Variations in the Quantum Efficiency of Multiple Exciton Generation for a Series of Chemically Treated PbSe Nanocrystal Films

Abstract

We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 ((0.2) at a photon energy of ∼4E g for films consisting of 3.7 nm NCs and between 1.1 and 1.6 ((0.1) at hν ∼ 5E g for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that a better understanding of the effects of surface chemistry on film doping, NC carrier dynamics, and inter-NC interactions is necessary to build solar energy conversion devices that can harvest the multiple carriers produced by MEG. Our results show that the MEG efficiency is very sensitive to the condition of the NC surface and suggest that the wide range of MEG efficiencies reported in the recent literature may be a result of uncontrolled differences in NC surface chemistry.Multiple exciton generation (MEG) in semiconductor nanocrystals (NCs) (also called quantum dots (QDs)) can produce n excitons for each absorbed photon possessing an energy of at least n multiples of the band gap energy (E g ), where n is an integer.1-3 If multiexciton formation, dissociation, and charge collection are simultaneously efficient, the resulting enhanced photocurrent can increase solar energy conversion efficiencies.4,5 Recently, we reported a Schottky-junction photovoltaic device based on a thin film of colloidal PbSe NCs that demonstrated a power conversion efficiency of >2% and a short-circuit current density, J SC , greater than 20 mA cm -2 . 6 The NC film in this device was treated with 1,2-ethanedithiol (EDT) in acetonitrile in a layer-by-layer procedure to produce a conductive NC film.7 Excitons are created, separated, and transported all within the singlecomponent NC film. The internal quantum efficiency (IQE), defined as the fraction of photons absorbed by the NCs that produce carriers in the external circuit, was found to be as high as 0.8, indicating efficient charge separation and transport. 8 However, no evidence was found in the IQE spectra to suggest that multiple charge carriers were collected per absorbed photon. To harvest MEG excitons from a NC film, the inter-NC charge transfer event that produces free electrons and holes must be faster than Auger recombination (nonradiative exciton annihilation), which typically occurs in 10-100 ps. In addition, the chemical treatments that are used to produce the c...

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