Observing enhanced phosphorescence and electroluminescence from triplet emitter by quantum dot doping

Abstract

Luminescent CdSe/ZnS core-shell-type quantum dots (QDs) were synthesized by chemical colloidal methods. The photoluminescence (PL) intensity of a triplet emitter [bis(4-trifluoromethyl)-2-phenylbenzothiazolatoacetylacetonate-iridium(III) (Ir complex 1) and bis(4-methyl)-2-phenylbenzothiazolatoacetylacetonate-iridium(III) (Ir-complex 2)] was dramatically enhanced when bluish-green emitting CdSe/ZnS QDs were incorporated into these compounds. Experimental results indicate that the emissive region of QDs substantially overlaps with the low-energy absorption bands of Ir-complexes, indicating that the photons were absorbed by both the QDs and the Ir-complexes and that the energy absorbed by the QDs was transferred efficiently to the Ir-complex triplet emitter, resulting in the observed enhancement of PL intensity. A slow quenching of QDs emission was observed in a prepared set of solutions with gradual increasing of Ir complex concentration at a fixed QDs concentration in thin film PMMA matrix, which supports the energy transfer from Ir complex to QDs. In the fabricated double-layer electroluminescent (EL) devices, the emitting layer contained either only Ir-complex or a mixture of Ir-complex and CdSe/ZnS QDs with a specific molar ratio [Ir-complex/QDs = 1/0 (D-I); 1/0.5 (D-II),..., 1/10 (D-V)]. The EL intensity and the luminance efficiency for D-II were higher (luminescence yield = 19.3cd A(-1)) than those of the other devices. The PL and EL enhancement of the triplet emitter were also strongly supported by using ZnSe QDs rather than CdSe/ZnS QDs.