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.
Singlet
fission of organic molecules has attracted recent attention
owing to its potential advantages in organic photovoltaic and electroluminescence
applications. Its microscopic mechanism however remains stymied. Large
couplings from charge-transfer (CT) state mediation were invoked to
explain the ultrafast singlet fission rate observed in crystalline
polyacene, but its experimental confirmation is still lacking. The
singlet fission and triplet fusion of amorphous rubrene were investigated
with time-resolved photoluminescence spectroscopy at different temperatures
to extract the rates of singlet fission, triplet fusion, and triplet
hopping. On the basis of the Marcus electron-transfer model, the deduced
electronic coupling constant of the singlet fission process was found
to be larger than that of the triplet fusion process, indicating that
the singlet fission process undertakes a CT-state-mediated channel
while the triplet fission process assumes a direct channel. This study
thus confers supporting evidence of the existence of the CT-state-mediated
channel for singlet fission of rubrene and offers an experimental
approach to study singlet fission dynamics.
Formation of ordered poly(3-hexylthiophene-2,5-diyl) (P3HT) molecular stacking during the freeze-drying process is tracked with in situ spectroscopy of Raman scattering, absorption, and photoluminescence. Raman spectra of pristine P3HT dissolved in 1,2dichlorobenzene show that P3HT polymers undergo drastic ordered aggregation upon being lower than 0 °C, at which the solubility of P3HT is reached, as evidenced by the emergence of pronounced red-shifted, narrow Raman peaks (1422 and 1435 cm −1 ) caused by intermolecular coupling. The absorption and photoluminescence spectra bear similar temperature dependence as the results of Raman. Aggregation of P3HT is further confirmed by coarse-grained molecular dynamics simulation showing the enhanced order parameters of distance and orientation between P3HT chains upon cooling. The incorporation of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) does not significantly alter the P3HT packing configuration, as verified by nearly identical Raman features observed in P3HT:PCBM mixing solution upon cooling. While optical spectroscopy and MD simulation portrayed the short-range order of P3HT aggregates, grazing-incident X-ray diffraction exposed the long-range order by the pronounced diffraction spots corresponding to the lamellar stacking of P3HT. This study demonstrates the ability of Raman spectroscopy to reveal the shortrange order of polymer packing, while the in situ monitoring illustrates that the ability of freeze-drying to separate molecular aggregation from solvent removal thus is advantageous for photovoltaic device fabrication without resorting to trial and error.
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