Understanding and Controlling Intermediates for Stable and Efficient Perovskite Solar Cells

Abstract

Metal halide perovskites have emerged as a next-generation photovoltaic material due to their remarkable optoelectronic properties and low-cost solution processability. However, despite achieving certified power conversion efficiencies exceeding 26%, the large-scale application of perovskite solar cells (PSCs) is still hindered by intrinsic instabilities associated with intermediates and the corresponding unwanted impurities generated during crystallization. These transient species govern nucleation, growth, and phase stability, ultimately determining device reproducibility and durability. The central objective of this work, entitled “Understanding and Controlling of Intermediates for Stable and Efficient Perovskite Solar Cells”, is to elucidate the formation mechanisms of these intermediates and impacts on final device to develop molecular design strategies that control them to achieve long- term stability and high efficiency. In the first part of this work, quasi‐2D scaffolding intermediates were introduced to stabilize 1.67 eV Cs‐rich pure‐iodide perovskites. By integrating low-dimensional phases within the 3D perovskite lattice, the formation of thermodynamically unstable δ-phase was effectively suppressed, while structural rigidity and carrier lifetime were simultaneously enhanced. This strategy demonstrated that controlled intermediate phases can act as beneficial scaffold for suppressing halide segregation, thus enabling more stable perovskite films suitable for tandem applications. In the second study, the role of cation heterogeneity and its influence on final devices were systematically explored. Through precise chemical tracking and spectroscopic analyses, it was revealed that the coexistence of formamidinium (FA⁺) and methylammonium (MA⁺) cations can induce the formation of a benign methylformamidinium (MFA⁺) species via a mild cation-exchange process. Rather than being detrimental, this MFA⁺ acts as a local structural buffer that suppresses δ-phase nucleation and reinforces the stability of the black α-perovskite phase. Moreover, the presence of MFA⁺ subtly modifies the local electrostatic environment and lattice symmetry, leading to reduced trap- assisted recombination and enhanced charge carrier mobility. This cooperative effect improves charge extraction at the perovskite/transporting layer and minimizes nonradiative losses during operation. Consequently, MFA+ not only plays a structural role in stabilizing the desired phase but also contributes electronically by promoting more efficient charge transport and longer carrier lifetimes. These findings highlight that selective cation-mediated interactions can be deliberately tuned to control crystallization pathways, passivate local defect states, and improve both structural and electronic properties of perovskite films. Finally, the third part of this thesis introduces a byproduct-free stabilization strategy employing a neutral amine additive, propylamine (PA). In contrast to conventional ammonium halide additives that trigger iodide oxidation and generate undesirable alkyl-formamidinium residues, PA establishes a dynamic and reversible hydrogen-bonded equilibrium with FA⁺ cations in the precursor solution. This molecular interaction effectively suppresses iodide oxidation in the liquid phase, thereby maintaining chemical integrity during solution aging. During the subsequent annealing process, the absence of chloride and the moderated FA⁺ reactivity prevents the formation of volatile or alkylated byproducts, leading to cleaner crystallization pathways. Consequently, the PA-assisted perovskite films exhibit markedly improved crystallinity, larger grain domains, and lower trap densities, which translate into enhanced charge transport, superior thermal robustness, and enhanced power conversion efficiency. This approach demonstrates that precise molecular control in the solution stage can dictate the solid-state quality of perovskite films, offering a chemically benign route toward highly stable and efficient solar cells. Collectively, these studies provide a unified chemical and mechanistic understanding of how intermediate chemistry governs perovskite crystallization and stability. The findings demonstrate that rational control of precursor interactions—whether through quasi-2D scaffolding, cation heterogeneity, or dynamic molecular buffering—can transform unstable solution chemistry into robust, high- performing materials. This work lays a foundation for the rational design of perovskite compositions and processing routes that decouple efficiency from instability, paving the way toward reliable, scalable photovoltaic technologies.