Controlling morphology in evaporating and solidifying perovskite thin films

Jesse Lev Starger

doi:10.17918/00010966

Perovskite semiconductors are a promising class of materials for the next-generation of thin-film photovoltaics (PV). However, large-scale deployment of perovskite PV has been frustrated by challenges in controlling film morphology during solution processing, a factor intrinsically linked to their optoelectronic performance. Scaling from lab techniques like spin-coating to industrial methods such as slot-die coating is challenging due to the complex dynamics and coupling of evaporation, nucleation, and crystal growth during film formation. As a result, the performance of large-area devices has lagged behind their lab-scale counterparts, and process development to date has typically relied on trial-and-error methods. This thesis describes novel drying models to understand and predict how thin (< 10 [mu]m) perovskite coatings dry and crystallize, to bridge the gap between lab-scale processes and manufacturing-scale coating techniques. First, a lumped-parameter drying model is developed and validated for various solvent systems and perovskite compositions. The model describes the evolution of film composition with time as a function of initial formulation, coating thickness, and drying environment. This simple model forms the basis for an improved framework for designing perovskite coating experiments that couples ink formulation to drying, and was successfully used to further improve the performance and reproducibility of previously optimized large-area blade-coated devices. A quasi-2D model is then introduced, integrating the validated evaporation model with diffusion-driven growth of a crystallizing domain in a thin film. The model reveals that the confining geometry of a thin film results in a complex relationship between evaporation, diffusion, nucleation, and growth that is not accounted for in classical growth models or the perovskite literature. Coating and drying experiments are used to validate the quasi-2D model and demonstrate a strong interdependence between evaporation and nucleation. Attempts to decouple evaporation from nucleation using temperature, convection, and solvent blends reveal fundamental limitations on achieving uniform domains and high substrate coverage in evaporating and crystallizing films using traditional coating methods. Preliminary data and scaling arguments suggest that successful devices demonstrated in the literature are formed in a limited processing window that allows for fast evaporation compared to nucleation and growth. Overall, the models and experiments presented here significantly advance our understanding of perovskite PV morphologies, and provide general insights into the relationship between drying dynamics and morphological features in the broader field of thin film manufacturing.

Controlling morphology in evaporating and solidifying perovskite thin films

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Controlling morphology in evaporating and solidifying perovskite thin films

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