Critical Progress in Quasi‐Planar Heterojunction Organic Solar Cells with Efficiency Over 18%

Abstract In this review, recent progress of the quasi‐planar heterojunction (QPHJ) organic solar cells (OSCs) is summarized from the perspective of device engineering. The foundational theory is introduced to clarify the critical factors behind the high efficiency of QPHJ‐OSCs, encompassing dynamic process dependence on active layers and the commonly employed characterization method. Several innovative approaches are subsequently expounded in detail to tackle key challenges in constructing efficient QPHJ‐OSCs, such as regulating intermolecular penetration between donor and acceptor layers, improving exciton utilization near electrodes, and ameliorating charge transport in donor and acceptor layers. Recent development of emitransparent QPHJ OSCs are introduced, including the superiority of the QPHJ structure and smart strategies for realizing high power conversion efficiency (PCE) and average visible transmittance (AVT) of semitransparent OSCs. Finally, the current challenges and future outlooks for the further optimization of QPHJ‐OSCs are discussed. Serving as a promising alternative, the QPHJ architecture has created new possibilities for the future advancement of OSCs. Through investigations on materials innovation and device engineering, QPHJ‐OSCs are expected to achieve higher PCE and better stability.