Chih-Teng Liao

Highly efficient double-doped solid-state white light-emitting electrochemical cells

We report highly efficient, solid-state, white light-emitting electrochemical cells (LECs) based on a double-doped strategy, which judiciously introduces an orange-emitting guest, [Ir(ppy)2(dasb)]+(PF6−), into a single-doped emissive layer comprised of an efficient blue-green emitting host, [Ir(dfppz)2(dtb-bpy)]+(PF6−), and a red-emitting guest, [Ir(ppy)2(biq)]+(PF6−), to improve the balance of carrier mobilities and, thus, to enhance the device efficiency. Photoluminescence (PL) measurements show that the single-doped (red guest) and the double-doped (red and orange guests) host–guest films exhibit similar white PL spectra and comparable photoluminescence quantum yields, while the device efficiencies of the double-doped white LECs are twofold higher than those of the single-doped white LECs. Therefore, such enhancement of the device efficiency is rationally attributed to the improved balance of carrier mobilities of the double-doped emissive layer. Peak external quantum efficiency and peak power efficiency of the double-doped white LECs reached 7.4% and 15 lm W−1, respectively. These efficiencies are amongst the highest reported for solid-state white LECs and, thus, confirm that the double-doping strategy is useful for achieving highly efficient white LECs.

Improving the balance of carrier mobilities of host–guest solid-state light-emitting electrochemical cells

We report efficient host-guest solid-state light-emitting electrochemical cells (LECs) utilizing a cationic terfluorene derivative as the host and a red-emitting cationic transition metal complex as the guest. Carrier trapping induced by the energy offset in the lowest unoccupied molecular orbital (LUMO) levels between the host and the guest impedes electron transport in the host-guest films and thus improves the balance of carrier mobilities of the host films intrinsically exhibiting electron preferred transporting characteristics. Photoluminescence measurements show efficient energy transfer in this host-guest system and thus ensure predominant guest emission at low guest concentrations, rendering significantly reduced self-quenching of guest molecules. EL measurements show that the peak EQE (power efficiency) of the host-guest LECs reaches 3.62% (7.36 lm W(-1)), which approaches the upper limit that one would expect from the photoluminescence quantum yield of the emissive layer (∼0.2) and an optical out-coupling efficiency of ∼20% and consequently indicates superior balance of carrier mobilities in such a host-guest emissive layer. These results are among the highest reported for red-emitting LECs and thus confirm that in addition to reducing self-quenching of guest molecules, the strategy of utilizing a carrier transporting host doped with a proper carrier trapping guest would improve balance of carrier mobilities in the host-guest emissive layer, offering an effective approach for optimizing device efficiencies of LECs.

Toward better full-color displays

Light-emitting electrochemical cells (LECs) and organic light emitting diodes (OLEDs) are frequently used in screens and displays. They are bright, thin, and efficient, and so are especially popular for mobile devices. LECs are solid-state devices that generate light from an electric current, a phenomenon called electroluminescence (EL), and are usually composed of a material containing mobile ions sandwiched between two metal electrodes. In general, LECs have several advantages over OLEDs, such as a simple single-layer configuration, solution-processing, and low operation voltages with air-stable electrodes.1 However, saturated deep-blue emission, which is essential for full-color displays, cannot be easily obtained from commonly used LEC materials, which include cationic transition metal complexes (CTMCs) and conducting polymers. Iridium-based CMTCs can cover a large color range to achieve full-color displays and white light emissions, but to date, the development of efficient saturated blue-emitting ionic iridium complexes has lagged behind those of other colors. Previous complexes with large optical band gaps have mainly exhibited emissions in the bluish-green region. The difficulty in colortuning toward the deep-blue region through molecular design of iridium-based CTMCs is largely due to intrinsically narrower energy gaps in such cationic complexes, relative to neutral complexes. LECs based on polymers such as polyfluorene (PF) suffer from significant green emission due to interchain aggregation, which deteriorates blue emission in these devices.2 To avoid the intrinsic tendency of aggregation that is widely observed for PF derivatives, we selected members of the terfluorene family—low-molecular-weight analogues of PFs—to create saturated blue-emitting LECs. We have used an ionic terfluorene (referred to as Compound 1: see Figure 1 for the structure) to achieve saturated deep-blue EL from LEC devices.3 Figure 2 compares the EL spectra of two LEC devices with the PL spectra of their emissive layers. In Device I, the Figure 1. Molecular structure of Compound 1, an ionic terfluorene derivative.