Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Abstract

Excited-state energy transfer from spherical green-light-emitting nanocrystals as donor to rod-shaped red-light-emitting nanocrystals as acceptor results in red fluorescence in mixed films. Uniaxial alignment of the quantum rods provides polarized emission, while the absorption remains unpolarized (see figure).

Blends of nanocrystalline zinc oxide nanoparticles (nc-ZnO) and regioregular poly(3-hexylthiophene) (P3HT) processed from solution have been used to construct hybrid polymer–metal oxide bulk-heterojunction solar cells. Thermal annealing of the spin-cast films significantly improves the solar-energy conversion efficiency of these hybrid solar cells to ∼ 0.9 %. Photoluminescence and photoinduced absorption spectroscopy demonstrate that charge-carrier generation is not quantitative, because a fraction of P3HT appears not to be in contact with or in close proximity to ZnO. The coarse morphology of the films, also identified by tapping-mode atomic force microscopy, likely limits the device performance.

We describe a new method towards bulk-heterojunction hybrid polymer solar cells based on composite films of zinc oxide (ZnO) and a conjugated polymer poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV). Spin-coating diethylzinc as a ZnO precursor and MDMO-PPV from a common solvent at 40 % humidity and annealing at 110 °C provides films in which crystalline ZnO is found to be intimately mixed with MDMO-PPV. Photoluminescence and photoinduced spectroscopy demonstrate that photoexcitation of these hybrid composite films results in a fast and long-lived charge transfer from the polymer as a donor to ZnO as ato be obtained n acceptor. Using the ZnO-precursor method, hybrid polymer solar cells have been made with an estimated air-mass of 1.5 (AM 1.5) energy conversion efficiency of 1.1 %. This new method represents a fivefold improved performance compared to similar hybrid polymer solar cells based on amorphous TiO₂.

Electron transfer in stacks: Hydrophobic effects, in combination with the tendency of π-conjugated donor and acceptor chromophores to form a charge-transfer complex (see figure), have been used to create coaggregates of these molecules in water with an alternating face-to-face structure. A fast, sub-picosecond photoinduced electron transfer has been identified in these alternating stacks by pump–probe spectroscopy.

The performance of bulk-heterojunction solar cells based on a phase-separated mixture of donor and acceptor materials is known to be critically dependent on the morphology of the active layer. Here we use a combination of techniques to resolve the morphology of spin cast films of poly(p-phenylene vinylene)/methanofullerene blends in three dimensions on a nanometer scale and relate the results to the performance of the corresponding solar cells. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and depth profiling using dynamic time-of-flight secondary ion mass spectrometry (TOF-SIMS) clearly show that for the two materials used in this study, 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) and poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV), phase separation is not observed up to 50 wt.-% PCBM. Nanoscale phase separation throughout the film sets in for concentrations of more than 67 wt.-% PCBM, to give domains of rather pure PCBM in a homogenous matrix of 50:50 wt.-% MDMO-PPV/PCBM. Electrical characterization, under illumination and in the dark, of the corresponding photovoltaic devices revealed a strong increase of power conversion efficiency when the phase-separated network develops, with a sharp increase of the photocurrent and fill factor between 50 and 67 wt.-% PCBM. As the phase separation sets in, enhanced electron transport and a reduction of bimolecular charge recombination provide the conditions for improved performance. The results are interpreted in terms of a model that proposes a hierarchical build up of two cooperative interpenetrating networks at different length scales.

Summary: Three polymers consisting of alternating perylene bisimide chromophores and flexible polytetrahydrofuran segments of different length have been studied using absorption and (time-resolved) photoluminescence spectroscopy. In o-dichlorobenzene, the chromophores self organize to form H-like aggregates. The photoluminescence spectra of the self-organized polymers consist of vibronically resolved monomeric perylene bisimde fluorescence (λ_max = 538 nm, τ = 3.9 ns) and unstructured excimer-type emission (λ_max = 635 nm, τ = 17 ns). An additional short-lived (τ ≈ 2 ns) luminescence component is observed and ascribed to the dynamic deactivation of the monomeric photoexcited state via excimer formation or energy transfer.

Two new donor–acceptor copolymers that consist of an enantiomerically pure oligo(p-phenylene vinylene) main chain with dangling perylene bisimides have been synthesized by using a Suzuki cross-coupling polymerization. Absorption and circular dichroism spectroscopy revealed that the transition dipole moments of the donor in the main chain and the dangling acceptor moieties of the copolymers are coupled and in a helical orientation in solution, even at elevated temperatures. A strong fluorescence quenching of both chromophores indicates an efficient photoinduced charge transfer after photoexcitation of either donor or acceptor. The formation and recombination kinetics of the charge-separated state were investigated in detail with femtosecond and near-steady-state photoinduced absorption spectroscopy. The charge-separated state forms within 1 ps after excitation, and recombination occurs with a time constant of 45–60 ps, both in solution and in the solid state. These optical characteristics indicate a short distance and appreciable interaction between the electron-rich donor chain and the dangling electron-poor acceptor chromophores.

