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Two donor–acceptor copolymers containing perylene diimide and oligothiophenes linked through triple bonds have been designed and synthesized by palladium catalyzed Sonogashira coupling reaction. The introduction of the triple bond spacer between the electron acceptor perylene moieties and the electron donating oligothiophene moieties produces an extended conjugation along the main chain, and produces soluble and high molecular weight copolymers.The thermal, optical and electrochemical properties of these copolymers were examined. The HOMO–LUMO energy levels of these copolymers, along with the quenching of their photoluminescence in 1:1 (w/w) blends with poly(3-hexylthiophene) (P3HT), indicate that they are suitable materials for application as electron acceptor and n-type components in bulk heterojunction solar cells with P3HT.

In this work we have tested in bulk heterojunction solar cells a polymer with a high oxidation potential, Poly-(dithieno[3,2-b:2′,3′-d]thiophene)-3-decylthiophene (P1), as electron donor together with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron acceptor. We have studied the photovoltaic characteristics of solar cells made with different P1:PCBM blend composition. The incident photon-to-current efficiency spectra reaches EQEs of 40%, evidencing good charge photogeneration. The best power conversion efficiency, η = 0.83 under AM1.5 illumination, is found for solar cells containing 67% in weight of PCBM. As expected for polymers with high oxidation potentials, the open circuit voltage V oc attains high values ( V oc ≥ 0.8 V), independently from the photoactive blend composition, but the fill factors FF of the cells are low (FF: 0.3–0.38), evidencing poor charge transport properties. The data here reported suggest that transport features, probably arising from poor polymer hole mobility, are the main limitation for cell performances.

The use of bipolar materials as an active layer in polymeric solar cells can be an alternative to ‘bulk-heterojunctions’ composed of donor–acceptor blends. We have prepared two series of polyalkylthiophene copolymers having acceptor molecules covalently grafted to the backbone. The random copolymerization of thiophene monomers containing an acceptor moiety (anthraquinone or tetracyanoanthraquinodimethane) and 3-alkylthiophene monomers is a powerful strategy to tune both the donor–acceptor content and the electronic properties of the material. Emission measurements show for both copolymer series a photoinduced electron transfer from the donor π-conjugated backbone to the acceptor substituents, and this process can be modulated as a function of the chemical composition.

Spectroelectrochemical studies of polydithienothiophenes (pDTTs), low band-gap conjugated polymers with polythiophene-like chain, using in situ FTIR-ATR and ESR spectroscopy, are shown. Spectral results obtained during p- and n-doping, as well as photoinduced spectral effects, are compared. The formation of paramagnetic positive and negative charge carriers with unusually high g-values is described.

The systems ITO/poly(4,4'-dipentoxy-2,2'bithiophene) and ITO/poly(3-butylthiophene)/aluminium (ITO=indium-tin oxide) are characterized by the rectifying properties of the junction polymer/aluminium. The same systems also showed an open circuit voltage in the dark and, under illumination, a photocurrent decreasing in time with a diffusion trend. The last properties have been explained on the basis of water absorption by the polymers, which therefore become ionic conductors. Finally, the aluminium oxidation at the anode and the proton (or polymer) reduction at the cathode lead to the formation of a galvanic cell. (C) 1998 Elsevier Science S.A. All rights reserved.

The processability and the solubility of organic materials are important in the fabrication of photoelectric conversion devices. A powerful strategy for chemical modification and for electronic property control of polymeric materials is the copolymerisation of different monomers. Polydithienothiophenes that exhibit low energy gap are promising materials for photovoltaic applications. To overcome their low processability we have designed copolymers containing either dithienothiophene or dithienothiophene-S,S-dioxide monomers in the backbone. The synthesis of these soluble copolymers is presented. Their use as photoactive materials in blends with buckminsterfullerene C60 or with TiO2 is discussed in terms of photo-induced charge transfer, probed by photoluminescence and photo-induced absorption measurements.

Since the discovery of the very fast photo-induced charge transfer between conducting polymers (as donors, D) and buckminsterfullerene (C 60 ) (as acceptors, A), interpenetrating D/A networks have been proposed to improve the collection of free charges at the electrodes. The morphology of the D/A blend is of crucial importance and embedding the photoactive materials in a plasticing matrix was suggested as a method to overcome phase separation, and to favor homogeneity. In this contribution we report on a morphological characterization of three-component blends of the type poly(3-octylthiophene)/plasticizer/C 60 , with different plasticizing materials. The morphological investigation was done with transmission electron microscopy. The use of a conjugated thiophene-based plasticizer produces dispersed fullerene domains interpenetrated in the polymeric matrix.

