Hai-Ching Su

Efficient solid-state host-guest light-emitting electrochemical cells based on cationic transition metal complexes

The authors demonstrate highly efficient solid-state light-emitting electrochemical cells (LECs) consisting of green-emitting [Ir(dFppy)2(SB)]+(PF6−) as the host and orange-emitting [Ir(ppy)2(SB)]+(PF6−) as the guest [where dFppy is 2-(2,4-difluorophenyl)pyridine, SB is 4,5-diaza-9,9′-spirobifluorene, and ppy is 2-phenylpyridine]. Photophysical studies show that with the optimized host-guest compositions, the emission is mainly from the guest and photoluminescence quantum yields are largely enhanced over those of pure host and guest films due to suppressed intermolecular interactions. Correspondingly, LECs based on such host-guest cationic complex systems show substantially enhanced quantum efficiencies (power efficiencies) of up to 10.4% (36.8lm∕W), representing a 1.5 times enhancement compared to those of pure host and guest devices. Such results indicate that the host-guest system is essential and useful for achieving highly efficient solid-state LECs.

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.

Solid-state light-emitting electrochemical cells employing phosphor-sensitized fluorescence

We report highly efficient phosphor-sensitized solid-state light-emitting electrochemical cells (LECs) utilizing a phosphorescent cationic iridium complex [Ir(dFppy)2(SB)]+(PF6−) as the host and a fluorescent cationic dye (R6G) as the guest. Photophysical studies show that R6G retains a high photoluminescence quantum yield (PLQY) in highly polar media, revealing its suitable use as an emitting guest in an ionic host matrix. Such solid-state LECs achieve quantum efficiency (cd A−1) efficiency, and power efficiency up to 5.5% photon/electron, 19 cd A−1 and 21.3 lm W−1, respectively. The device quantum efficiency achieved is among the highest reported for fluorescent LECs and is higher than one would expect from the PLQY of the R6G fluorescent dye in the host film, thus indicating that phosphor-sensitization is useful for achieving highly efficient fluorescent LECs. Moreover, using narrow-band fluorescent emitters, such as R6G (FWHM, ∼50 nm), is effective in improving the color saturation of solid-state LECs based on cationic complexes.

Bis(diphenylamino)-9,9 '-spirobifluorene functionalized Ir(III) complex: a conceptual design en route to a three-in-one system possessing emitting core and electron and hole transport peripherals

Conceptual design of a three-in-one (luminescence chromophore with electron and hole transports) system was demonstrated by a functionalized Ir(III) complex 3, in which 4,5-diazafluorene and bis(diphenylamino) serve as electron and hole transporting sites, respectively. The poor emission quantum yield of 3 was systematically examined via a series of photophysical studies in combination with theoretical approaches. The far lifting of the π-electron from -NPh2 renders virtually no 3MLCT contribution to the lowest transition in the triplet manifold as compared with that of the parent model 2 without amino substituents. With an empirical approach, we conclude that an energy gap law may account for the major deactivation process. A light-emitting electrochemical cell (LEC) device based on 3 shows peak EQE, peak current efficiency and peak power efficiency at 2.4 V of 0.020%, 0.013 cd A−1 and 0.017 lm/W, respectively. The low device efficiencies are in accordance with the low PL quantum yield, stemming from the ligand-centered radiationless deactivation. The conceptual design presented here should provide valuable information for future progress en route to an ideal three-in-one system suited for OLEDs.

Efficient solid-state white light-emitting electrochemical cells employing embedded red color conversion layers

Recently, white light-emitting electrochemical cells (LECs) have attracted scientific attention due to their advantages of simple device structure, low operation voltage and compatibility with solution processes. In this work, efficient white emission from blue-emitting LECs with embedded red color conversion layers (CCLs) is reported. The red CCL is embedded between an indium tin oxide (ITO) layer and a glass substrate. The blue electroluminescence (EL) trapped in the waveguided mode, i.e. in the ITO layer, shows an evanescent tail in the CCL layer and excites the red-emitting dye, generating red photoluminescence (PL). Because the trapped blue EL can be partially transferred to the out-coupled red PL, the doping concentration of the red dye in the CCL can be reduced, and thus the absorption loss of the blue EL in the forward direction (air mode) can be mitigated, rendering enhanced device efficiency. The peak external quantum efficiency and power efficiency obtained in the blue-emitting LECs with embedded red CCLs are up to 12.5% and 27 lm W−1, respectively. These values are among the highest reported for white LECs, and thus confirm that blue-emitting LECs containing embedded red CCLs are capable of providing efficient white emission.

