光电所所讯

Si and SiC quantum dots co-doped Si-rich SiC p-i-n junction diode with enhanced blue-green light emission

Professor Gong-Ru Lin's Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所林恭如教授

Si quantum dots (Si-QDs) and SiC quantum dots (SiC-QDs) co-doped Si-rich silicon carbide (Si_xC_1-x) based p-i-n junction light-emitting diodes (LEDs) with blue-green light emission is demonstrated. By growing the Si_xC_1-x with plasma-enhanced chemical vapor deposition (PECVD) at relatively low temperature, the turn-on voltage of Si-rich Si_xC_1-x LEDs can be reduced to 6.1 V when thinning i-Si_xC_1-x layer to 25 nm because of higher tunneling probability and lower series resistance. The electroluminescent (EL) power increases to 136 nW, however, which inversely attenuates due to the reduced SiC-QDs if the i-Si_xC_1-x thickness further shrinks to 25 nm. The principle EL peak at 500 nm with narrower shape and blue-green emission pattern is attributed to the self-trapped excitons at surface states among SiC-QDs. The external quantum efficiency (EQE) of the Si-rich Si_xC_1-x LEDs with i-Si_xC_1-x thickness of 50 nm is up to 1.58×10^-1% with enhanced carrier tunneling probability. The carrier injection efficiency is enhanced to 46% by increasing the doping concentration to 10¹⁶cm^-3, leading to almost one order of magnitude improvement on the EQE of Si-rich Si_xC_1-x LEDs.

Figure

Left: The TEM images (Upper) of the Si-rich Si_xC_1-x films (Left) with co-doped SiC-QDs and Si-QDs in average sizes of 2.5 nm and 4.3 nm, respectively with the size distribution (Lower) of SiC-QDs and Si-QDs in the Si-rich Si_xC_1-x film with the thickness of 50 nm.

Middle: The I-V (Lower) and P-I (Upper) characteristics of the Si-rich Si_xC_1-x p-i-n junction LEDs with different i-SiC thickness grown with the fluence ratio of 60%.

Right: The EL spectra (Upper) of Si-rich Si_xC_1-x p-i-n junction LEDs with the different i-Si_xC_1-x thickness.

Inset: the EL emission pattern. The carrier injection efficiency (Lower) of the Si-rich Si_xC_1-x p-i-n junction LED as a function of n-type dopant concentration under fixed B₂H₆ doping concentration.

Reference: Hung-Yu Tai, Chih-Hsien Cheng, and Gong-Ru Lin, “Blue-Green Light Emission from Si and SiC quantum dots co-doped Si-rich SiC p-i-n Junction Diode,” IEEE J. Sel. Top. Quantum Electron., Vol. 20, Issue 4, 8200507, Jul.-Aug. 2014.

ITO-free low-index transparent electrodes for enhancing light out-coupling of organic light-emitting devices

Professor Chung-Chih Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所吴忠帜教授

The transparent conductor indium tin oxide (ITO) has been widely used in organic light-emitting devices (OLEDs) and other organic/inorganic optoelectronics as the transparent electrode. ITO, however, is rather expensive and thus not cost-effective due to scarcity of the indium element on earth. ITO is also brittle and usually requires high-temperature processing, limiting its compatibility with flexible plastic substrates and thus its use for flexible devices. Accordingly, in recent years, alternative (ITO-free) transparent conductors/electrodes for replacement of ITO are being intensively pursued. Through our studies, we have developed high-conductivity conducting polymers having excellent mechanical flexibility, good transmittance, solution processing capability, and low cost. Most importantly, they possess high conductivity of up to ~1000 S/cm, similar to that conventional ITO and sufficient to replace ITO as alternative transparent electrodes in various optoelectronic devices. Intriguingly, such high-conductivity conducting polymers also possess an optical refractive index significantly lower than those of ITO and typical organic layers in OLEDs and well matching those of typical OLED substrates. Optical simulation reveals that by replacing ITO with such a low-index transparent electrode, the guided modes trapped within the organic/ITO layers in conventional OLEDs can be substantially suppressed, leading to more light coupled into the substrate than the conventional ITO device (Fig. 1). By applying light out-coupling structures onto outer surfaces of substrates to effectively extract radiation into substrates, OLEDs using such low-index transparent electrodes achieve much enhanced optical out-coupling and electroluminescence efficiencies (EQE up to 45%, luminous efficiency up to185 lm/W) in comparison with conventional OLEDs using ITO (Fig. 2).

Fig. 1. Calculated mode distributions (fraction of radiation coupled into different modes) of OLED internally generated radiation.

Fig. 2. External quantum efficiencies and luminous efficiencies of various devices.