Article pubs.acs.org/cm
Defect States and Charge Transport in Quantum Dot Solids Nicholas P. Brawand,*,†,¶ Matthew B. Goldey,*,†,¶ Márton Vörös,*,‡ and Giulia Galli*,†,‡ †
Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States Argonne National Laboratory, Lemont, Illinois 60439, United States
‡
S Supporting Information *
ABSTRACT: Defects at the surface of semiconductor quantum dots (QDs) give rise to electronic states within the gap, which are detrimental to charge transport properties of QD devices. We investigated charge transport in silicon quantum dots with deep and shallow defect levels, using ab initio calculations and constrained density functional theory. We found that shallow defects may be more detrimental to charge transport than deep ones, with associated transfer rates differing by up to 5 orders of magnitude for the small dots (1− 2 nm) considered here. Hence, our results indicate that the common assumption, that the ability of defects to trap charges is determined by their position in the energy gap of the QD, is too simplistic, and our findings call for a reassessment of the role played by shallow defects in QD devices. Overall, our results highlight the key importance of taking into account the atomistic structural properties of QD surfaces when investigating transport properties.
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neglect orbital relaxation beyond the single particle picture,24 the effect of polarization caused by localized charges, and environmental effects, such as image charges due to electrodes. In this work, we focused on charge transport properties of silicon QDs, which are promising systems for photovoltaics,25−30 LEDs,31 and optical labels for multiplexed imaging and drug delivery,32 due to the natural abundance of silicon, a nontoxic material, and the widespread industrial expertise in its manufacturing. We carried out ab initio calculations within density functional theory (DFT), using constrained DFT (CDFT) which includes polarization and orbital relaxation effects. CDFT, though originally pioneered in 1984,33 has seen more extensive use in the modern formulation introduced by Wu and Van Voorhis and colleagues.34−39 This methodology has been applied to the simulation of charge transport in a variety of materials, including molecules,18,40,41 molecular solids,42,43 oxides,44 solvated ions,45,46 and organic photovoltaic materials.47 We studied QDs with intrinsic defects and impurities and analyzed how deep and shallow defects may affect charge transport properties of QD films. We explicitly took into account the effect of image charges which may arise from electrodes or surfaces of substrates where films are deposited. Our results indicate that the common assumption, that the ability of defects to trap charges is mainly dictated by their position in the energy gap of the film, is too simplistic and that the presence of shallow levels may be detrimental to charge transport.
INTRODUCTION Advances in the synthesis of colloidal quantum dots have led to the fabrication of monodisperse (size distribution
