title: Finely Tuned Polymer Interlayers Enhance Solar Cell Efficiency.
abstract: Three 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.
authors.affiliation: Polymer Science and Engineering Department, University of Massachusetts Amherst, 120 Governors Drive, Amherst, MA 01003 (USA).
authors.forename: Yao
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Polymer Science and Engineering Department, University of Massachusetts Amherst, 120 Governors Drive, Amherst, MA 01003 (USA).
authors.forename: Zachariah A
authors.initials: ZA
authors.lastname: Page
authors.affiliation: Polymer Science and Engineering Department, University of Massachusetts Amherst, 120 Governors Drive, Amherst, MA 01003 (USA). russell@mail.pse.umass.edu.
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Russell
authors.affiliation: Polymer Science and Engineering Department, University of Massachusetts Amherst, 120 Governors Drive, Amherst, MA 01003 (USA). tsemrick@mail.pse.umass.edu.
authors.forename: Todd
authors.initials: T
authors.lastname: Emrick
keywords: dipoles; interfacial layers; polymers; power conversion efficiency; solar cells