Within a decade, there has been a surge of interest into halide perovskite semiconductors. With tunable bandgaps, versatile and facile deposition processes and excellent optoelectronic properties, these materials have found applications in photovoltaics (PV), broad-spectrum photodetectors, light-emitting diodes and lasers. While research into using these materials for emission and X-ray detection applications is now beginning to surge; currently, halide perovskites are most well known for their remarkable PV performance, with perovskite solar cells achieving certified power conversion efficiencies exceeding 25% in a remarkably short developmental timescale. Despite the truly impressive device performance, these materials have not yet reached their full potential. The major obstacle to this is an incomplete understanding of crystallization processes, as well as defects which exists on surfaces and at interfaces in perovskite thin-films. Deficiencies at these interfaces, likely formed during the crystallization process, are responsible for the major losses in perovskite-based optoelectronic devices; limiting charge extraction, increasing non-radiative recombination, contributing to hysteresis and increasing the voltage-loss of perovskite photovoltaics.
Herein, I will present various effective strategies to reduce the defect densities of perovskite films and thus improve the performance of perovskite optoelectronic devices. First, I will discuss how manipulating precursor solutions can have significant impact on the quality of the perovskite films, through enabling reduced defect density. Consequently, this is one approach which can be used to boost device performance. Next, I will focus on a strategy in which we extrinsically modify the bottom interface of perovskite solar cells such that the crystallization of the perovskite yields films with fewer defects. Specifically, we show that ionic liquids can have a dramatic effect on the interfacial trap density in a perovskite solar cell. And finally, I will illustrate an approach in which we manipulate the charge-carrier density in the perovskite thin-film after it is fully crystallized. We utilized molecular dopants to significantly alter the interfacial energetics in order to suppress non-radiative recombination losses. As a result, we observe a boost in both performance and stability. Importantly, these interface modification strategies can be used in tandem, resulting in further improvements in device efficiency. The utility of these defect mitigation strategies can readily be applied beyond perovskite PV and is likely to also improve the performance of a range of other perovskite-based optoelectronic devices.
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Bio - Dr. Nakita K. Noel obtained her undergraduate degrees in Chemistry and Physics at the University of the West Indies, St. Augustine. In 2011, she joined the laboratory of Prof. Henry J. Snaith at the University of Oxford where she undertook her PhD in Condensed Matter Physics. During her PhD, Dr. Noel focused on tailoring the composition and surface chemistry of metal halide perovskites with the goals of reducing toxicity and improving the efficiency of perovskite-based photovoltaics. After completing her PhD in 2014, she stayed on at Oxford as a postdoctoral researcher where her main focus was understanding the fundamental chemistry of the perovskite precursor solutions and its impact on the crystallisation on perovskite films. Based on the insights gained from this work, she developed a new low-boiling point solvent for the deposition of perovskite thin films and linked a fundamental solvent decomposition process to the dissolution of colloids in perovskite precursor solutions, providing a simple route to fabricating perovskite solar cells with record low voltage losses. Dr. Noel then moved to Princeton University to take up a Materials Science Postdoctoral Research Fellowship in the Princeton Center for Complex Materials where she studied the detailed chemical composition of perovskite precursor inks and its impact on crystallisation kinetics, and the role of interfaces and defects on the performance of perovskite optoelectronics. In 2020, she was awarded an Early Career Research Fellowship from the Engineering and Physical Sciences Research Council, and recently returned to Oxford to establish an independent research group.