Mixed Lead-Tin Halide Perovskites for Optoelectronic Applications

Change log

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.

Stranks, Sam
Halide perovskites, Lead-tin perovskites, perovskite optoelectronics, Perovskite solar cells, Perovskite field effect transistors
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
Cambridge Trusts, Cambridge Philosophical Society, Churchill College, European Research Council, Institution of Engineering & Technology (IET), The Armourers and Brasiers' Gauntlet Trust, SuperGen SuperSolar