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Mixed lead-tin (Pb-Sn) perovskites are unique materials in the family of halide perovskites. Unlike Pb perovskites, these mixed-metal systems can demonstrate bandgaps below 1.3 eV and are therefore essential constituents for low bandgap bottom subcell in all-perovskite tandem solar cells as well as for near-infrared light emitting diodes (LEDs), lasers and photodetectors. Although the air stability of these Sn-containing perovskites are relatively poor due to the facile oxidation of Sn2+ to Sn4+, these materials do possess certain bright aspects in their optoelectronic properties which have received less attention in the community and this forms the foundation of this thesis. Chapter 1 provides a bigger picture of the need to explore sustainable alternatives to energy generation and consumption and the role of emerging semiconductor materials, especially metal halide perovskites, in that pursuit. Chapter 2 provides a general background to semiconductors and outlines the operating principles of solar cells and FETs. It also presents the current understanding of the optoelectronic properties and degradation mechanisms of mixed Pb-Sn halide perovskites. All the experimental techniques used in the thesis are introduced in Chapter 3. Chapter 4 summarises the optimization strategies of mixed Pb-Sn halide perovskite systems for demonstrating reliable and hysteresis-free p-type perovskite FETs with high hole mobility reaching 5.4 cm2/Vs and ON/OFF ratio approaching 106, which are among the best metrics in the field of perovskite FETs. We also rationalize these findings of long-range lateral transport with the support of theoretical calculations, film morphology studies and chemical analysis of defects in these materials. We then extend the above work to probe the lateral charge transport mechanism in mixed Pb-Sn perovskite FETs in Chapter 5. Through temperature-dependent field-effect mobility measurements, aided further with photoluminescence microscopy under bias, we show that ionic screening effects are greatly suppressed in mixed Pb-Sn devices when compared to their Pb-based analogues. We also demonstrate that dipolar disorder (associated with methylammonium, MA+ cation) induced lowering of FET mobility near room temperature can also be seen for mixed Pb-Sn perovskites and hence further efforts need to be invested in going MA-free in future. Next, we generalize the above findings of suppressed ion dynamics in mixed Pb-Sn systems by fabricating optoelectronic device stacks with vertical charge transport in Chapter 6, which are relevant for solar cells and LEDs. We reconcile these findings through first principles calculations, which reveal the key role played by Sn vacancies (with low formation energy) in increasing the migration barrier for iodides due to severe local structural distortion in the lattice. In Chapter 7, we show that the partial or complete incorporation of Sn in the metal (B) site of mixed halide perovskites offer very promising intrinsic stability to halide segregation under a host of processing and operational conditions. We further study the optoelectronic properties of these mixed halide Pb-Sn perovskites to understand the impact of light soaking on the charge carrier recombination and transport in these materials. We also assess the device performance of these mixed halide perovskite materials by fabricating single single junction solar cells. Chapter 8 summarises the key findings of this thesis and proposes several potential directions of research involving these mixed Pb-Sn perovskites. All the work presented herein provides an important advance to the fundamental understanding and applied device integration of mixed lead-tin perovskite materials and can be leveraged for demonstrating a ‘perovskite optoelectronic universe’ with high performance and stability.

Photodetectors with multiple spectral response bands have shown promise to improve imaging and communications through the switchable detection of different photon energies. However, demonstrations to date have been limited to only two bands and lack capability for fast switching in situ. Here, we exploit the band gap tunability and capability of all-perovskite tandem solar cells to demonstrate a new device concept realizing four spectral bands of response from a single multijunction device, with fast, optically controlled switching between the bands. The response to monochromatic light is highly selective and narrowband without the need for additional filters and switches to broader response bands on applying bias light. Sensitive photodetection above 6 × 1011 Jones is demonstrated in all modes, with rapid switching response times of, This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (HYPERION, grant agreement number 756962 and PEROVSCI, grant agreement number 957513). ODIM acknowledges funding from a departmental awarded Engineering and Physical Sciences Research Council (EPSRC) Doctoral Training Studentship. S.D.S. acknowledges the Royal Society and Tata Group (grant no. UF150033). The authors acknowledge the EPSRC (EP/R023980/1, EP/S030638/1, EP/T02030X/1) for funding. Part of this work was undertaken using equipment facilities provided by the Henry Royce Institute, via the grant Henry Royce Institute, Cambridge Equipment: EP/P024947/1 with additional funding from the “Centre for Advanced Materials for Integrated Energy Systems (CAM-IES)” (EP/P007767/1). For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) license to any Author Accepted Manuscript version arising from this submission

Riding on the coat tails of rapid developments in single-junction halide perovskite solar cells, all-perovskite multijunction solar cells have recently garnered significant attention, with the highest power-conversion efficiency already reaching 25.6%. Much of this progress has been fueled by the rapid rise in the photovoltaic performance of low-bandgap halide perovskite absorbers, materials, which, to date, have only been achievable by the partial or complete substitution of lead with tin. However, much room still exists to develop a more critical understanding of key material properties in these low-bandgap perovskites. Herein, the key optoelectronic properties of absorption, carrier generation, recombination, and transport in these tin-containing perovskites are discussed, showing that intrinsic doping distinctively impacts many of these properties, thereby rendering this class of halide perovskites unique within the family. Current understanding of the mechanisms that degrade optoelectronic performance in these materials and the corresponding devices are also summarized. These collective results highlight an important interplay between doping, defects, and degradation that will need to be controlled. Finally, the current gaps in understanding of these low-bandgap perovskites are outlined, thereby providing guidelines for further research, which will unlock their full potential for realizing a plethora of high-performance optoelectronic devices.

Funder: Cambridge Trust, Mixed valence Sn doping of Cs3Bi2Br9 leads to broad visible light absorption.

It is common practice in the lead halide perovskite solar cell field to add a small molar excess of lead iodide (PbI2) to the precursor solution to increase the device performance. However, recent reports have shown that an excess of PbI2 can accelerate performance loss. In addition, PbI2 is photoactive (band gap ∼2.3 eV), which may lead to parasitic absorption losses in a solar cell. Here we show that devices using small quantities of excess PbI2 exhibit better device performance as compared with stoichiometric devices, both initially and for the duration of a stability test under operating conditions, primarily by enhancing the charge extraction. However, the photolysis of PbI2 negates the beneficial effect on charge extraction by leaving voids in the perovskite film and introduces trap states that are detrimental for device performance. We propose that although excess PbI2 provides a good template for enhanced performance, the community must continue to seek other additives or synthesis routes that fulfill the same beneficial role as excess PbI2, but without the photolysis that negates these beneficial effects under long-term device operation.

Mixed valence Sn doping of Cs3Bi2Br9 leads to broad visible light absorption.

Investigation of the inherent field-driven charge transport behaviour of three-dimensional lead halide perovskites has largely remained challenging, owing to undesirable ionic migration effects near room temperature and dipolar disorder instabilities prevalent specifically in methylammonium-and-lead-based high-performing three-dimensional perovskite compositions. Here, we address both these challenges and demonstrate that field-effect transistors based on methylammonium-free, mixed metal (Pb/Sn) perovskite compositions do not suffer from ion migration effects as notably as their pure-Pb counterparts and reliably exhibit hysteresis-free p-type transport with a mobility reaching 5.4 cm2 V-1 s-1. The reduced ion migration is visualized through photoluminescence microscopy under bias and is manifested as an activated temperature dependence of the field-effect mobility with a low activation energy (~48 meV) consistent with the presence of the shallow defects present in these materials. An understanding of the long-range electronic charge transport in these inherently doped mixed metal halide perovskites will contribute immensely towards high-performance optoelectronic devices.