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Three-dimensional organic–inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties^,,,,, which have led to power conversion efficiencies exceeding 20 per cent^,, with the prospect of further improvements towards the Shockley–Queisser limit for a single‐junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions^,,,,,,. In contrast to their three-dimensional counterparts, Ruddlesden–Popper phases—layered two-dimensional perovskite films—have shown promising stability, but poor efficiency at only 4.73 per cent^,,. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting …