Electrical Injection and Transport in Films of Semiconductor Nanocrystals
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Although a large body of work exists on the size-dependent optical properties of nanocrystals, 7 ,8 relatively few investigations have focused on the electrical properties of structures containing large numbers of nanocrystals. To date, electrical studies of colloidally prepared quantum dots have generally focused either on the transport of charge through single nanocrystals 9 ,10 or on charge transport through nanocrystal monolayers. 1l,12 In addition, a few studies have also investigated the conductivity of 3-dimensional arrays of nanoparticles. 13-15 These studies have demonstrated a number of effects that can be expected to play important roles in charge transport through disordered nanocrystal arrays, including coulomb charging effects, barriers associated with the organic surfactants surrounding the particles, and the presence of 16 deep traps. In order to address these issues, we have used spin coating to fabricate sandwich structures of colloidally grown, organically passivated CdSe nanocrystals between metal electrodes. We find that these devices are free of pinholes, such that meaningful current-voltage characteristics can be measured. In this paper, we investigate the current-voltage (I-V) characteristics of such devices as a function of electrode material, nanocrystal size, illumination, and temperature.
185 Mat. Res. Soc. Symp. Proc. Vol. 571 © 2000 Materials Research Society
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Figure 1. I-V curve for a 200 nm thick
Figure 2. Photocurrent spectra from two
ITO/CdSe/AI device with 4.5 nm diameter nanocrystals. Inset shows device structure.
devices of different diameter nanocrystals; 4.5 nm (solid line) and 2.7 nm (dashed).
EXPERIMENT CdSe nanocrystals were synthesized by the tri-n-octylphosphine oxide (TOPO) method of Murray et al.17, as modified by Katari et al.18 The nanocrystals were washed three times with methanol to remove excess TOPO, and then dissolved in toluene at high concentrations (>100 mg/mL). The resulting solutions were filtered through 0.45 gm PTFE filters. Films were prepared by spin-coating the toluene solutions onto indium-tin oxide (ITO) coated glass substrates at speeds in excess of 2000 rpm. Prior to spin-coating, the ITO substrates were treated for 10 minutes under an oxygen plasma. This treatment has been shown to decrease the ITO surface roughness, and to increase its work function.1 9 Metal contacts were deposited on top of the nanocrystal film by thermal evaporation at a rate of -2 A /s, with typical areas of 3 mm2 . The final device structure is illustrated in the inset of Figure 1. Spin-coating from concentrated toluene solutions has been found to yield disordered arrays of close-packed 5 nanocrystals
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