Guangrui Yao

Performance enhancement of blue light-emitting diodes with AlGaN barriers and a special designed electron-blocking layer

In this study, the characteristics of the nitride-based blue light-emitting diode (LED) with AlGaN barriers are analyzed numerically and experimentally. The emission spectra, carrier concentrations in the quantum wells (QWs), energy band diagrams, electrostatic fields, and internal quantum efficiency are investigated. The results indicate that the LED with AlGaN barriers has a better hole-injection efficiency and an enhanced carrier confinement in its active region over the conventional counterpart with GaN barriers. The results also show that the AlGaN electron-blocking layer (EBL) with a gradual variation of Al mole fraction has a significantly enhanced electron blocking capability as well as a greatly improved hole-injection efficiency. When Al0.08Ga0.92N QW barriers and the special designed EBL are used, the electroluminescence emission intensity is increased greatly by 69% at 200 A/cm2 and the efficiency droop is reduced markedly to 8.7% from 85% at 400 A/cm2 compared with those of the conventional LED.

Performance enhancement of blue light-emitting diodes without an electron-blocking layer by using special designed p-type doped InGaN barriers

In this study, the characteristics of the nitride-based blue light-emitting diode (LED) without an electron-blocking layer (EBL) are analyzed numerically. The emission spectra, carrier concentrations in the quantum wells (QWs), energy band diagrams, electrostatic fields, and internal quantum efficiency (IQE) are investigated. The simulation results indicate that the LED without an EBL has a better hole-injection efficiency and smaller electrostatic fields in its active region over the conventional LED with an AlGaN EBL. The simulation results also show that the LED without an EBL has severe efficiency droop. However, when the special designed p-type doped InGaN QW barriers are used, the efficiency droop is markedly improved and the electroluminescence (EL) emission intensity is greatly enhanced which is due to the improvement of the hole uniformity in the active region and small electron leakage.

First-principles prediction of a new class of photovoltaic materials: I-III-IV2-V4 phosphides

A new class of quaternary I-III-IV2-V4, including CuAlGe2P4, CuGaGe2P4, CuAlSn2P4, and CuGaSn2P4 are studied by density functional theory and beyond for potential photovoltaic application. We found that CuAlGe2P4 and CuGaGe2P4 have a ground state of kesterite (KS) structure, while CuAlSn2P4 and CuGaSn2P4 are nearly energetically degenerated for KS and stannite structures. Interestingly, the band gaps of all the studied quaternary compounds are predicted to be in the range of 1.1–1.7 eV by the hybrid functional calculation and Δ-sol approach [M. K.Y. Chan and G. Ceder, Phys. Rev. Lett. 105, 196403 (2010)]. In particular, CuAlSn2P4 in KS structure is predicted to be a potential high-efficiency photovoltaic material since it contains no rare or toxic elements with a direct gap around 1.52 eV.

Performance improvement of InGaN blue light-emitting diodes with several kinds of electron-blocking layers

The performance of InGaN blue light-emitting diodes (LEDs) with different kinds of electron-blocking layers is investigated numerically. We compare the simulated emission spectra, electron and hole concentrations, energy band diagrams, electrostatic fields, and internal quantum efficiencies of the LEDs. The LED using AlGaN with gradually increasing Al content from 0% to 20% as the electron-blocking layer (EBL) has a strong spectrum intensity, mitigates efficiency droop, and possesses higher output power compared with the LEDs with the other three types of EBLs. These advantages could be because of the lower electron leakage current and more effective hole injection. The optical performance of the specifically designed LED is also improved in the case of large injection current.