Moritz Riede

Abstract:

Efficient synthesis of a series of terminally dicyanovinyl (DCV)-substituted oligothiophenes, DCVnT 1-6, without solubilizing side chains synthesized via a novel convergent approach and their application as electron donors in vacuum-processed m-i-p-type planar and p-i-n-type bulk heterojunction organic solar cells is described. Purification of the products via gradient sublimation yields thermally highly stable organic semiconducting materials in single crystalline quality which allows for X-ray structure analysis. Important insights into the packing features and intermolecular interactions of these promising solar cell materials are provided. Optical absorption spectra and electrochemical properties of the oligomers are investigated and valuable structure-property relationships deduced. Photovoltaic devices incorporating DCVnTs 4-6 showed power conversion efficiencies up to 2.8% for planar and 5.2% for bulk heterojunction organic solar cells under full sun illumination (mismatch corrected simulated AM 1.5G sunlight). The 5.2% efficiency shown here represents one of the highest values ever reported for organic vacuum-deposited single heterojunction solar cells. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

A series of novel aza-diisoindolmethine dyes 9 with six different aryl and heteroaryl groups at the indole moiety have been synthesized by the addition of aryl Grignard compounds to phthalodinitrile and subsequent reaction with formamide. A plausible reaction mechanism, through a Leuckart-Wallach-type reduction has been confirmed by means of DFT calculations of the related transition and intermediate states. The corresponding boron difluoride complexes (10) of 9 were prepared in a subsequent reaction step and the spectroscopic and electrochemical properties of 9 and 10 have been investigated both experimentally and theoretically. The aza-diisoindolmethines 9 exhibit an absorption maximum in the range from 615 to 720 nm, whereas the complexes 10 show a bathochromically shifted absorption maximum between 681 and 793 nm. Measurements of 9 and 10 by cyclic voltammetry display fully reversible redox waves for the reduction and oxidation with higher potentials for 10. From the measured redox potentials, the HOMO and LUMO energy levels were calculated for 9 and 10. The frontier orbital energies, the energies of the absorption bands, as well as the orbitals involved in the absorption process were calculated with DFT and compared to the measured results of 9 and 10. The absorption maximum can be related to an intense HOMO-LUMO transition and the more-pronounced stabilization of the LUMO upon complexation is the origin of the bathochromic shift of the absorption. Additionally, single-crystal structures for two species, 10 d and 10 f, are reported.

Abstract:

We introduce the material hexaazatriphenylene hexacarbonitrile (HATCN) as electron conducting window layer for separating the photoactive region from the cathode in organic pin type solar cells. HATCN has a wide band gap of 3.3 eV and is thus transparent in the visible range of the solar spectrum. Its electrical properties can be tuned by means of molecular n-doping which leads to an increase of electron conductivity by several orders of magnitude up to 2.2×10-4S/cm. However, an application in photovoltaic devices is restrained by its exceptionally high electron affinity, estimated 4.8 eV, which introduces an electron injection barrier to the photoactive acceptor material C60. Here, we present a strategy to remove this barrier by means of introducing doped and undoped C60 intermediate layers, thus demonstrating the importance of energy level matching in a multiple layer structure and the advantages of Fermi level control by doping. © 2010 Elsevier B.V.All rights reserved.

Abstract:

The change of morphology in mixed layers due to different substrate temperature T of organic solar cells containing C60 and zinc phthalocyanine (ZnPc) is studied. Heating the substrate during deposition of the bulk heterojunction C60:ZnPc leads to a significant improvement of solar cell performance, mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved charge carrier percolation pathways within the C60:ZnPc blend. Using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, organic field effect transistor, X-ray diffraction, and absorption measurements, we observe aggregation, cluster-like, and polycrystalline growth of the heated bulk layer. This provides better transport percolation paths by inducing a phase separation of the molecules. Heated blend layer with thickness of 60 nm shows high performance without loss in FF. When heating the substrate to the optimum temperature of 110 °C, a power conversion efficiency of 3.0% is achieved, compared to 1.4% for an identical device prepared on a substrate held at room temperature. © 2010 Elsevier B.V. All rights reserved.

Abstract:

We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carrier mobilities (hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno [1,2,3-cd: 1′, 2′, 3′ -lm]perylene), with the acceptors C60 and N, N′ -dimethylperylene-3,4:9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interface effects but also the photoactive material itself can cause S-kinks. © 2011 American Institute of Physics.