A Donor–Acceptor Polymer with a Peculiar Negative‐Charge‐“Trapping” Characteristic

Voltammetric and spectrophotometric measurements of poly(3,3″‐dipentoxy‐3′‐dicyanoethenyl‐2,2′:5′,2″‐terthiophene) (polyCN) films, in connection with other experimental evidence, reveal a normal oxidative, but a peculiar reductive behavior consisting of trapping of the negative charge during the cathodic scan. Another interesting property of polyCN films is the tendency to form strong intramolecular and intermolecular associations, probably charge‐transfer (CT) complexes. These properties could account for the fact that the photovoltaic performance does not improve when polyCN is blended with a polythiophene donor.     

Synthesis of 2,7‐Carbazolenevinylene‐Based Copolymers and Characterization of Their Photovoltaic Properties

New electroactive and photoactive conjugated copolymers consisting of alternating 2,7‐carbazole and oligothiophene moieties linked by vinylene groups have been developed. Different oligothiophene units have been introduced to study the relationship between the polymer structure and the electronic properties. The resulting copolymers are characterized by UV‐vis spectroscopy, size‐exclusion chromatography, and thermal and electrochemical analyses. Bulk heterojunction photovoltaic cells from different copolymers and a soluble fullerene derivative, [6,6]‐phenyl‐C₆₁ butyric acid methyl ester, have been fabricated, and promising preliminary results are obtained. For instance, non‐optimized devices using poly(N‐(4‐octyloxyphenyl)‐2,7‐carbazolenevinylene‐alt‐3″,4″‐dihexyl‐2,2′;5′,2″;5″,2″′;5″′,2″″‐quinquethiophenevinylene 1″,1″‐dioxide) as an absorbing and hole‐carrier semiconductor exhibit power conversion efficiency up to 0.8 % under air mass (AM) 1.5 illumination. These features make 2,7‐carbazolenevinylene‐based and related polymers attractive candidates for solar‐cell applications.     

Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells

We investigate thin poly(3‐hexylthiophene‐2,5‐diyl)/[6,6]‐phenyl C₆₁ butyric acid methyl ester (P3HT/PCBM) films, which are widely used as active layers in plastic solar cells. Their structural properties are studied by grazing‐incidence X‐ray diffraction (XRD). The size and the orientation of crystalline P3HT nanodomains within the films are determined. PCBM crystallites are not detected in thin films by XRD. Upon annealing, the P3HT crystallinity increases, leading to an increase in the optical absorption and spectral photocurrent in the low‐photon‐energy region. As a consequence, the efficiency of P3HT/PCBM solar cells is significantly increased. A direct relation between efficiency and P3HT crystallinity is demonstrated.     

ELECTROCHEMICAL COPOLYMERIZATION OF DIBENZOFURAN AND 3-METHYLTHIOPHENE IN BORON TRIFLUORIDE DIETHYL ETHERATE

The copolymerization of dibenzofuran(DBF)and 3-methylthiophene(MET)was successfully achieved electrochemically by direct anodic oxidation of the monomer mixtures in boron trifluoride diethyl etherate.The effects of applied polymerization potential and the monomer concentration ratios on the copolymerization were investigated by linear sweep voltammetry and cyclic voltammetry(CV).The structure of copolymer films were investigated by UV-Vis,infrared spectroscopy,thermal analysis.As-formed novel copolymers ...     

High Molecular Weights,Polydispersities, and Annealing Temperatures in the Optimization of Bulk‐Heterojunction Photovoltaic Cells Based on Poly(3‐hexylthiophene) or Poly(3‐butylthiophene)

A series of poly(3‐hexylthiophene)s (P3HTs) and poly(3‐butylthiophene)s (P3BTs) with predetermined molecular weights and varying polydispersities are prepared using a simplified Grignard metathesis chain‐growth polymerization. Techniques were elaborated to prepare extremely high molecular weight P3HT (number‐average molecular weight of around 280 000 g mol^–1) with a low polydispersity (< 1.1) without resorting to fractionation. Optimization of the annealing of a series of solar cells based on blends of poly(3‐alkylthiophene)s (P3ATs) and [6,6]‐phenyl C₆₁ butyric acid methyl ester indicates that the polydispersities, molecular weights, and degrees of conjugation of the P3ATs all have an important impact not only on cell characteristics but also on the most effective annealing temperature required. The results indicate that each cell requires annealing treatments specific to the type of polymer and its molecular weight distribution.     

Preparation and optical characterization of photosensitive multilayered structures based on polythiophenes and tetracyanoquinodimethane

Organic double layers obtained by electrodeposition of a substituted polythiophene followed by spin-coating of a Tetracyanoquinodimethane (TCNQ) electron acceptor layer, are characterized by means of spectrophotometric measurements carried out in the UV-VIS-IR range. According to the optical data, the electron acceptor molecules reduce to the singly ionized state as a consequence of a charge-transfer reaction that takes place at the interface with the electrodeposited polymer, which acts as an electron donor. Rectifying heterojunctions are obtained by depositing the double layers on the top of monocrystalline silicon wafers, and on the top of monocrystalline silicon wafers coated with electrochemically de-doped poly(3,4-ethylendioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). Current versus voltage measurements performed in the dark and under halogen light irradiation show that the developed heterojunctions exhibit photosensitive charge transport features, promising in view of photovoltaic applications.     

