Improved power conversion efficiency of bulk-heterojunction organic photovoltaic cells using neat C70 as an effective acceptor for an amorphous π-conjugated polymer

Abstract

Low-cost neat C70 was successfully utilized to fabricate solution-processable bulk heterojunction (BHJ) organic photovoltaics (OPVs) by combining the neat C70 with an amorphous polymer, PEDOTNDIF, which was also synthesized at low cost by using a two-step direct arylation reaction. The optimized BHJ OPV using PEDOTNDIF:C70 exhibited a short-circuit current density (Jsc) of 8.70mAcm−2, an open-circuit voltage of 0.75V, a fill factor of 0.36, and a power conversion efficiency (PCE) of 2.33%, as compared to 1.46mAcm−2, 0.80V, 0.26, and 0.30%, respectively, for the BHJ OPV fabricated using PEDOTNDIF:PC70BM. The much-improved PCE (Jsc) is mainly due to the strong acceptor nature of neat C70, which needs to efficiently separate photo-excitons into free carriers. A good miscibility in the PEDOTNDIF:C70-based BHJ film with a large donor/acceptor interfacial area was also confirmed by characterizing atomic force microscopy images of the BHJ films and comparing PCEs of planar-heterojunction OPVs with PEDOTNDIF/C70 bilayers.