| Summary: | <p>The work presented in this thesis principally investigates and develops the processing, properties and photovoltaic performance of formamidinium lead triiodide perovskite (FAPbI<sub>3</sub>). Since its first use in perovskite solar cells (PSCs) by Eperon, et al., in 2014 this composition has often been overlooked by the perovskite research community in favour of alloying formamidinium (FA<sup>+</sup>) with methylammonium (MA<sup>+</sup>) or cesium (Cs<sup>+</sup>), or iodide (I<sup>-</sup>) with bromide (Br<sup>-</sup>). This is principally due to the thermodynamic instability of the desirable photoactive α-phase under ambient conditions. α-FAPbI<sub>3</sub> has been shown to be kinetically metastable with respect to transformation to the photoinactive hexagonal δ-phase. However, typically this metastability is only on the order of days. By contrast, to be commercially relevant PSCs will need to remain stable under operation over at least several years. Often perovskite stability is attributed directly and solely to its composition. Here we explore mechanisms by which it is possible to substantially increase the stability of α-FAPbI<sub>3</sub>, and critically use these to highlight that the process by which this perovskite is fabricated is integral to its resulting stability.</p>
<p>First, the motivation behind this work is presented (Chapter 1), followed by an overview of the relevant scientific theory underpinning our findings and inventions (Chapter 2). An in-depth description of the synthetic methods and the characterisation techniques employed in this work follows (Chapter 3).</p>
<p>In Chapter 4 we present a novel process for the deposition of thin layers of α-FAPbI<sub>3</sub> that employs volatile, low-toxicity solvents compatible with large scale processing of perovskite materials. By first depositing a 2D perovskite material we are able to template the growth of highly crystalline α-FAPbI<sub>3</sub>, which we show out-performs other state-of-the-art FAPbI<sub>3</sub>-based materials.</p>
<p>Chapter 5 takes the α-FAPbI<sub>3</sub> material developed in the preceding chapter and spotlights the substantially improved ambient and thermal stability it displays in comparison to other state-of-the-art perovskites. We explore the mechanisms behind these enhancements and report preliminary work on the integration of our α-FAPbI<sub>3</sub> perovskite into highly stable PSCs. By comparison with a wide array of other perovskite materials we are able to directly link, and begin to explain, the process by which the perovskite is made with its resultant stability.</p>
<p>Finally, Chapter 6 explores the role methylenediammonium dichloride (MDACl<sub>2</sub>) plays in stabilising α-FAPbI<sub>3</sub>. We show that previous reports claiming that the MDA2+ is incorporated into the perovskite lattice are erroneous, and that instead a complex array of chemical reactions in solution lead to the inclusion of tetrahyrdrotriazinium (THTZ-H+) in the structure of FAPbI<sub>3</sub>, resulting in a substantial increase in the ambient phase stability of this perovskite.</p>
<p>Having established that metal halide perovskites can function as highly effective photoabsorber materials in next-generation photovoltaics, research effort has now turned to developing perovskite materials that can deliver stable power output over a number of years. Pristine FAPbI<sub>3</sub> has, for the most part, been left out of this ongoing work. Perhaps it is time to reconsider?</p>
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