Scalable vacuum-based deposition methods for halide perovskites
- Autores: Nathan Jacques Rodkey
- Directores de la Tesis: Henk Jan Bolink (dir. tes.), Mónica Morales Masis (codir. tes.)
- Lectura: En la Universitat de València ( España ) en 2024
- Idioma: español
- Número de páginas: 149
- Tribunal Calificador de la Tesis: Pablo Pérez Boix (presid.), Efrén Navarro Moratalla (secret.), Annalisa Bruno (voc.)
- Programa de doctorado: Programa de Doctorado en Nanociencia y Nanotecnología por la Universidad de Alicante; la Universidad de Castilla-La Mancha; la Universidad de La Laguna; la Universidad Jaume I de Castellón y la Universitat de València (Estudi General)
- Calificación: Sobresaliente Cum Laude
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
This thesis is about thin film deposition processes and characterization, but also dives into the design and engineering needed to create these processes. It was the aim of this thesis to understand the processes involved, often gauging their quality via device fabrication. Joining these processes is the theme of scalable deposition methods, always kept in mind during the pursuit of meaningful and impactful research questions. Below is a summary of the work presented in each chapter of this thesis.
In Chapter 3, we introduce the use of pulsed laser deposition (PLD) for making Cs2AgBiBr6 thin films. This material was chosen to demonstrate the precise control PLD has on film stoichiometry, depositing such films from preformed powders, pressed into a single-source pellet. The mechanism behind near-stoichiometric growth is associated to a thermalization process, related to the working pressure of the system. The quality of films grown was then enhanced via substrate heating, characterized with photoluminescence and x-ray diffraction to gauge film quality. It was the aim of this chapter to promote PLD as a viable technique for depositing perovskites, regardless of complexity. Following the achievement described in this chapter, fellow PhD students in the Morales group have demonstrated perovskite based photovoltaic device achieving power conversion efficiencies >19%. As such, this recent achievement as well as the results presented in this chapter demonstrate the potential quality of perovskite films and devices made by PLD, opening doors to its future scalable applications. In Chapter 4, thermal co-evaporation was used for the fabrication of 1 µm thick, high-efficiency bifacial perovskite solar cells (PCE >20%). Optimized anti-reflection coatings (ARC) based on detailed analysis of the optical constants were employed. Due to these ARCs the performance of bifacial solar cells exceeds that of the monofacial counterparts due to the better index-matching of ARC coatings at the perovskite-air interface. Losses in the device were thoroughly analyzed, noting that transmittance and charge-carrier collection (due to poorly conducting contacts) were the most significant problems accounting for nearly 3 mA/cm2 of lost current-density. To counteract this, cells in the future need to be made even thicker (> 2 µm) or possibly textured to scatter the incoming light and enhance its optical length. Additionally, this may require increased care and study of diffusion lengths, which may begin to hamper cell performance beyond 1 µm. For charge-carrier collection losses, passivating contacts need to be as thin as possible and followed by highly doped contacts. It is also important to comment that bifacial modules in an industrial context would be sold with encapsulation on both sides; the ARC coatings optimized here would have to be revised considering the refractive index of said encapsulation. As such, bifacial perovskite solar cells need continued investigation by the community to elevate them to the same standards as their monofacial counterparts, and simultaneously compete with silicon.
In Chapter 5, the exploration of single-source deposition processes as described in chapter 3 using PLD continues via thermal flash evaporation. State-of-the-art flash evaporation processes were improved upon by the engineering of a pulsed, powder dispenser which introduces preformed powder into a heated crucible. Three materials were used as a case-study: MAPbI3 as a reference, FAPbBr3 due to the hard to control sublimation of FABr, and MAxFA1-xPb0.6Sn0.4I3:SnF2 (10%) as a demonstration of complex six-component transfer. Unlike the PLD process, films grown by this technique were demonstrated in devices to gauge their quality with MAPbI3 and FAPbBr3 reaching 9.9% and 4.0% power conversion efficiencies respectively. Device performances were promising yet could be further improved and the conceptual demonstration of flash evaporated films grown in this way let to the design of a vapor transport deposition system and process.
In Chapter 6 close-space sublimation (CSS) was investigated to tackle the difficult question of stable organic sources. For context, typical high-vacuum thermal sublimation systems do not reuse MA- and FA-based sources because of degradation of the source and dopants used to control the sublimation. In the work presented here, organic MAI and FAI sources are reused many times to sequentially convert inorganic precursor layers. The sublimation behavior of MAI in particular is investigated, noting "sticking" and "sublimation" limited regimes that occur during the CSS process, more akin to chemical vapor processes than thermal sublimation. Quality of the process was gauged via device fabrication showing a 12.0% champion PCE for MAPbI3. Targeting a composition reported in record vacuum-processed cells, FA0.9Cs0.1PbI3:Cl devices were fabricated, reaching a champion PCE of >18%, and thermal stability under nitrogen at 85 oC for >650 hours. A marked improvement was observed during this annealing process, explained by the reduction of bulk traps via drift-diffusion simulations.
Work in the field of halide perovskite was challenging, fast-paced, and at times rewarding. The topics presented here are broad, and each merit a deeper investigation, at both a fundamental and device-engineering level. This work presents several examples of successful deposition strategies, all of which could be scaled for the deposition of halide perovskites. As such, I believe this work has provided insight to the community, benefiting the future of the community and industry.
