A lot of research has been lately conducted on perovskites, which show promising properties for many applications, ranging from optoelectronics to photovoltaics. Recently, a new class of perovskite nanocrystals (NCs) has been synthesized, namely all-inorganic cesium lead halide perovskite NCs (CsPbX3, X = Cl, Br, I), showing high photoluminescence quantum yields (50–90%), narrow emission bands, and tunable emission. In this study, we investigate the ultrafast carrier dynamics in CsPbBr3 perovskite NCs using pump–probe transient-induced absorption spectroscopy. We demonstrate that depending on the excitation fluence, hot carriers with carrier temperatures up to 800 K are created upon photon excitation. We also report, for the first time, lifetimes of higher order multiexciton complexes, next to that of the biexciton. These results have implications for the application prospects of these materials for lasers, light-emitting devices, and photovoltaic devices, among others.

We studied the dynamics of photogenerated carriers in Cu2ZnSnS4 (CZTS) single crystals using femtosecond transient reflectivity (TR) and optical pump-THz probe transient absorption (THz-TA) spectroscopy. The TR and THz-TA decay dynamics consistently showed that free carriers have long lifetimes of up to a few nanoseconds. The excitation-photon-energy-dependent TR measurements revealed a slow picosecond energy relaxation of free carriers to the band edge in CZTS. The relaxation and recombination dynamics of free carriers were affected by nonradiative recombinations at the surface. Our results revealed a global feature of energy relaxation and recombination processes of free carriers in CZTS single crystals.

We investigated the composition-dependent photocarrier dynamics in Cu 2 ZnSn(S x Se 1−x ) 4 (CZTSSe) single crystals using various types of steady-state and time-resolved optical spectroscopy. Photoluminescence spectroscopy shows that the band-tail states formed below the band edge decrease monotonically with increasing Se content. THz time-resolved spectroscopy clarifies that an increase in the Se content leads to a shorter lifetime of the free photocarriers. A trade-off between the composition-dependent band-tail density and the free-carrier lifetime occurs in CZTSSe single crystals. Our experimental results provide insights into the physics behind the low and composition-dependent conversion efficiency of CZTSSe-based solar cells.

We investigated the effects of sodium doping on the photocarrier dynamics in Cu 2 ZnSnS 4 (CZTS) single crystals using optical pump-THz probe transient reflectivity (THz-TR) and time-resolved photoluminescence (PL) spectroscopy. The THz-TR and PL decay dynamics are influenced by sodium doping, and their sodium-induced changes are consistent with each other. These time-resolved measurements revealed that the lifetime of photocarriers increases with sodium doping. This result indicates that a part of defects is suppressed by doping sodium into CZTS and implies that sodium doping improves the charge transport properties of CZTS, leading to an improvement in the performance of CZTS-based solar cells.

We estimated the carrier multiplication efficiency in the most common solar-cell material, Si, by using optical-pump/terahertz-probe spectroscopy. Through close analysis of time-resolved data, we extracted the exact number of photoexcited carriers from the sheet carrier density 10 ps after photoexcitation, excluding the influences of spatial diffusion and surface recombination in the time domain. For incident photon energies greater than 4.0 eV, we observed enhanced internal quantum efficiency due to carrier multiplication. The evaluated value of internal quantum efficiency agrees well with the results of photocurrent measurements. This optical method allows us to estimate the carrier multiplication and surface recombination of carriers quantitatively, which are crucial for the design of the solar cells.