Improved Performance from Multilayer Quantum Dot Light‐Emitting Diodes via Thermal Annealing of the Quantum Dot Layer

Abstract

Colloidal nanocrystal quantum dots (QDs) are solution-processable chromophores with size-tunable bandgaps, high photoluminescence (PL) quantum efficiency (QE), excellent photostability, narrow emission line widths (< 30 nm), and large spin-orbit coupling. These factors make them good candidates for use in next-generation thin-film optoelectronic devices. Indeed, colloidal QDs are currently being explored for use in photovoltaics, [1][2][3][4] photodetectors, [5,6] and light emitting diodes, [7][8][9][10][11][12][13][14][15][16][17][18] often in hybrid structures that incorporate both QDs and conjugated polymers or small-molecule organic semiconductors. Despite the potential advantages of using QDs as emitters, early QD light-emitting diodes (QD-LEDs) exhibited low efficiencies, and often produced broad voltage-dependent emission with spectral contributions from both the QDs and the organic host materials. However, drawing from lessons learned from the field of all-organic LEDs, the MIT group reported a multilayer LED structure incorporating a monolayer of CdSe/ZnS core/shell QDs sandwiched between small molecule hole and electron transport layers. These devices exhibited a maximum external quantum efficiency (Q ext ) of ∼ 0.5 % and a luminous efficiency (LE) of 1.9 cd/ A at a brightness of 100 cd/ m 2 , although pure emission spectra at high brightness were not achieved in the initial report. [8,16] With subsequent refinements, the same authors have achieved maximum Q ext of > 2 % and luminous power efficiency (LPE) > 1 lm/W.[17]Recently, we reported an alternative strategy for QD-LED fabrication that allows for independent control of the QD and hole-transport layer (HTL) thicknesses by spin-coating the QD layer onto a thermally cross-linked HTL.[18] Using this flexible fabrication strategy, we demonstrated that graded structures comprising multiple hole-transport and injection layers could be used to further improve Q ext of the devices. The best devices exhibited good efficiency (Q ext > 0.8 % at 100 cd/ m 2 ), narrow EL spectra (∼ 30 nm FWHM) and maximum brightness in excess of 1000 cd/ m 2 . However, because of the high turn-on voltage for our first QD-LEDs, the LPE was not high.Herein, we describe how a substantial improvement in QD-LED performance, especially the LPE, can be obtained both by using an improved polymer hole-injection layer (HIL)/ HTL structure and by performing a thermal annealing of the QD layer prior to the final deposition of the organic electrontransport layer. In particular, the annealing step results in a significant performance improvement with these devices. In order to lay the scientific groundwork for future improvements in QD-LED performance, we characterize the changes in the chemical, photophysical, and electronic properties of the structures that occur due to the annealing process.

Flexible quantum dot light-emitting diodes for next-generation displays

“…To solve these issues, the type (ii) device (QD layers sandwiched between organic HTLs and ETLs) was proposed. [79][80][81][82][83][84] The peak EQE of the initial type (ii) device was~0.5% and has been improved to 6% (Fig. 3d, e).…”

Electroluminescent Cu‐doped CdS Quantum Dots

“…[6] Recently an h ext of $2.1% was obtained by annealing the QD layer at temperatures up to 180 8C, which removes the organic ligands from the surface of the QD, leading to improved charge injection and higher efficiency. [7] The efficiencies of these devices remain limited, mainly because of ineffective charge injection into the QDs. Several factors contribute to the poor carrier injection.…”

Energy transfer in hybrid quantum dot light-emitting diodes

“…In the first device layout, a thin QD layer is sandwiched between a hole and electron injection layer such that excitons are formed directly in the QD layer. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] In the second layout, the active layer consists of a blend of QDs dispersed in a polymer [29][30][31][32][33][34][35][36][37][38][39] or small molecule matrix. 40,41 The QDs in this composite material serve as emissive traps for ͑migrating͒ excitons that are generated in the polymer matrix by charge carrier recombination.…”