Benzodithiophene-based wide-bandgap small-molecule donors for organic photovoltaics with large open-circuit voltages

초록

We have developed new small-molecule electron donors, SMBDT-S and SMBDT-SF, based on benzo[1,2-b:4,5-b'] dithiophene (BDT) with thiophene side chains as a core unit and rhodanine end-capping units to enhance the open-circuit voltage (V-oc), which is one of the factors limiting performance of small-molecule organic photovoltaics (OPVs). The SMBDT-S and SMBDT-SF donor molecules exhibited excellent thermal stability and high crystallinity. SMBDT-S was found to have a wide bandgap of 1.85 eV with the highest occupied molecular orbital (HOMO) energy level of 5.56 eV. The fluorination in SMBDT-SF led to the same bandgap of 1.86 eV with an even lower-lying HOMO energy level of 5.72 eV. Both SMBDT-S and SMBDT-SF have large thin-film absorption coefficients of (1.4-1.5) x 10(5) cm(-1). Bulk heterojunction OPV devices based on pairing SMBDT-S and SMBDT-SF respectively with PC71BM electron acceptor showed V-oc as high as 1.18 eV, which is among the highest Voc reported for all-small-molecule solar cells. These results demonstrate that introducing fluorine atoms and alkylthio side chains are effective strategies to downshift the HOMO energy level and enhance the V-oc in OPVs.

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