Efficient and low-cost solutions to green energy production are vital to alleviate the damaging effects of climate change. To this end, lead halide perovskites have recently been highlighted as a novel class of materials with remarkable optoelectronic properties arising from relatively simple and cheap synthesis routes. In order to further optimise perovskite-based devices, a better understanding of their underlying photophysics is required.
Herein, the intraband relaxation of above-gap charge-carriers (“hot carrier cooling”) is used as a probe of the fundamental interactions operating in bulk and nanoscale perovskites. This is achieved by employing a novel ultrafast optical approach, “pump-push-probe” spectroscopy, to isolate carrier cooling from the various other dynamics occurring on similar timescales, and separate cooling pathways mediated by carrier, phonon and trap scattering.
A long-standing debate exists over the effect of quantum confinement on hot carrier dynamics in low-dimensional perovskites. The results herein, obtained via a near-infrared probe, reveal the significant role of carrier-carrier scattering, which becomes increasingly dominant with greater excitonic character of the excited state. The defect tolerance of perovskite nanocrystals is then extended to its effect on hot carrier dynamics using a visible light probe. Here, it is shown that hot carriers are not impervious to trap states in bromide-based systems, but retain this beneficial property in their iodide analogues. Finally, hot carrier mobility in a bulk perovskite single-crystal is assessed via a terahertz probe. It is shown that the response is dominated by localised lattice heating, which suppresses carrier mobility for a time period after intraband relaxation has occurred.
The presented research clarifies the spectroscopic response of an important and broadly studied material class across a wide range of the electromagnetic spectrum. This will contribute to the rational design of next-generation optoelectronic devices, while aiding interpretation of overlapping photophysical processes that are otherwise difficult to disentangle.