Publication date: 27th June 2014
Semiconductor nanocrystals (NCs) are promising materials for next-generation photovoltaic technologies and other various optoelectronic applications. NCs can be coupled yet still quantum confined into arrays with intriguing properties utilizing recent developments on surface ligand manipulation. Despite being organized into highly disordered arrays, the charge carrier transport characteristics have been characterized as bulk-like while still maintaining discrete excitonic optical transitions. Thus, understanding the peculiarities of the optoelectronic properties of coupled quantum dot (QD) arrays will further enable unique applications for NCs. In this talk, I will describe recent work where we have measured carrier trapping occurring in band tail states extending from the band edges into the gap. The band tails have surprisingly low characteristic energies near 14 meV, similar to those found in larger grain, polycrystalline bulk semiconductors, rather than the large Urbach energies normally associated with amorphous, or porous nanocrystalline films. Then, utilizing ultrafast cross-polarized transient grating (CPTG) spectroscopy we measured electron–hole wave function overlap in CdSe QD films with chemically modified surfaces for tuning inter-QD electronic coupling. By comparing the CPTG decays with those of isolated QDs, we find that excitons coherently delocalize to form excited states more than 200% larger than the QD diameter. Furthermore, we have used transient microwave conductivity experiments to measure charge carrier mobilities within isolated NCs in solution. We find that the (optical frequency) mobility and lifetime of photo-induced carriers are controlled by the NC shape and dimensions. When cast into films, carrier mobilities are highly dependent on the capping ligand for the CdTe NCs measured. We identify specific Te-based ligands that improve transport characteristics (i.e. diffusion lengths) over that of bulk CdTe films. The combination of these unique energetic and pump-probe techniques further the understanding of such complex materials and provide new insight into coupling strategies.
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