Research by EPFL shows that a new engineered passivator, 4‐tert‐butyl‐benzylammonium iodide (tBBAI), has bulky tert‐butyl groups that prevent unwanted aggregation by steric repulsion. It was found that simple surface treatment with tBBAI significantly accelerates the charge extraction from the perovskite into the spiro‐OMeTAD hole‐transporter, while retarding the nonradiative charge carrier recombination.

This boosts the power conversion efficiency (PCE) of the PSC from ≈20% to 23.5%, reducing the hysteresis to barely detectable levels.

Importantly, the tBBAI treatment raises the fill factor from 0.75 to the very high value of 0.82, which concurs with a decrease in the ideality factor from 1.72 to 1.34, confirming the suppression of radiation‐less carrier recombination.

The tert‐butyl group also provides a hydrophobic umbrella protecting the perovskite film from attack by ambient moisture. As a result, the PSCs show excellent operational stability retaining over 95% of their initial PCE after 500 h full‐sun illumination under maximum‐power‐point tracking under continuous simulated solar irradiation.