P. C. Ku

Photoluminescence study of semipolar {101¯1}InGaN∕GaN multiple quantum wells grown by selective area epitaxy

Semipolar InGaN∕GaN multiple quantum wells (MQWs) were fabricated on the {101¯1} facets of GaN pyramidal structures by selective area epitaxy. Optical properties of the MQWs were investigated by photoluminescence (PL) in comparison with (0001) MQWs. Compared with (0001) MQWs, the internal electric field in the {101¯1} MQWs was remarkably reduced, the PL peak redshifted monotonically with the increasing temperature, and the internal quantum efficiency was estimated to be improved by a factor of 3. These results suggest that the {101¯1} planes are promising for improving the performance of III-nitride light emitters owing to their surface stability and suppression of polarization effects.

Multiple Wavelength Emission From Semipolar InGaN/GaN Quantum Wells Selectively Grown by MOCVD

Multiple wavelength emission is experimentally observed from semipolar InGaN/GaN quantum wells selectively grown by MOCVD. Selective growth rates on different mask opening areas result in a multiple wavelength emission from the same wafer.

Plasmonic backscattering enhanced inverted photovoltaics

A plasmonic nanoparticle incorporated inverted organic photovoltaic structure was demonstrated where a monolayer of Ag nanoparticles acted as a wavelength selective reflector. Enhanced light harvesting via plasmonic backscattering into the photovoltaic absorber was observed, resulting in a two-fold improvement in the photocurrent and increased open-circuit voltage. Further, utilizing an optical spacer, the plasmonic backscattering was spectrally controlled, thereby modulating the external quantum efficiency and the photocurrent. Unlike a regular thin-film metallic back reflector, excellent off-resonance optical transmission in excess of 80% was observed from the Ag nanoparticles, making this structure highly suitable for semi-transparent and multi-junction photovoltaic applications.

Angular selective semi-transparent photovoltaics.

Conventional semi-transparent photovoltaics suffer from an inherent tradeoff between the amount of visible light transmitted versus absorbed, reducing energy conversion efficiency when higher transparency is desired. As a solution to lift this tradeoff, we propose a wavelength and angular selective reflector and demonstrate a potential implementation utilizing high aspect ratio metal nanoparticles. Using the anisotropy in the localized surface plasmon resonance wavelength, the proposed device can selectively harness sunlight incident at an elevated angle, increasing the power conversion efficiency by a factor of 1.44, while maintaining 70 percent optical transparency at normal incidence.

Angular selective backreflector for semitransparent photovoltaics

A nanoporous anodized aluminum oxide template is used as an incident-angle selective backreflector to lift the transparency vs. light absorption tradeoff in semi-transparent photovoltaics. Light incident at angles greater-than 45° from the template normal is backscattered at greater-than 35% intensity to a semitransparent photovoltaic increasing absorption and photocurrent. Meanwhile, light incident orthogonal to the template is transmitted with minimum attenuation (>85% at 600u2009nm), limited only by a single-pass absorption within the active layer. Using a vertical semitransparent photovoltaic proposed here, efficient capture of direct sunlight is possible while maintaining transparency at viewing angles close to the substrate normal.

Broadband characteristics of surface plasmon enhanced solar cells

The surface plasmon characteristics of metal nanostructures have the potential to improve optical absorption in thin film photovoltaics. We present a novel method for simulating and analyzing absorption in the active region of metal nanoparticle enhanced solar cells, capable of determining broad spectrum behavior from a single simulation and accurately accounting for optical losses in the metal. This method has several advantages over alternative simulations, and can be applied to modelling any number of thin film material systems.

Fabrication of site-controlled, highly uniform, and dense InGaN quantum dots

We report fabrication of site-controlled, highly uniform and dense (>=1e10/cm2) InGaN multiple stacks of quantum dots using selective area epitaxy in MOCVD. The dot height and lateral dimension are 3 nm and 30 nm, respectively.

Monolithically integrated multi-color InGaN/GaN nanopillar light emitting diodes

A lot of works have been done to explore the possibility of integrating multiple colors in one LED chip [1,2,3]. It has great potential in displays, imaging, sensors, and plenty of possible applications. Multi-color nanoLED arrays have been achieved by molecular-beam-epitaxy grown InGaN dot-in-wire structures [1,3] and colloidal quantum dot arrays [2]. However, they requires complicated manufacturing processes that could not be easily adopted for commercial products. Here we reported a scalable fabrication scheme to make monolithically integrated and electrically driven multi-color nanoLED arrays. The InGaN/GaN nanopillars can be easily fabricated from quantum well structures by lithography and dry etching, which are highly compatible to current LED technologies, and the emission wavelength can be tuned by manipulating nanopillar diameter and engineering strain relaxation effect. The proposed structure of multi-color nanoLED arrays is shown in Fig. 1. In addition, the nanopillar structure could improve the emission efficiency [4] and reduce efficiency droop [5]. All these advantages make nanopillars one of the most promising candidate for next-generation light emitting devices.

MOCVD-Grown InGaN nanowires for photovoltaic applications

Dislocation-free, vertically-aligned, and strain-relaxed InGaN nanowires were realized using metal-organic chemical vapor deposition. The indium composition is continuously tunable from 3.4 to 2.5 eV, with the latter being significant for a four-junction photovoltaic device. Growth mechanism, morphological evolution, and optical as well as structural properties were analyzed. Photovoltaic response was also shown.

Plasmonic nanostructures for transparent photovoltaic facades

Building-integrated photovotaic facades can capture a significant amount of solar energy that is currently underused. Semitransparent photovoltaic devices are one of the most promising solutions for such applications. However, in conventional transparent photovoltaic structures, transparency inevitably translates to poor light harvesting and hence low power conversion efficiency. In this paper, we propose a plasmonic nanostructure based wavelength and angle selective back reflector that can provide excellent see-through clarity while providing effective light harvesting for efficient solar energy conversion. The proposed back reflector is independent from the solar cell design and can be integrated with nearly every type of transparent solar cell structures, ranging from amorphous to polymer absorbers and from single-junction to tandem cells. Uniquely, the plasmonic nanostructures can be integrated with a large-area thin-film solar cell device on flexible substrates, enabling “photovoltaic film” that can be retrofitted to the existing window systems. This paper presents the proposed concept and its validation using finite-difference-time-domain simulations. Results indicate that plasmonic light trapping can improve photovoltaic absorption of angled light by a factor of 1.7 on resonance while maintaining good transparency for normally incident light.