Highly conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) films treated with an amphiphilic fluoro compound as the transparent electrode of polymer solar cells

Abstract

Flexible transparent electrode materials are strongly needed for optoelectronic devices. We report a novel method to significantly enhance the conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films through treatment with a fluoro compound, hexafluoroacetone (HFA). HFA hydrolyzes with water into a geminal diol, 1,1,1,3,3,3-hexafluoropropane-2,2-diol (HFP2OH) that has two –OH groups connected to the middle carbon atom. The conductivity increased from 0.3 to 1164 and 1325 S cm⁻¹ after the treatment with HFA once and four times, respectively. The highly conductive HFA-treated PEDOT:PSS films can have a sheet resistances of 46 Ω □⁻¹ and a transparency of around 83% at 550 nm. These values are comparable to those of indium tin oxide (ITO) on polyethylene terephthalate (PET). The conductivity enhancement is attributed to the HFP2OH-induced phase segregation of some hydrophilic PSSH chains from PEDOT:PSS and the conformational change of the conductive PEDOT chains, driven by the interactions between amphiphilic HFP2OH and PEDOT:PSS. The hydrophobic –CF₃groups of HFP2OH preferentially interact with the hydrophobic PEDOT chains of PEDOT:PSS, while the hydrophilic –OH groups preferentially interact with hydrophilic PSS chains. The highly conductive PEDOT:PSS films were used to replace ITO as the transparent anode of polymer solar cells. Polymer solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) exhibited a photovoltaic efficiency of 3.57% under simulated AM1.5G illumination, comparable to the control devices with ITO as the anode.