Carrier Dynamics in Perovskite Nanocrystals
In this thesis, we describe our work discovering perovskite nanocrystals with desirable optoelectronic properties for high-efficiency solar cells. Ultrafast carrier-phonon interactions in semiconductors represent a major loss of the excess energy of photoexcitations above the bandgap. We demonstrate approaches to suppress the relaxation process, which may allow the full photon energy to be usefully extracted before carrier relaxation takes place. Two distinct strategies are presented to achieve this: a hot phonon bottleneck effect and a phonon bottleneck effect. First of all, we demonstrate the synthesis of lead-based perovskite nanocrystals, tin-based perovskite nanocrystals, and tin-lead alloy perovskite nanocrystals where the electronic structure and optical properties can be tuned by composition engineering. The replacement of the toxic lead with tin may also provide a less negative impact on the environment and human health in applications. Having synthesised perovskite nanocrystals, we focus on understanding the electronic structure of nanocrystal quantum dots and the interactions between excitons, carriers and phonons. By using femtosecond spectroscopic methods, we follow the dynamics of photoexcitations inside perovskite nanocrystals. A hot phonon bottleneck effect that leads to suppressed cooling at high carrier densities is observed in both hybrid and inorganic tin-lead alloy nanocrystals. Tin addition is confirmed to prolong the carrier cooling time in nanocrystal alloys, which we attribute to suppressed Klemens decay. Since a hot phonon bottleneck only takes place at extremely high fluence which is not achievable under one sun illumination, we further explore a slow relaxation occurring at low carrier densities in tin-based perovskite nanocrystals, making it practically relevant. We attribute the slow relaxation to a phonon bottleneck effect where a decreased density of states due to quantum confinement is realised between the high-energy and the low-energy states. Photodegradation measurements reveal that the high-energy signal in transient absorption spectroscopy results from two spectrally-overlapping energy levels, both of which may contribute to the slow cooling. This makes the process complicated but more attractive. The slow relaxation we observe offers new insights into the intrinsic photophysics of perovskite nanocrystals, with direct implications for photovoltaic applications where suppressed relaxation could lead to high-efficiency solar cells.
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