Directed assembly of cathode interlayers enables efficient binary organic solar cells

Cathode interlayers create interfacial dipoles in organic solar cells to maximize device efficiency. However, the dipoles are highly dependent on the molecular orientation of the interlayers, which remain challenging to be controlled. Here, based on the amphiphilic nature of the interlayers, we show a stepwise film deposition approach using a polar solvent for active layer pretreatment and a bipolar solvent for subsequent interlayer self assembly. Sum-frequency generation vibrational spectroscopy reveals a preferred perpendicular orientation of the interlayer molecules, thereby enhancing the surface potential and reducing the charge transport resistance. As a result, we achieve a certified efficiency of 20.65% and fill factor of 82.7% in binary organic solar cells. This method is universally applicable to different cathode interlayers and active layers. Our work highlights the importance of controlling interlayer molecular orientation for unlocking significant performance gains, paving the way for further advancements in device optimization.