Underlying Mechanisms of Halide Perovskites Properties and Metal Insulator Transitions in Vanadium Dioxide

Halide perovskites, as potential substitutes for silicon, stand out as one of the most promising photovoltaic materials for next-generation solar cells, owing to their superior properties such as high light-absorption ability, high carrier mobilities, easy synthesis, thin-film fabrication, and low production costs. The efficiency of perovskite solar cells has rapidly increased, reaching 32.5% in 2023 from a modest 3.8% in 2009. Despite this rapid growth, the underlying mechanisms driving the high light-absorption and small effective masses in halide perovskites are not fully understood. Here, we show that these properties mainly arise from the multi-centred X-B-X bonding formed by the linearly aligned B-p and X-p orbitals, resulting in large matrix elements, especially in the visible spectrum. The normalized matrix elements are twice that of typical two-centre bonds, and the transition probabilities are four times larger, leading to high light absorption. Additionally, strong coupling between B-p and X-p orbitals in the X-B-X bonds leads to broad valence and conduction bands, along with the small band gap, resulting in small effective masses for both carriers. Another challenge is the long-term instability of perovskite solar cells due to the unstable cubic phase of halide perovskites and poor interfacial quality with the hole transport layer. We propose that Co₃O₄ offers greater benefits as a hole transport material with perovskites compared to NiO, due to smaller valence band offsets, higher interfacial adhesion energies, and greater formation energies of interfacial metal vacancies. Additionally, FA_0.4MA_0.6PbI₃ exhibits superiority over MAPbI₃ as a light-absorbing layer with Co₃O₄ or NiO due to higher adhesion energy and better band alignment without defects or dopant facilitation. Thus, the Co₃O₄/FA_0.4MA_0.6PbI₃ combination holds promise for efficient and stable perovskite solar cells. Vanadium dioxide (VO₂) undergoes a metal-insulator transition (MIT) at 340K from its semiconducting monoclinic phase to the metallic rutile phase. In polycrystalline VO₂, the MIT is less sharp due to grain boundaries (GBs). Despite the crucial role of GBs in the MIT, the mechanisms driving them to be semiconducting or metallic are not fully understood. Here, we demonstrate that V-V pairing, which renders monoclinic VO₂ semiconducting in bulk, also causes band gap opening at GBs. Polycrystalline monoclinic VO₂ thin films are expected to have more metallic GBs without V-V dimerization due to energetic favour, leading to decreased resistivity and thus less sharp MIT. Additionally, the energy of twin GBs is primarily determined by re-binding across the GB, rather than by surface energy. Although introducing oxygen vacancies is a potential method to reduce the transition temperature, semiconducting GBs become metallic upon induction, resulting in less sharp MIT.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Zhang, Xuewei

Advisor dc:contributor.advisor

• Chu, Daping

• Density functional theory
• Perovskite
• Vanadium Dioxide

Licence

• https://www.repository.cam.ac.uk/bitstreams/fc55880d-8493-4b22-abb9-d608a0f0a1a5/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.111514

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/372855