Enhancing device efficiencies of solid-state white light-emitting electrochemical cells by employing waveguide coupling

Solid-state white light-emitting electrochemical cells (LECs) have received intense scientific attention owing to their potential applications in display and lighting. Although device efficiencies of white LECs have been significantly improved in recent years, further improvements are still required for their practical applications. In this work, we demonstrate the enhancement of device efficiencies of white LECs by employing waveguide coupling. Two transparent photoresist (TPR) layers doped with TiO2 nanoparticles (NPs) are inserted between the indium tin oxide (ITO) layer and the glass substrate. By tuning the doping concentration of 25 nm TiO2 NPs in the upper TPR layer to adjust the refractive index, effective waveguide coupling between the ITO layer and the lower TPR layer can be achieved. Since the lower TPR layer contains 250 nm TiO2 NPs, electroluminescence (EL) outcoupled from the ITO layer can be scattered and redirected into the forward direction. Furthermore, the EL trapped in the glass substrate can also transmit into the lower TPR layer and then is scattered to the forward direction. When the EL trapped in the ITO layer and the glass substrate can be effectively recycled into the forward direction, the peak external quantum efficiency and power efficiency obtained in white LECs employing waveguide coupling are up to 19.4% and 34.1 lm W−1, respectively. These efficiencies are among the highest reported for white LECs and thus confirm that waveguide coupling would be useful for realizing highly efficient white LECs. In addition to the enhanced device efficiencies, improved color migration of EL spectra, which is desired for lighting applications, can be obtained in white LECs with scattering waveguide layers since EL of different angles can be mixed in the forward direction.

Host-only solid-state near-infrared light-emitting electrochemical cells based on interferometric spectral tailoring.

Solid-state near-infrared (NIR) light-emitting electrochemical cells (LECs) possess great potential in applications of NIR light sources due to their simple device structure, compatibility with large area and flexible substrates and low operating voltage. However, common host-guest NIR LECs suffer from the problem of significantly enhanced residual host emission upon increasing the bias voltage to achieve a higher NIR light output. A higher NIR light output can only be obtained at the expense of spectral purity in host-guest NIR LECs. To enhance the NIR light output of LECs without sacrificing the spectral purity significantly, a novel approach to generate NIR EL from host-only red-emitting LECs by adjusting the device thickness to modify the microcavity effect is proposed. NIR EL from host-only red-emitting LECs can be realized by adjusting the device thickness to shift the peak wavelength for constructive interference at the NIR spectral region. NIR EL resulting from the microcavity effect is relatively insensitive to bias voltage. Therefore, without losing spectral purity significantly, a 20× enhancement in the NIR output has been obtained in comparison to the previously reported value from host-guest NIR LECs. These results reveal that tailoring the EL spectra of host-only red-emitting LECs via modifying the microcavity effect would be a promising way to generate a higher NIR light output without suffering from the residual host emission problem of host-guest NIR LECs.

In situ electrochemical doping enhances the efficiency of polymer photovoltaic devices

In this study, we found that the formation of a p–i–n junction through in situ electrochemical doping is a promising way to enhance the performance of polymer photovoltaic devices. We applied a pre-bias to metal triflate [LiOTf, KOTf, Ca(OTf)2, Zn(OTf)2]/poly(ethylene oxide) (PEO)–incorporated poly[5-(2′-ethylhexyloxy)-2-methoxy-1,4-phenylenevinylene] (MEH-PPV)/{6}-1-(3-(methoxycarbonyl)propyl)-{5}-1-phenyl-[5,6]-C61 (PCBM) photovoltaic devices to form p–i–n junctions in their active layers. Auger depth profile analyses and alternating-current capacitance analyses of these doped devices revealed that the positive and negative ions were distributed unequally to form an asymmetrical p–i–n structure in a thin layer of ca. 100 nm of the polymer, and the intrinsic layer became thinner when formed under a higher pre-bias voltage. Atomic force microscopy and transmission electron microscopy revealed that the addition of metal triflate/PEO to MEH-PPV/PCBM improved the morphology of the composite films. Among the various doped devices, the MEH-PPV/PCBM device incorporating a LiOTf/PEO mixture exhibited the highest power conversion efficiency, a 40% increase relative to that of the reference device (MEH-PPV/PCBM).

A demonstration of solid-state white light-emitting electrochemical cells using the integrated on-chip plasmonic notch filters

In this study, we demonstrate solid-state white light-emitting electrochemical cells (LECs) using an integrated plasmonic notch filter to tailor the electroluminescence (EL) spectrum of non-doped blue-green emissive material. The plasmonic notch filter is composed of randomly distributed silver nanoparticles (Ag-NPs) embedded in the anode contact of indium tin oxide (ITO). This plasmonic notch filter strongly absorbs green light due to local surface plasmon (LSP) resonance of the Ag-NPs embedded in ITO. Thus, the emission green light of the solid-state LEC is strongly suppressed, leaving the blue and red light output to generate a white EL emission. Moreover, the duration of white EL can be maintained for a longer time under operation, which overcomes the issues regarding the short lifetime of white EL generated by the microcavity effect. In addition, the Ag-NPs can be readily fabricated by the thermal annealing of Ag film, which is compatible with current fabrication technologies typically used in light-emitting diode (LED) industry. Therefore, solid-state white LECs using an integrated on-chip plasmonic notch filter have great potential for applications in solid-state lighting.