Easily Attainable Phenothiazine-Based Polymers for Polymer Solar Cells: Advantage of Insertion of <i>S</i>,<i>S</i>-dioxides into its Polymer for Inverted Structure Solar Cells

Abstract

Two donor– (D−) acceptor (A) type polymers based on a soluble chromophore of phenothiazine (PT) unit that is a tricyclic nitrogen–sulfur heterocycle, have been synthesized by introducing an electron-deficient benzothiadiazole (BT) building block copolymerized with either PT or phenothiazine-<i>S</i>,<i>S</i>-dioxide (PT-<i>SS</i>) unit as an oxidized form of PT. The resulting polymers, <b>PPTDTBT</b> and <b>PPTDTBT-</b><i><b>SS</b></i> are fully characterized by UV–vis absorption, electrochemical cyclic voltammetry, X-ray diffraction (XRD), and DFT theoretical calculations. We find that the maximum absorption of <b>PPTDTBT</b> is not only markedly red-shifted with respect to that of <b>PPTDTBT-</b><i><b>SS</b></i> but also its band gap as well as molecular energy levels are readily tuned by the insertion of <i>S</i>,<i>S</i>-dioxides into the polymer. The main interest is focused on the electronic applications of the two polymers in organic field-effect transistors (OFETs) as well as conventional and inverted polymeric solar cells (PSCs). <b>PPTDTBT</b> is a typical p-type polymer semiconductor for OFETs and conventional PSCs based on this polymer and PC₇₁BM show a power conversion efficiency (PCE) of 1.69%. In case of <b>PPTDTBT-</b><i><b>SS</b></i>, the devices characteristics result in: (i) 1 order of magnitude higher hole mobility (μ = 6.9 × 10^–4 cm² V^–1 s^–1) than that obtained with <b>PPTDTBT</b> and (ii) improved performance of the inverted PSCs (1.22%), compared to its conventional devices. Such positive features can be accounted for in terms of closer packing molecular characteristics owing either to the effects of dipolar intermolecular interactions orientated from the sulfonyl groups or the relatively high coplanarity of <b>PPTDTBT-</b><i><b>SS</b></i> backbone