Siyu Yan

Abstract:

Lattice dynamics are critical to photovoltaic material performance, governing dynamic disorder, hot-carrier cooling, charge-carrier recombination, and transport. Soft metal-halide perovskites exhibit particularly intriguing dynamics, with Raman spectra exhibiting an unusually broad low-frequency response whose origin is still much debated. Here, we utilize ultra-low frequency Raman and infrared terahertz time-domain spectroscopies to provide a systematic examination of the vibrational response for a wide range of metal-halide semiconductors: FAPbI3, MAPbI x Br3–x , CsPbBr3, PbI2, Cs2AgBiBr6, Cu2AgBiI6, and AgI. We rule out extrinsic defects, octahedral tilting, cation lone pairs, and “liquid-like” Boson peaks as causes of the debated central Raman peak. Instead, we propose that the central Raman response results from an interplay of the significant broadening of Raman-active, low-energy phonon modes that are strongly amplified by a population component from Bose–Einstein statistics toward low frequency. These findings elucidate the complexities of light interactions with low-energy lattice vibrations in soft metal-halide semiconductors emerging for photovoltaic applications.

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:

Metal-halide perovskites have exhibited excellent optoelectronic properties and are widely used as the photoactive layers in photovoltaics. While predominantly a research innovation which showed significant promise on a lab-scale, as of 2024, perovskite solar cells are moving towards large-scale commercialisation. While vapour phase deposition processes dominate the established thin-film manufacturing, perovskite solar cells are still predominantly developed using solution-based methods. To enable scalable fabrication and industry adoption, it is imperative to advance the understanding of metal-halide perovskite thin-films fabricated by the vapour deposition technique. This thesis advances the understanding of vapourdeposited perovskite thin films and establishes several key strategies to reduce bulk and interfacial defect densities, thereby enhancing the performance and stability of vapour-deposited devices.

A major advance presented in this work is the elucidation of how organic impurities in commercial formamidinium iodide precursors influence film formation in both solution- and vapour-deposited systems. It is demonstrated that, while such impurities can beneficially passivate defects in solution-processed films, they induce altered sublimation behaviour in vapour deposition, leading to off-stoichiometric compositions and the formation of non-photoactive polytype phases. These findings establish impurity control as a critical requirement for achieving phase-pure, highquality perovskite layers by vapour deposition.

A second key contribution is the introduction of a templating strategy that decouples perovskite nucleation from the underlying charge-transport substrate. This approach enables consistent film morphology, crystallographic orientation, and optoelectronic quality across diverse substrates. By providing a means to tune initial perovskite stoichiometry and suppress unwanted interfacial reactions, this templating method offers a scalable route toward reproducible, high-performance vapour-deposited perovskite devices.

Furthermore, this thesis demonstrates the use of aromatic ammonium halides as vapour-deposited passivation agents and reveals that their structural and functional properties depend strongly on the deposition method and post-deposition treatment. The work establishes that measurement atmosphere and thermal annealing can drive surface reconstruction and modulate passivation efficiency, thereby highlighting new levers for interface engineering in vapour-deposited perovskites.

Collectively, this research provides a comprehensive understanding of impurity effects, interfacial templating, and molecular passivation in vapour-deposited perovskite solar cells. These insights pave the way toward scalable, stable, and highefficiency perovskite photovoltaics compatible with industrial thin-film manufacturing.