Wegen der starken Absorption von sichtbarem Licht durch [70]Fulleren-Derivate erreicht man mit ihnen als Elektronenacceptor in Polymer-Dünnschichtsolarzellen hohe Photoströme. Ein Poly(p-phenylenvinylen) mit eingelagertem [70]PCBM erzielt bei der Umwandlung einfallender Photonen in Strom eine Effizienz von 66 %. Der Grund für diese hohen Photoströme ist ein ultraschneller Ladungstransfer zwischen Fulleren und Polymer nach der Photoanregung.

Owing to their increased absorption of light in the visible region, [70]fullerene derivatives provide a high photocurrent when incorporated as the electron acceptor in thin-film polymer photovoltaic cells. An incident-photon-to-current efficiency of 66 % is obtained when [70]PCBM (see picture) is used in combination with a poly(p-phenylene vinylene). These high currents originate from an ultrafast charge transfer that occurs upon photoexcitation of either the fullerene or the polymer.

Bulk-heterojunction photovoltaic cells consisting of a photoactive layer of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and a C₆₀ derivative, (1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene), (PCBM), sandwiched between an indium tin oxide (ITO) anode covered with poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and an aluminum cathode have been analyzed using transmission electron microscopy (TEM) and cryogenic Rutherford backscattering spectrometry (RBS) to assess the structural and elemental composition of these devices. TEM of cross sections of fully processed photovoltaic cells, prepared using a focused ion beam, provide a clear view of the individual layers and their interfaces. RBS shows that during preparation diffusion of indium into the PEDOT:PSS occurs while the diffusion of aluminum into the polymer layers is negligible. An iodinated C₆₀ derivative (I-PCBM) was used to determine the concentration profile of this derivative in the vertical direction of a 100 nm active layer.

Monodisperse stearic acid coated titanium dioxide nanoparticles (< 3 nm) have been prepared and characterized. The nanoparticles are soluble in common apolar organic solvents and bind to carboxylic acid functionalized π-conjugated oligomers to form heterosupramolecular assemblies. When [2,2′;5′,2″]terthiophene-5-carboxylic acid was coupled to the nanoparticles, the fluorescence of the terthiophene moiety of the heterosupramolecular system was completely quenched due to a photoinduced electron transfer from terthiophene to titanium dioxide. Time-resolved fluorescence and Stern–Volmer analysis revealed that the quenching is predominantly static rather than dynamic and occurs in apolar solvents.

Two quaterthiophene–[60]fullerene dyads in which C₆₀ is singly (4TsC) or doubly (4TdC) connected to the inner β-position of the terminal thiophene rings have been synthesized. The electronic properties of these donor–acceptor compounds were analyzed by UV/Vis spectroscopy and cyclic voltammetry, and their photophysical properties in solution and in the solid state by (time-resolved) photoluminescence (PL) and photoinduced absorption (PIA) spectroscopy. Both the flexible and geometrically constrained 4TsC and 4TdC dyads exhibit photoinduced charge transfer from the quaterthiophene to the fullerene in toluene and o-dichlorobenzene (ODCB). In toluene, charge transfer occurs in both dyads by an indirect mechanism, the first step of which is a singlet-energy transfer from the 4T(S₁) state to the C₆₀(S₁) state. In the more polar ODCB, direct electron transfer from 4T(S₁) competes with energy transfer, and both direct and indirect charge transfers are observed. The geometrical fixation of the donor and acceptor chromophores in 4TdC results in rate constants for energy and electron transfer that are more than an order of magnitude larger than those of the flexible 4TsC system. For both dyads, charge recombination is extremely fast, as inferred from picosecond-resolved temporal evolution of the excited state absorption of the 4T^.+ radical cation both in toluene and ODCB.

A novel donor–acceptor–donor molecule consisting of two oligo(p-phenylene vinylene) (OPV4) units attached to a central perylene bisimide (PERY) core is described. This OPV4–PERY–OPV4 is the first mesogenic molecule that incorporates both p- and n-type semiconducting properties and possesses a liquid-crystalline mesophase, in which donor and acceptor functionalities self-assemble into an ordered material. Upon photoexcitation of the donor, a subpicosecond electron-transfer reaction occurs in OPV4–PERY–OPV4, both in solution and in (ordered) thin solid films. The lifetime of the charge-separated state is significantly longer in (ordered) thin films than in solution as a result of a reduction of geminate recombination by migration and spatial separation of charges in the film.

