Tailoring Self‐Assembled Monolayers for High‐Performance Polymer Solar Cells with Improved Stability

Self‐assembly monolayers (SAMs) of small molecules are attractive alternatives of traditional transporting materials to reconcile interfaces with tunable interface properties in polymer solar cells (PSCs). Herein, it is found that benzylphosphonic acid (BnPA)/pentafluorobenzylphosphonic acid (F 5 BnPA) mixture could form an ordered SAM on indium tin oxide (ITO) due to the strong arene–perfluoroarene interaction, thus the hole‐transport‐layer‐free PSCs based on poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐fluoro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]‐dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PM6): BO‐4Cl achieves a power conversion efficiency (PCE) of 18.0%. The high performance is attributed to the improved energy level alignment, excellent carrier‐extraction ability, and reduced recombination. The device also shows much better stability compared with the devices based on BnPA‐ or F 5 BnPA‐modified ITO, and shows comparable stability to the device based on ITO/poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Furthermore, the device with an area of 1.05 cm 2 shows a PCE of 15.3%, which is among the highest reported values. Herein, the potential of SAMs is highlighted for highly stable and high‐performance PSCs toward commercialization.