Colloidal quantum dot light-emitting devices

Abstract

Colloidal quantum dot light-emitting devices (QD-LEDs) have generated considerable interest for applications such as thin film displays with improved color saturation and white lighting with a high color rendering index (CRI). We review the key advantages of using quantum dots (QDs) in display and lighting applications, including their color purity, solution processability, and stability. After highlighting the main developments in QD-LED technology in the past 15 years, we describe the three mechanisms for exciting QDs - optical excitation, Förster energy transfer, and direct charge injection - that have been leveraged to create QD-LEDs. We outline the challenges facing QD-LED development, such as QD charging and QD luminescence quenching in QD thin films. We describe how optical downconversion schemes have enabled researchers to overcome these challenges and develop commercial lighting products that incorporate QDs to achieve desirable color temperature and a high CRI while maintaining efficiencies comparable to inorganic white LEDs (>65 lumens per Watt). We conclude by discussing some current directions in QD research that focus on achieving higher efficiency and air-stable QD-LEDs using electrical excitation of the luminescent QDs.

Keywords: displays; lighting; nanocrystals; optoelectronics.

Fig. 1

(a) Photoluminescence spectra of CdSe/ZnS and PbS/CdS core/shell colloidal QDs showing the possibility for narrowband emission across the visible and near-IR wavelength regions. The inset schematic shows a colloidal quantum dot (QDs). (b) CIE chromaticity diagram showing that the spectral purity of QDs enables a color gamut larger than the HDTV standard.

Fig. 2

(a) Plot showing the efficiency and color rendering index for different lighting solutions. The first commercial QD-based solid-state lighting solution developed by QD Vision and Nexxus Lighting has a high color rendering index while maintaining high luminescence efficiency. The Quantum Light platform consists of a quantum dot optic (see inset photograph in upper right) that is backlit by a blue LED. (b) Photographs comparing the illumination of objects with an incandescent lamp, a cool white LED, and the Quantum Light solution. (c) Long-term efficiency testing data on QD Vision/Nexxus Lighting LED shows the stability of the QD optic. (Figures courtesy of QD Vision Inc.)

Fig. 3

(a) Photographs and spectra of multicolor QD-LEDs that sandwich the QD monolayer using the same organic charge transport layers on either side of the having the cross-sectional geometry shown in the inset. Microcontact printing of QDs enables mixing of red, green, and blue QDs within a single monolayer to create a single white light-emitting active layer (b) or side-by-side deposition of 25×25 mm red, green, and blue pixels (c). (Figure courtesy of Polina Anikeeva.)

Fig. 4

(a) Electroluminescence spectra of the first QD-LED to use metal oxide charge transport layers (23). (b) Plot of the QD-LED external quantum efficiency (EQE) measured from the front face of the device as a function of current density. The inset shows a photograph of a bright and uniform pixel at 6 V applied bias operating in ambient conditions, unpackaged.