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