Semiconducting Polymer Thin Films Used in Organic Solar Cells: A Scanning Tunneling Microscopy Study

International audienceThe widely used semiconducting polymers poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) (PH1000), poly(3-hexylthiophene-2,5-diyl) (P3HT), and Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) for organic photovoltaic (OPV) cells, are deposited on highly ordered pyrolytic graphite substrates and investigated by scanning tunneling microscopy (STM) under the liquid/solid interface technique. PEDOT:PSS film morphology shows an ellipsoidal shape of PEDOT surrounded by PSS with 4 nm size. STM images of P3HT reveal that films tend to form well-defined crystalline domains with an average interchain distance around 1.41 nm and chain length ≈41 nm. PTB7 chains are self-aggregated by using chlorobenzene as solvent; however, when using 1-phenyloctane with 3% of 1,8-diiodooctane, these chains show a worm-like pattern with a distance of ≈2 nm between backbone chains and a chain length of ≈90 nm. For P3HT and PTB7 based OPVs, local J–V plots are determined from photoconductive atomic force microscopy (pc-AFM) measurements and fitted with the modified Mott−Gurney equation, the reached parameters are correlated with those achieved from the macroscopic J–V graphs. Active films of OPVs cells based on PTB7 are also analyzed by using AFM phase imaging showing domains size of 80–120 nm. This current STM analysis can elucidate some new important insights on polymer film formation and morphology and therefore, their influence on the OPV performance

Nano-films of carbo-benzene derivatives: Scanning probe microscopy analysis and prospects of use in organic solar cells

International audienceThree carbo-mer derivatives based on a C18Ph4 core decorated with two identical electro-active groups X, i.e. two aromatic carbo-benzenes (1 and 2, X = 4-anilinyl) and one pro-aromatic carbo-quinoid (the carbo-TTF 3, X = 1,3-dithiol-2-ylidene) were studied through Scanning Probe Microscopies (SPMs). Self-Assembled Monolayers (SAMs) were fabricated (thickness ~160 pm), for the two centrosymmetric representatives 1 and 3, the organization of which on the HOPG substrate was found to be structure-specific. Electrical/electronic properties of the three carbo-mers were determined by using Atomic Force Microscopy (AFM) and its electrical modes: Kelvin Probe Force Microscopy (KPFM) and conductive Atomic Force Microscopy (c-AFM). Measurements of the work function (∅) through KPFM result in a ∅ = 5.60 eV value for 1, 4.97 eV for 2 and 4.82 eV for 3. Hole mobility (µ) values extracted from local I-V plots by using c-AFM are 15×10- 8cm2V- 1s- 1 for 1, 3×10- 6cm2V-1s-1 for 2 and 87×10- 8cm2V- 1s- 1 for 3. A concept test of the possible application of carbo-mers in self-assembled hole transporting monolayer (SA-HTM), with the view to replacing the most common p-type contact used in organic solar cells (OSCs), PEDOT:PSS, is also reported

Expanding the carbo ‐Benzene Chemical Space for Electron‐Accepting Ability: Trifluorotolyl/Tertiobutyl Substitution Balance

International audienceWith the view to altering the lipophilicity and electron accepting ability of the tetraphenyl-carbo-benzene scaffold, peripheral fluorination of the C18 ring through aromatic linkers was envisaged from the C18Ph6 and o-tBu2C18Ph4 references, by replacement of two Ph substituents with two p-CF3-C6H4 counterparts (FTol). The synthesis relied on a [8+10] macrocyclization involving a common bis(trifluorotolyl)-tetraynedione, followed by reductive aromatization of the resulting [6]pericyclynediols. While p-FTol2C18Ph4 proved to be hardly tractable due to an extremely low solubility, p-FTol2-o-tBu2C18Ph2 could be extensively studied by X-ray crystallography, NMR and UV/Vis spectroscopy, voltammetry, STM imaging of monolayers, and AFM imaging of binary films with P3HT or PC71BM fabricated by spin-coating for organic photovoltaic cells and J−V curve measurement thereof. The electronic and polarity properties are correlated with moderate but consistent electron-withdrawing effects of the CF3 groups, in agreement with the DFT-calculated frontier orbitals and multipole moments. The results provide guidelines for optimization of fluorinated carbo-benzene targets

Core <i>carbo</i>‐mer of an Extended Tetrathiafulvalene:Redox‐Controlled Reversible Conversion to a <i>carbo</i>‐Benzenic Dication

International audienceCarbo -benzene is an aromatic molecule devised by inserting C 2 units within each C-C bond of the benzene molecule. By integrating the corresponding carbo -quinoid core as bridging unit in a p -extended tetrathiafulvalene (exTTF), it is shown that a carbo -benzene ring can be reversibly formed by electrochemical reduction or oxidation. The so-called carbo -exTTF molecule was thus experimentally prepared and studied by UV-visible absorption spectroscopy and cyclic voltammetry, as well as by X-ray crystallography and by scanning tunneling microscopy (STM) on a surface of highly oriented pyrolytic graphite (HOPG). The molecule and its oxidized and reduced forms were subjected to a computational study at the density functional theory (DFT) level, supporting carbo -aromaticity as a driving force for the formation of the dication, radical cation, and radical anion. By allowing co-planarity of the dithiolylidene rings and carbo -quinoidal core, carbo -exTTFs present a promising new class of redox-active systems