  The development of scalable 1.25 eV mixed Pb-Sn perovskite solar modules by
blade coating lags behind Pb-based perovskites due to limited understanding of
solution-substrate interaction of the perovskite ink on PEDOT:PSS and
subsequent gas quenching. To address this challenge, we systematically studied
the wet film deposition and quenching process to better understand narrow
bandgap perovskite film formation on PEDOT:PSS. We found, the wetting of Pb-Sn
perovskite ink on PEDOT:PSS is highly unstable over relevant coating time
scales, causing the contact angles to decrease rapidly from 42{\deg} to
16{\deg} within seconds. This instability leads to localized irregularities in
the wet film, resulting in uneven solvent extraction and inhomogeneous nuclei
density. As a result, rough perovskite films with voids at the buried interface
are obtained. To overcome this problem, we developed a quasi-static wetting
process by reducing the blade coating speed, thereby stabilizing the wetting
behavior of Pb-Sn perovskite precursor ink on PEDOT:PSS. This optimized process
facilitates the deposition of high-quality, void-free Pb-Sn perovskite films
with uniform thickness over 8 cm of coating length using moderate (1.4 bar) N2
quenching. We achieved 20 % efficient narrow bandgap perovskite solar cells and
mini-modules with 15.8 % active area efficiency on 15.9 cm2.
