利用单层密排的纳米球提高发光二极管的出光效率

在发光二极管(LED)的透明电极层上制作单层六角密排的聚苯乙烯(polystyrene, PS) 纳米球, 研究提高GaN基蓝光LED的出光效率. 采用自组装的方法在透明电极铟锡氧化物层上制备了直径分别约为250, 300, 450, 600和950 nm的PS纳米球, 并且开展了电致发光的研究. 结果表明, 在LED的透明电极层上附有PS纳米球能有效地提高LED的出光效率; 当PS纳米球的直径与出射光的波长比较接近时, LED的出光效率最优. 与参考样品相比, 在20 mA和150 mA工作电流下, 附有PS纳米球的样品的发光效率分别增加1.34倍和1.25倍. 三维时域有限差分方法计算表明, 该出光增强主要归因于附有PS纳米球的LED结构可以增大LED结构的光输出临界角, 从而提高LED的出光效率. 因此, 这是一种低成本的实现高效率LED的方法.

Light-extraction enhancement of GaN-based LEDs by closely-packed nanospheres monolayer

GaN based light-emitting diodes (LEDs) have been attracting a great deal of interest due to their capability in emitting a spectrum from ultraviolet to green and their applications in traffic signals, displays and solid-state lighting. However, the high efficiency of LED is still obstructed by light-extraction efficiency. In this work, we propose that light-extraction efficiency of GaN-based blue LED should be improved by a self-assembled monolayer of polystyrene spheres. The GaN-based LED grown on sapphire substrate emits the light mainly from the indium tin oxide (ITO) transparent electrode. And the hexagonal closely-packed polystyrene sphere monolayer is formed onto the ITO layer. In order to study the light-extraction efficiency affected by the size of nanosphere, nanosphere monolayers of different sizes are prepared onto the ITO layer, and the diameters of the polystyrene spheres are 250, 300, 450, 600 and 950 nm, respectively. The electroluminescence results show that using polystyrene sphere monolayer can improve the light-extraction efficiency compared with using the conventional LEDs, and the light-extraction efficiency reaches a maximum when the average size of spheres (450 nm) approximates to the wavelength (465 nm) of that light. The light output power of the LED with polystyrene sphere of the optimum size is experimentally enhanced by 1.34 and 1.25 times under the injection currents of 20 and 150 mA, respectively. In order to explain the physical mechanism of the light-extraction enhancement, we carried out the three-dimensional finite difference time-domain simulation thereby calculate the transmission spectrum of the structure. The results of simulation show that the incident light beyond the critical angle can be partly extracted when the surface of LED has a polystyrene sphere monolayer, leading to an enhanced light-extraction efficiency. So the nanosphere monolayer acts as a two-dimensional diffraction lattice which behaves as a light scattering medium for the light propagating in a waveguiding mode within the LED. Furthermore, the polystyrene nanosphere has the advantages of low-cost and high-precision, and is very suitable for large area preparation on LEDs. So this method is a simple and cost-effective method to improve the light-extraction efficiency from LED.