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Hot wall epitaxy works close to thermodynamic equilibrium and is therefore most applicable for materials with Van der Waals binding character. The hot wall epitaxy system can be described as semiclosed growth reactor, consisting of a vertically mounted quartz cylinder, which is heated by three separately controllable ovens, and is closed on top by the substrate. The first oven heats the source material and controls the growth rate. The second heats the hot wall between source and substrate, which guaranties the closed character of the reactor and helps to avoid loss of valuable material. The third oven controls the substrate temperature and allows to influence the growth process on the substrate surface.We achieved crystalline perfection for C60 layers on mica substrates and fabricated Ba-doped n-type layers with mobilities of 6000 cm2/V s at room temperature. [Copyright &y& Elsevier]

This study focuses on structural properties of para-sexiphenyl (PSP) epitaxial thin films grown on freshly cleaved mica (0 0 1) substrates. The layers were prepared by hot wall epitaxy (HWE) technique resulting in highly ordered organic structures with a needle-like morphology on the substrates. X-ray diffraction (XRD) pole figure technique, transmission electron diffraction (TED) and atomic force microscopy were used to characterise the epitaxial growth. However, on not perfectly cleaved mica substrates the needle direction can change around 120° when the films are grown on different terraces of mica (0 0 1) which are separated by cleavage steps. This behaviour can be referred to the properties of the monoclinic crystal structure of mica which is described in detail. [Copyright &y& Elsevier]

For efficiently harvesting the terrestrial solar spectrum in conjugated polymer based solar cells, low bandgap polymers with a bandgap <1.8 eV are needed. The photophysics of such low band gap conjugated polymers as well as their excited state interactions with electron acceptors such as fullerenes are of importance when using them in photovoltaic devices. In this work we present a device structural study on the soluble low bandgap polymer PTPTB, consisting of alternating electron-rich N-dodecyl-2,5-bis (2′-thienyl)pyrrole (TPT) and electron-deficient 2,1,3-benzothiadiazole (B) units. The bandgap of this polymer, determined by electrochemistry and by optical absorption, is 1.6 eV. The performance of the photovoltaic devices is discussed in terms of spectrally resolved photocurrent measurements, AM1.5 measurements and temperature dependent I–V spectroscopy. Strategies to utilize this polymer for bulk heterojunction tandem solar cells with either a wide band gap polymer or in conjunction with strongly absorbing small molecular dyes are discussed. [Copyright &y& Elsevier]

Conjugated polymer/fullerene plastic solar cells of the first generation were consisting of two distinct layers, made of the donor polymer and of the acceptor fullerene, respectively, sandwiched between two metal contacts. By mixing the polymer and the fullerene components, thus replacing a single flat junction with an interpenetrating network bulk-heterojunction, the device efficiency was dramatically improved. As a further step to proceed with the development of plastic solar cells, we developed the bulk diffusion bilayer approach, allowing the creation of donor-acceptor diffused interfaces with less restrictions to the phase compatibility of the two components. For a novel series of fullerenes, the bulk diffusion bilayer approach is shown to yield devices with comparable efficiencies as the blend bulk heterojunction approach. Bulk-heterojunction devices show unusually high open circuit voltage (VOC) values. These values cannot be explained by the metal-insulator-metal (MIM) model that has been often used for organic light emitting diodes. In order to investigate the origin of the VOC in bulk-heterojunction plastic solar cells, we have prepared PPV based devices varying both the metal negative contact and the fullerene acceptor. Fullerene derivatives with varying acceptor strength, (i.e. the first reduction potential) were used as electron acceptors in bulk-heterojunction plastic solar cells produced with the blend as well as the diffusion bilayer approach. The open circuit voltage of the devices was found to correlate directly with the acceptor strength of the fullerenes, while it was rather insensitive to variations in the workfunction of the negative electrode metal. These results suggest that the quasi-Fermi level of the fullerene pins the Fermi level of the evaporated negative metal contact. [Copyright &y& Elsevier]

