We report multilayer nanocrystal quantum dot light-emitting diodes (QD-LEDs) fabricated by spin-coating a monolayer of colloidal CdSe/CdS nanocrystals on top of thermally polymerized solvent-resistant hole-transport layers (HTLs). We obtain high-quality QD layers of controlled thickness (down to submonolayer) simply by spin-coating QD solutions directly onto the HTL. The resulting QD-LEDs exhibit narrow ( approximately 30 nm, fwhm) electroluminescence from the QDs with virtually no emission from the organic matrix at any voltage. Using multiple spin-on HTLs improves the external quantum efficiency of the QD-LEDs to approximately 0.8% at a brightness of 100 cd/m(2) (with a maximum brightness over 1,000 cd/m(2)). We conclude that QD-LEDs could be made more efficient by further optimization of the organic semiconductors.

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.

In order to fulfill the promise of organic electronic devices, performance‐limiting factors, such as the energetic discontinuity of the material interfaces, must be overcome. Here, improved performance of polymer light‐emitting diodes (PLEDs) is demonstrated using self‐assembled monolayers (SAMs) of triarylamine‐based hole‐transporting molecules with phosphonic acid‐binding groups to modify the surface of the indium tin oxide (ITO) anode. The modified ITO surfaces are used in multilayer PLEDs, in which a green‐emitting polymer, poly[2,7‐(9,9‐dihexylfluorene)‐co‐4,7‐(2,1,3‐benzothiadiazole)] (PFBT5), is sandwiched between a thermally crosslinked hole‐transporting layer (HTL) and an electron‐transporting layer (ETL). All tetraphenyl‐diamine (TPD)‐based SAMs show significantly improved hole‐injection between ITO and the HTL compared to oxygen plasma‐treated ITO and simple aromatic SAMs on ITO. The device performance is consistent with the hole‐transporting properties of triarylamine groups (measured by electrochemical measurements) and improved surface energy matching with the HTL. The turn‐on voltage of the devices using SAM‐modified anodes can be lowered by up to 3 V compared to bare ITO, yielding up to 18‐fold increases in current density and up to 17‐fold increases in brightness at 10 V. Variations in hole‐injection and turn‐on voltage between the different TPD‐based molecules are attributed to the position of alkyl‐spacers within the molecules.

We report the use of a novel UV-sensitive self-assembled monolayer to selectively deposit single-walled carbon nanotubes from solution using heterogeneous surface wettability. This process combines ubiquitous photopatterning techniques with simple solution processing to yield highly selective and densely packed carbon nanotube patterns. The essential concept behind this process is the change in surface chemistry caused by the UV-induced monolayer reaction. Selective deposition of carbon nanotubes was achieved by drop-casting, and the resulting films show local ordering, indicating that further development of this process will lead to simple technique for large-scale integration.

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