Organic-inorganic metal halide perovskite semiconductors have become one of the most promising photovoltaic materials. They can be used as the main absorber layer in solar cells or in tandem with silicon solar cells, boosting overall efficiency. However, these compounds have many challenges, including current-voltage hysteresis and their susceptibility to degradation under light, moisture, air, and electron beams. In an effort to address these challenges, many mixed halide perovskites compositions have been considered with the alloying of inorganic metals (Cs and Rb) and organic components (methylammonium (MA) and formamidinium (FA)) for the cation A sites and different halides (I, Br and Cl) for the anion X sites.
The addition of chlorine anions (Cl-) to the perovskite mixture has been shown to improve the morphology, crystallinity, grain size, and optoelectronic properties as well as the stability of the perovskite solar cells (PSCs). [ 1, 2, 3] PSC devices using alkylammonium chlorides (RACl) added to FAPbI 3 have reached a record of 26.08% (certified 25.73%) efficiency under standard illumination. [ 3] Recently, PSC devices using templated FA 0.9Cs 0.1PbI 3–xCl x demonstrate significant enhancements, achieving a steady-state power conversion efficiency exceeding 19.8%. [ 4] Inserting a templating layer between the perovskite film and the substrate produced highly oriented co-evaporated perovskite films with consistent morphology, structure, and optoelectronic properties across various materials. [ 4] However, the underlying mechanisms of how Cl- anions and templating layer affect the morphology, grain orientation, structure, composition, defects, and improve the optical properties of the perovskite photo-absorber are still unclear.
In this study, we systematically investigated the impact of Cs + cations and Cl- anions upon the morphology, grain boundary, grain orientation, crystal structure, defects, chemical composition, and optical properties of FAPbI 3-based PSCs ( Fig.1).
Fig. 1.
(A) Schematic diagram for studying microstructure of FAPbI 3-based perovskites. (B) Low-dose low-angle annular dark field (LAADF) STEM image of FA 0.9Cs 0.1PbI 3–xCl x perovskite film is filtered with a Bragg filter to effectively enhance contrast.
