Space charge capacitance and the physical mechanism of negative capacitance in organic light-emitting diodes (OLEDs) by transient current response analysis are investigated for the first time. Space charge capacitance is found to be a fixed value as voltage increases for each device. The inflection points in capacitance–voltage curves correspond to carrier injection, transportation and combination processes in organic layers. A negative capacitance effect at low frequency relates to the internal accumulated carrier state in the OLEDs. The nonsynchronicity between the phases of the internal accumulated carriers’ states changing and the small alternating current leads to electric field reversal. Only the electric field reversal at low frequency results in a negative capacitance effect.

•The dependency relation between transmission rate and electron transport layer is revealed.•The critical temperature points for the influence of luminescent materials and injection barriers on delay time are found.•The influence of light-emitting material and injection layer on carrier accumulation is quantified.In this work, the organic light-emitting diodes (OLEDs) based on Alq3 are fabricated. In order to make clear the transport mechanism of carriers in organic light-emitting devices at low temperature, detailed electroluminescence transient response and the current-voltage–luminescence (I–V–L) characteristics under different temperatures in those OLEDs are investigated. It founds that the acceleration of brightness increases with increasing temperature is maximum when the temperature is 200 K and it is mainly affected by the electron transport layer (Alq3). The MoO3 injection layer and the electroluminescent layer have great influence on the delay time when the temperature is 200 K. Once the temperature is greater than 250 K, the delay time is mainly affected by the MoO3 injection layer. On the contrary, the fall time is mainly affected by the electroluminescent material. The Vf is the average growth rate of fall time when the temperature increases 1 K which represents the accumulation rate of carriers. The difference between Vf caused by the MoO3 injection layer is 0.52 us/K and caused by the electroluminescent material Ir(ppy)3 is 0.73 us/K.

Detailed transient current response characteristics under pulsed voltage in MoO3-based organic light-emitting diodes (OLEDs) are first investigated. The transient current can be divided into three parts: positive current spike (IP), steady-state current (IS), and negative current spike (IN). It is found that IS is the operating current of OLED devices. IP and IN show a linear relationship with pulsed voltage amplitude. Higher IN than IP is induced by the difference of anode Fermi level and highest occupied molecular orbital (HOMO) of NPB. We also derive an expression for duty ratio with space charges. Results show that IP and IN correspond to the charging and discharging processes of space charge, respectively. The current spikes and threshold duty ratio are found only to be affected by the injection barrier and independent of internal device structure.

In this work, an organic–inorganic hybrid optical upconverter that can convert irradiated 980 nm IR light to 510 nm green phosphorescence sensitively was fabricated and studied. fac-Tris(2-phenylpyridine) iridium (Ir(ppy)3) doped 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) was used as emitting layer in the phosphorescent organic light-emitting diode (OLED) unit. The upconverter using a phosphorescent OLED as display unit can achieve a higher upconversion efficiency and a low power consumption when compared with the one using fluorescent. An upconversion efficiency of 4.8% can be achieved for phosphorescent device at 15 V, much higher than that of fluorescent one (2.0%). The upconverter’s transient optical and electric response to IR pulse were also investigated for the first time. The response time was found to be influenced by IR intensity and applied voltage. It has a response time as short as 60 μs. The rapid response property of the upconverter makes it feasible to be applied to high-speed IR imaging systems.Keywords: fast response; infrared photo detector; OLED; optical upconverter; organic−inorganic hybrid device; phosphorescentKeywords: 42.79.Pw; 85.60.Bt; 85.60.Jb

An organic/inorganic hybrid up-conversion device was demonstrated in this work, which can convert near-infrared light (NIR) to visible green at high conversion efficiency. The upconverter was fabricated by integrating an In0.12Ga0.88As/GaAs multiquantum wells (MQWs) photodetector (PD) with an organic light emitting diode (OLED). The up-conversion efficiency of 4.0 W/W % was obtained at 20 V under NIR illumination of 1mW/mm2 at room temperature by optimizing the structure of the PD unit and adding MoO3 doped perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) as interfacial layer of OLED. Meanwhile, the green light output induced by NIR achieved 6050 cd/m2, which proves that the organic/inorganic hybrid upconverter an excellent candidate that can be applied in light converter field.Keywords: hybrid up-conversion device; interfacial layer; MoO3-doped PTCDA; near-infrared; organic light-emitting diode; photodetector;

