Effect of ligand-exchange on CdSe/CdS dot-in-rods based light emitting diodes

Among colloidal semiconductor nanocrystals (NCs), CdSe/CdS dot-in-rods (DiRs) [1] are very promising candidates for light emitting application such as lasers [2] and light emitting diodes (LEDs)[3] due to their unique optical... [ view full abstract ]

Among colloidal semiconductor nanocrystals (NCs), CdSe/CdS dot-in-rods (DiRs) [1] are very promising candidates for light emitting application such as lasers [2] and light emitting diodes (LEDs)[3] due to their unique optical properties such as high quantum yield and low Auger recombination. In this work, we investigate the effect of the ligands present on the nanorod surface on the LED performance. In addition to the native octadecylphosphonic (ODPA) ligands we tested shorter molecules such as 3-mercaptopropionic acid (MPA), 2-Aminoethanethiol hydrochloride (AET), and ammonium thiocyanate (SCN). Here ligand exchange was performed after film deposition. Figure 1a shows the structure of the LED where poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and Poly [(9,9- dioctylfluorenyl- 2,7- diyl)-co- (4,4'- N- (4- sec- Butylphenyl) diphenylamine)] (TFB) together serve as a hole injecting/transport layer and ZnO nanoparticles as an electron injecting layer. The ligands affect the position of the energy bands (Fig. 1b) and thus provide a means to tune the band offsets in the LED structure. Furthermore, exchanging the long chain ligands to shorter ones has been shown to improve significantly charge injection and transport. However, the LED performance will also depend on a compromise between charge injection enhancement and the possible decrease of photoluminescence quantum yield caused by the formation of surface traps following the ligand modification. We elucidate in detail the impact of surface ligands on luminance, current density and external quantum efficiency (Fig. 1c,d), which gives valuable insight for the development of high performance LEDs.
Keywords: Dot-in-rods, LEDs, ligand-exchange

References:

1) L. Carbone et al., Nano Lett. 2007, 7, 2942.
2) F. Di Stasio et al., Small 2015, 11, 1328.
3) A. Castelli et al., Nano Lett. 2015, 15, 5455.