Finely Tuned Polymer Interlayers Enhance Solar Cell Efficiency

Abstract

AbstractThree conjugated polymer zwitterions (CPZs), containing thiophene‐, diketopyrrolopyrrole‐ (DPP), and naphthalene diimide (NDI) backbones, were synthesized with pendant zwitterions, specifically sulfobetaine groups. Diboronate‐ester‐functionalized bithiophene and benzothiadiazole monomers were copolymerized with zwitterion‐substituted dibromothiophene, DPP, and NDI monomers by A2+B2 Suzuki polymerization. The CPZs were incorporated into polymer solar cells (PSCs) as interlayers between the photoactive layer and Ag cathode. The thiophene‐based CPZs gave power conversion efficiencies (PCEs) of about 5 %, while the narrow‐energy‐gap DPP‐ and NDI‐based CPZs performed exceptionally well, giving PCEs of 9.49 % and 10.19 %, respectively. The interlayer thickness had only a minor impact on the device performance for the DPP‐ and NDI‐CPZs, a finding attributed to their electron‐transport properties. Ultraviolet photoelectron and reflectance spectroscopies, combined with external quantum efficiency measurements, provided structure–property relationships that lend insight into the function of CPZ interlayers in PSCs. NDI‐based CPZ interlayers provide some of the best performing organic solar cells reported to date, and prove useful in conjunction with high‐performing polymer‐active layers and stable, high‐work‐function, metal cathodes.