The Role of Organic Amidiniums in Perovskite Photovoltaics

Abstract

Clean energy forms the foundation of sustainable development, and among various technologies, photovoltaics─directly converting sunlight into electricity─stand out as one of the most promising and impactful. In recent years, it has garnered significant attention and undergone rapid development. Notably, Organic–inorganic Lead Halide Perovskites (OLHPs) have emerged as a breakthrough in this field. After just a decade of research and development, OLHP-based solar cells have achieved power conversion efficiencies (PCEs) exceeding 26%. OLHPs offer a unique combination of solution-based processing, low-cost production, and high efficiency, making them strong competitors to traditional inorganic semiconductor technologies such as silicon-based photovoltaics.

OLHPs are described by the chemical formula ABX₃, where “A″ is a monovalent cation, “B″ is the divalent lead cation (Pb²⁺ or Sn²⁺), and “X″ is a halide anion. In the early stages of OLHP development, the choice of the A cation was largely limited to methylammonium (MA⁺), formamidinium (FA⁺), and cesium (Cs⁺), as these cations were small enough to fit into the crystal lattice of the perovskite structure based on size and structural requirements. Moreover, progress in recent years discovered that incorporating oversized A cations as additives or passivators could significantly fine-tune the perovskite properties, leading to major advancements in performance. As the focus of OLHP research gradually shifted from methylaminium lead triiodide (MAPbI₃) to formamidinium lead triiodide (FAPbI₃), with a more suitable band gap and longer carrier lifetime, recent studies have highlighted the critical influence of the oversized amidiniums. Compared to traditional oversized ammoniums, oversized amidiniums demonstrating a more pronounced effect on optoelectronic properties.

In this Account, we explore key advancements brought about by the expanded role of amidiniums in OLHP research. These include: (i) the nucleation thermodynamic and kinetic regulation toward desirable OLHP phases; (ii) the modulation of bulk-phase electronic states through strain-induced effects; and (iii) the tuning of surface electronic states via low-dimensional phases and multifunctional groups. These areas are now at the cutting edge of OLHP research, playing a pivotal role in determining the utility, function, performance, and long-term stability of OLHP-based optoelectronic devices. In the future, further development of amidinium compounds will be essential, and the discovery of new amidiniums or novel applications is highly anticipated. As perovskite solar cells move toward commercialization, amidiniums are expected to play a crucial role in the fabrication of large-area, uniform, and high-quality perovskite films with consistent passivation to mitigate transverse carrier recombination. Additionally, amidiniums will be key in addressing the challenge of long-term operational instability in both solar cells and the module.