Quantum efficiency enhancement optimization in colloidal semiconductor quantum dot solids using nonradiative energy transfer

Abstract

Abstract:We investigate quantum efficiency (QE) enhancement via recycling of trapped excitons in energy-gradient of quantum dot solids using nonradiative energy transfer. The maximum QE increase of 17% is achieved when the donor-acceptor ratio is 1:1. ©2010 Optical Society of AmericaOCIS codes: (160.4236) Nanomaterials; (160.4760) Optical properties Colloidal semiconductor quantum dots (QDs) possess favorable characteristics to serve as color-converters in hybrid white light emitting diodes (LEDs). For instance, red-emitting QDs accomplish a photon conversion efficiency of 52.8%, which is higher than that of their phosphor-based counterparts [1]. These QDs exhibit strong absorption of incident blue/UV pump light and in turn make photon emission for color conversion at the required photon energy that is precisely tuneable. Furthermore, such QDs cover the entire emission spectrum from visible to infrared using the size effect. Their high-quality solid films can easily be formed using standard techniques including layer-by-layer film deposition and spin casting [2]. Because of these advantages, QD integrated LEDs achieved important steps forward in solid state lighting such as highly color-rendered, warm lighting. However, these QDs experience a reduction in quantum efficiency (QE) in solid film with respect to their higher QE in solution [3]. To address this problem, trapped excitons can be recycled into QDs via nonradiative energy transfer (ET) (i.e., Förster-type energy transfer), which offers a potential increase in the QE of these QD solid films. Here we present the optimization of the donor-acceptor (D-A) QD ratios to investigate QE enhancement in these colloidal QD solids [4]. To have nonradiative ET, an energy gradient QD system is needed. For quantum efficiency measurements, a monochromator (Newport) with a broad-emitting light source is used to pump QDs and an integrating sphere (Newport) that collects the entire luminescence of these QDs is connected to the monochromator. For time-resolved luminescence decay measurements, a spectroscopy system (Picoquant) is used with a calibrated time-resolution of 16 ps. Figure 1 presents the QE enhancement measurements of these QDs dependent on their D-A ratios compared to the control groups (which are represented as the base line at the far edges of the plot for easier visualization). In all of these energy gradient QD structures with different donor-acceptor a1285_1.pdf OSA / CLEO/QELS 2010CTuNN6.pdf 978-1-55752-890-2/10/$26.00 ©2010 IEEE