Dr Qimu Yuan

Abstract:

Metal halide perovskite semiconductors have shown significant potential for use in photovoltaic (PV) devices. While fabrication of perovskite thin films can be achieved through a variety of techniques, thermal vapor deposition is particularly promising, allowing for high-throughput fabrication. However, the ability to control the nucleation and growth of these materials, particularly at the charge-transport layer/perovskite interface, is critical to unlocking the full potential of vapor-deposited perovskite PV. In this study, we explore the use of a templating layer to control the growth of coevaporated perovskite films and find that such templating leads to highly oriented films with identical morphology, crystal structure, and optoelectronic properties independent of the underlying layers. Solar cells incorporating templated FA0.9Cs0.1PbI3–xClx show marked improvements with steady-state power conversion efficiency over 19.8%. Our findings provide a straightforward and reproducible method of controlling the charge-transport layer/coevaporated perovskite interface, further clearing the path toward large-scale fabrication of efficient PV devices.

Abstract:

Vacuum deposition is a solvent-free method suitable for growing thin films of metal halide perovskite (MHP) semiconductors. However, most reports of high-efficiency solar cells based on such vacuum-deposited MHP films incorporate solution-processed hole transport layers (HTLs), thereby complicating prospects of industrial upscaling and potentially affecting the overall device stability. In this work, we investigate organometallic copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) as alternative, low-cost, and durable HTLs in all-vacuum-deposited solvent-free formamidinium-cesium lead triodide [CH(NH₂)₂]_0.83Cs_0.17PbI₃ (FACsPbI₃) perovskite solar cells. We elucidate that the CuPc HTL, when employed in an “inverted” p–i–n solar cell configuration, attains a solar-to-electrical power conversion efficiency of up to 13.9%. Importantly, unencapsulated devices as large as 1 cm² exhibited excellent long-term stability, demonstrating no observable degradation in efficiency after more than 5000 h in storage and 3700 h under 85 °C thermal stressing in N₂ atmosphere.

Abstract:

As perovskite-based photovoltaics near commercialization, it is imperative to develop industrial-scale defect-passivation techniques. Vapor deposition is a solvent-free fabrication technique that is widely implemented in industry and can be used to fabricate metal-halide perovskite thin films. We demonstrate markably improved growth and optoelectronic properties for vapor-deposited [CH(NH2)2]0.83Cs0.17PbI3 perovskite solar cells by partially substituting PbI2 for PbCl2 as the inorganic precursor. We find the partial substitution of PbI2 for PbCl2 enhances photoluminescence lifetimes from 5.6 ns to over 100 ns, photoluminescence quantum yields by more than an order of magnitude, and charge-carrier mobility from 46 cm2/(V s) to 56 cm2/(V s). This results in improved solar-cell power conversion efficiency, from 16.4% to 19.3% for the devices employing perovskite films deposited with 20% substitution of PbI2 for PbCl2. Our method presents a scalable, dry, and solvent-free route to reducing nonradiative recombination centers and hence improving the performance of vapor-deposited metal-halide perovskite solar cells.

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.