Abstract Interfacial engineering is a vital strategy to enable high‐performance perovskite solar cells (PSCs). To develop efficient, low‐cost, and green biomass interfacial materials, here, a bifunctional cellulose derivative is presented, 6‐O‐[4‐(9H‐carbazol‐9‐yl)butyl]‐2,3‐di‐O‐methyl cellulose (C‐Cz), with numerous methoxy groups on the backbone and redox‐active carbazole units as side chains. The bifunctional C‐Cz shows excellent energy level alignment, good thermal stability and strong interactions with the perovskite surface, all of which are critical for not only carrier transportation but also potential defects passivation. Consequently, with C‐Cz as the interfacial modifier, the PSCs achieve a remarkably enhanced power conversion efficiency (PCE) of 23.02%, along with significantly enhanced long‐term stability. These results underscore the advantages of bifunctional cellulose materials as interfacial layers with effective charge transport properties and strong passivation capability for efficient and stable PSCs.
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