Lingling Deng

Recent Developments in Top‐Emitting Organic Light‐Emitting Diodes

Organic light-emitting diodes (OLEDs) have rapidly progressed in recent years due to their unique characteristics and potential applications in flat panel displays. Significant advancements in top-emitting OLEDs have driven the development of large-size screens and microdisplays with high resolution and large aperture ratio. After a brief introduction to the architecture and types of top-emitting OLEDs, the microcavity theory typically used in top-emitting OLEDs is described in detail here. Then, methods for producing and understanding monochromatic (red, green, and blue) and white top-emitting OLEDs are summarized and discussed. Finally, the status of display development based on top-emitting OLEDs is briefly addressed.

Effect of gold nanorods and nanocubes on electroluminescent performances in organic light-emitting diodes and its working mechanism

In this manuscript we investigated the influence of Au nanoparticles on electrical and electroluminescent (EL) performances in organic light-emitting diodes (OLEDs) via doping as-synthesized Au nanorods (NRs) or nanocubes (NCs) into hole transport layer (HTL). Through accurately controlling the distance between the Au NRs and the emitting layer, altering the guest emitter’s lifetime, and replacing Au NRs with Au NCs to satisfy a better spectrum overlap with the emission guest, we got a conclusion that doping Au NRs or NCs into HTL has no significant influence on the device’s electrical and EL performances, although we observed an increase in the spontaneous emission rate in a fluorescent material by the exciton-surface plasmon-coupling. Our results suggest that a further research on emission mechanism in surface plasmon-enhanced OLEDs is still in process.

Negative differential resistance and hysteresis in graphene-based organic light-emitting devices

Here, we report the experimental observation of negative differential resistance (NDR) and hysteresis phenomena in graphene-based OLEDs and their effects on device performance. Our results reveal that the NDR and hysteresis mainly originate from the poly(methyl methacrylate) residue resting on graphene. We further demonstrate that current annealing is a facile and effective technique to remove the polymeric residue and eliminate the NDR, leading to the dramatically enhanced luminous efficiency from 30.9 to 41.6 cd A−1, and to 89.2 cd A−1 when equipped with a high index half-ball lens. The demonstrated pretreatment process of graphene establishes a new path for the construction of a wide variety of high performance optoelectronic devices.

Influences of wide-angle and multi-beam interference on the chromaticity and efficiency of top-emitting white organic light-emitting diodes

Wide-angle interference (WI) and multi-beam interference (MI) in microcavity are analyzed separately to improve chromaticity and efficiency of the top-emitting white organic light-emitting diodes (TWOLEDs). A classic electromagnetic theory is used to calculate the resonance intensities of WI and MI in top-emitting organic light-emitting diodes (TOLEDs) with influence factors (e.g., electrodes and exciton locations) being considered. The role of WI on the performances of TOLEDs is revealed through using δ-doping technology and comparing blue and red EML positions in top-emitting and bottom-emitting devices. The blue light intensity significantly increases and the chromaticity of TWOLEDs is further improved with the use of enhanced WI (the blue emitting layer moving towards the reflective electrode) in the case of a weak MI. In addition, the effect of the thicknesses of light output layer and carrier transport layers on WI and MI are also investigated. Apart from the microcavity effect, other factors, e.g., carrier balance and carrier recombination regions are considered to obtain TWOLEDs with high efficiency and improved chromaticity near white light equal-energy point.

High performance flexible top-emitting warm-white organic light-emitting devices and chromaticity shift mechanism

Flexible warm-white top-emitting organic light-emitting devices (TEOLEDs) are fabricated onto PET substrates with a simple semi-transparent cathode Sm/Ag and two-color phosphors respectively doped into a single host material TCTA. By adjusting the relative position of the orange-red EML sandwiched between the blue emitting layers, the optimized device exhibits the highest power/current efficiency of 8.07 lm/W and near 13 cd/A, with a correlated color temperature (CCT) of 4105 K and a color rendering index (CRI) of 70. In addition, a moderate chromaticity variation of (-0.025, +0.008) around warm white illumination coordinates (0.45, 0.44) is obtained over a large luminance range of 1000 to 10000 cd/m2. The emission mechanism is discussed via delta-doping method and single-carrier device, which is summarized that the carrier trapping, the exciton quenching, the mobility change and the recombination zone alteration are negative to color stability while the energy transfer process and the blue/red/blue sandw...

The investigation of light outcoupling in blue top-emitting OLEDs

A classic electromagnetic theory is used in this paper to investigate the light outcoupling in blue top-emitting organic light-emitting devices (TEOLEDs) with a samarium/silver (Sm/Ag) bilayer cathode. With the method, the outcoupling efficiency and the spectra of the devices with different top-electrodes and outcoupling layers were simulated. The calculated results demonstrate that in the devices, the increasing thickness of the Ag film would result in the redshift of blue emission and the decrease of emission intensity. While the thickness of the Sm film only influences the emission intensity of the devices. The thickness of the outcoupling layer is varied to obtain the saturated and efficient blue emission and then the optimal thickness is determined. The microcavity effect induced mainly by the bilayer cathode with a relatively high reflectivity is considered to explain the optical characteristics of the blue TEOLEDs, including some abnormal phenomena. The simulated results show good agreement with the measured data.

Interfacial engineering of graphene for highly efficient blue and white organic light-emitting devices

Graphene as anodes of flexible organic light-emitting devices (OLEDs) has intrinsic drawbacks of a low work function and a high sheet resistance although it can eliminate the brittle feature of ITO. Chemical doping as a conventional approach is universally used to decrease the sheet resistance and adjust the work function of graphene electrodes, but it suffers from instability problems due to the volatility of chemical species. Here, an insulated poly(4-styrenesulphonate) (PSS) modification layer is firstly coated on the graphene surface along with improved air-stability and hole-injection ability via interfacial dipoles. Besides, the utilization of PSS is beneficial to reduce the leakage current of OLEDs. Then a gradient injection layer of poly(3,4-ethylenedioxythiophene):PSS (PEDOT:PSS)/tetrafluoroethyleneperfluoro-3,6-dioxa-4-methyl-7-octenesulphonic acid copolymer-doped PEDOT:PSS is covered onto the PSS-modified graphene to further promote hole injection and improve carrier balance inside OLEDs. With above interfacial modification technique, very high efficiencies of 201.9 cd A−1 (76.1 lm W−1, 45.2%) and 326.5 cd A−1 (128.2 lm W−1, 99.5%) for blue and white emissions are obtained, which are comparable to the most efficient display and lighting technologies so far.

Enhanced performances for top-emitting white organic light-emitting diodes by utilizing green phosphor as energy transfer medium

In top-emitting white organic light-emitting diodes (TWOLEDs), the device performances attribute to the several important factors, such as exciton profile, energy transfer, and microcavity effect. In this paper, a TWOLED containing a heterojunction blue emission layer (EML) and a red EML is reported. A host material with high triplet energy level is employed for the adjacent blue and red EML, while the inefficient red emission reduces the emission efficiency of the TWOLED. In order to enhance the red emission efficiency, mixed-host and co-doping technologies are used in the red EML. By mixing the hole transporting and electron transporting host materials, the exciton recombination zone extends to the red EML to increase the red emission intensity and reduce the efficiency roll-off. And by co-doping a green phosphor into the red EML as the energy transfer medium, the energy transfer rate is enhanced, and then the current efficiency increases. Besides, both the mixed-host and co-doping change the carrier transport and the exciton recombination zone, which further affects the microcavity resonance in the devices. Due to the enhancement on the red emission intensity and the shift of resonant wavelength, the chromaticity of the TWOLED is improved.