Abstract
In band-like semiconductors, charge carriers form a thermal energy distribution rapidly after optical excitation. In hybrid perovskites, the cooling of such thermal carrier distributions occurs on timescales of about 300 fs via carrier-phonon scattering. However, the initial build-up of the thermal distribution proved difficult to resolve with pump–probe techniques due to the requirement of high resolution, both in time and pump energy. Here, we use two-dimensional electronic spectroscopy with sub-10 fs resolution to directly observe the carrier interactions that lead to a thermal carrier distribution. We find that thermalization occurs dominantly via carrier-carrier scattering under the investigated fluences and report the dependence of carrier scattering rates on excess energy and carrier density. We extract characteristic carrier thermalization times from below 10 to 85 fs. These values allow for mobilities of 500 cm2 V−1 s−1 at carrier densities lower than 2 × 1019 cm−3 and limit the time for carrier extraction in hot carrier solar cells.
Highlights
In band-like semiconductors, charge carriers form a thermal energy distribution rapidly after optical excitation
These time scales are fast compared to GaAs where carrier thermalization times have been measured in the range of 100 fs to 4 ps at room temperature[12, 15, 17]
The carrier thermalization times we observe for perovskites, show a strong dependence on both excess energy and carrier density
Summary
In band-like semiconductors, charge carriers form a thermal energy distribution rapidly after optical excitation. The cooling of such thermal carrier distributions occurs on timescales of about 300 fs via carrier-phonon scattering. The initial buildup of the thermal distribution proved difficult to resolve with pump–probe techniques due to the requirement of high resolution, both in time and pump energy. For the prototypical semiconductor GaAs, studies of thermalization dynamics reported timescales ranging from 100 fs to 4 ps[12, 15, 17], and provided insights into dephasing times[12], band structure[15] and carrier-carrier scattering processes[18], which affect subsequent carrier cooling and recombination processes. Ultrafast transient absorption spectroscopy has been used to study the carrier cooling process, which was found to occur on 100 s of femtoseconds timescales[19,20,21,22], with a strong contribution from a hot phonon effect at high fluences[23]. Taking the FT of the nonlinear optical signal over t1 provides the excitation frequency axis, with a resolution that is only limited by the t1 scanning range
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