Advent of alkali metal doping: a roadmap for the evolution of perovskite solar cells

Metal-halide hybrid perovskites have prompted the prosperity of the sustainable energy field and simultaneously demonstrated their great potential in meeting both the growing consumption of energy and the increasing social development requirements. Their inimitable features such as strong absorption ability, direct photogeneration of free carriers, long carrier diffusion lengths, ease of fabrication, and low production cost triggered the development of perovskite solar cells (PSCs) at an incredible rate, which soon reached power conversion efficiencies up to the commercialized level. During their evolution process, it has been witnessed that alkali metal cations play a pivotal role in the crystal structure as well as intrinsic properties of hybrid perovskites, thus enabling the unique positioning of the correlated doping strategy in the development history of PSCs in the past decade. Herein, we summarize the growth and progress of the state-of-the-art alkali metal cation (Cs+, Rb+, K+, Na+, Li+) doping in the field of hybrid perovskite-based photovoltaics. To start with, the accurate identification of different alkali metal-occupied locations in the perovskite crystal lattice are discussed in detail with highlighted advanced characterization methods. Beyond that, the location-dependent functions induced by alkali metal doping are intensely focused upon and comprehensively assessed, indicating their versatile and special effects on perovskites in terms of bottleneck issues such as crystallinity modulation, crystal structure stabilization, defect passivation, and ion-migration inhibition. Thereafter, we are committed to analyze their responsible working mechanisms so as to unveil the relationship between occupied locations and crucial roles for each doped cation. The systematical overview and in-depth understanding of the superiorities of such strategies together with their future challenges and prospects would further boost the advancement of perovskite-related fields.