Sequential Processing for Organic Photovoltaics: Design Rules for Morphology Control by Tailored Semi-Orthogonal Solvent Blends

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Design rules are presented for significantly expanding sequential processing (SqP) into previously inaccessible polymer:fullerene systems by tailoring binary solvent blends for fullerene deposition. Starting with a base solvent that has high fullerene solubility, 2-chlorophenol (2-CP), ellipsometry-based swelling experiments are used to investigate different co-solvents for the fullerene-casting solution. By tuning the Flory-Huggins χ parameter of the 2-CP/co-solvent blend, it is possible to optimally swell the polymer of interest for fullerene interdiffusion without dissolution of the polymer underlayer. In this way solar cell power conversion efficiencies are obtained for the PTB7 (poly[(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)(3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl)]) and PC61BM (phenyl-C61-butyric acid methyl ester) materials combination that match those of blend-cast films. Both semicrystalline (e.g., P3HT (poly(3-hexylthiophene-2,5-diyl)) and entirely amorphous (e.g., PSDTTT (poly[(4,8-di(2-butyloxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)-alt-(2,5-bis(4,4'-bis(2-octyl)dithieno[3,2-b:2'3'-d]silole-2,6-diyl)thiazolo[5,4-d]thiazole)]) conjugated polymers can be processed into highly efficient photovoltaic devices using the solvent-blend SqP design rules. Grazing-incidence wide-angle x-ray diffraction experiments confirm that proper choice of the fullerene casting co-solvent yields well-ordered interdispersed bulk heterojunction (BHJ) morphologies without the need for subsequent thermal annealing or the use of trace solvent additives (e.g., diiodooctane). The results open SqP to polymer/fullerene systems that are currently incompatible with traditional methods of device fabrication, and make BHJ morphology control a more tractable problem

Impact of supercritical CO[sub 2] drying on roughness of hydrogen silsesquioxane e-beam resist

Surface roughness (SR) and, especially, the closely related line-edge roughness (LER) of nanostructures are important issues in advanced lithography. In this study, the origin of surface roughness in the negative tone electron resist hydrogen silsesquioxane is shown to be associated with polymer aggregate extraction not only during resist development but also during resist drying. In addition, the impact of exposure dose and resist development time on SR is clarified. Possibilities to reduce SR and LER of nanostructures by optimizing resist rinsing and drying are evaluated. A process of supercritical CO(2) resist drying that delivers remarkable reduction of roughness is presented. (c) 2006 American Vacuum Society.</p

Effect of dissolution pathways of polyacrylonitrile on the solution homogeneity: Thermodynamic- or kinetic-controlled dissolution

Dissolution behavior of polyacrylontrile (PAN) in N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO) is investigated in terms of dipole-dipole interactions at polymer-solvent interface. Fourier-transformed infrared (FTIR) imaging of polymer-solvent interface under static dissolution condition at 25 ??C and 60 ??C exhibits the gel-like swollen layer in the early stage of dissolution for both solvents. This layer readily disappears in PAN-DMSO and DMSO molecules penetrate deep into the polymer phase, whereas that in PAN-DMF is still observed until 12 h even at 60 ??C. The better dissolving ability of DMSO is interpreted by higher intrinsic viscosity and lower Huggins??? constant. The solubility (solution homogeneity) is also evaluated by rheological analysis in a wide concentration range of 8???20 wt%. At above 15 wt%, DMSO produces homogeneous solution at 60 ??C even without stirring, showing the lower Newtonian flow region. On the other hand, it is necessary to apply vigorous stirring and heating for PAN-DMF system in order to prepare concentrated solution. DMSO provides greater slope of the Cole-Cole plot by rheological measurement than DMF, indicating better solution homogeneity in PAN-DMSO. The FTIR spectra and 2D correlation analysis show that both DMSO and DMF dissolve PAN in a way of sequential contributions of enthalpic and entropic factors. However, DMSO exhibits predominant contribution by enthalpic factor, suggesting that enthalpy change of mixing is a driving force of PAN dissolution in DMSO, whereas dissolution of PAN-DMF is governed by kinetic control