Engineering carrier dynamics in lead halide perovskites

• Main Author: Lim, Swee Sien
• Other Authors: Sum Tze Chien
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2018
• Subjects: DRNTU::Science::Physics::Optics and light; DRNTU::Engineering::Materials::Photonics and optoelectronics materials
• Online Access: http://hdl.handle.net/10356/74854
• Institution: Nanyang Technological University

Organic-inorganic hybrid perovskites have attracted immense attention primarily due to its outstanding photovoltaic and light emission properties. Specifically, certified power conversion efficiencies exceeding 20 % have been demonstrated in perovskite solar cells – attributed to their large absorption coefficients and long, balanced, ambipolar diffusion lengths, large grains and its unique defect tolerance. Careful morphological control is needed to form dense, uniform films essential for high performance devices. However, the resultant fundamental optoelectronic properties of such process controls are not well understood. In addition, the higher excited states that can aid in breaking the detailed balance limit is also unexplored. This thesis reports on a series of studies using ultrafast optical spectroscopy on the archetypal bulk 3D perovskite, MAPI, to provide insights on the photophysics and recombination dynamics of this class of materials. Here, process controls in fabricating perovskite thin films refers to treatments to the film, in the form of solvent engineering and additives, and to the substrate for hydrophilicity. Our findings reveal that solvent engineering of MAPI, i.e., dripping of toluene during spincoating which is key to improving film morphologies and subsequent solar cell efficiencies, had resulted in increased trap densities. We attribute this anomalous behaviour to an interplay of factors where the improved film morphology had also resulted in the lengthening of the carrier recombination lifetimes. Interestingly, depending on the combination of treatments, the charge extraction interface can go from injecting to non-injecting. Exceptionally low carrier recombination rates in lead halide perovskites is crucial for its high performance. By careful optimisation of the additive concentration, improvements to device performance were observed when H2O was added to MAPI as an additive. Trace amounts of H2O passivates the trap states leading to reduced recombination rates, markedly improved carrier lifetimes and Jsc. At the optimal additive concentration of 1 vol% H2O, higher order carrier recombination is suppressed, and the greatly reduced monomolecular and bimolecular recombination rates was correlated with an increase in power conversion efficiencies. Process controls have all but ensured the unprecedented growth in record efficiencies of perovskite solar cells. However, it slows as it approaches the detailed balance limit of solar cells. One way to surpass this limit is to exploit the concept of hot carriers -- photoexcited carriers in higher excited states. A technique to directly probe these states is presented, and through this, the broad photoinduced absorption band was revealed to be attributed to the promotion of photoexcited carriers to higher energy states. However, the observed sub-picosecond thermalisation times of these higher excited states may prove difficult for hot carrier extraction. Importantly, our results underscores the importance of judiciously choosing process controls to optimise optoelectronic properties of perovskite devices.