A new concept for organic light-emitting diodes (OLEDs) is presented, which is called exciplex-sensitized triplet-triplet annihilation (ESTTA). The exciplex formed at the organic heterojunction interface of 4,4',4″-tris(N-3-methyphenyl-N-phenyl-amino) triphenylamine and 9,10-bis(2'-naphthyl) anthracene (ADN) is used to sensitize the triplet-triplet annihilation (TTA) process on the ADN molecules. This results in a turn-on voltage (2.2 V) of the blue emission from the OLED below the bandgap (2.9 eV). From the transient electroluminescence measurement, blue emission totally came from the TTA process without direct recombination on the ADN molecules. The blue singlet exciton from the TTA process can be quenched by energy transfer to the exciplex, as revealed by transient photoluminescence measurements. This can be prevented by blocking the energy transfer path and improving the radiative recombination rate of blue emission. With the insertion of the "triplet diffusion and singlet blocking (TDSB)" layer and the incorporation of the dopant material, an ESTTA-OLED with external quantum efficiency of 5.1% was achieved, which consists of yellow and blue emission coming from the exciplex and ESTTA process, respectively.
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.
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