Notwithstanding the success of lead-halide perovskites in emerging solar energy conversion technologies, many of the fundamental photophysical phenomena in this material remain debated. Here, the initial steps following photogeneration of free charge carriers in lead-iodide perovskites are studied, and timescales of charge carrier cooling and polaron formation, as a function of temperature and charge carrier excess energy, are quantified. It is found, using terahertz time-domain spectroscopy (THz-TDS), that the observed femtosecond rise in the photoconductivity can be described very well using a simple model of sequential charge carrier cooling and polaron formation. For excitation above the bandgap, the carrier cooling time depends on the charge carrier excess energy and lattice temperature, with cooling rates varying between 1 and 6 meV fs , depending on the cation. While carrier cooling depends on the cation, polaron formation occurs within ≈400 fs in CH NH PbI (MAPbI ), CH(NH ) PbI (FAPbI ), and CsPbI . Its formation time is independent of temperature between 160 and 295 K. The very similar polaron formation dynamics observed for the three perovskites points to the critical role of the inorganic lattice, rather than the cations, for polaron formation.

The ultrafast dynamics and conductivity of photoexcited graphene can be explained using solely electronic effects.

Semiconductor nanoplatelets exhibit spectrally pure, directional fluorescence. To make polarized light emission accessible and the charge transport effective, nanoplatelets have to be collectively oriented in the solid state. We discovered that the collective nanoplatelets orientation in monolayers can be controlled kinetically by exploiting the solvent evaporation rate in self-assembly at liquid interfaces. Our method avoids insulating additives such as surfactants, making it ideally suited for optoelectronics. The monolayer films with controlled nanoplatelets orientation (edge-up or face-down) exhibit long-range ordering of transition dipole moments and macroscopically polarized light emission. Furthermore, we unveil that the substantial in-plane electronic coupling between nanoplatelets enables charge transport through a single nanoplatelets monolayer, with an efficiency that strongly depends on the orientation of the nanoplatelets. The ability to kinetically control the assembly of nanoplatelets into ordered monolayers with tunable optical and electronic properties paves the way for new applications in optoelectronic devices.

Van der Waals heterostructures consisting of graphene and transition metal dichalcogenides have shown great promise for optoelectronic applications. However, an in-depth understanding of the critical processes for device operation, namely, interfacial charge transfer (CT) and recombination, has so far remained elusive. Here, we investigate these processes in graphene-WS2 heterostructures by complementarily probing the ultrafast terahertz photoconductivity in graphene and the transient absorption dynamics in WS2 following photoexcitation. We observe that separated charges in the heterostructure following CT live extremely long: beyond 1 ns, in contrast to ~1 ps charge separation reported in previous studies. This leads to efficient photogating of graphene. Furthermore, for the CT process across graphene-WS2 interfaces, we find that it occurs via photo-thermionic emission for sub-A-exciton excitations and direct hole transfer from WS2 to the valence band of graphene for above-A-exciton excitations. These findings provide insights to further optimize the performance of optoelectronic devices, in particular photodetection.