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.

A novel starlike polyfluorene derivative, PFO-SQ, was synthesized by the Ni(0)-catalyzed reaction of octa(2-(4-bromophenyl)ethyl)octasilsesquioxane (OBPE-SQ) and polydioctylfluoroene (PFO). The incorporation of the silsesquioxane core into polyfluorene could significantly reduce the aggregation as well as enhance the thermal stability. The DSC study showed an elimination of the glass transition and crystallization as well as a significant reduction of the melting enthalpy in PFO-SQ. The UV−vis absorption spectra in a different solvent combination or solid-state film showed an intensity reduction of the aggregation peak for PFO-SQ in comparison with that of PFO. The stability of the photoluminescence spectra of PFO-SQ could be up to 150 °C, while that of PFO showed a significant green emission at 530 nm. A single-layer LED device using PFO-SQ showed a turn on voltage of 6.0 V, a brightness of 5430 cd/m2 (at a drive voltage of 8.8 V), and a current density of 0.844 A/cm2. The maximum luminescence intensity and quantum efficiency of PFO-SQ were almost twice as good as those of the PFO electroluminescent device. Hence, the incorporation of the inorganic silsesquioxane core into polyfluorenes could provide a new methodology for preparing organic light-emitting diodes with improved thermal and optoelectronic characteristics.

Efficient UV-blue polymer light-emitting diodes based on a fluorene-based nonconjugated polymer, poly[2,7-(9,9-dihexylfluorene)-alt-4,4′-phenylether] (PFPE), are fabricated. The device with PFPE as emitting layer shows a very narrow ultraviolet-blue electroluminescence emission with a peak at 397nm and a maximal external quantum efficiency of 1.07%. By blending PFPE into poly(N-vinylcarbazole) (PVK), the device performance can be further improved. A maximum external quantum efficiency of 1.81%, with a maximum irradiance power density of 1223μW∕cm2, was reached by using a blend of PVK and PFPE in the weight ratio of 95:5 as emitting layer.

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