The relative permittivity of the materials constituting heterojunction solar cells is usually not considered as a design parameter when searching for novel combinations of heterojunction materials. In this work, we investigate whether such an approach is valid. Specifically, we show the effect of the materials permittivity on the physics and performance of the solar cell by means of numerical simulation supported by analytical relations. We demonstrate that, depending on the specific solar cell configuration and materials properties, there are scenarios where the relative permittivity has a major influence on the achievable conversion efficiency, and scenarios where its influence can be safely ignored. In particular, we argue that high-permittivity materials should always be the preferred choice as heterojunction partners of the absorber layer when prototyping new materials combinations. When the heterojunction partner has a high permittivity, solar cells are consistently more robust against several non-idealities that are especially likely to occur in early-stage development, when the device is not yet optimized.

. (2017) Structural stability of naphthyl end-capped oligothiophenes in organic field-effect transistors measured by grazingincidence X-ray diffraction in operando. Organic Electronics. Permanent WRAP URL:http://wrap.warwick.ac.uk/89989 Copyright and reuse:The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available.Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP URL' above for details on accessing the published version and note that access may require a subscription. AbstractWe report on microstructural durability of 5,5'-bis(naphth-2-yl)-2,2'-bithiophene (NaT2) in organic field effect transistors (OFETs) in operando monitored by grazing-incidence X-ray diffraction (GIXRD). NaT2 maintains its monoclinic bulk motif in operating OFETs with a = 20.31 ± 0.06Å, b = 6.00 ± 0.01Å, c = 8.17 ± 0.04Å and β = (96.64 ± 0.74)• . Crystallites appear as a mosaic of single crystals reaching through the whole 50 nm thick active layer. The lattice parameters variation (<1%) falls within the statistical error of structure refinement when the OFET gate voltage is varied from 0 V to -40 V; or when the OFET is continuously cycled within this voltage interval over more than 10 h period. Within the first few cycles, both the hole mobility and threshold voltage are changing but then reach stable levels with an average mobility of (3.25 ± 0.04) × 10 −4 cm 2 /Vs and an average threshold voltage of −13.6 ± 0.2 V, both varying less than 4% for the remainder of the 10 h period. This demonstrates crystalline stability of NaT2 in operating OFETs.

We report on the microstructure, morphology, and growth of 5,5´-bis(naphth-2yl)-2,2´-bithiophene (NaT2) thin films deposited on graphene, characterized by grazingincidence X-ray diffraction (GIXRD) and complemented by atomic force microscopy (AFM) measurements. NaT2 is deposited on two types of graphene surfaces: custom-made samples where CVD-grown graphene layers are transferred onto a Si/SiO 2 substrate by us and common commercially transferred CVD graphene on Si/SiO 2 . Pristine Si/SiO 2 substrates are used as a reference. The NaT2 crystal structure and orientation depend strongly on the underlying surface, with the molecules predominantly lying-down on the graphene surface (face-on orientation) and standing nearly out-of-plane (edge-on orientation) on the Si/SiO 2 reference surface. Post growth GIXRD and AFM measurements reveal that the crystalline structure and grain morphology differ depending on whether there is polymer residue left on the graphene surface. In situ GIXRD measurements show that the thickness dependence of the intensity of the (111) reflection from the crystalline edge-on phase does not intersect zero at the beginning of the deposition process, suggesting that an initial wetting layer, corresponding to 1-2 molecular layers, is formed at the surface-film interface. By contrast, the (111) reflection intensity from the crystalline face-on phase grows at a constant rate as a function of film thickness during the entire deposition.

M. (2018). Structural basis for a naphthyl end-capped oligothiophene with embedded metallic nanoparticles for organic field-effect transistors. Applied Physics Letters, 113(25), [251903]. https://doi.

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The structure of two naphthylene-capped oligothiophene, 5,5 -bis(naphth-2-yl)-2,2bi-and tri-thiophene, thin-film field-effect transistor assemblies has been studied using modeling in conjunction with grazing incidence x-ray diffraction. Although the well known herringbone molecular packing motif is observed in these films for both compounds, density functional calculations and molecular mechanics modeling give evidence for a local polymorphic ordering in which these molecules can be flipped 180 • about the long axis. In one case, that of the oligothiophene trimer, a disordered surface induced phase is observed. Prospective structural models are tested and refined using 1

Organic semiconductors have seen widespread application in thin‐film devices, such as organic field‐effect transistors (OFETs), whose performance is closely linked to the molecular‐level microstructure and crystalline orientation. In actual OFETs, the microstructure varies significantly based on the local environment, for example, in the proximity of contact electrodes. This account highlights recent examples where microfocused grazing‐incidence wide‐angle X‐ray scattering (μGIWAXS) maps structural information in between the OFET electrodes. Also shown are results where μGIWAXS is used to study the microstructure of naphthyl end‐capped oligothiophenes across interdigitated electrode arrays in a bottom‐contact OFET identifying lateral proximity effects of the contact electrodes in terms of crystalline misorientation, crystallite size, and disorder. The results together with those highlighted, classify essential structural parameters on and in between the electrodes and demonstrate capabilities of microfocused X‐rays to map microstructures in actual devices. The ideas presented herein bring us toward guidelines for understanding electrode proximity and device performance in molecular semiconductors. It is also believed that they are readily expanded from OFETs to other devices and from small molecules to polymers and other materials.

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