ABSTRACT
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.
ABSTRACT
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',4â³-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.
ABSTRACT
Applications of quantum dots (QDs) are often obstructed by the associated surface electronic states that quench photoluminescence (PL) and hinder charge transport. Preventing this is still largely being stymied owing to the lack of means to regulate their presence. Dispersing PbS QDs in toluene, we show that varying the solvent temperature offers a way of modulating their surface electronic state. A comprehensive energy-transfer model explains all the anomalous temperature-dependent behavior of the absorption and PL, explicitly revealing the PL quenching dynamics of isolated QDs due to the induced surface state by imposing solvent stress on their surface ligands. This study demonstrates that the local stress induced by a solvent can serve as a 'switch' for the surface electronic states of QDs, which is enabled by the well-studied thermo-physical properties of a liquid solvent.
