(Invited) Improving the Performance of Quantum Dot Light-Emitting Diodes through Nanoscale Engineering

Semiconductor nanocrystal quantum dots (QDs) are extremely attractive fluorophores for use in lighting and display applications due to their narrow-band, highly efficient and widely tunable emission, and amenability toward low-cost processing and flexible substrates. As QDs begin to enter the display market as downconverters in back-lit displays, attention has increasingly turned to high-performance electrically pumped QD-based light-emitting diodes (QD-LEDs). Tremendous progress has been made since the first demonstration of QD-LEDs, as there have been numerous attempts to improve the device performance by optimizing both the materials synthesis and the device architecture.¹

Even so, there is substantial room for improvement, particularly in terms of device stability, overall quantum efficiency, and in efficiency “roll-off”, in which efficiency falls at high drive currents. While device architectures can always be further optimized through sheer iteration/evolution, larger advances may require more fundamental insight.  Indeed, accelerating the development of essentially all optoelectronic devices based on QDs is the increasingly deep understanding of the physics of quantum-confined nanomaterials and how they interact within devices. For instance, as the outcome of years of detailed study, the multicarrier phenomenon known as “carrier multiplication”² has advanced beyond the laboratory to be observed in functional solar cells.³ However, even as the subtleties of QD-LED physics become clearer, the role of multicarrier effects including non-radiative Auger recombination, is still largely unclear. While Auger recombination has been established as an important factor in efficiency roll-off in GaN LEDs,⁴ its relevance to QD-LEDs remains the subject of ongoing controversy.

Here we analyze the role of Auger recombination in QD-LEDs by conducting a systematic characterization of device performance in conjunction with studies of the dynamics of photoexcited carriers directly within the device structure.⁵ In our LEDs, we use a series of structurally engineered core/shell QDs that exhibit very similar single-exciton properties, but distinctly different rates of non-radiative Auger recombination.⁶ By correlating device performance with the multi-carrier photophysical properties of the QDs, we find that both LED efficiency and the onset for efficiency roll-off are strongly influenced by Auger recombination processes. We further show that device efficiency can be improved by use of either of two distinct types of specifically engineered heterostructured QDs, designed either to reduce the efficiency of Auger recombination, or to improve charge injection balance in the QD layer during operation. We demonstrate how both types of QD enhance peak emission efficiency, and significantly increases the threshold current of efficiency roll-off, and discuss how this insight can be applied to achieve future performance improvements.

References 

(1) Bae, W. K.; Brovelli, S.; Klimov, V. I.  MRS BULLETIN 2013, 38, 721.

(2) Schaller, R. D.; Klimov, V. I.  Physical Review Letters 2004, 92, 186601.

(3) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C.  Science 2011, 334, 1530.

(4) Iveland, J.; Martinelli, L.; Peretti, J.; Speck, J. S.; Weisbuch, C.  Physical Review Letters 2013, 110, 177406.

(5) Bae, W. K.; Park, Y.-S.; Lim, J.; Lee, D.; Padilha, L. A.; McDaniel, H.; Robel, I.; Lee, C.; Pietryga, J. M.; Klimov, V. I.  Nat Commun 2013, 4.

(6) Bae, W. K.; Padilha, L. A.; Park, Y.-S.; McDaniel, H.; Robel, I.; Pietryga, J. M.; Klimov, V. I.  ACS Nano 2013, 7, 3411.