To obtain photovoltaic devices based on electron donating conjugated polymers with a higher efficiency, a major breakthrough was realised by mixing the polymers with a suitable electron acceptor, thereby enhancing the rate for photo-induced charge generation by several orders. State-of-the-art organic bulk hetero-junction photovoltaic cells are based on an interpenetrating donor–acceptor network in the bulk to form efficient nanostructured p–n junctions in the organic materials. Devices made with ‘Gilch’ poly(2-methoxy-5-(3′,7′-dimethyl-octyloxy))-p-phenylene vinylene, (MDMO-PPV), as an electron donor and (6,6)-phenyl-C61-butyric-acid (PCBM) (a soluble C60 derivative) as an electron acceptor yielded the highest efficiency until now in this class of devices. A power conversion efficiency of approximately ηe≥2.5% (electrical power out/incident light power) under AM 1.5 illumination was reported. The ‘gilch’ route is a direct synthetic route. The ‘sulphinyl’ route is a promising, indirect precursor-route towards MDMO-PPV. Due to the non-symmetric monomer, so-called ‘head-to-head’ and ‘tail-to-tail’ additions are excluded to a higher level in comparison to the ‘gilch’ route. This difference between both materials makes them interesting candidates to compare them in the state-of-the-art photovoltaic devices. Preliminary results indicate that the ‘sulphinyl’ MDMO-PPV/PCBM bulk hetero-junction solar cells attain a power conversion efficiency of nearly ηe=3% (electrical power out/incident light power), have a higher fill factor, incident photon per converted electron value (IPCE) and short circuit current. It is indicated that the observed solar cell characteristics are related to the defect level of the conjugated polymer used. [Copyright &y& Elsevier]

A spectroscopical study of composites made with a new conjugated polymer, polydithienothiophene, and nanocrystalline TiO2 is carried out by optical absorption, photoluminescence and steady state photo-induced absorption spectroscopy. Sintered nanoporous TiO2 films have been stained by dipping the inorganic semiconductor films into dithienothiophene polymer solutions. The optical absorption spectra indicate that this polymer is well absorbed into nanoporous TiO2 films, showing a remarkable interaction with the inorganic semiconductor as compared to other alkyl substituted thiophene based conjugated polymers. The photoluminescence quenching and the steady state photo-induced absorption spectra evidence that TiO2 acts as an efficient electron acceptor toward the polymer in the excited state. These features indicate that this conjugated polymer is a promising material to develop new hybrid organic/inorganic photovoltaic cells. [Copyright &y& Elsevier]

Abstract: Small band gap semiconducting π-conjugated polymers may, in principle, be more efficient as the active media in solar cells, due to their better adaptability to the solar spectrum. In luminescent π-conjugated polymers, interband photon absorption results in the radiative recombination of the photogenerated excitons, which competes with charge separation. The photoluminescence (PL) quantum efficiency is largely determined by the relative energies of the optically allowed lowest odd parity (LOP, e.g. 1Bu) excited state and the optically forbidden lowest even parity (LEP, e.g. 2Ag) excited state. If E(LEP)&#60; E(LOP), the quantum efficiency is small and charge separation following photoexcitation is more likely than in efficient PL polymers. The determination of the relative position of LEP and LOP states is therefore essential for the possible utilization of small band gap π-conjugated polymers in practical devices. It was previously shown that resonant Raman scattering dispersion might serve as a spectroscopic tool for the determination of the LEP states, since the dispersion is solely determined by the dependence of the LEP state on the conjugation length, rather than that of the optical gap, or LOP state. In this work, we use the Raman spectra measured in poly(dithieno-thiophene) (PDTT1) in order to estimate the LEP energy level. We find that the LEP energy level is in the vicinity of the optical gap. [Copyright &y& Elsevier]

Abstract: In this work we use atomic force microscopy and X-ray diffraction to study morphology and growth kinetics of para-sexiphenyl layers deposited by hot wall epitaxy on crystalline KCl(001) substrates. It is shown that the growth on KCl(001) is characterized by co-existence of long needles (“laying” molecules) and terraces structured islands (“standing” molecules). The former are the initial growth stage of the films, generating a rectangular network in accordance with surface symmetry of the substrate. It is in contrast to mica substrates, where para-sexiphenyl forms only needles with one preferential direction. [Copyright &y& Elsevier]