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

Herein, the synthesis of methylammonium-free and bromide-free Cs(x)FA(1-x)Pbl(3) (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 (Pbl(2)) precursor solubilization and results in a purer PVK phase. NH4Cl allows the control ofthe 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 -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 Cs(0.1)FA(0.9)Pbl(3) cells with a stabilized power conversion efficiency (PCE) of 20.02% and with superior stability.

Automated summary

The synthesis of Cs(x)FA(1-x)Pbl(3) perovskite photovoltaic layers without methylammonium and bromide, using a coadditives approach with potassium chloride and ammonium chloride additives, results in enhanced crystallinity and stability. The addition of KCl and NH4Cl plays a key role in producing a pure PVK phase and controlling ionic mobility, leading to improved performance in Cs(0.1)FA(0.9)Pbl(3) cells with stabilized power conversion efficiency of 20.02%.