A Coadditive Strategy for Blocking Ionic Mobility in Methylammonium‐Free Perovskite Solar Cells and High‐Stability Achievement

Abstract

Herein, the synthesis of methylammonium‐free and bromide‐free CsxFA1−xPbI3 (FA for formamidinium) perovskite (PVK) photovoltaic layers with intrinsic outstanding properties in terms of crystallinity, defect waiving/passivation, and ionic mobility blocking by using a coadditives approach is described. It consists in mixing two chloride additives in the PVK precursor solution: potassium chloride (KCl) and ammonium chloride (NH4Cl). KCl favors the lead iodide (PbI2) precursor solubilization and results in a purer PVK phase. NH4Cl allows the control of the crystallization growth speed, leading to the formation of large crystal grains and well‐crystallized layers. By the glow‐discharge optical emission spectroscopy (GD‐OES) technique, it is directly visualized that potassium incorporated in the whole film blocks the iodide mobility by defect passivation. Also, the reduction (or suppression) of iodide mobility and the reduction (or suppression) of the J–V curve hysteresis is clearly correlated. It is found that blocking the ionic mobility is insufficient to fully stabilize the halide perovskite material which also requires the crystallization monitoring carried out in parallel by the second additive. It resulted in Cs0.1FA0.9PbI3 cells with a stabilized power conversion efficiency (PCE) of 20.02% and with superior stability.

Controlling the Formation Process of Methylammonium‐Free Halide Perovskite Films for a Homogeneous Incorporation of Alkali Metal Cations Beneficial to Solar Cell Performance

“…Different components can bring desired properties, such as improved optical properties or structural stability. To achieve better performance of PSC, perovskite materials composition has been complexified by combining several A + monovalent elemental and organic cations, from double-cation (e.g., MA-FA, [13,14] Cs/FA, [15,16] Rb/FA, [17] MA + being methylammonium and FA + being formamidinium) to triple-cation (e.g., Cs/MA/ FA, [8,18,19] Rb/MA/FA, [20] K/Cs/FA [21] ), and quadruple-cation (Rb/Cs/MA/FA [18,22] ). The incorporation of other cations, namely MA + , Cs + , Rb + and K + , aims at alleviating the formation of photo inactive δ-FAPbI 3 phase and/or suppress defects in perovskite film.…”

Chlorides, other Halides, and Pseudo‐Halides as Additives for the Fabrication of Efficient and Stable Perovskite Solar Cells

“…In inorganic chloride additives, the cation includes alkali metal (K + , Na + , Li + ), [50] Pb 2+ , Mn 2+ , [44] and ammonia species NH 4 + [50] . Normally, inorganic chloride additives present in the PVK precursor solution increase the grain size as well as enhance the grain orientation and finally often increase the PCE of the device.…”

“…Zheng et al. have clearly shown that K + from KCl suppresses iodide mobility in PVK and linked it to J ‐ V curve hysteresis cancellation [50] …”

“…Under the condition of ambient air, the PSCs maintained 96 % of the initial efficiency after 576 h due to the triple mesoporous scaffold [36] and PVK material quality. Pauporté and co‐workers [50] demonstrated the benefit of employing NH 4 Cl additive combined with KCl for the preparation of methylammonium‐free bromide‐free Cs 0.1 FA 0.9 PbI 3 perovskite layers. They showed that KCl favors the PbI 2 precursor solubilization, and it resulted in pure perovskite phase.…”

“…Chloride additives have been employed to passivate the charged defects of the surface and grain boundaries as well as to control of crystallization and formation of PVK films for attaining efficient and stable photovoltaics. K + counter cation passivates trap states caused by defects and blocks the iodide mobility [50] . Many groups have investigated the possible mechanism of chloride additives engineered PVK crystallization process.…”