Naphtho[1,2-b:5,6-b′]dithiophene-Based Small Molecules for Thick Film Organic Solar Cells with High Fill Factors

Two novel acceptor-donor- acceptor type small molecules based on a two-dimensional conjugated naphtho[1,2b:5,6-b']dithiophene (NDT) unit containing alkylthienyl or alkylphenyl side chains, named as NDTT-CNCOO and NDTP-CNCOO, respectively, were designed and synthesized. Compared with NDTT-CNCOO, NDTP-CNCOO-based bulk heterojunction (BHJ) solar cells exhibited higher fill factors, higher short current density, and higher power conversion efficiencies. Such performance benefitted from highly ordered structures, excellent charge transport property, and good film formation capability. The fill factors of BHJ solar cells using NDTP-CNCOO as a donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor were all above 70% with thicknesses of active layers ranging from 100 to 300 nm. In particular, the optimal power conversion efficiency of the solar cell devices reached 7.20% with an active layer thickness of 300 nm, which is the best result reported for NDT-based small molecule solar cells.