Compared with inverted 3D perovskite solar cell (PSCs), inverted quasi‐2D PSCs have advantages in device stability, but the device efficiency is still lagging behind. Constructing polymer hole‐transporting materials (HTMs) with passivation functions to improve the buried interface and crystallization properties of perovskite films is one of the effective strategies to improve the performance of inverted quasi‐2D PSCs. Herein, two novel side‐chain functionalized polymer HTMs containing methylthio‐based passivation groups are designed, named PVCz‐SMeTPA and PVCz‐SMeDAD, for inverted quasi‐2D PSCs. Benefited from the non‐conjugated flexible backbone bearing functionalized side‐chain groups, the polymer HTMs exhibit excellent film‐forming properties, well‐matched energy levels and improved charge mobility, which facilitates the charge extraction and transport between HTM and quasi‐2D perovskite layer. More importantly, by introducing methylthio units, the polymer HTMs can enhance the contact and interactions with quasi‐2D perovskite, and further passivating the buried interface defects and assisting the deposition of high‐quality perovskite. Due to the suppressed interfacial non‐radiative recombination, the inverted quasi‐2D PSCs using PVCz‐SMeTPA and PVCz‐SMeDAD achieve impressive power conversion efficiency (PCE) of 21.41% and 20.63% with open‐circuit voltage of 1.23 and 1.22 V, respectively. Furthermore, the PVCz‐SMeTPA based inverted quasi‐2D PSCs also exhibits negligible hysteresis and considerably improved thermal and long‐term stability.

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