Sjoerd Veenstra

Sexithiophene-C60 blends as model systems for photovoltaic devices

Large photovoltaic responses have been recently observed in devices based on donor-acceptor blends. Their enhanced performance, which relies on the formation of a bicontinuous network of internal heterojunctions, is very sensitive to the morphology of the blend. In this paper, we propose sexithiophene-C60 blends, prepared by coevaporation, as a model system for the study of these devices. Preliminary results suggest a microphase-separated morphology with a domain size comparable to the exciton diffusion range. Photovoltaic cells based on the blends are demonstrated.

Energy level alignment at the conjugated phenylenevinylene oligomer/metal interface

In this letter we report an investigation of the interfacial electronic structure formed by metals and conjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metal substrates for two five-ring phenylenevinylene oligomers: unsubstituted p-bis[(p-styryl)styryl]benzene (P5V4) and the analogous oligomer with 2-methoxy-5-(2’-ethyl-hexyloxy) substitution on the central ring (MEH-P5V4). We found for all interfaces a lowering of the energy levels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably due to interface dipole layers, was always such as to keep the hole injection barrier nearly constant and therefore at most weakly sensitive to the work function of the metal or the ionization potential of the oligomer.

Analytical study of PPV-oligomer- and C60-based devices for optimising organic solar cells

A blend of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) and C60, sandwiched between two electrodes, has been used as the active layer for an organic solar cell. It delivered external quantum efficiencies up to 60% in the visible and 70% in the UV part of the spectrum. To unambiguously determine which parts of the bulk heterojunction structure are responsible for the rectifying behaviour and which can be considered as ohmic, the I/V characteristics of several other devices were investigated. It is found that the presence of C60 in a bulk heterojunction solar cell introduces fill factor reducing shunting paths.

Structural studies on thin films of an unsubstituted oligo(para-phenylenevinylene)

Abstract Typically, thin films of the conjugated oligomer p-bis[(p-styryl)styryl]benzene (P5V4) prepared by vacuum deposition on substrates like silicon wafers and quartz plates are polycrystalline. Atomic force and electron microscopy reveal elongated clusters with characteristic dimensions of 20×80 nm2. X-ray and electron diffraction (ED) measurements indicate a close correspondence between the crystal structure of these micro-crystallites and the reported structure of single crystals grown from solution. However, we argue on the basis of ED measurements that P5V4 micro-crystallites found in thin films vapor-deposited on amorphous carbon substrates adopt a morphology that differs from the single-crystal structure. In the proposed polymorph, the molecular axis is nearly perpendicular to the substrate plane, though the molecular arrangement is otherwise very similar to that in the single crystal. We suggest that several polymorphs may coexist in the thin films.

Preparation of photovoltaic cells from sexithiophene-C-60 blends

Large photovoltaic responses have been recently observed in devices based on conjugated polymer-C60 blends. Their enhanced performance, which relies on the formation of a bicontinuous network of donor-acceptor heterojunctions, is very sensitive to the morphology of the blend. In this paper, we propose sexithiophene-C60 blends, prepared by coevaporation, as a model system for the study of these devices. Preliminary results suggest a microphase-separated morphology with a domain size comparable to the exciton diffusion length. A photovoltaic device with quantum efficiency of 1% is demonstrated.