Effects of doping molybdenum oxide (MoO3) in copper phthalocyanine (CuPc) as hole injection layer in OLEDs are studied. A green OLED with structure of ITO/MoO3-doped CuPc/NPB/10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H, 11H-(1)-benzopyropyrano(6,7,8-i,j) quinolizin-11-one (C545T): tris(8-hydroxyquinoline) aluminum (Alq3)/Alq3/LiF/Al shows the driving voltage of 4.4 V, and power efficiency of 4.3 lm/W at luminance of 100 cd/m2. The charge transfer complex between CuPc and MoO3 plays a decisive role in improving the performance of OLEDs. The AFM characterization shows that the doped film owns a better smooth surface, which is also in good agreement with the electrical performance of OLEDs.Highlights► MoO3-doped CuPc layer as hole injection layer is effective for OLEDs. ► The charge transfer complex is important to improve the performance of OLEDs. ► The amount of charge transfer complex depends on MoO3 doping concentration.

We report a hybrid up-conversion device integrating a In0.2Ga0.8As/GaAs MQWs photodetector with an organic light emitting diode (OLED), that converts input 980 nm infrared light to output 520 nm green light. Devices with different interface layer, used as the hole injection layer (HIL) in OLEDs were fabricated and tested. It was found that the device with an HIL of MoO3-doped CuPc exhibited a lowest turn-on voltage of 2.6 V. The maximum external up conversion efficiency of 0.81 W/W% is achieved at the bias of 20 V under an input 980 nm NIR power density of 1 mW/mm2.Graphical abstractSchematic diagram of up conversion device.Highlights► We designed and fabricated the organic-inorganic hybrid light up-conversion devices. ► Up-convertor is composed of an OLED and In0.2Ga0.8As MQWS photodetector. ► We examine three different interface layers for upconversion device performance. ► That the appropriate interface layer is crucial for the up conversion device.

The tandem organic light-emitting diodes (OLEDs) with an effective charge-generation connection structure of Mg-doped tris(8-hydroxyquinoline) aluminum (Alq3)/Molybdenum oxide (MoO3)-doped 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) were presented. At a current density of 50 mA/cm2, the current efficiency of the tandem OLED with two standard NPB/Alq3 emitting units is 4.2 cd/A, which is 1.7 times greater than that of the single EL device. The tandem OLED with the similar connection structure of Mg-doped PTCDA/ MoO3-doped PTCDA was also fabricated and the influences of the different connection units on the current efficiency of the tandem OLED were discussed as well.

The tandem organic light-emitting diodes (OLEDs) with an effective charge-generation connection structure of Mg-doped tris(8-hydroxyquinoline) aluminum (Alq3)/Molybdenum oxide (MoO3)-doped 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) were presented. At a current density of 50 mA/cm2, the current efficiency of the tandem OLED with two standard NPB/Alq3 emitting units is 4.2 cd/A, which is 1.7 times greater than that of the single EL device. The tandem OLED with the similar connection structure of Mg-doped PTCDA/ MoO3-doped PTCDA was also fabricated and the influences of the different connection units on the current efficiency of the tandem OLED were discussed as well.

As one of the most environmentally important cations, mercury(II) iron has the biological toxicity which impacts wild life ecology and human health heavily. A Hg2+ biosensor based on AlGaAs/InGaAs high electron mobility transistors with high sensitivity and short response time is demonstrated experimentally. To achieve highly specific detection of Hg2+, an one-end thiol-modified ssDNA with lots of T thymine is immobilized to the Au-coated gate area of the high electron mobility transistors by a covalent modification method. The introduction of Hg2+ to the gate of the high electron mobility transistors affects surface charges, which leads to a change in the concentration of the two-dimensional electron gas in the AlGaAs/InGaAs high electron mobility transistors. Thus, the saturation current curves can be shifted with the modification of the gate areas and varied concentrations of Hg2+. Under the bias of 100 mV, a detection limit for the Hg2+ as low as10 nM is achieved. Successful detection with minute quantity of the sample indicates that the sensor has great potential in practical screening for a wide population. In addition, the dimension of the active area of the sensor is 20×50 μm2 and that of the entire sensor chip is 1×2 mm2, which make the Hg2+ biosensor portable.