Effect of Wetting Surfactants on the Work Function of PEDOT:PSS for Organic Solar Cells

Solution-processable poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a widely used hole transporting layer (HTL) in organic solar cells (OSCs). However, it is a challenge to adopt it as the HTL on hydrophobic active layers to fabricate inverted OSCs. Surfactants are generally added to improve the wettability of aqueous PEDOT:PSS. In this work, we report that surfactants could induce a change in the work function (WF) of PEDOT:PSS and significantly influence the device performance. Fluorine-containing surfactants could deliver higher device performance than fluorine-free surfactants because the fluorine-free surfactants decrease the WF of PEDOT:PSS, which results in poor electrical contact between the WF of PEDOT:PSS and the highest occupied molecular orbital (HOMO) level of the polymer donor of the active layer. With the fluorine-containing surfactants, we deposited aqueous PEDOT:PSS on the hydrophobic active layer with good film quality and high WF and further fabricated inverted OSCs with comparable performance to the reference device with evaporated MoO3 as the HTL.