New D–A–A-Configured Small-Molecule Donors for High-Efficiency Vacuum-Processed Organic Photovoltaics under Ambient Light

Four
new donor–acceptor–acceptor (D–A–A)
type molecules (DTCPB, DTCTB, DTCPBO, and DTCTBO), wherein benzothiadiazole or benzoxadiazole
serves as the central A bridging triarylamine (D) and cyano group
(terminal A), have been synthesized and characterized. The intramolecular
charge-transfer character renders these molecules with strong visible
light absorption and forms antiparallel dimeric crystal packing with
evident π–π intermolecular interactions. The characteristics
of the vacuum-processed photovoltaic device with a bulk heterojunction
active layer employing these molecules as electronic donors combining
C70 as electronic acceptor were examined and a clear structure–property–performance
relationship was concluded. Among them, the DTCPB-based
device delivers the best power conversion efficiency (PCE) up to 6.55%
under AM 1.5 G irradiation. The study of PCE dependence on the light
intensity indicates the DTCPB-based device exhibits superior
exciton dissociation and less propensity of geminated recombination,
which was further verified by a steady photoluminescence study. The DTCPB-based device was further optimized to give an improved
PCE up to 6.96% with relatively high stability under AM 1.5 G continuous
light-soaking for 150 h. This device can also perform a PCE close
to 16% under a TLD-840 fluorescent lamp (800 lux), indicating its
promising prospect for indoor photovoltaic application.