Photoconduction in Annealed and Chemically Treated CdSe/ZnS Inorganic Nanocrystal Films

Abstract

We present a study of photoconductivity in close-packed films of CdSe/ZnS core/shell nanocrystals, from which the majority of the organic ligands are removed. The ZnS inorganic shells separate the CdSe cores, and passivate the surface of the CdSe nanocrystals, slowing the nonradiative exciton decay rate. These films retain the size-dependent quantum-confined electronic properties of the nanocrystals, including the tunability of their band gap. We demonstrate that replacing the organic ligands, which form the tunnel barrier between nanocrystals in films of CdSe nanocrystals, with the inorganic ZnS shell results in photoconduction with near unity internal quantum efficiency at room temperature.

Annealing-Induced Morphological Changes in Nanocrystalline Quantum Dots and Their Impact on Charge Transport Properties

“…The potential barrier height decreases to $0.7 eV due to the loss of the organic ligands. 9 The current density plot for the annealed QDs is depicted in Fig. 6.…”

“…With core-shell nanocrystals (with a thin shell layer), this temperature dependence of the photoresponse can be improved. 9 Increased photoresponse with annealing was observed with core-shell NCs as well. 9,11,13 Although significant improvements have been observed, this technique introduces additional surface states which may act as traps for charge carriers.…”

“…9 Increased photoresponse with annealing was observed with core-shell NCs as well. 9,11,13 Although significant improvements have been observed, this technique introduces additional surface states which may act as traps for charge carriers. 14 These surface states on the NCs could result in an appreciable nonradiative decay rate of excitons, thereby limiting the room-temperature internal quantum efficiency.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] NC-based optoelectronic devices are of immense interest due to their solution processability, low cost, flexibility, and scalability. Extensive research is in progress to improve the external quantum efficiency (the number of charges extracted in the external circuit per absorbed photon) of such NC devices.…”

“…10 Core-only nanocrystals, on the other hand, have poor photoresponse due to the increased nonradiative recombination rates at room temperature. 9 For practical device application, efficient operation at room temperature is desirable. With core-shell nanocrystals (with a thin shell layer), this temperature dependence of the photoresponse can be improved.…”