Slow Hot‐Carrier Cooling in Halide Perovskites: Prospects for Hot‐Carrier Solar Cells

Abstract

rapidly (within hundreds of femtoseconds), making HCs extraction extremely challenging. A reduced HC cooling rate in solar absorbers is therefore a key material criterion for realizing HCSC.Halide perovskites possess novel slow HC cooling properties favorable for development as HCSC. Since the first reports of slow HC cooling (≈0.4 ps) in MAPbI 3 polycrystalline thin films, [7,8] there have been growing interests about this novel phenomenon. Li et al. reported a drastic slowdown of HC cooling by a further two orders in MAPbBr 3 colloidal nanocrystals (≈30 ps) at high pump fluence and demonstrated efficient (≈83%) extraction of HCs with an energy-selective organic layer. [9] Long HC transport lengths (≈600 nm) in MAPbI 3 thin films were visualized by Huang et al. [10] These exciting findings forebode the potential of perovskite HCSCs that could dramatically boost perovskite solar cell efficiencies beyond their SQ limits. Presently, the origins and mechanisms of slow HC cooling in halide perovskites remain fragmented and confusing with disparate models being proposed. A clear understanding of the intrinsic photophysics of HC cooling is essential for further technological developments.In this review, we examine the milestones and advancements of slow HC cooling in halide perovskites and distill their photophysical mechanisms. We begin with a brief introduction of the operation principles of HCSCs and carrier relaxation processes, followed by highlighting the seminal experimental and theoretical works on slow HC cooling in halide perovskites, before explicating their origins and mechanisms. A developmental toolbox for engineering slow HC cooling in halide perovskites and developing new perovskite materials with slower HC cooling will also be discussed. Lastly, we highlight the challenges and opportunities for perovskite HCSCs. How Hot Carriers Can Be Used?We begin with a quick overview of the several concepts for highefficiency solar cells. Figure 1a illustrates the energy band diagram of a typical single junction solar cell with its major energy loss processes following light illumination. [11,12] In all solar cells, photons possessing energies greater than the semiconductor bandgap can create free carriers or excitons with excess energies above the bandgap. These carriers or excitons with a temperature higher than the lattice temperature are termed "hot Rapid hot-carrier cooling is a major loss channel in solar cells. Thermodynamic calculations reveal a 66% solar conversion efficiency for single junction cells (under 1 sun illumination) if these hot carriers are harvested before cooling to the lattice temperature. A reduced hot-carrier cooling rate for efficient extraction is a key enabler to this disruptive technology. Recently, halide perovskites emerge as promising candidates with favorable hot-carrier properties: slow hot-carrier cooling lifetimes several orders of magnitude longer than conventional solar cell absorbers, longrange hot-carrier transport (up to ≈600 nm), and highly efficient hot-carrier extractio...

Giant two- to five-photon absorption in CsPbBr27I03 two-dimensional nanoplatelets

“…2(c) and 2(d)]. The carrier cooling times of the 2D NPs were comparable to the relevant values for many materials used for solar cells [16], including CH 3 NH 3 PbI 3 films [17] and CdS microplates [18]. The slow hot carrier cooling in 2D NPs indicates their small energy loss in optoelectronic device applications [16].…”

Excitons in Carbonic Nanostructures

“…In semiconductor QDs, such cooling rates, which are due to the coupling with lattice vibrations, are observed in early picoseconds, which is faster than the emission rates by approximately three orders of magnitude. Meanwhile, for special cases of QDs [299] and halide, perovskites [300] were described for which the electron carrier cooling was greatly retarded. In these cases, a slower carrier cooling was achieved when the spacing among their discrete electronic levels was much larger than the phonon energy.…”

Direct evidence of weakly dispersed and strongly anharmonic optical phonons in hybrid perovskites

“…This mechanism is an alternative to the Rashba effect, which was proposed to lead to an inversion of dark and bright exciton states in CQDs [22]. Further, the potential of perovskite CQDs for hot-carriers solar cell applications has also been stressed [23]: the suppression of the LO relaxation process to longitudinal acoustic (LA) phonons was attributed to an optical phonon bottleneck effect [24], and later on related to the anharmonicity of the acoustic modes [25]. Then, a direct measurement of optical phonons branches with the same methodology as for the acoustic modes [18,19] becomes a necessary step to completely uncover carrier-phonon coupling dynamics and to assess the fundamental intrinsic limit of the mobility of charge carriers in these materials.…”