Finely Tuned Polymer Interlayers Enhance Solar Cell Efficiency

Three conjugated polymer zwitterions (CPZs), containing thiophene-, diketopyrrolopyrrole- (DPP), and naphthalene diimide (NDI) backbones, were synthesized with pendant zwitterions, specifically sulfobetaine groups. Diboronateester-functionalized bithiophene and benzothiadiazole monomers were copolymerized with zwitterion-substituted dibromothiophene, DPP, and NDI monomers by A(2)+B-2 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.