The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n=1–4) and their β,β′-unsubstituted analogues (Tn, n=1–4) are described. Both series are end capped with phenyl groups to prevent irreversible α-coupling reactions during oxidative doping. Absorption and fluorescence spectra of both series reveal a significantly higher degree of intrachain conformational order in the EDOTn oligomers. Oxidation potentials (E_PA1 and E_PA2) determined by cyclic voltammetry reveal that those of EDOTn are significantly lower than the corresponding Tn oligomers as a consequence of the electron-donating 3,4-ethylenedioxy substitution. Linear fits of E_PA1 and E_PA2 versus the reciprocal number of double bonds reveal significantly steeper slopes for the EDOTn than for the Tn oligomers. This could indicate a more effective conjugation for the EDOTn series, confirmed by the fact that coalescence of E_PA1 and E_PA2 is reached already at relatively short chain lengths (≈5 EDOT units) in contrast to the Tn series (>10 thiophene units). The stepwise chemical oxidation of the EDOTn and Tn oligomers in solution was carried out to obtain radical cations and dications. The energies of the optical transitions of the radical cations and dications as determined by UV/Vis/NIR spectroscopy were similar for the two series. These spectroscopic observations are consistent with quantum-chemical calculations performed on the singly charged molecules. Cooling solutions containing T2^.+, T3^.+, EDOT2^.+, and EDOT3^.+ revealed the reversible formation of dimers, albeit with a somewhat different tendency, expressed in the values for the dimerization enthalpy.

We have synthesized three new donor–acceptor-type monomers to achieve soluble and processable low-band gap polymers, 4,7-bis(4-octyl-2-thienyl)-2,1,3-benzothiadiazole (B4TB), 4,7-bis(3-octyl-2-thienyl)-2,1,3-benzothiadiazole (B3TB), and 4-(3-octyl-2-thienyl)-7-(4-octyl-2-thienyl)-2,1,3-benzothiadiazole (B34TB), by the Suzuki coupling reaction. Using B4TB and B3TB, two soluble high molecular weight regioregular head-to-head and tail-to-tail polymers poly[4,7-bis(4-octyl-2-thienyl)-2,1,3- benzothiadiazole] (PB4TB) and poly[4,7-bis(3-octyl-2-thienyl)-2,1,3-benzothiadiazole] (PB3TB) were prepared via iron(III) chloride-mediated oxidative polymerization. The structures of the polymers were confirmed by ¹H and ¹³C NMR, and the molecular weights were determined by size exclusion chromatography. The optical properties (absorbance and fluorescence) of the monomers and polymers were studied and compared with unsubstituted analogues. The monomers and polymers bearing octyl substituents on the thiophene rings pointing away from the benzothiadiazole units (B4TB and PB4TB) possess a more planar structure, and their optical spectra appear redshifted as compared with those having the octyl chain nearer to the benzothiadiazole (B3TB and PB3TB). The optical band gaps of PB3BT (E_g = 2.01 eV) and PB4BT (E_g = 1.96 eV), however, are at much higher energy levels than that of the unsubstituted electrochemically polymerized PBTB material (E_g = 1.1–1.2 eV) as a result of steric effects of the octyl chains. The electrochemical properties of the monomers and polymers were examined using cyclic voltammetry and reflect the effect of alkyl substitution. B4TB and PB4TB were oxidized at a lower potential than B3TB and PB3TB, whereas their reduction potentials were less negative. The electrochemical band gap calculated from the onset of the reduction and oxidation process agreed with the optical band gap calculated from the absorption edges. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 251–261, 2002

Novel alternating conjugated copolymers (P1–P6) consisting of an electron-deficient benzothiadiazole and a variety of electron-rich thiophene-arene-thiophene units were synthesized by palladium-catalyzed polycondensations (Stille and Suzuki reactions), aiming at processable materials with a reduced optical band gap. The structures of P1–P6 were confirmed by ¹H NMR and ¹³C NMR, and their molecular weights were determined by size exclusion chromatography. In the Suzuki polycondensation, the role of the catalyst [Pd(PPh₃)₄ and Pd(OAc)₂] on the resulting molecular weight was investigated. Pd(OAc)₂ enhances the molecular weight of the polymers for both thiophene and phenylene bis-boronic esters as compared with Pd(PPh₃)₄. The optical properties of the polymers were examined in solution and the solid state. The polymers with n-octyl substituents (P1, P4, P5, and P6) on the thiophene rings possessed less-planar structures as a result of torsional steric hindrance, and their absorption spectra appeared blueshifted as compared with their unsubstituted analogues (P2 and P3). The electrochemical properties of the polymers were studied using cyclic voltammetry. Although the alkyl substitution affects the oxidation potential, only marginal differences in the reduction potentials were observed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2360–2372, 2002

