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• Multifunctional interlayer for highly stable and efficient P3HT-based PSCs. • Simultaneously passivating the defects of perovskite and improving the interface contaction and charge extraction. • Inducing the oriented self-assembly of P3HT and promoting efficient charge transfer. • Enhancing the thermal and moisture resistance of the perovskite film. Poly (3-hexylthiophene) (P3HT) has been recognized to be a potential hole-transport material (HTM) for stable and efficient perovskite solar cells (PSCs) due to its low cost, high hole mobility, and solution processability. However, the uncontrolled configuration of the polymer chains and the poor contact on the perovskite film restrain the charge extraction and transport in the devices. Here, we demonstrate a simple and versatile strategy for the fabrication of stable and efficient P3HT-based PSCs, using 1-hexyl-2,5-dimethyl-1H-pyrrole-3-carboxylic acid (HPCA) as interlayer between perovskite and P3HT layers. The carboxyl group in HPCA can effectively suppress anion-vacancy defects at the grain boundaries and surface of the perovskite film, reducing the trap state density and improving the carrier lifetimes. Moreover, the peripheral hexyl-pyrrole in HPCA can improve the interface contact between the perovskite and P3HT, and regulate P3HT orientation and improve charge extraction and transport in the device. As a result, the PSC using the dopant-free P3HT HTM yields a power conversion efficiency (PCE) of 20.8%, along with superior stability at a relative humidity of 60%, heat at 80 °C, or under continuous light illumination. [ABSTRACT FROM AUTHOR]