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Abstract:

We present the material α-ω -bis-(dicyanovinylen)-sexithiophen (DCV6T) as donor material in organic solar cells. A systematic study on the potential of DCV6T is given for different active layer concepts. DCV6T is a member of a class of acceptor-substituted oligothiophenes, which showed efficiencies of up to 3.4% and open circuit voltages (Voc) of 1.0 V, which were recently reported [K. Schulze Adv. Mater. (Weinheim, Ger.) 18, 2875 (2006)]. To verify the potential of the material (DCV6T), organic solar cells with planar heterojunctions, bulk heterojunctions, and a hybrid-planar-mixed heterojunction are investigated. The planar heterojunction solar cells of DCV6T and C60 show the highest Voc of 0.90 V. The mixed heterojunction solar cells have improved currents but a lower Voc of 0.82 V. The solar cell using the hybrid-planar-mixed heterojunction achieves the best combination of parameters. It has a Voc of 0.88 V, a short circuit current (jsc) of 5.7±0.4 mA cm-2, a fill factor of 41.6%, and a power conversion efficiency of 2.1±0.2%. © 2009 American Institute of Physics.

Abstract:

We present metal multilayers consisting of aluminum and silver in different combinations serving as semitransparent top contacts for organic solar cells. Scanning electron microscopy, atomic force microscopy, and optical spectroscopy are used to illustrate how ultrathin Al interlayers influence the morphology of Ag layers evaporated on top of organic materials and how closed layers with good conductivity can be achieved. Multilayer metal contacts are used to fabricate top-illuminated small-molecule organic solar cells (SM-OSCs) which reach efficiencies comparable to conventional SM-OSCs that employ tin-doped indium oxide as electrode. It is shown that combinations of Al and Au lead to similar results, suggesting a similar mechanism for the influence on morphological development of both Ag and Au. © 2009 American Institute of Physics.

Abstract:

We present zinc phthalocyanine (ZnPc): C60 bulk-heterojunction top-illuminated organic solar cells using ultrathin metal layers as transparent top contacts. We show that solar cell performance sensitively depends on the interface and morphology of the cathode, which can be influenced by varying the composition and layer structure of the metal contact. We investigate various metal combinations, such as 3 nm Al/8 nm Ag and 7 nm Al/14 nm Ag, to illustrate the necessity to find a suitable combination of morphology and electrical and optical properties. Solar cells using standard materials and a 1 nm Al/14 nm Ag cathode exhibit promising efficiencies of over 2.2%. © 2009 American Institute of Physics.

Abstract:

Poly(4-vinylpyridine)-block-poly(4-iodo-styrene), P4VP-b-PS(I), block copolymers obtained by iodination of readily available P4VP-b-PS block copolymers strongly adhere to variety of polar substrates including Si wafers, glasses, or metal oxide surfaces by a polar P4VP block, forming polymer brushes of moderately stretched PS(I) chains. Kumada catalyst-transfer polycondensation (KCTP) from the P4VP-b-PS(I) brushes results into planar brushes of the graft copolymer in which relatively short ( approximately 10 nm) poly(3-hexylthiophene), P3HT, grafts emanate from the surface-tethered PS(I) chains. Grafting of the P3HT leads to significant stretching of the PS(I) backbone as a result of increased excluded volume interactions. Specific adsorption of the P4VP block to polar surfaces was utilized in this work to pattern the P4VP(25)-b-PS(I)(350) brush. The microscopically structured P4VP(25)-b-PS(I)(350) brush was converted into the respectively patterned P4VP-PS(I)-g-P3HT one using KCTP. We also demonstrated that KCTP from functional block copolymers is an attractive option for nanostructuring with polymer brushes. P4VP(75)-b-PS(I)(313) micelles obtained in selective solvent for the PS(I) block form a quasi-ordered hexagonal array on Si wafer. The P4VP(75)-b-PS(I)(313) monolayer preserves the characteristic quasi-regular arrangement of the micelles even after extensive rinsing with various solvents. Although the grafting of P3HT from the nanopatterned P4VP(75)-b-PS(I)(313) brush destroys the initial order, the particulate morphology in the resulting film is preserved. We believe that the developed method to structured brushes of conductive polymers can be further exploited in novel stimuli-responsive materials, optoectronic devices, and sensors.