Conjugated Thiophene-Containing Polymer Zwitterions: Direct Synthesis and Thin Film Electronic Properties

We report a direct and facile synthesis of novel conjugated polymeric zwitterions (CPZs) as a simple route to electronically active homopolymers and copolymers containing dipole-inducing pendent zwitterions. Sulfobetaine-containing polythiophenes (<b>PTSB-1</b> and <b>PTSB-2</b>) and alternating thiophene–benzothiadiazoles (<b>PTBTSB-1</b> and <b>PTBTSB-2</b>) were prepared and characterized relative to alkylated polymer analogues (<b>POT-</b><i><b>a</b></i><b>-T</b> and <b>POT-</b><i><b>a</b></i><b>-BT</b>). The polar zwitterionic side chains make these polymers hydrophilic and salt-responsive, with interesting electronic properties that depend on zwitterion distance from the conjugated polymer backbone (tether length), as characterized by UV–vis absorption and ultraviolet photoelectron spectroscopy (UPS). Close proximity (CH₂ spacer) of the sulfobetaine groups to the polymer backbone results in increased ionization potential and enlarged band gaps of 2.19 and 2.04 eV for <b>PTSB-1</b> and <b>PTBTSB-1</b>, respectively. On Au and Ag surfaces, the zwitterionic pendent groups significantly alter the work function due to the presence of an interfacial dipole, with the largest interfacial dipoles measuring −1.29 eV (<b>PTBTSB-1</b>/Au) and −0.69 eV (<b>PTBTSB-1</b>/Ag)

Bulk Charge Carrier Transport in Push–Pull Type Organic Semiconductor

Operation of organic electronic and optoelectronic devices relies on charge transport properties of active layer materials. The magnitude of charge carrier mobility, a key efficiency metrics of charge transport properties, is determined by the chemical structure of molecular units and their crystallographic packing motifs, as well as strongly depends on the film fabrication approaches that produce films with different degrees of anisotropy and structural order. Probed by the time-of-flight and grazing incidence X-ray diffraction techniques, bulk charge carrier transport, molecular packing, and film morphology in different structural phases of push–pull type organic semiconductor, 7,7′-(4,4-bis­(2-ethylhexyl)-4H-silolo­[3,2-b:4,5-b′]­dithiophene-2,6-diyl)­bis­(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5yl)­benzo­[c]­[1,2,5] thiadiazole), one of the most efficient small-molecule photovoltaic materials to-date, are described herein. In the isotropic phase, the material is ambipolar with high mobilities for a fluid state. The electron and hole mobilities at the phase onset at 210.78 °C are 1.0 × 10^–3 cm²/(V s) and 6.5 × 10^–4 cm²/(V s), respectively. Analysis of the temperature and electric field dependences of the mobilities in the framework of Gaussian disorder formalism suggests larger energetic and positional disorder for electron transport sites. Below 210 °C, crystallization into a polycrystalline film with a triclinic unit cell symmetry and high degree of anisotropy leads to a 10-fold increase of hole mobility. The mobility is limited by the charge transfer along the direction of branched alkyl side chains. Below 90 °C, faster cooling rates produce even higher hole mobilities up to 2 × 10^–2 cm²/(V s) at 25 °C because of the more isotropic orientations of crystalline domains. These properties facilitate in understanding efficient material performance in photovoltaic devices and will guide further development of materials and devices

N‑Doped Zwitterionic Fullerenes as Interlayers in Organic and Perovskite Photovoltaic Devices

The efficient operation of polymer- and perovskite-based photovoltaic devices depends on selective charge extraction layers that are placed between the active layer and electrodes. Herein, we demonstrate that integration of a tetra-<i>n</i>-butyl ammonium iodide-doped zwitterionic fulleropyrrolidine derivative, C₆₀-SB, as a cathode modification interlayer significantly improves the photovoltaic device performance. Compared to the intrinsic (undoped) zwitterionic material, which is an efficient interlayer itself, the doped interlayers further improve average power conversion efficiencies from 8.37% to 9.68% in polymer-based devices and from 12.53% to 15.31% in perovskite-based devices. Ultraviolet photoelectron spectroscopy revealed that doping increases the interfacial dipole at the C₆₀-SB/Ag interface, i.e., reduces the effective work function of the resultant composite cathode. This effect originates from the population of negative polaron states in C₆₀-SB by extrinsic charges that prevent directional charge transfer from Ag to the integer charge-transfer states in C₆₀-SB, pinning the Fermi level at higher energy. The reduced resistivity of the doped interlayer, as measured by impedance spectroscopy, enables efficient device operation with a broad range of interlayer thicknesses, thus simplifying the solution-based device fabrication process

Crystallinity and Morphology Effects on a Solvent-Processed Solar Cell Using a Triarylamine-Substituted Squaraine

2,4-Bis­[4′-(<i><i>N,N</i></i>-di­(4″-hydroxyphenyl)­amino)-2′,6′-dihydroxyphenyl]­squaraine (Sq-TAA-OH, optical bandgap 1.4 eV, HOMO level −5.3 eV by ultraviolet photoelectron spectroscopy) is used as an active layer material in solution processed, bulk-heterojunction organic photovoltaic cells with configuration ITO/PEDOT:PSS/Sq-TAA-OH:PC₇₁BM/LiF/Al. Power conversion efficiencies (PCEs) up to 4.8% are obtained by a well-reproducible procedure using a mixture of good and poor Sq-TAA-OH solubilizing organic solvents, with diiodooctane (DIO) additive to make a bulk heterojunction layer, followed by thermal annealing, to give optimized <i>V</i>_OC = 0.84–0.86 V, <i>J</i>_SC = 10 mA cm^–2, and FF = 0.53. X-ray diffraction and scattering studies of pristine, pure Sq-TAA-OH solution-cast films show <i>d</i>-spacing features similar to single-crystal packing and spacing. The DIO additive in a good solvent/poor solvent mixture apparently broadens the size distribution of Sq-TAA-OH crystallites in pristine films, but thermal annealing provides a narrower size distribution. Direct X-ray diffraction and scattering morphological studies of “as-fabricated” active layers show improved Sq-TAA-OH/PC₇₁BM phase separation and formation of crystallites, ∼48 nm in size, under conditions that give the best PCE