Enhanced Hole Transport Research Articles

Efficiency retention at high current injection levels in m-plane InGaN light emitting diodes

We investigated the internal quantum efficiency (IQE) and the relative external quantum efficiency (EQE) of m-plane InGaN light emitting diodes (LEDs) grown on m-plane freestanding GaN emitting at ∼400 nm for current densities up to 2500 A/cm2. IQE values extracted from intensity and temperature dependent photoluminescence measurements were consistently higher, by some 30%, for the m-plane LEDs than for reference c-plane LEDs having the same structure, e.g., 80% versus 60% at an injected steady-state carrier concentration of 1.2×1018 cm−3. With increasing current injection up to 2500 A/cm2, the maximum EQE is nearly retained in m-plane LEDs, whereas c-plane LEDs exhibit approximately 25% droop. The negligible droop in m-plane LEDs is consistent with the reported enhanced hole carrier concentration and light holes in m-plane orientation, thereby enhanced hole transport throughout the active region, and lack of polarization induced field. A high quantum efficiency and in particular its retention at high injection levels bode well for m-plane LEDs as candidates for general lighting applications.

Fabrication and Characterization of Temperature Insensitive 660-nm Resonant-Cavity LEDs

InGaP/AlGaInP 660-nm resonant-cavity light-emitting diodes (RCLEDs) with stable temperature characteristics have been achieved by extending the resonant cavity length from one optical wavelength (1 lambda) to three optical wavelengths (3 lambda) and tripling the number of quantum wells. When the operation temperature increases from 25degC to 95degC, the degree of power variation at 20 mA is reduced from -2.1 dB to -0.6 dB for the conventional 1- lambda cavity RCLEDs and 3- lambda cavity RCLEDs, respectively. In order to interpret the temperature-dependent experimental results, advanced device simulation is applied to model the RCLEDs with different cavity designs. According to the numerical simulation results, we deduce that the stable temperature-dependent output performance should originate from the reduction of electron leakage current and thermally enhanced hole transport for the 3- lambda cavity AlGaInP RCLEDs.

Enhanced hole transport in poly(p-phenylene vinylene) planar metal-polymer-metal devices

In planar metal-poly(p-phenylene vinylene) (PPV)-metal devices the experimental current is five to six orders of magnitude larger as compared to the expected space-charge limited current. Comparing these measurements with field-effect transistors demonstrates that the enhanced current originates from a high surface charge carrier density at the polymer/substrate interface. This surface charge is found to be only weakly dependent on the substrate, device geometry, and chemical treatment of the substrate. The presence of such a conducting channel due to charging of the surface obscures the intrinsic in-plane conducting properties of PPV.

Enhanced hole transport in C60-doped hole transport layer

The carrier mobility of C60-doped hole transport layer was investigated according to the C60 content. The addition of C60 in the 1, 3, 5-tris (N, N-bis-(4, 5-methoxyphenyl)-aminophenyl) benzol (TDAPB) hole transport material resulted in the increase of hole mobility of TDAPB. The C60 molecule acted as a p-type dopant in the hole transport layer, resulting in the improved hole transport in TDAPB. The hole mobility of C60-doped TDAPB was 9.0×10−4cm2∕Vs compared with 1.0×10−4cm2∕Vs for nondoped TDAPB.

Enhanced hole transport from Au to Zn-phthalocyanine by an insertion of a thin n-type layer

A thin C60 or bathocuproine (BCP) layer of 5 nm thickness is inserted to the Au/Zn-phthalocyanine (ZnPc) interface of a Au/ZnPc/In/Al Schottky-junction cell, and the effect of the insertion on the dark current is investigated. The forward dark current density is increased from 0.5mA∕cm2 to 9.7 and 9.2mA∕cm2 by the insertions of the C60 and BCP layers, respectively. The increase is not attributed to a change in the barrier height for the hole injection, but to a decrease of the concentration of trapped negative charges on the ZnPc surface.