Fig. 1. Schematic diagram. (a) AM1.5 solar spectrum in Earth’s atmosphere, inset is electronic band structure with the separation of the quasi-Fermi levels; (b) theoretical Shockley-Queisser detailed-balance efficiency limit as a function of the bandgap^[25]; (c) promising strategy for a high-efficiency solar cell assisted with hot carrier effects^[24]; (d) comparison of I-V curve between hot carrier battery and traditional solar battery; (e)(f) schematic illustration of the carrier cooling kinetics in semiconductors [thermal equilibrium state (0), non-thermal equilibrium distribution after light excitation (1), carrier-carrier scattering (2), carrier-phonon scattering (3), after carrier and lattice temperature reach equilibrium, the carriers recombine back to the initial thermal equilibrium state (0)]

Fig. 2. Time-resolved fluorescence spectrum detection method. (a) PL spectra at different delay time after excitation; (b) PL intensity decay at 1.65, 1.41, and 1.38 eV; (c) extracted carrier temperature from the hot carrier distribution as a function of the delay time^[38]; (d)(e) pseudo-color images of time-resolved photoluminescence spectra of single crystal FAPbBr₃ and single crystal CsPbBr3[37]; (f) schematic diagram of the principle of THz excitation-PL detection experiment; (g) PL intensity dynamics dependent on THz excitation intensity^[46]

Fig. 3. Transient absorption spectrum detection method. (a) Schematic illustration of the transient absorption spectroscopy; (b) transient absorption spectra of CH₃NH₃PbI₃ for a series of pump energies^[35]; (c) normalized transient absorption signal dynamics (dashed lines) and carrier cooling dynamics (solid lines), under different initial carrier densities; (d) normalized photoinduced changes in transmitted probe signal, under a series of carrier densities; (e) dynamic two-dimensional electronic spectroscopy (2DES) under 655 nm wavelength pumping and 695 nm wavelength detection^[47]; (f) ultrafast temperature maps of an isolated perovskite particle at different pump powers and time delays^[48]

Fig. 4. Hot phonon bottleneck effect. (a) Pseudo-color representation of methylammonium lead halide perovskite transient absorption spectra; (b) T_e changes with time, under different n₀, with pump light photon energy constant^[20]

Fig. 5. Rotation of organic cations. (a) Schematic representation of the wobbling-in-a-cone motion (top) and the 90°angle jumps (bottom) of the MA⁺ ion; (b) time resolved cation anisotropy for mixed and pure perovskite^[54]

Fig. 6. Band filling effect. (a) Schematic of the generation, cooling, and band filling of excess carriers; (b) normalized transient absorption spectra in CH₃NH₃PbI₃ (5 ps) after 387 nm pump excitation of varying intensity^[58]; (c) time-resolved photoluminescence at various energies in FASnI₃(SnF₂) thin film^[38]

Fig. 7. Phonon bottleneck effect in perovskite nanocrystals. (a) Phonon bottleneck effect in quantum dots, the quantized phonon energy level weakens the phonon assisted relaxation process^[62]; (b) absorptive 2DES spectra with satisfactory time and energy resolutions simultaneously, recorded at time delays of T=0, 8, 50, and 3000 fs^[63]; (c) size-dependent lifetime for hot carrier, with different excess energy ΔE; (d) multiple exciton generation quantum yield as a function of relative pump photon energies for FAPbI₃ nanocrystals of different edge lengths and bulk material films^[64]

Fig. 8. Ultrafast spectral detection of hot carrier transport process. Detecting the transient absorption microscopic image of CH₃NH₃PbI₃ film with light with 1.58 eV photon energy at different time delays, the pump photon energy is (a) 3.14 eV (creating very hot carriers) and (b) 1.97 eV (creating fewer carriers), respectively; (c) variance corresponding to carrier spatial distribution in the Gaussian function fitting Fig. (a) and (b) at different time delays^[39]; (d) variation of effective carrier diffusion coefficient with time delay; (e) transient absorption microscopy images of carrier transport in perovskite at different time delays; (f) Gaussian function fits the variance corresponding to the spatial distribution of carriers in different kinds of perovskites at different time delays^[65]

Fig. 9. Ultrafast spectral detection of hot carrier extraction process . (a) Influence of push pulses of varying photon energies on the hot carrier extraction; (b) push photon energy-dependent ΔT in pump-push-probe measurements^[66]; (c) level diagram for illustration of the hot-electron extraction from perovskites nanocrystals to bphen with competing hot-electron cooling pathways; (d) normalized transient absorption spectra for perovskite nanocrystals with/without bphen, inset shows the un-normalized transient absorption spectra at 0.8 ps; (e) transmission electron microscopy image of the suspended Gr/CH₃NH₃PbI3[68]; (f) evolution of integrated electron population on the graphene orbitals under different times after deep-band photoexcitation, with fitted charge collection time of 30 fs^[69]; energy level characteristics (g) with and (h) without C₆₀ at the interface of TiO₂ and CH₃NH₃PbI₃; (i) normalized absorption kinetic traces probing at 780 nm of the CH₃NH₃PbI₃ films with/without cTiO₂ and cTiO₂/C₆₀ using λ_exc=695 nm^[69]