Tunable electroluminescence properties in CdSe/PVK guest-host based light-emitting devices

The photo- (PL) and electroluminescence (EL) properties of CdSe-TOPO quantum dots (QDs) doped into a PVK:PBD host matrix with a double-layer device configuration [PEDOT:PSS/PVK:PBD/QDs/LiF/Al] are investigated. Depending on the concentrations of CdSe-TOPO QDs, EL properties are changed including the emitted colors. At a low QD concentration (1 wt%), QD-light emitting diode (QD-LED) shows a broad EL band with dominant QD emission and minor emissions (electromer from PVK and exciplex from the PVK:PBD blend). However, at a higher QD concentration (5 wt%) there are two competitive emission peaks observed in the EL spectrum, arising from exciplex emission in the PVK:PBD blend (shorter wavelength) and QD emission at CdSe-TOPO. These EL peaks are closely related to Förster energy transfer and charge transfer, respectively. With increasing bias voltage, the EL peak at shorter wavelength gradually decreases and the emitted color is changed from blue to dark green in the QD-LED. As a result, both the concentration and applied bias voltage are important tunable factors in the emitted colors.