Planar heterojunction perovskite solar cells (PSCs) provide great potential for fabricating high‐efficiency, low‐cost, large‐area, and flexible photovoltaic devices. In planar PSCs, a perovskite absorber is sandwiched between hole and electron transport materials. The charge‐transporting interlayers play an important role in enhancing charge extraction, transport, and collection. Organic interlayers including small molecules and polymers offer great advantages for their tunable chemical/electronic structures and low‐temperature solution processibility. Here, recent progress of organic interlayers in planar heterojunction PSCs is discussed, and the effect of chemical structures on device performance is also illuminated. Finally, the main challenges in developing planar heterojunction PSCs based on organic interlayers are identified, and strategies for enhancing the device performance are also proposed.

This review provides an up‐to‐date review about the small molecule interlayers (SMIs) in organic solar cells (OSCs). Compared to polymer interlayers, SMIs exhibit intrinsic advantages such as easy synthesis and purification, monodispersity, well‐defined chemical structure, and high batch‐to‐batch reproducibility. Recently, various SMIs have been reported with landmark efficiencies of over 10% in both conventional and inverted OSCs, exhibiting promising potential in commercial application. In this review, the authors summarize the progress of SMIs from a device fabrication point of view, paying particular attention to the material categories, molecular design, preparation process, and applicable device structure. In addition, the working mechanisms of different SMIs are also discussed, including the structure–property relationships and the corresponding impact on device performance. Finally, a brief outlook is provided that includes opportunities and challenges in this emerging area.