Efficient as-cast bulk-heterojunction solar cells based on a tert-butyl substituted methanofullerene acceptor

We report on the implementation of a modified methanofullerene PCBtB ([6,6]-phenyl-C-61 butyric acid tert-butylester) as electron acceptor for solution-processable bulk-heterojunction (BHJ) solar cells based on P3HT (poly(3-hexylthiophene)). The results demonstrate how a small modification of the steric hindrance of the benchmark molecule PCBM, leaving the electronic structure unchanged, significantly influences the main properties of the BHJ active layer. In particular, we investigate the effect of the replacement of the PC61BM methyl end-group with a tert-butyl substituent on the optical properties, OPV parameters, nanoscale morphology, D: A phase segregation, and charge carrier mobility in bulk-heterojunction with P3HT. We find that the PCBtB-based blends present a finer self-organization, higher D: A phase segregation and good charge carrier mobility, affording interesting photovoltaic responses and highlighting a marked potential of PCBtB to tailor the active BHJ layer. The best BHJ solar cells based on as-cast P3HT: PCBtB blend films exhibit power conversion efficiencies (PCEs) of similar to 3.1%, under a simulated AM1.5G illumination of 100 mW cm(-2), which are greater than the analogous as-cast P3HT: PCBM based devices.