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The surface ligands on colloidal nanocrystals (NCs) play an important role inthe performance of NCs based optoelectronic devices such as photovoltaic cells,photodetectors and light emitting diodes (LEDs). On one hand, the NC emissiondepends critically on the passivation of the surface to minimize trap statesthat can provide non-radiative recombination channels. On the other hand, theelectrical properties of NC films are dominated by the ligands that constitutethe barriers for charge transport from one NC to its neighbor. Therefore,surface modifications via ligandexchange have been employed to improve theconductance of NC films. However, in light emitting devices, such surfacemodifications are more critical due to their possible detrimental effects onthe emission properties. In this work, we study the role of surface ligandmodifications on the optical and electrical properties of CdSe/CdS dot-in-rods(DiRs) in films, and investigate their performance in all-solution processedLEDs. The DiR films maintain high PLQY, around 40-50%, and theirelectroluminescence in the LED preserves the excellent color purity of the PL.In the LEDs, the ligand-exchange boosted the luminance, reaching a four-foldincrease from 2200 cd/m2 for native surfactants to 8500 cd/m2 for the exchangedaminoethanethiol ligands. Moreover, the efficiency roll-off, operationalstability, and shelf life are significantly improved, and the external quantumefficiency is modestly increased from 5.1 % to 5.4 %. We relate theseimprovements to the increased conductivity of the emissive layer, and to thebetter charge balance of the electrically injected carriers. In this respect,we performed ultraviolet photoelectron spectroscopy (UPS) to obtain deeperinsight in the band alignment of the LED structure.

Enhancing the Performance of CdSe/CdS Dot-in- Rod Light Emitting Diodes via Surface Ligand Modification