High work function with reduced phase separation of PSS in metal oxide modified PEDOT:PSS interlayers for organic photovoltaics

Abstract

A simple and effective approach to reduce leakage currents in organic photovoltaics (OPVs) is achieved by doping a commonly used hole transporting interlayer (HTL), poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), PEDOT:PSS, with a metal oxide. The effects of three metal oxide dopants, WO_x, VO_x, and NiO_x, are investigated. From Raman spectra analysis it is found that conversion of the benzoid structure of pristine PEDOT:PSS to a quinoid structure increases after metal oxide doping during thin-film processing. X-ray photoelectron spectroscopy also reveals that metal oxides reduce phase separation between PEDOT⁺ and PSS⁻ polymeric chains, consistent with the decrease in thickness of the insulating phase separated PSS thin-film that forms on the surface of pristine PEDOT:PSS, from 62 Å to 40 ± 5, 57 ± 5, and 55 ± 5 Å, respectively for WO_x, VO_x, and NiO_x incorporation. For the case of WO_x doping, ultraviolet photoelectron spectroscopic depicts work function enhancement from 4.84 eV (PEDOT:PSS) to 5.15 eV (PEDOT:PSS:WO_x). By employing metal oxide incorporated PEDOT:PSS thin films as hole transporting layers in a bulk heterojunction (BHJ) organic photovoltaic (OPV) (poly(3-hexylthiophene) (P3HT) and C₆₀ derivative, [6,6]-phenyl-C₆₀-butyric acid methyl ester (IC₆₀BA)), leakage current decreased from ∼30 × 10⁻⁸ to ∼4 × 10⁻⁸ A cm⁻². The reproducibility of these effects of metal oxide doped PEDOT:PSS on OPV performance is also confirmed in two other BHJs (PTB7:PC₇₀BM and PBDTTT:EFT:PC₇₀BM).