Challenges and solutions for high-efficiency quantum dot-based LEDs
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Challenges in QD-LEDs Colloidal quantum dot (QD)-based light-emitting devices (QD-LEDs) are of considerable interest for applications such as thin-film displays and white lighting with improved and selectable color.1 One metric for defining the performance of a QD-LED is the external quantum efficiency (EQE), which is the number of photons emitted from the device per injected electron. The red-emitting QD-LED with 18% EQE, recently demonstrated by QD Vision Inc., underscores the potential for QD-LEDs to compete and eventually surpass the efficiency of organic LED (OLED) technology.2 However, the EQE of most QD-LEDs, particularly those emitting in blue or green, is significantly less.3,4 Understanding what limits efficiency is critical for the systematic development of QD chemistries and device architectures for high-performance QD-LEDs.
Efficiency in QD-LEDs Efficient electron and hole injection, balance of charge carriers arriving at the QD active layer, and minimization of the electric field across the QDs are all important design criteria for ensuring high-performance QD-LEDs.2,5,6 However, these design guidelines are highly device specific and difficult to achieve in the same device for different color emitters with various chemistries and sizes.3,6
Given the extensive discussion of optimization in device structure in prior literature, in this article, we do not consider the challenges of bringing charge carriers to the QD layer and forming excitons on the QDs; rather, we examine efficiency in the last step of the light generation process in a QD-LED. Namely, when an exciton is present on the QD, what is the probability that it will recombine to emit a photon, which can be quantified by the luminescent quantum yield (QY). The device EQE can thus be assumed to be proportional to the QY of the emitters in the device structure, which depends on the exciton nonradiative (knr) and radiative (kr) recombination rates: EQE α QY = k r / ( k nr + k r ) .
(1)
As illustrated schematically in Figure 1, knr and kr are determined by the QDs themselves and the interaction of the QDs with the electric field (F) and charge (Q) resulting from the voltage and current needed to operate the LED. In a QD-LED, the two major contributors to the non-radiative rate are electronic trap states and free-charge carriers. For example, if the QD emitter has surface state defects, trapassisted recombination can occur, whereby the electron or hole in an exciton relaxes to a trap state, and the two carriers subsequently recombine without emission of a photon.7 This
Deniz Bozyigit, ETH Zürich, Switzerland; [email protected] Vanessa Wood, ETH Zürich, Switzerland; [email protected] DOI: 10.1557/mrs.2013.180
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • SEPTEMBER 2013 • www.mrs.org/bulletin
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CHALLENGES AND SOLUTIONS FOR HIGH-EFFICIENCY QUANTUM DOT-BASED LEDS
the surface of the QD (right side of Figure 2). This results in a decrease in the non-radiative trap-assisted recombination rate (kt), thereby improving the luminescence QY of the Q
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