While semiconductor nanocrystals offer a versatile platform for active components in optoelectronic devices due to their size-tunable properties and solution processability, they have seen limited integration into commercially available technology. Creating conductive nanocrystal solids remains a substantial challenge due to the properties of the “native ligands” that passivate their surfaces following synthesis. Native ligands are typically long (12-30 carbons) alkyl chains that create large inter-nanocrystal separation in close-packed films and act as an insulating shell around the nanocrystal, impeding carrier transport at the nanocrystal-ligand interface. To overcome these issues researchers have developed ligand exchange methods to improve conductivity by exchanging these insulating native ligands for shorter ones to decrease inter-particle separation. Recently, efforts have focused on developing ligand systems that can act as a conductive pathway to electronically couple neighboring nanocrystals, reducing the energetic barrier at the nanocrystal-ligand interface. One class of ligands known as “exciton-delocalizing ligands” have been shown to exhibit this property by coupling strongly to nanocrystal electronic states, sharing carrier density between. Using an exciton-delocalizing ligand known as phenyldithiocarbamate (PDTC), we employ a facile, room temperature exchange procedure which alters the electronic structure of the film and improves exciton mobility in CdSe nanocrystal solids. A combination of femtosecond optical spectroscopies and Kinetic Monte Carlo simulations were used to extract the relevant parameters to determine how ligand exchange with PDTC affects the transport properties of the films. Using femtosecond transient absorption we measure rapid downhill energetic migration of excitons that persists for tens of picoseconds, while this effect is not present in the native ligand, oleic acid capped films. Kinetic Monte-Carlo simulations are used to extract both rates and length scales for exciton diffusion in the system and are compared to native ligand capped nanocrystal films. Finally, principles of ligand design for the formation of strongly coupled nanocrystal arrays will be discussed.

Improving energy migration in nanocrystal solids ligand exchanged with exciton-delocalizing ligands

255th National Meeting (2018)

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