Dongge Ma

Spatial exciton allocation strategy with reduced energy loss for high-efficiency fluorescent/phosphorescent hybrid white organic light-emitting diodes

Due to the poor operational lifetime of blue phosphorescent dopants and blue thermally activated delayed fluorescent (TADF) materials, hybrid white organic light-emitting diodes (WOLEDs) with conventional blue fluorescent emitters are still preferred for commercial applications in general lighting. However, the improvement in the overall efficiency of hybrid WOLEDs has been limited due to energy losses during the energy transfer process and exciton quenching after the spatial separation of the singlet and triplet excitons. Here we demonstrate the development of a Spatial Exciton Allocation Strategy (SEAS) to achieve close to 100% internal quantum efficiency (IQE) in blue-yellow complementary color hybrid WOLEDs. The employed blue fluorophore not only has a resonant triplet level with the yellow phosphor to reduce energy loss during energy transfer processes and triplet–triplet annihilation (TTA), but also has a resonant singlet level with the electron transport layer to extend singlet exciton distribution and enhance both singlet and triplet exciton utilization. The resulting hybrid WOLEDs exhibited 104 lm W−1 efficacy at 100 cd m−2 and 74 lm W−1 at 1000 cd m−2 with CIE coordinates of (0.42, 0.44) which is warm white and suitable for indoor lighting.

Deep ultraviolet-to-NIR broad spectral response organic photodetectors with large gain

We developed organic photodetectors with high photocurrent gain by enhancing the electron tunneling from the electrode owing to the effect of interface charge trapping. Furthermore, we utilized a down-conversion material to enhance the photoresponse in the deep-ultraviolet (UV) range. As a result, the photodetectors achieved an external quantum efficiency (EQE) of as high as 4000% while the photoresponsivity remained at 9 A W−1 at the UV wavelength of 250 nm. Finally, a high-performance organic photodetector with broad-photoresponse covering the spectral range from deep-UV (200 nm) to near-infrared (1000 nm) was successfully realized. The maximum EQE exceeds 10000% at a wavelength of 780 nm, and the photoresponsivity and specific detectivity are up to 70 A W−1 and 4 × 1012 Jones, respectively.

Vapour-assisted multi-functional perovskite thin films for solar cells and photodetectors

In this work, we manufactured high quality perovskite thin films via a vapour-assisted process. The film morphologies are qualified by XRD and SEM images. It can be seen that not only high efficiency solar cells, but also high detectivity photodetectors can be fabricated. The solar cells show a power conversion efficiency (PCE) of approximately 10% (with C60 as electron-transport layer). When PCBM serves as the electron-transport layer, the value exceeds 12%. In addition, the photodetectors also achieve a photodetectivity as high as 3 × 1012 Jones.

C70/C70:pentacene/pentacene organic heterojunction as the connecting layer for high performance tandem organic light-emitting diodes: Mechanism investigation of electron injection and transport

A high performance tandem organic light-emitting diode (OLED) is realized by employing a C70/C70:pentacene/pentacene organic heterojunction as the efficient charge generation layer (CGL). Not only more than two time enhancement of external quantum efficiency but also significant improvement in both power efficiency and lifetime are well achieved. The mechanism investigations find that the electron injection from the CGL to the adjacent electron transport layer (ETL) in tandem devices is injection rate-limited due to the high interface energy barrier between the CGL and the ETL. By the capacitance-frequency (C-F) and low temperature current density-voltage (J-V) characteristic analysis, we confirm that the electron transport is a space-charge-limited current process with exponential trap distribution. These traps are localized states below the lowest unoccupied molecular orbital edge inside the gap and would be filled with the upward shift of the Fermi level during the n-doping process. Furthermore, both t...

