Qingxun Guo

Organic semiconductor heterojunctions: electrode-independent charge injectors for high-performance organic light-emitting diodes

Organic light-emitting diodes (OLEDs) are driven by injected charges from an anode and a cathode. The low and high work function metals are necessary for the effective injection of electrons and holes, respectively. Here, we introduce a fully novel design concept using organic semiconductor heterojunctions (OSHJs) as the charge injectors for achieving highly efficient OLEDs, regardless of the work functions of the electrodes. In contrast to traditional injected charges from the electrodes, the injected charges originate from the OSHJs. The device performance was shown to be significantly improved in efficiency and stability compared to conventional OLEDs. Attractively, the OLEDs based on OSHJs as charge injectors still exhibited an impressive performance when the low work function Al was replaced by air- and chemistry-stable high work function metals, such as Au, Ag, and Cu, as the cathode contact, which has been suggested to be difficult in conventional OLEDs. This concept challenges the conventional design approach for the injection of charges and allows for the realization of practical applications of OLEDs with respect to high efficiency, selectable electrodes, and a long lifetime.

Charge generation mechanism of tandem organic light emitting diodes with pentacene/C70 organic heterojunction as the connecting layer

The working mechanism of a planar organic heterojunction based on pentacene/C70 under reverse voltage is studied through current–voltage (I–V) and capacitance–voltage (C–V) measurements. It is found that the pentacene/C70 heterojunction generates large amounts of charges and the charge generation is a tunneling process. The proposed Fowler–Nordheim (F–N) model theoretically demonstrates the I–V properties of the pentacene/C70 heterojunction-based device at different temperatures. The heterojunction interface energy diagram is also well determined by ultraviolet photoemission spectroscopy (UPS) measurements, further elucidating this tunneling process. Moreover, by taking advantage of the large charge generation property of the pentacene/C70 heterojunction, a high efficiency green tandem organic light emitting diode (OLED) is successfully fabricated, where not only the current efficiency is doubled, but also the power efficiency is greatly enhanced, proving the excellent performance of the pentacene/C70 heterojunction as charge generation layer (CGL). This work highlights the working mechanism of such heterojunction and provides us a new guide to design high performance tandem OLEDs.

High Tg small-molecule phenanthroline derivatives as a potential universal hole-blocking layer for high power-efficiency and stable organic light-emitting diodes

Electron-transport/hole-blocking materials are beneficial for improving OLED efficiency through promoting hole/electron recombination. In this contribution, we present a new phenanthroline derivative Phen-DFP by appending 3-(3,5-bis(2,4-difluorophenyl)phenyl)phenyl to 1,10-phenanthroline, based on the recent triarylphosphine oxide–phenanthroline molecular conjugate Phen-m-PhDPO. Phen-DFP possesses a Tg of 95 °C, a LUMO/HOMO level of ca. −3.0/−6.6 eV and a μe of ca. 2.5 × 10−5 cm2 V−1 s−1 @ E = 5 × 10 5 V cm−1. It was evaluated as a potential hole blocker for pin sky blue fluorescent, and green and red phosphorescent OLEDs, in comparison with Phen-m-PhDPO and TPBi. Remarkably, the modified DSA-Ph based fluorescent OLEDs that contained Phen-m-PhDPO and Phen-DFP produced a luminous and power efficiency of ∼16 cd A−1 and 13 lm W−1 @1000 cd m−2 with a primitive lifetime t90 ≈ 200 h @1000 cd m−2 under constant current driving. In contrast with the sky blue fluorescent and red phosphorescent OLEDs, the inferior luminous efficiency of the green Phen-DFP phosphorescent OLEDs was attributed to the lower triplet energy of Phen-DFP with respect to Phen-m-PhDPO and TPBi. These comprehensive studies may contribute to the understanding of the complex trade-offs among electron injection/transport, triplet energy, hole blocking, OLED luminous/power efficiency and operational stability, huddled around a hole blocker.

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...