Xianfeng Qiao

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.

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...

Efficient Room-Temperature Phosphorescence from Organic-Inorganic Hybrid Perovskites by Molecular Engineering

Solution-processed organic-inorganic hybrid perovskites are promising emitters for next-generation optoelectronic devices. Multiple-colored, bright light emission is achieved by tuning their composition and structures. However, there is very little research on exploring optically active organic cations for hybrid perovskites. Here, unique room-temperature phosphorescence from hybrid perovskites is reported by employing novel organic cations. Efficient room-temperature phosphorescence is activated by designing a mixed-cation perovskite system to suppress nonradiative recombination. Multiple-colored phosphorescence is achieved by molecular design. Moreover, the emission lifetime can be tuned by varying the perovskite composition to achieve persistent luminescence. Efficient room-temperature phosphorescence is demonstrated in hybrid perovskites that originates from the triplet states of the organic cations, opening a new dimension to the further development of perovskite emitters with novel functional organic cations for versatile display applications.

Molecular engineering of two-dimensional hybrid perovskites with broadband emission for white light-emitting diodes

Two-dimensional (2D) hybrid perovskites exhibiting broadband light emission are attractive as down-converting phosphors in white light-emitting diodes (WLEDs). Despite active exploration of new members of this family of materials, fine-tuning of their emission through structural variation for realizing high color-rendering white light remains largely untapped. Here we report a series of (100)-oriented 2D perovskites whose structures are templated by the organic cations. By controlling the tilting of the inorganic octahedra, we were able to shift the broadband emission from blue to white. A photophysical study further suggests that the coexistence of self-trapped excitons and free excitons contributes to a double-peak broad emission, covering the entire visible spectrum. Using the broad-emitting perovskites as down-converting phosphors, we fabricated WLEDs with white-light emission having a correlated color temperature (CCT) of 6600 K and a high color rendering index (CRI, Ra) of 86.

Magnetic field effects on the quenching of triplet excitons in exciplex-based organic light emitting diodes

Triplet excited states in exciplex-based organic light emitting diodes (OLEDs) can be wasted by transferring their energy to the host material in a system with smaller triplet energy levels. This triplet loss process is investigated using magnetic field effects (MFEs). Magnetophotoluminescence (MPL) and magnetoelectroluminescence (MEL) are comprehensively studied and the results suggest that this energy transfer can influence the lineshape or amplitude of MPL and the current dependence of MEL. Transient MEL is also studied to examine the influence of the magnetic field on exciton dynamic processes. The experimental results show that the time-resolved MELs at the rising and falling pulse stages will be affected by this triplet energy transfer based on different exciton interactions under an external magnetic field.

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.