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

• Published in: Organic electronics 2021-01, Vol.88, p.105996, Article 105996
• Main Authors: Lee, Eunhee, Tran, Duyen K., Park, Jihun, Ko, Wonyoung, Jenekhe, Samson A., Hwang, Ye-Jin
• Format: Article
• Language: English
• Subjects: Alkylthiolation; Benzodithiophene; Fluorination; Organic photovoltaics; Small-molecule donor
• ISSN: 1566-1199; 1878-5530
• DOI: 10.1016/j.orgel.2020.105996

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 (Voc), 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) × 105 cm−1. Bulk heterojunction OPV devices based on pairing SMBDT-S and SMBDT-SF respectively with PC71BM electron acceptor showed Voc 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 Voc in OPVs. [Display omitted] •New wide-bandgap small-molecule electron donors, SMBDT-S and SMBDT-SF, were developed based on benzo [1,2-b:4,5-b’]dithiophene (BDT) with thiophene side chains as a core unit.•By introducing alkylthio-groups and fluorine atoms, wide bandgaps (>1.8 eV) with low-lying HOMO energy levels (