBE film during annealing and aging, while phase separation was observed in the P3HT:PCBM film after annealing, and it progressed upon aging. These results suggest that active layers with higher stability at the nanometer scale afford thermally stable OPV cells.

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