Bright infrared quantum-dot light-emitting diodes through inter-dot spacing control

Abstract

Infrared light-emitting diodes are currently fabricated from direct-gap semiconductors using epitaxy, which makes them expensive and difficult to integrate with other materials. Light-emitting diodes based on colloidal semiconductor quantum dots, on the other hand, can be solution-processed at low cost, and can be directly integrated with silicon 1 . However, so far, exciton dissociation and recombination have not been well controlled in these devices, and this has limited their performance [2][3][4][5][6][7][8] . Here, by tuning the distance between adjacent PbS quantum dots, we fabricate thin-film quantumdot light-emitting diodes that operate at infrared wavelengths with radiances (6.4 W sr 21 m 22 ) eight times higher and external quantum efficiencies (2.0%) two times higher than the highest values previously reported. The distance between adjacent dots is tuned over a range of 1.3 nm by varying the lengths of the linker molecules from three to eight CH 2 groups, which allows us to achieve the optimum balance between charge injection and radiative exciton recombination. The electroluminescent powers of the best devices are comparable to those produced by commercial InGaAsP light-emitting diodes. By varying the size of the quantum dots, we can tune the emission wavelengths between 800 and 1,850 nm.Colloidal quantum dots have been proposed for the development of low-temperature solution-processed quantum-dot devices, including next-generation photovoltaics, photodetectors and lightemitting diodes (LEDs) [1][2][3][4][5][6][7][8][9][10][11] . In particular, the development of high-power, efficient and low-cost infrared LEDs will further progress in applications such as night vision, optical communications and sensing. Early efforts to exploit quantum dots in LEDs were based on hybrid device structures in which the quantum dots were interfaced with conjugated polymers. Quantum dots with long capping ligands were either mixed with an organic host or directly sandwiched between organic carrier-transporting layers to form the LED structure 4,5,8 . The operating mechanism of such devices is based mainly on Förster transfer, in which exciton energy transfers from the organic host to the quantum dots by means of a dipole-dipole interaction. Owing to the long capping ligands and low carrier mobility of the organic materials, these devices suffer from low current density, charge injection imbalance and exciton ionization caused by large applied bias voltages 12 .Recently, an infrared quantum-dot LED based on direct exciton generation through carrier injection achieved 1.15% external quantum efficiency (EQE), but the organic carrier-injection layer limited the current density and, as a result, the radiance 4). In visiblewavelength quantum-dot LEDs, inorganic charge-transport layers (ZnO:SnO 2 alloy for electrons and NiO for holes) have recently been used to increase the current density to a few amperes per square centimetre, with a consequent significant improvement in radiance 6 . These results directly reflect the impro...

Colloidal Quantum Nanostructures: Emerging Materials for Display Applications

“…The right spacing between adjacent NCs is needed to obtain a delicate balance between the charge injection and undesired exciton dissociation. An optimum distance was indeed demonstrated by tuning the distance between adjacent PbS quantum dots through modifying the lengths of the linker molecules from three to eight CH 2 groups 149. In addition, an increase in the EQE by replacing the oleic acid ligands of the as‐synthesized SCNCs with shorter 1‐octanethiol ligands was reported for violet‐blue‐emitting ZnCdS/ZnS graded core–shell QDs 150…”

Controlling the spatial distribution of quantum dots in nanofiber for light-harvesting devices

“…[8][9][10][11][12][13][14] For instance, QD light-emitting diodes or photovoltaic devices suffer from the inter-dot interactions of aggregates, which cause a spectral shift and a broadening in the photoluminescence of QDs as well as a decrease in the quantum yield. 10,13 Therefore, control of inter-dot spacing is critical in ensuring device performance.…”

“…[8][9][10][11][12][13][14] For instance, QD light-emitting diodes or photovoltaic devices suffer from the inter-dot interactions of aggregates, which cause a spectral shift and a broadening in the photoluminescence of QDs as well as a decrease in the quantum yield. 10,13 Therefore, control of inter-dot spacing is critical in ensuring device performance. [8][9][10]13,15 Researchers have long sought methods to control the spatial distribution of functional moieties in devices and prevent the formation of aggregates, both for fundamental research and the potential technological applications.…”

“…10,13 Therefore, control of inter-dot spacing is critical in ensuring device performance. [8][9][10]13,15 Researchers have long sought methods to control the spatial distribution of functional moieties in devices and prevent the formation of aggregates, both for fundamental research and the potential technological applications. In solid-state films in particular, it has been challenging to increase film thickness while keeping functional moieties in the monomeric state.…”

Theory of a field-effect transistor based on a semiconductor nanocrystal array

“…However, due to intrinsic difficulties of doping of NCs by impurity atoms 10 , so far there are only a limited number of experimental works with successful bulk doping [11][12][13] . On the other hand, introducing carriers via field-effect is a more successful approach 11,[14][15][16][17][18] . The whole recent discussion of record band-like mobilities of NC arrays is centered around recent field-effect data 15,[19][20][21][22] .…”