Terahertz photonics

Abstract:

Metal halide perovskites are a prominent class of semiconductors as highly promising photovoltaic and light-emission materials. Vapour deposition, a technique of subliming precursor materials under high vacuum, is a solvent-free and industry-applicable method for depositing uniform and crystalline perovskite thin films. This thesis focusses on the development of vapour co-deposition techniques, applied to three perovskite compositions of formamidinium-caesium lead triiodide (FA_0.83Cs_0.17PbI₃), caesium lead triiodide (CsPbI₃), and caesium lead tribromide (CsPbBr₃).

An all-vacuum-processed perovskite solar cell device stack with FA_0.83Cs_0.17PbI₃ as the intrinsic layer is developed, with two metal phthalocyanine hole transport layer candidates scrutinised. It is elucidated that the copper phthalocyanine (CuPc) exhibits enhanced compatibility than zinc phthalocyanine for hole extraction, when employed in an p–i–n planar heterojunction solar cell, and attains a solar-to-electrical power conversion efficiency up to 13.9%. Device performance is further improved to 15.5% with the modification of the CuPc-FA_0.83Cs_0.17PbI₃ interface by inserting an electron blocking layer of aluminium oxide. These unencapsulated devices also demonstrate excellent long-term stability, such that minimal change in efficiency after more than 5000 hours in storage and 3700 hour under 85°C heat-testing in N₂ atmosphere is observed.

The co-deposition of phase-stable γ-CsPbI₃ is probed through crystallographic, atomic-scale structural, and photo-physical studies. From optimising nominal CsI:PbI₂ precursor ratios, thin films with improved crystallinity and tolerance to thermal stressing are obtained in the Cs-rich parameter space. The presence of Ruddlesden-Popper (RP) planar defects is uncovered in these Cs-rich films whilst intensified trap-mediated recombination dynamics are revealed, which correlate to the number density of RP defects in γ-CsPbI₃.

Finally, the application of CsPbBr₃ as a gain medium for perovskite lasing is examined. To address challenges of effective thermal management and power scalability, the design of a thin-disc perovskite laser is presented. Successful fabrication of the thin-disc gain medium with vapour co-deposited CsPbBr₃ is demonstrated, which room-temperature amplified spontaneous emission is observed with a low threshold of 27.3 μJ cm⁻², elucidating the favourable prospect for realising optically-driven lasing in a free-space cavity.