Hot-carrier energy-loss rates in alloy semiconductors

Cooling and diffusion characteristics of a hot carrier in the monolayer WS2

The characteristics of a hot carrier distributed in the C excitonic state of the monolayer WS₂ is investigated by exploiting the transient absorption (TA) spectroscopy. The hot carrier cooling lifetime gradually prolongs from 0.58 ps to 2.68 ps with the absorbed photon flux owing to the hot phonon bottleneck effect, as the excitation photon energy is 2.03 eV. Meanwhile, the normalized TA spectra shows that the spectral feature of hot carriers is different from that of normal carriers. Based on the modified Lennard-Jones model, the average distance among hot carriers can be estimated according to the peak shift of TA spectra and the diffusion velocity can also be calculated simultaneously. The hot carrier limits the diffusion of the photo-generated carrier at the initial several picoseconds. These results help people to elucidate the hot carrier dynamics in 2D TMDCs and give guidance on the designing and optimizing the TMDC-based electronic devices of high performance.

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

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, long-range hot-carrier transport (up to ≈600 nm), and highly efficient hot-carrier extraction (up to ≈83%). This review presents the developmental milestones, distills the complex photophysical findings, and highlights the challenges and opportunities in this emerging field. A developmental toolbox for engineering the slow hot-carrier cooling properties in halide perovskites and prospects for perovskite hot-carrier solar cells are also discussed.

Hot carrier cooling mechanisms in halide perovskites

Halide perovskites exhibit unique slow hot-carrier cooling properties capable of unlocking disruptive perovskite photon-electron conversion technologies (e.g., high-efficiency hot-carrier photovoltaics, photo-catalysis, and photodetectors). Presently, the origins and mechanisms of this retardation remain highly contentious (e.g., large polarons, hot-phonon bottleneck, acoustical-optical phonon upconversion etc.). Here, we investigate the fluence-dependent hot-carrier dynamics in methylammonium lead triiodide using transient absorption spectroscopy, and correlate with theoretical modeling and first-principles calculations. At moderate carrier concentrations (around 1018 cm-3), carrier cooling is mediated by polar Fröhlich electron-phonon interactions through zone-center delayed longitudinal optical phonon emissions (i.e., with phonon lifetime τ LO around 0.6 ± 0.1 ps) induced by the hot-phonon bottleneck. The hot-phonon effect arises from the suppression of the Klemens relaxation pathway essential for longitudinal optical phonon decay. At high carrier concentrations (around 1019 cm-3), Auger heating further reduces the cooling rates. Our study unravels the intricate interplay between the hot-phonon bottleneck and Auger heating effects on carrier cooling, which will resolve the existing controversy.