Hengda Sun

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.

Rosin-enabled ultraclean and damage-free transfer of graphene for large-area flexible organic light-emitting diodes

The large polymer particle residue generated during the transfer process of graphene grown by chemical vapour deposition is a critical issue that limits its use in large-area thin-film devices such as organic light-emitting diodes. The available lighting areas of the graphene-based organic light-emitting diodes reported so far are usually <1 cm2. Here we report a transfer method using rosin as a support layer, whose weak interaction with graphene, good solubility and sufficient strength enable ultraclean and damage-free transfer. The transferred graphene has a low surface roughness with an occasional maximum residue height of about 15 nm and a uniform sheet resistance of 560 Ω per square with about 1% deviation over a large area. Such clean, damage-free graphene has produced the four-inch monolithic flexible graphene-based organic light-emitting diode with a high brightness of about 10,000 cd m−2 that can already satisfy the requirements for lighting sources and displays.

Achieving Extreme Utilization of Excitons by an Efficient Sandwich-Type Emissive Layer Architecture for Reduced Efficiency Roll-Off and Improved Operational Stability in Organic Light-Emitting Diodes.

It has been demonstrated that the efficiency roll-off is generally caused by the accumulation of excitons or charge carriers, which is intimately related to the emissive layer (EML) architecture in organic light-emitting diodes (OLEDs). In this article, an efficient sandwich-type EML structure with a mixed-host EML sandwiched between two single-host EMLs was designed to eliminate this accumulation, thus simultaneously achieving high efficiency, low efficiency roll-off and good operational stability in the resulting OLEDs. The devices show excellent electroluminescence performances, realizing a maximum external quantum efficiency (EQE) of 24.6% with a maximum power efficiency of 105.6 lm W(-1) and a maximum current efficiency of 93.5 cd A(-1). At the high brightness of 5,000 cd m(-2), they still remain as high as 23.3%, 71.1 lm W(-1), and 88.3 cd A(-1), respectively. And, the device lifetime is up to 2000 h at initial luminance of 1000 cd m(-2), which is significantly higher than that of compared devices with conventional EML structures. The improvement mechanism is systematically studied by the dependence of the exciton distribution in EML and the exciton quenching processes. It can be seen that the utilization of the efficient sandwich-type EML broadens the recombination zone width, thus greatly reducing the exciton quenching and increasing the probability of the exciton recombination. It is believed that the design concept provides a new avenue for us to achieve high-performance OLEDs.

Interconnectors in Tandem Organic Light Emitting Diodes and Their Influence on Device Performance

In this paper, we present a comprehensive review on recent progress of interconnectors in tandem organic light emitting diodes, from structure design to performance and their working principles. We will introduce the most commonly used interconnectors, discuss in detail the main features of them, as well as the advantages and disadvantages of each type. Especially, organic heterojunction type interconnectors are highlighted.