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

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

Semicrystalline D–A Copolymers with Different Chain Curvature for Applications in Polymer Optoelectronic Devices

Thiophene- and thienothiophene-based donor–acceptor (D–A) type semicrystalline copolymers with different backbone curvatures, <b>PTBT14</b> and <b>PTTBT14</b>, were designed and synthesized. Both the polymers exhibit a nearly planar structure via noncovalent S···O and C–H···N attractive interactions, etc., in the polymer chain. <b>PTTBT14</b> is linear, whereas <b>PTBT14</b> is curved owing to ∼160° bond angle of the thiophene linkage. <b>PTTBT14</b> showed the higher degree of interchain ordering with edge-on orientation, resulting in efficient charge transport (0.26 cm² V^–1 s^–1 for <b>PTTBT14</b> compared to 0.02 cm² V^–1 s^–1 for <b>PTBT14</b>) in PFETs with remarkable morphological stability and no deterioration in device properties at temperatures up to 250 °C. On the other hand, the curved shape of <b>PTBT14</b> attributed to its improved photovoltaic properties with a power conversion efficiency of 5.56%. The linear <b>PTTBT14</b> showed much stronger self-interactions with negligible morphological changes and little miscibility with PC₆₁BM, showing the poor photovoltaic characteristics

Organic Solar Cells Fabricated by One-Step Deposition of a Bulk Heterojunction Mixture and TiO₂/NiO Hole-Collecting Agents

Organic solar cells (OSCs) were fabricated using a one-step deposition of a mixture of NiO nanoparticles, region-regular poly­(3-hexylthiophene) (P3HT), and [6,6]-phenyl-C₆₁-butyric methyl ester (PCBM) without poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS). Although the intended NiO layer was successfully formed at the interface between indium tin oxide (ITO) and the photoactive layer, only a marginal increase in the power conversion efficiency (PCE) of the OSCs (from 0.773 to 1.171%) was found by addition of NiO nanoparticles to the solution of the P3HT/PCBM mixture. Using X-ray photoelectron spectroscopy, it was evidenced that P3HT was oxidized at interfaces of P3HT and NiO, which can decrease the photovoltaic performance of an OSC. Ultrathin TiO₂ wrapping layers (thickness ∼ 2 nm) on the surface of NiO nanoparticles prepared by atomic layer deposition quenched oxidation of P3HT resulted in a significant increase in PCE up to 2.684%. Our result shows that, in OSCs, oxidation of active polymers at oxide/polymer interfaces should be of concern, and a strategy for avoiding such degradation of polymers is required. Fabrication of various core–shell nanostructures as oxide buffers can be useful for quenching the oxidation of active polymers and increasing photovoltaic performances