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 Frohlich 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. Harvesting excess energy from above-bandgap photons can break solar cells’ conventional efficiency limits. Using transient spectroscopy, modelling and ab-initio calculations, Fu et al., unravel the interplay between hot phonon bottleneck and Auger heating effects on hot-carrier cooling in halide perovskites.

/pdf/hot-carrier-cooling-mechanisms-in-halide-perovskites-2e190f4hjd.pdf

Hot carrier cooling mechanisms in halide perovskites.

A fundamental understanding of hot-carrier dynamics in photo-excited metal nanostructures is needed to unlock their potential for photodetection and photocatalysis. Despite numerous studies on the ultrafast dynamics of hot electrons, so far, the temporal evolution of hot holes in metal–semiconductor heterostructures remains unknown. Here, we report ultrafast (t < 200 fs) hot-hole injection from Au nanoparticles into the valence band of p-type GaN. The removal of hot holes from below the Au Fermi level is observed to substantially alter the thermalization dynamics of hot electrons, reducing the peak electronic temperature and the electron–phonon coupling time of the Au nanoparticles. First-principles calculations reveal that hot-hole injection modifies the relaxation dynamics of hot electrons in Au nanoparticles by modulating the electronic structure of the metal on timescales commensurate with electron–electron scattering. These results advance our understanding of hot-hole dynamics in metal–semiconductor heterostructures and offer additional strategies for manipulating the dynamics of hot carriers on ultrafast timescales. Photo-excited gold nanoparticles are shown to provide ultrafast and efficient hot-hole injection to the valence band of p-type GaN, substantially altering hot-electron dynamics in the nanoparticles and forming a basis to design hot-hole-based optoelectronics.

/pdf/ultrafast-hot-hole-injection-modifies-hot-electron-dynamics-404tx5qfxx.pdf

Ultrafast hot-hole injection modifies hot-electron dynamics in Au/p-GaN heterostructures

Precise optical modeling of blue light-emitting diodes (LEDs) is constructed by reasonable optical parameters and Monte Carlo ray-tracing with the capability of precisely predicting light extraction and radiation pattern for both bare LED and packaged LED. Refractive indices and absorption coefficients of LED materials are determined by abundant references and comparisons between simulations and experiments. Surface roughness is considered in the optical model to improve the simulation precision. The simulation precisions are excellent for both bare blue LEDs (>96.5% for light extraction and >99% for radiation pattern) and packaged blue LEDs (>98.5% for both light extraction and radiation pattern).

Precise optical modeling of blue light-emitting diodes by Monte Carlo ray-tracing

Subpicosecond resolution differential transmission measurements of an InN epilayer have been employed to probe the carrier recombination dynamics and hot carrier relaxation processes in these materials at room temperature. We observed a fast initial hot carrier cooling followed by a slower recombination process with characteristic decay times of 300–400 ps. At short times after pulsed excitation, modeling of the observed relaxation suggests that the dominant energy relaxation process is longitudinal optical phonon scattering modified by a strong hot phonon effect. At longer times, a redshift of the peak energy in the differential transmission spectra was observed. This redshift is consistent with a reduction of the bandfilling effect that occurs as the photoexcited carriers recombine.

http://www.ee.buffalo.edu/faculty/cartwright/research/publications_files/Journals/2003_APL_v83_24_4984.pdf

Time-resolved spectroscopy of recombination and relaxation dynamics in InN

GaN is a key material for lighting technology. Yet, the carrier transport and ultrafast dynamics that are central in GaN light-emitting devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron–phonon (e-ph) scattering and the cooling and nanoscale dynamics of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons and that for hot carriers with an initial 0.5–1 eV excess energy, holes take a significantly shorter time (∼0.1 ps) to relax to the band edge compared to electrons, which take ∼1 ps. The asymmetry in the hot carrier dynamics is shown to originate from the valence band degeneracy, the heavier effective mass of holes compared to electrons, and the details of the coupling to different phonon modes in the valence and conduction bands. We show that the slow cooling of hot electrons and their long ballistic mean free paths (over 3 nm at room temperature) are a possible cause of efficiency...

/pdf/ultrafast-hot-carrier-dynamics-in-gan-and-its-impact-on-the-4a3wemlwck.pdf

Ultrafast Hot Carrier Dynamics in GaN and Its Impact on the Efficiency Droop.

The hot electron relaxation time is studied in an n-type GaN film grown by molecular beam epitaxy on sapphire. A femtosecond pump–probe technique is used in which the electrons are excited by an infrared pump and the carrier dynamics are monitored by a tunable near ultraviolet probe. Complex transients, showing bleaching or induced absorption, are observed. The data are fitted by a model in which the longitudinal optical (LO)-phonon emission is the dominant energy relaxation process. The LO-phonon emission time is measured to be 0.2 ps.

Hot electron relaxation time in GaN

The hot electron and hot hole dynamics have been studied in n-type and p-type GaN films grown by molecular beam epitaxy on sapphire A novel non-degenerate femtosecond pump-probe spectroscopic technique has been used, in which the electrons (or holes) are excited by a strong infrared pump, and carrier thermalization and cooling are monitored by a tunable UV probe From the complex transients, which show bleaching and induced absorption, we have been able to determine several parameters of importance, including the hot electron and the hot hole relaxation times The experimental results are compared to the theoretical predictions obtained assuming that LO-phonon emission is the dominant energy relaxation process

Electron and hole dynamics in GaN

The hot hole relaxation dynamics is studied in a Mg-doped p-type GaN film grown by molecular-beam epitaxy on sapphire. A nondegenerate femtosecond pump-probe technique is used, in which the holes are excited by an infrared pump and the hole dynamics is monitored by a tunable near ultraviolet probe. Complex transients, showing bleaching or induced absorption, are observed. A hot hole energy relaxation time of 0.6 ps has been obtained. Modeling suggests that longitudinal optical phonon emission modified by hot phonon effects is the dominant energy relaxation process.

Hot hole relaxation dynamics in p-GaN

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Hot electron relaxation time in GaN

Electron and hole dynamics in GaN

Hot hole relaxation dynamics in p-GaN