The Shockley-Queisser limit for solar cell efficiency can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. We report direct visualization of hot-carrier migration in methylammonium lead iodide (CHNHPbI) thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites.

Two-dimensional (2D) atomically thin perovskites with strongly bound excitons are highly promising for optoelectronic applications. However, the nature of nonradiative processes that limit the photoluminescence (PL) efficiency remains elusive. Here, we present time-resolved and temperature-dependent PL studies to systematically address the intrinsic exciton relaxation pathways in layered (CHNH)(CHNH)PbI (n = 1, 2, 3) structures. Our results show that scatterings via deformation potential by acoustic and homopolar optical phonons are the main scattering mechanisms for excitons in ultrathin single exfoliated flakes, exhibiting a T (γ = 1.3 to 1.9) temperature dependence for scattering rates. We attribute the absence of polar optical phonon and defect scattering to efficient screening of Coulomb potential, similar to what has been observed in 3D perovskites. These results establish an understanding of the origins of nonradiative pathways and provide guidelines for optimizing PL efficiencies of atomically thin 2D perovskites.

Charge carrier diffusion coefficient and length are important physical parameters for semiconducting materials. Long-range carrier diffusion in perovskite thin films has led to remarkable solar cell efficiencies; however, spatial and temporal mechanisms of charge transport remain unclear. Here we present a direct measurement of carrier transport in space and in time by mapping carrier density with simultaneous ultrafast time resolution and ∼50-nm spatial precision in perovskite thin films using transient absorption microscopy. These results directly visualize long-range carrier transport of ∼220 nm in 2 ns for solution-processed polycrystalline CH3NH3PbI3 thin films. Variations of the carrier diffusion coefficient at the μm length scale have been observed with values ranging between 0.05 and 0.08 cm2 s−1. The spatially and temporally resolved measurements reported here underscore the importance of the local morphology and establish an important first step towards discerning the underlying transport properties of perovskite materials.

Singlet fission presents an attractive solution to overcome the Shockley-Queisser limit by generating two triplet excitons from one singlet exciton. However, although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. Here, we report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. These measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion.

The thermal conductivities of β-Ga 2 O 3 single crystals along four different crystal directions were measured in the temperature range of 80-495K using the time domain thermoreflectance (TDTR) method. A large anisotropy was found. At room temperature, the [010] direction has the highest thermal conductivity of 27.0±2 .0 W/mK, while that along the [100] direction has the lowest value of 10.9±1.0 W/mK. At high temperatures, the thermal conductivity follows a ~1/T relationship characteristic of Umklapp phonon scattering, indicating phonon-dominated heat transport in the β-Ga 2 O 3 crystal. The measured experimental thermal conductivity is supported by first-principles calculations which suggest that the anisotropy in thermal conductivity is due to the differences of the speed of sound along different crystal directions.

By systematically comparing experimental and theoretical transport properties, we identify the polar optical phonon scattering as the dominant mechanism limiting electron mobility in β−Ga 2 O 3 to <200 cm 2 /V·s at 300 K for donor doping densities lower than ∼10 18 cm −3 . In spite of similar electron effective mass of β−Ga 2 O 3 to GaN, the electron mobility is ∼10× lower because of a massive Fröhlich interaction, due to the low phonon energies stemming from the crystal structure and strong bond ionicity. Based on the theoretical and experimental analysis, we provide an empirical expression for electron mobility in β−Ga 2 O 3 that should help calibrate its potential in high performance device design and applications.β−Ga 2 O 3 has recently emerged as an ultra widebandgap semiconductor E g = 4.6 − 4.9 eV 1,2 with 300 K electron mobility µ ∼150 cm 2 /V·s, 3 attractive enough to potentially offer high-voltage electronic device performance 4 that is beyond the reach of the currently successful GaN and SiC platforms. With the recent success in the synthesis of large-area bulk single crystal substrates and availability of nanomembranes, 4-6 β−Ga 2 O 3 becomes a transparent conductive oxide (TCO) with significant potential. It advances the field of oxide electronics from the traditional IGZO, perovskites (SrTiO 3 , BaSnO 3 , etc), and ZnO. [7][8][9] In this work, we have explored the physics of the intrinsic electron mobility limits in this material system and obtained expressions that should prove useful in device design.Since the Drude electron mobility µ = eτ /m c is determined by the conduction band minimum (CBM) effective mass m c , electron charge e, and the low-field scattering rate τ , we investigate m c term first. From standard k · p theory, m c of sp 3 -bonded direct gap semiconductors with the CBM at the Γ point is related to E g by, where p 0 ≈ h/a 0 is the deBroglie momentum of electrons at the Brillouin-zone edge k = 2π/a 0 with k the electron wavevector, a 0 the lattice constant, h = 2π the Planck's constant, and m 0 the free electron rest mass. Figure 1 (a) shows m * c for various compound semiconductors as a function of E g .11,12 The solid blue line shows the k·p prediction with 2p 2 0 /m 0 ∼14 eV. Variations in lattice constant, Landé g factor, slight indirectness of the bandgap, and the ionicity of the semiconductor can explain the slight deviations, 10 but the overall fit and the trend it predicts overrides these details -with a large E g ∼ 4.6 -4.9 eV, 1,2 β−Ga 2 O 3 boasts a relatively low m c ∼ 0.23−0.28m 0 , 2,13 as indicated in Fig. 1

Conventional wisdom tells us that interfacial thermal transport is more efficient when the interface adhesion energy is enhanced. In this study, it is demonstrated that molecular bridges consisting of small molecules chemically absorbed on solid surfaces can enhance the thermal transport across hard-soft material interfaces by as much as 7-fold despite a significant decrease in the interface adhesion energy. This work provides an unconventional strategy to improve thermal transport across material interfaces.

By direct imaging of singlet and triplet populations with ultrafast microscopy, it is shown that the triplet diffusion length and singlet fission yield can be simultaneously optimized for tetracene and its derivatives, making them ideal structures for application in bilayer solar cells.