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We performed a comprehensive study of the charge carrier diffusion in the CsMAFA perovskite, one of the state-of-the-art perovskites for photovoltaic applications, starting from the diffusion of hot carriers to the eventual trapping at the surfaces and grain boundaries. We discovered evidence of non-ambipolar diffusion by using ultrafast transient reflectance spectroscopy: by comparing the transient reflectance signals of perovskite samples to a GaAs reference sample, we saw that the electrons diffused at their maximum mobility without being hindered by the slower holes. These findings dispute the common assumption of ambipolar diffusion in these materials. Then, after a few tens of picoseconds, the diffusion decelerated from 1.6 cm2 s−1 to 0.2 cm2 s−1 due to traps and grain boundaries. The hot-carrier dynamics of perovskites have also garnered great interest thanks to their reported quasi-ballistic mobility and the slow cooling caused by the hot-phonon bottleneck. However, our findings dispute the ultrafast diffusion and provide an alternative explanation for the prolonged cooling. Based on our analysis, we recommend paying great attention to the carrier distribution when conducting hot-carrier studies. publishedVersion

Hot charge carriers (HC) are photoexcited electrons and holes that exist in nonequilibrium high-energy states of photoactive materials. Prolonged cooling time and rapid extraction are the current challenges for the development of future innovative HC-based optoelectronic devices, such as HC solar cells (HCSCs), hot energy transistors (HETs), HC photocatalytic reactors, and lasing devices. Based on a thorough analysis of the basic mechanisms of HC generation, thermalization, and cooling dynamics, this review outlines the various possible strategies to delay the HC cooling as well as to speed up their extraction. Various materials with slow cooling behavior, including perovskites and other semiconductors, are thoroughly presented. In addition, the opportunities for the generation of plasmon-induced HC through surface plasmon resonance and their technological applications in hybrid nanostructures are discussed in detail. By judiciously designing the plasmonic nanostructures, the light coupling into the photoactive layer and its optical absorption can be greatly enhanced as well as the successful conversion of incident photons to HC with tunable energies can also be realized. Finally, the future outlook of HC in optoelectronics is highlighted which will provide great insight to the research community., In photoactive materials, the fundamental understandings of hot charge carriers and a successful device design are the current challenges for the development of highly efficient hot carrier optoelectronic devices.

We have developed a new noninvasive optical method for monitoring charge carrier diffusion and mobility in semiconductor thin films in the direction perpendicular to the surface which is most relevant for devices. The method is based on standard transient absorption measurements carried out in reflectance and transmittance modes at wavelengths below the band gap where the transient response is mainly determined by the change in refractive index, which in turn depends on the distribution of photogenerated carriers across the film. This distribution is initially inhomogeneous because of absorption at the excitation wavelength and becomes uniform over time via diffusion. By modeling these phenomena we can determine the diffusion constant and respective mobility. Applying the method to a 500 nm thick triple cation FAMACs perovskite film revealed that homogeneous carrier distribution is established in few hundred picoseconds, which is consistent with mobility of 66 cm2 (V s)-1. publishedVersion