Additive-Modulated Evolution of HC(NH₂)₂PbI₃ Black Polymorph for Mesoscopic Perovskite Solar Cells

Abstract

Formamidinium lead triiodide (HC(NH 2 ) 2 PbI 3 or FAPbI 3 ) is gaining increasing interest in the field of mesoscopic perovskite solar cells (PSCs) for its broader light absorption compared with the more widely studied CH 3 NH 3 PbI 3 (MAPbI 3 ). Since FAPbI 3 has two polymorphs ('black' α-FAPbI 3 and 'yellow' δ-FAPbI 3 ) at ambient temperature, where α-FAPbI 3 is the desirable photoactive perovskite phase, it becomes particularly important to suppress the formation of the non-perovskite δ-FAPbI 3 for achieving high efficiency in FAPbI 3 -based mesoscopic PSCs. In this study, we demonstrate that the judicious use of low-volatility additives in the precursor solution assists in the evolution of α-FAPbI 3 through the formation of non-δ-intermediate phases, which then convert to α-FAPbI 3 during thermal annealing. The underlying mechanism involved in the additive-modulated evolution of α-FAPbI 3 upon mesoporous TiO 2 substrates is elucidated, which suggests guidelines for developing protocols for the fabrication efficient FAPbI 3 -based mesoscopic PSCs.

Thiocyanate assisted performance enhancement of formamidinium based planar perovskite solar cells through a single one-step solution process

“…Some other additives, including HI, NH4Cl, MACl and FACl, have also been reported, among which HI and FACl are also helpful to get pure α-FAPbI3. 18,33 Unlike these fully-volatile additives along with easily-evaporated anions, some SCNfrom NH4SCN additive in our case still remains in the FAPbI3 film after annealing, which may be ascribed to the strong interaction between Pb 2+ and SCN -.…”

Hybrid Perovskites: Effective Crystal Growth for Optoelectronic Applications

“…[132] Increasing interest for FAPbI3 perovskite as an alternative to conventional MAPbI3 perovskite is attribute to the lower optical band-gap (1.43 -1.48 eV) and better thermal stability. [7,154,155] However, the control of phase remains a key issue due to the polymorphic nature of FAPbI3 at ambient; α-FAPbI3 is trigonal perovskite with the band-gap of 1.5 eV and δ-FAPbI3 is non-perovskite phase with the band-gap of 2.48 eV.…”

Combination of Hybrid CVD and Cation Exchange for Upscaling Cs‐Substituted Mixed Cation Perovskite Solar Cells with High Efficiency and Stability

“…At room temperature, FAPbI 3 tends to form the non-photoactive phase (yellow phase) with a bandgap of 2.48 eV other than 1.53 eV in its photoactive perovskite phase (black phase). [34,35] To maintain FAPbI 3 in the black perovskite phase, a mixed perovskite material with 10% FA + substituted by Cs + , that is, FA 0.9 Cs 0.1 PbI 3 has been reported, showing significantly improved phase and device stability. [31] The improved phase stability was interpreted by the optimal effective tolerance factor using mixed cations.…”