Stability of low bandgap Pb-Sn perovskites

: (Alternative Format Thesis)

Abstract

Metal-halide perovskites are considered one of the most promising emerging solar technologies being the fastest ever evolved with a certified efficiency of 25.7%, surpassing all other thin-film technologies in only one decade of development. Beyond their excellent optoelectronic properties enabling such astonishing efficiencies, metal-halide perovskites also have the unique property of bandgap tuning via compositional alterations. This enables the bandgap optimization not only for single junction solar cells but also for multijunction or tandem solar cells, which promise efficiencies beyond anything a single junction can produce making them particularly interesting for any future photovoltaic technology. All-perovskite tandem solar cells require a low and a wide bandgap perovskite to be employed as the main absorbers. So far, the best efficiencies have been obtained when mixtures of lead (Pb) and tin (Sn) are employed as the narrow bandgap component. In addition to efficiencies though, long-term stability and operational lifetime also comprise major requirements for the commercialization of a PV technology. In this thesis, the intrinsic material stability of Pb-Sn perovskites is investigated in order to gain a deeper understanding of the fundamental driving forces for degradation, to identify the different degradation pathways and ultimately the most stable Pb-Sn composition suitable for all-perovskite tandem solar cells.
Initially, Chapter 3 contextualizes perovskite-based tandem solar cells by elucidating the advantages of Si/Perovskite and all-perovskite tandem solar cells from a materials, manufacturing, sustainability and business perspective. While the merits of perovskite-based tandem solar cells fuel the expectation of a perovskite-driven revolution in the PV market, a certain amount of uncertainty remains with regards to the perovskite lifetime, which in case of tandems is even more complex as both narrow and wide bandgap components must be equally durable, since the degradation of one subcell will significantly impact the performance of the entire tandem device. Although significant progress has been made for wide bandgap perovskites, low bandgap Pb-Sn perovskites are far behind in terms of stability, a fact stemming mainly from the inherent tendency of Sn to oxidize. In Chapter 4, a comparative study of the practical stability of the frontrunning low bandgap candidates under thermal and humidity stressing is presented, focusing on the effect of composition on the degradation pathways. Although the multivalent nature of Sn is known as the weak point of Sn-based compositions, it is still unclear whether the A-site cation has a strong effect on the intrinsic stability for Pb-Sn perovskites as well. Here, specific focus was devoted on the role of the A-site cation on the degradation of Pb-Sn perovskites. The more fundamental aspects of the composition in degradation are explored in Chapter 5. DFT calculations were performed to investigate the changes induced in the structure due compositional variations along with the effect of molecular oxygen (O2) presence in the lattice. Furthermore, various types of conductivity measurements were conducted in order to capture the evolution of Sn-oxidation and to gain a more profound understanding of the changes of the fundamental optoelectronic properties. Nano-XRF measurements were also performed to directly monitor the presence of inhomogeneities or their formation upon stressing that can affect long-term stability and what is the impact of composition. Finally, alternative A-cation combinations in Pb-Sn alloys were studied in Chapter 6. Using calculated tolerance factor values as guiding principle, the effect of larger organic cations occupying the A-site in the lattice on the structural and optical properties of the respective thin films was investigated, along with their respective device performance and intrinsic stability.
This work highlights the impact of the materials’ composition on the degradation pathway of Pb-Sn perovskites and paves the way for the investigation of alternative low bandgap Pb-Sn compositions
In particular, it was found that the A-cation dictates the thermal stability of the Pb-Sn perovskites and exerts a strong impact on the degradation pathways under humidity exposure. DFT studies also confirmed the influence of composition on structural distortions and highlighted the existence of preferential and composition–dependent starting points of degradation. In terms of optoelectronic properties, conductivity measurements were identified as a useful tool to capture the degradation process and assess the initial quality of the material, while the A-cation was proved to not only the initial optoelectronic properties but also the rate they deteriorate upon air exposure. Preliminary n-XRF results revealed the presence of inhomogeneities in nanoscale, both in pristine and stressed samples, a phenomenon heavily correlated with composition that could stimulate degradation, phase segregation and device performance deterioration. Furthermore, alternative low bandgap Pb-Sn materials were designed by introducing larger A-cations in the perovskite lattice leading to highly oriented perovskite films and functional devices.
These insights enhance our understanding on the the driving forces of degradation and the effect of the A-cation on stability and structural cohesion and essentially contributes to the compositional selection and future stability improvement strategies for the development of commercialisable all-perovskite tandems.

Date of Award	16 Nov 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Alison Walker (Supervisor) & Petra Cameron (Supervisor)