Enhanced carrier multiplication in engineered quasi-type-II quantum dots

Abstract

One process limiting the performance of solar cells is rapid cooling (thermalization) of hot carriers generated by higher-energy solar photons. In principle, the thermalization losses can be reduced by converting the kinetic energy of energetic carriers into additional electron-hole pairs via carrier multiplication (CM). While being inefficient in bulk semiconductors this process is enhanced in quantum dots, although not sufficiently high to considerably boost the power output of practical devices. Here we demonstrate that thick-shell PbSe/CdSe nanostructures can show almost a fourfold increase in the CM yield over conventional PbSe quantum dots, accompanied by a considerable reduction of the CM threshold. These structures enhance a valence-band CM channel due to effective capture of energetic holes into long-lived shell-localized states. The attainment of the regime of slowed cooling responsible for CM enhancement is indicated by the development of shell-related emission in the visible observed simultaneously with infrared emission from the core.

Novel silicon phases and nanostructures for solar energy conversion

“…[116][117][118] In addition, quantum-dot sensitized solar cells with type-II interfaces have been reported in the literature. [119][120][121] However, in the context of a NP and its embedding matrix, clear strategies to design band offsets are not yet available.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Interband hot-carrier emission is due to recombination of a hot carrier in one band (e.g., an electron in the conduction band), with a carrier in the other band. This emission is blue-shifted with respect to that from the ground-state exciton, and decays on a timescale of picoseconds or faster [73,172,181]. In addition, the possibility of intraband hot-carrier emission has recently been demonstrated in Cd-based and Hgbased NCs [182,183].…”

“…The possibility of multi-exciton generation (MEG), also called carrier multiplication (CM), has been investigated for several years, most commonly in Pb-chalcogenide NCs [166][167][168][169][170][171][172], but also for other NC materials [173][174][175][176]. In the process of multi-exciton generation, a hot carrier with excess energy higher than the bandgap can relax to the ground state while generating an additional electron-hole pair (Fig.…”

Multiple Exciton Generation and Dynamics in InP/CdS Colloidal Quantum Dots

“…13 In this work, we study MEG in dispersions of quasi-Type II CQDs formed by the growth of a CdS shell around a InP core; in this CQD structure, the electron is delocalised over the core and shell whilst the hole is confined to the InP core, 18 as illustrated in Figure 1. InP CQDs form with a cubic crystal structure (lattice constant 5.87 Å) 19 but the thermodynamically stable phase 4 of CdS is hexagonal 18 .…”

“…6 MEG in CQDs was first proposed in 2001 7 and first experimentally demonstrated in 2005 for PbSe CQDs. 8 Since then MEG has been demonstrated in CQDs composed of a range of single materials, including PbS, 9 HgTe, 10 CdHgTe 11 and CuInSe 2 , 12 as well as in other forms of colloidal nanostructures such as PbSe nanorods 13 and PbS nanosheets 14 . One of the most useful aspects of the solution-phase synthesis of CQDs is that it allows a core-shell structure to be readily produced by the growth of a shell or shells of different material(s) around the original CQD.…”