Highly efficient inverted organic light-emitting diodes using composite organic heterojunctions as electrode-independent injectors

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode have attracted increasing attention for display applications because of their easy integration with the n-type transistors based on low-cost and highly-uniform amorphous silicon (a-Si), and transparent amorphous oxide semiconductors (TAOSs). Up to date, indium tin oxide (ITO) has been widely used as the transparent electrode, but the dogged issue of using ITO as the cathode in IOLEDs is the large energy barrier for electron injection due to its high work function. In this work, a kind of organic charge generation layer (CGL), comprising of a p-type semiconductor/bulk heterojunction (BHJ)/n-type semiconductor (p/BHJ/n), is introduced on the ITO cathode to fabricate high performance red, green and blue IOLEDs. It is found that the utilization of the composite organic heterojunction CGL as an electron injector greatly enhanced the electron injection, thus significantly improving the electroluminescence efficiency of the resulting IOLEDs. More importantly, their performances are independent of the work function of the used cathode. It is experimentally demonstrated that the electrons injected into the emitting layer are from the generated charges in composite organic heterojunction CGLs, which are completely different in the injection manner from electrodes in conventional OLEDs. It is believed that our studies provide a promising method to fabricate high performance IOLEDs regardless of the choice of electrodes, which will benefit to integrate IOLEDs on the n-type TFTs.

Managing excitons for high performance hybrid white organic light-emitting diodes by using a simple planar heterojunction interlayer

High performance hybrid white organic light-emitting diodes (WOLEDs) were fabricated by inserting a planar heterojunction interlayer between the fluorescent and phosphorescent emitting layers (EMLs ...

Highly efficient charge generation and electron injection of m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction on ITO cathode for high efficiency inverted white organic light-emitting diodes

The charge generation and electron injection characteristics of m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction made of 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) p-type organic semiconductor and 1,4,5,8,9,12-hexaazatriphenylene-hexacarbonitrile (HAT-CN) n-type semiconductor were well studied. It was found that m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction showed better charge generation ability than m-MTDATA/HAT-CN organic heterojuntion, and realized highly efficient electron injection when using it as charge generator on indium tin oxide (ITO) cathode. The investigations of capacitance-frequency and current density-voltage characteristics of the electron-only devices based on m-MTDATA/m-MTDATA:HAT-CN/HAT-CN organic heterojunction demonstrated that the amounts of the injected electrons were dependent on the properties of the used n-doping electron transporting layer (n-ETL). Therefore, by optimization, high efficiency inverted white organic light-emitting diodes...

Near infrared to visible light organic up-conversion devices with photon-to-photon conversion efficiency approaching 30%

Near infrared to visible light organic up-conversion devices have drawn more and more attention for their advantages of low cost, large area, flexible and easy processing. However, the photon-to-photon conversion efficiency is still a challenge for practical application. Here, high-performance near infrared to visible light organic up-conversion devices were developed by integrating a near-infrared polymer photodetector and a green tandem organic light emitting diode. The integrated devices showed a peak photon-to-photon conversion efficiency up to 29.6%, which is among the highest reported values. Comprehensive studies on the both optical and electrical interaction between the light absorption unit and the light emission unit in the integrated devices revealed that the more than doubled electron-to-photon conversion quantum efficiency was obtained through the photo-multiplication effect of tandem OLEDs. Furthermore, large area and flexible up-conversion devices with the same structure were also fabricated, showing a better imaging performance than the commercial IR detection card.

Improvement of efficiency and its roll-off at high brightness in white organic light-emitting diodes by strategically managing triplet excitons in the emission layer

Although white organic light-emitting diodes (WOLEDs) have grown dramatically in solid-state lighting, realizing high efficiency and low efficiency roll-off simultaneously is still a prerequisite for practical applications. Here, high efficiency and low efficiency roll-off WOLEDs are presented by strategically managing triplet excitons in the emission layer. In our devices, the emission layer is constructed by strategically introducing non-doped ultrathin orange and green phosphorescent emitters in the blue phosphorescence-doped exciplex host. It can be seen that the triplet excitons within the emission layer are effectively regulated through such an engineering method. As a result, the resulting WOLED operates at a low turn-on voltage of ∼2.3 V due to well-matched energy alignment, and exhibits a maximum forward-viewing power efficiency (PE) and external quantum efficiency (EQE) of 95.3 lm W−1 and 22.8% which remain at 55.5 lm W−1 and 21.9% at 1000 cd m−2 without using light out-coupling techniques. Remarkably, at luminances of 5000 and 10 000 cd m−2, the EQEs are still as high as 20.4% and 18.9%, exhibiting extremely low efficiency roll-off. It is believed that the strategies of emission layer engineering are beneficial for the future development of high efficiency and low roll-off WOLEDs.