PbI₂ Crystallization for Perovskite Solar Cell Fabrication Using Spin-Coating and APCVD

Abstract

This doctoral thesis investigates the optimization of the two-step deposition method for metal halide perovskite solar cells (PSCs), a critical technology for next-generation photovoltaics. The research focuses on the morphological and structural evolution of the lead iodide (PbI2) precursor layer and its subsequent conversion into the photoactive perovskite phase. A significant portion of the work is dedicated to the wet-chemical optimization of Cs-doped PbI2 films. Furthermore, the thesis presents a novel, scalable approach for depositing PbI2 using Atmospheric Pressure Chemical Vapor Deposition (APCVD), marking the first successful application of an industrial APCVD tool for this material class. In the first phase, the study systematically analyzes the annealing process of spin-coated Cs0.05PbI2.05 precursor films. The solvent system utilized a mixture of DMF and DMSO. A key finding is that standard annealing temperatures (≥ 80°C) lead to incomplete substrate coverage and the formation of holes, which persist in the final perovskite layer. Consequently, a reduced annealing temperature of 50°C was established. This lower temperature regime allows for precise control over the crystallization kinetics. Initially, excess unbound DMSO evaporates within the first 3 minutes. Subsequently, PbI2-DMSO complexes dissociate, releasing PbI2 for crystallization. It was demonstrated that amorphous precursor films—resulting from short annealing durations (1–3 min)—retain a higher DMSO content. This facilitates a more reliable conversion into high-quality perovskite films via an intermolecular exchange mechanism between DMSO and formamidinium iodide (FAI). The second step, converting the precursor to the perovskite composition Cs0.05MA0.28FA0.67Pb(I0.96Br0.04)3, revealed significant temperature sensitivities. The study highlights a trade-off during the conversion annealing at 150°C. While necessary for phase formation, this temperature induces the degradation of the perovskite via the volatilization of the organic cation methylammonium (MA+), leaving behind a crystalline PbI2 phase and reducing the bandgap energy. Contrarily, reducing the conversion annealing time to 1 minute prevented decomposition while achieving complete conversion. A remarkable phenomenon was observed when converting highly crystalline PbI2 precursors (annealed for 30 min): the high conversion rate led to the formation of exceptionally large perovskite grains (up to 7.4 µm). However, these large grains were found to be thermally unstable, fragmenting into smaller domains upon prolonged annealing. Simulations confirmed that the grain size is inversely proportional to the nucleation density, which is governed by the conversion rate. A major contribution of this work is the introduction of APCVD for the deposition of PbI2 layers. This method operates at atmospheric pressure, offering a distinct advantage over vacuum-based Physical Vapor Deposition (PVD) regarding throughput and cost. The APCVD process produced high-crystallinity PbI2 films characterized by large, hexagonal platelets (> 1 µm) oriented along the (001) direction. A deposition rate of (55.5 ± 2.3) nm min-1 was achieved, surpassing many existing vapor-phase techniques. The deposition mechanism suggests growth primarily at the crystal edges due to the high collision rate at atmospheric pressure. While deposition on the hole-selective layer NiO resulted in unfavorable vertical growth due to poor adhesion, deposition on MeO-2PACz yielded planar films suitable for device integration. Subsequent conversion of these APCVD-PbI2 layers into perovskite resulted in absorber films with implied open-circuit voltages (iVoc) of up to 1.13 V, demonstrating the high potential of this industrial manufacturing route. The thesis provides a comprehensive understanding of the interplay between precursor morphology, solvent evaporation, and thermal processing in the two-step fabrication of perovskites. It identifies MA+ evaporation as a critical degradation pathway during fabrication and proposes optimized, low-thermal-budget protocols. Finally, the successful implementation of APCVD for PbI2 paves the way for scalable, high-throughput manufacturing of perovskite solar cells.