Poly-3-[5-(mentholate)-pentyl]thiophenes in both enantiomeric forms and with a high degree of regioregularity have been synthesized. Circular Dichroism spectra have been taken in the UV and in the IR ranges in chloroform solutions. While the UV-CD spectra are quite weak, the IR-CD spectra have shown many signals, most of them mirror images for the two enantiomers.

A general photochemical synthesis of large thiohelicenes composed containing 5, 7, 9 and 11 rings is presented. The absorption and emission properties of the racemic molecules are discussed along with the chirooptical properties of two enantiomers.

The electrical and photoelectrical characteristics of sandwich-type devices based. on regioregular head-to-tail (HT) poly(3-alkylthiophenes) (PATs) have been investigated and compared with those obtained with regiorandom polymers. Better performances have been achieved in sandwich-type devices made with regioregular polymers having a short alkyl substituent. A spectroscopic and morphological investigation of the materials have been also done.

The electrical characteristics of Al/poly(4,4′-dipentoxy-2,2′bithiophene)/ITO and of Al/poly(3-butylthiophene)/ITO Schottky diodes were investigated in the dark and under white light illumination. In both cases a battery was found superimposed to the rectifying junctions, leading to a negative current for positive bias applied in the dark. Under illumination, the build-up of a photoelectrochemical cell was demonstrated, which prevents the photovoltaic parameters inherent to the Schottky junctions being observed. In the case of the Al/poly(4,4′-dipentoxy-2,2′bithiophene)/ITO system a dependence of the photocurrent on the relative environmental humidity was revealed. All these findings were explained on the basis of the ion conductivity induced by the water absorbed by the polymers and by little amounts of ions included in them. The formation of an aluminium hydroxide layer, electronic insulator but ion conductor, at the Al/polymer interface was hypothesised to justify that both a battery in the dark and a photoelectrochemical cell can be established.

We report a photophysical study of a polythiophene with a wide distribution of conjugation lengths, obtained by the random copolymerization of 3-butyl and 3,4-butyl thiophenes in a 1/3 ratio. The photoinduced absorption spectra shift in energy upon varying the excitation wavelength, showing a localization threshold within the absorption tail. The data are explained in terms of triplet energy migration to the longest conjugated segments.

In situ UV-VIS-NIR and Raman spectroelectrochemical studies of polybenzimidazobenzophenanthroline (BBL), a conjugated ladder polymer, are reported. BBL films were reduced potentiostatically in spectroelectrochemical cells suitable for in situ UV-VIS-NIR and in situ Raman spectroscopy. The polymer shows four reversible reduction processes during n-doping in contrast to most of the other conjugated polymers. The spectral changes, obtained ill situ during the individual processes, are correlated with in situ ATR-FTIR spectroelectroelectrochemical measurements.

Resonant Raman scattering of polydithieno(3,4-b;3′,4′-d)thiophene (PDTT') has been studied throughout the visible and near infrared region. Our results are consistent with the existence of a quasi bidimensional conjugated network, which is possibly responsible for the low energy gap in this polymer.

Different approaches for the preparation of improved-order samples of polyalkylthiophenes (PAT) have been attempted. Thermal annealing and subsequent cooling gave high-quality samples. PAT blends have been oriented. Mixed LB films of PAT and arachidic acid, have been prepared aiming to obtain PAT in a well defined molecular arrangement. These samples were studied with UV-vis. spectroscopy and X-ray diffraction: some hypothesis on the order-degree are suggested.

Conductive polyheterocyclic systems exhibiting different properties may be obtained by modifying the molecular architecture of the monomer units. Lowering the band gap value one can also achieve an intrinsically conducting polymer without the need of doping. Pursuing these objectives and on the basis of the experience on the fused thiophena-ring series, we have chosen the dithieno-[3,4-b:3′,4′-d]thiophene molecule as the building block for the following reasons: a) the large delocalization of π-electrons as compared to thiophene based monomers. b) the availability of specific and equivalent sites for the polymer propagation (α-positions with respect to the sulphur atom). The monomer undergoes to electropolymerization at low oxidation potential (1.04 V vs SCE), as described in ref. 1, the observed conductivity of as grown films is 1 Ω −1 cm −1 , while the band gap of the neutral polymer is 1.1 eV (0.9 eV lower than polythiophene). The intermolecular packing together with polymer enchainment is discussed in terms of X-ray diffraction analysis and molecular modelling calculations.