Low optical bandgap conjugated polymers may improve the efficiency of organic photovoltaic devices by increasing the absorption in the visible and near infrared region of the solar spectrum. Here we demonstrate that condensation polymerization of 2,5-bis(5-trimethylstannyl-2-thienyl)-N-dodecylpyrrole and 4,7-dibromo-2,1,3-benzothiadiazole in the presence of Pd(PPh₃)₂Cl₂ as a catalyst affords a novel conjugated oligomeric material (PTPTB), which exhibits a low optical bandgap as a result of the alternation of electron-rich and electron-deficient units along the chain. By varying the molar ratio of the monomers in the reaction and fractionation of the reaction product, two different molecular weight fractions (PTPTB-I and PTPTB-II, see Experimental section) were isolated, containing 5–17 and 13–33 aromatic units respectively, as inferred from matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Thin films of PTPTB-I and PTPTB-II exhibit an optical bandgap of 1.60 and 1.46 eV, respectively. Photoinduced absorption (PIA) and photoluminescence spectroscopy of blends of PTPTB-I and a methanofullerene (1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C₆₁, PCBM) gave direct spectral evidence of the photoinduced electron-transfer reaction from PTPTB-I as a donor to the fullerene derivative as an acceptor. Thin PTPTB-I:PCBM composite films were sandwiched between indium tin oxide/poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (ITO/PEDOT:PSS) and Al electrodes to prepare working photovoltaic devices, which show an open circuit voltage of 0.67 V under white-light illumination. The spectral dependence of the device shows an onset of the photocurrent at 1.65 eV (750 nm).

The synthesis and full characterisation of three novel regioregular polythiophenes 8, 9, and 15 is presented. By varying the nature of the substituents on the polythiophene backbone in these materials it is possible to increase their first oxidation potential by almost 0.5 V. Because the first reduction potential follows the same trend, the electrochemical band gap of these polymers is essentially constant. The constant band gap inferred from the electrochemical data is confirmed by the onset of the optical absorption spectrum of the polythiophenes, which shows only small changes.

A combination of the well-defined character of π-conjugated oligomers and the material properties of polymers is displayed by the supramolecular hydrogen-bonded π-conjugated architectures of the title compounds (see scheme). These compounds have been successfully used for the first time in electronic devices.

Eine Kombination aus den wohldefinierten Merkmalen π-konjugierter Oligomere und den Materialeigenschaften von Polymeren weisen die supramolekularen Wasserstoffbrücken-gebundenen π-konjugierten Architekturen der Titelverbindung auf (siehe Bild). Diese Verbindungen wurden erstmals erfolgreich in elektronischen Bauelementen eingesetzt.

The electronic structure of successive redox states of two series of thienylenevinylene oligomers (nTVs, n=4, 5, 6, 8, and 12) that carry hexyl substituents at the α- or β-positions in order to increase the solubility was investigated in detail by means of UV/Vis/near-IR and ESR spectroscopy in solution. The nTV redox states have been fully characterized up to the dication for the shorter oligomers (n≤6) and up to the tetracation for the longer oligomers (n≥8). While the monocation radicals of the nTVs exhibit two dipole-allowed electronic transitions in the Vis/near-IR region, all the higher oxidized states invariably show a single strong absorption in the near-IR region. The electronic transitions of the various oxidized states and those of their vibronic replicas shift to lower energies with increasing conjugation length. The ESR spectra, recorded as a function of the degree of oxidation, provide evidence for the presence of electron spin in the odd-charged and the absence of spin in even-charged cations. Variable-temperature UV/Vis/near-IR and ESR spectroscopy establish that the tendency of the nTV monocation radicals to form spin-less π dimers in solution strongly depends on the number of solubilizing hexyl groups. While the oligomers that carry two hexyl chains at the α-positions of the terminal thiophenes (α-nTVs) readily form π dimers at low temperature, the oligomers that carry hexyl groups on the β- and β′-positions of every thiophene ring (β-nTVs) do not form π dimers. Low-temperature UV/Vis/near-IR and ESR experiments on solutions in which neutral and singly oxidized nTVs are simultaneously present, reveal the occurrence of interchain interactions between these two species, accompanied by a pronounced change in the existing disproportionation equilibrium.