Effect of Molecular Weight on the Structure and Morphology of Oriented Thin Films of Regioregular Poly(3‐hexylthiophene) Grown by Directional Epitaxial Solidification

Regioregular head‐to‐tail (HT)‐coupled poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) with a weight‐average molecular weight (M_w) in the 7.3–69.6 kDa range is crystallized by directional epitaxial solidification in 1,3,5‐trichlorobenzene (TCB) to yield highly oriented thin films. An oriented and periodic lamellar structure consisting of crystalline lamellae separated by amorphous interlamellar zones is evidenced by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both the overall crystallinity as well as the orientation of the crystalline lamellae decrease significantly with increasing M_w. The total lamellar periodicity is close to the length of “fully extended” chains for M_w = 7.3 kDa (polystyrene‐equivalent molecular weight, eq. PS) and it saturates to a value of ca. (25–28) ± 2 nm for M_w ≥ 18.8 kDa (eq. PS). This behavior is attributed to a transition from an oligomeric‐like system, for which P3HT chains are essentially in a fully extended all‐trans conformation and do not fold, to a semicrystalline system that involves a periodic alternation of crystalline lamellae separated by extended amorphous interlamellar zones, which harbor chain folds, chain ends, and tie molecules. For P3HT with M_w of ca. 7.3 kDa (eq. PS), epitaxial crystallization on TCB allows for the growth of both “edge‐on” and “flat‐on” oriented crystalline lamellae on the TCB substrate. The orientation of the lamellae is attributed to 1D epitaxy. Because of the large size of the “flat‐on” crystalline lamellae, a characteristic single‐crystal electron diffraction pattern corresponding to the [001] zone was obtained by selected area electron diffraction (SAED), indicating that P3HT crystallizes in a monoclinic unit cell with a = 16.0 Å, b = 7.8 Å, c = 7.8 Å, and γ = 93.5°.     

Organic Thin‐Film Photovoltaic Cells Based on Oligothiophenes with Reduced Bandgap

The best polymeric solar cells reported so far are based on a so‐called bulk heterojunction of a polythiophene as donor and a soluble fullerene derivative as acceptor. However, these cells still suffer from an unsatisfying photovoltage, typically below 0.7 V. Here, we show that we can achieve higher photovoltages using a new terthiophene end‐capped with electron withdrawing dicyanovinyl groups (DCV3T) that increase both the ionization energy and even more strongly the electron affinity of the compound. The new material is tested in cells using a photoactive heterojunction to separate the excitons generated in the oligomer and a p‐doped wide‐gap transport layer. The solar cells show an open circuit voltage of up to 1.04 V and a broad spectral sensitivity band ranging from 420 nm to 650 nm. Solar cells based on such oligothiophenes are promising candidates for stacked organic solar cells tailored to the sun‐spectrum. Moreover, we present first examples of a new concept for organic solar cells: By blending DCV3T with fullerene C₆₀, an enhanced generation of triplet excitons on the oligomer can be achieved via a back and forth transfer of excitons (ping‐pong‐effect).     

Benzodithiophene Copolymer—A Low‐Temperature,Solution‐Processed High‐Performance Semiconductor for Thin‐Film Transistors

Poly(4,8‐didodecyl‐2,6‐bis‐(3‐methylthiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene) self‐assembled on appropriate substrates from solution and formed highly structured thin films at low temperatures. As an as‐prepared thin‐film semiconductor without thermal annealing, it exhibited excellent field‐effect transistor properties with mobility of ～ 0.15 cm² V^–1 s^–1 in thin‐film transistors.     

Evolution in crystal structure and electrical performance of thiophene-based polymer field effect transistors: A remarkable difference between thermal and solvent vapor annealing

We report the various conformational structures of long pendant side-chains, and the effects of thermal and solvent vapor annealing (SVA) with the corresponding charge carrier mobilities of thiophene-based conjugated polymers, poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT-12) and poly(4,4′-bis-decyloxymethylquaterthiophene) (POQT), by correlated study of their extraordinary polymorphic crystal structures. In substitution for alkyl chains in polythiophenes, ether alkyl chains induce a favorable non-covalent interaction between the oxygen and sulfur atoms and help the polymer chains planar with lower torsion angles between conjugated backbone units showing a reduced π–π stacking distance. However, the flexibility and conformational freedom with such long side-chains dominantly induce polymorphic crystallites from bent and extended side-chains. Especially, POQT exhibit two polymorphic crystallite phases in a similar ratio probably due to the increased freedom of ether alkyl chains. Therefore, the field effect mobility of POQT is decreased gradually with the increase of annealing temperature from 0.024 (at 80 °C) to 3.96 × 10⁻⁴ cm²/V s (at 170 °C). Contrary to the thermal annealing method, solvent-vapor-annealed POQT films show highly ordered and single-phase crystallites with edge-on orientation to the substrate, which ultimately provides an effectively improved charge carrier mobility from 0.023 (pristine) to 0.076 cm²/V s after adequate solvent vapor exposure.