Abstract
In traditional solar cells, photogenerated energetic carriers (so-called hot carriers) rapidly relax to band edges via emission of phonons, prohibiting the extraction of their excess energy above the band gap. Quantum confined semiconductor nanocrystals, or quantum dots (QDs), were predicted to have long-lived hot carriers enabled by a phonon bottleneck, i.e., the large inter-level spacings in QDs should result in inefficient phonon emissions. Here we study the effect of quantum confinement on hot carrier/exciton lifetime in lead halide perovskite nanocrystals. We synthesized a series of strongly confined CsPbBr3 nanocrystals with edge lengths down to 2.6 nm, the smallest reported to date, and studied their hot exciton relaxation using ultrafast spectroscopy. We observed sub-ps hot exciton lifetimes in all the samples with edge lengths within 2.6-6.2 nm and thus the absence of a phonon bottleneck. Their well-resolved excitonic peaks allowed us to quantify hot carrier/exciton energy loss rates which increased with decreasing NC sizes. This behavior can be well reproduced by a nonadiabatic transition mechanism between excitonic states induced by coupling to surface ligands.
Highlights
In typical semiconductors, photogenerated carriers with excess energy above band edges, the so-called hot carriers, rapidly relax to band edges mainly via emission of phonons
Quantum dots (QDs), were predicted to have longlived hot carriers enabled by a phonon bottleneck, i.e., the large inter-level spacings in quantum dots (QDs) should result in inefficient phonon emissions
Transmission Electron Microscopy (TEM) images of these NCs are presented in Fig. S1 in the Electronic supplementary information (ESI).‡ The edge lengths (L) of these cube-shaped NCs are in the range of 2.6 to 6.2 nm (Table 1), which are smaller than the Bohr exciton diameter of CsPbBr3 ($7 nm), and all samples fall in the strong quantum con nement regime.[34]
Summary
In typical semiconductors, photogenerated carriers with excess energy above band edges, the so-called hot carriers, rapidly relax to band edges mainly via emission of phonons. The relaxation o en occurs on a sub-ps timescale,[1,2,3,4] making the excess energy of hot carriers very difficult to harvest. As a result, this part of energy is wasted as heat in conventional optoelectronic devices such as solar cells, which is a major reason for the Shockley– Queisser limit for single-junction solar cells.[5] If hot carriers can be efficiently extracted to selective contacts,[6,7,8] the efficiency of solar cells can be pushed to as high as 66%.6,9. Lead halide perovskites (APbX3; with A 1⁄4 Cs, MA, and FA, and X 1⁄4 Cl, Br, I) have recently been proven to be technologically important for many light-harvesting and -emitting applications.[10,11,12] Hot carrier dynamics in these materials have been intensively studied because of recent reports of long-lived
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