Ambipolar charge carrier transport in mixed organic layers of phthalocyanine and fullerene

Mixed layers of copper-phthalocyanine (p-conductive) and fullerene (n-conductive) are used for the fabrication of organic field-effect transistors (OFETs) and inverters. Regarding the electrical characteristics of these donor-acceptor blends they show ambipolar charge carrier transport, whereas devices made from only one of the materials show unipolar behavior. Such mixed films are model systems for ambipolar transport with adjustable field-effect mobilities for electrons and holes. By variation of the mixing ratio it is possible to balance the transport of both charge-carrier types. In this paper we discuss the variation of mobility and threshold voltage with the mixing ratio and demonstrate ambipolar inverters as a leadoff application. The gained results were analyzed by simulations using an analytical model for ambipolar transistors and subsequently compared to complementary inverters

Differences of interface and bulk transport properties in polymer field-effect devices

The influence of substrate treatment with self-assembled monolayers and thermal annealing was analysed by electrical and structural measurements on field-effect transistors (FETs) and metal-insulator-semiconductor (MIS) diodes using poly(3-hexylthiophene) (P3HT) as a semiconducting polymer and Si/SiO2 wafers as a substrate. It is found that surface treatment using silanising agents like hexamethyldisilazane (HMDS) and octadecyltrichlorosi-lane (OTS) can increase the field-effect mobility by up to a factor of 50, reaching values in saturation of more than 4E-2 cm^2/Vs at room temperature. While there is a clear correlation between the obtained field-effect mobility and the contact angle of water on the treated substrates, X-ray diffraction and capacitance measurements on MIS diodes show that structural and electrical properties in the bulk of the P3HT films are not influenced by the surface treatment. On the other hand, thermal annealing is found to cause an increase of grain size, bulk relaxation frequency and thereby of the mobility perpendicular to the SiO2/P3HT interface, but has very little influence on the field-effect mobility. Temperature dependent investigations on MIS diodes and FETs show that the transport perpendicular to the substrate plane is thermally activated and can be described by hopping in a Gaussian density of states, whereas the field-effect mobility in the substrate plane is almost temperature independent over a wide range. This investigations reveal significant differ-ences between interface and bulk transport properties in polymer field-effect devices.Comment: accepted at Organic electronic

Approaching disorder-free transport in high-mobility conjugated polymers.

Conjugated polymers enable the production of flexible semiconductor devices that can be processed from solution at low temperatures. Over the past 25 years, device performance has improved greatly as a wide variety of molecular structures have been studied. However, one major limitation has not been overcome; transport properties in polymer films are still limited by pervasive conformational and energetic disorder. This not only limits the rational design of materials with higher performance, but also prevents the study of physical phenomena associated with an extended π-electron delocalization along the polymer backbone. Here we report a comparative transport study of several high-mobility conjugated polymers by field-effect-modulated Seebeck, transistor and sub-bandgap optical absorption measurements. We show that in several of these polymers, most notably in a recently reported, indacenodithiophene-based donor-acceptor copolymer with a near-amorphous microstructure, the charge transport properties approach intrinsic disorder-free limits at which all molecular sites are thermally accessible. Molecular dynamics simulations identify the origin of this long sought-after regime as a planar, torsion-free backbone conformation that is surprisingly resilient to side-chain disorder. Our results provide molecular-design guidelines for 'disorder-free' conjugated polymers.We gratefully acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) through a programme grant (EP/G060738/1) and the Technology Strategy Board (TSB) (PORSCHED project). D. Venkateshvaran acknowledges financial support from the Cambridge Commonwealth Trust through a Cambridge International Scholarship. K. Broch acknowledges post-doctoral fellowship support from the German Research Foundation (DFG). Mateusz Zelazny acknowledges funding from the NanoDTC in Cambridge. The work in Mons was supported by the European Commission / Région Wallonne (FEDER – Smartfilm RF project), the Interuniversity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 7/05), Programme d’Excellence de la Région Wallonne (OPTI2MAT project) and FNRS-FRFC. D.B. and J.C. are FNRS Research Fellows.This is the accepted manuscript. The final version's available from Nature at http://www.nature.com/nature/journal/vaop/ncurrent/full/nature13854.html

Complex dielectric response of the CDW ground state in o-TaS3

We report the temperature evolution of the low-frequency (f<10 MHz) dielectric response of the CDW material orthorombic TaS3 between 150 K and 10 K. There are two distinctive processes observed bellow and above 50 K that define different ground states of CDW. The first one due to the weakly pinned elastic CDW freezes at about 50 K and a second one due to the dynamics of topological defects of frozen CDW appears below. Similar change of regime is observed in linear conductivity in the same temperature range. This second process we detect also in thermally stimulated discharge, an alternative technique of low frequency dielectric measurement, which allows access to effective frequencies down to 0.1 mHz

Neue Einsatzmoeglichkeiten natuerlicher Oele und Fette II. Teilvorhaben 5: Aufgabe 4.1: Analytik von Dimerfettsaeuren Bildung und Struktur 'Dimerer Fettsaeuren'

Available from TIB Hannover: DtF QN1(29,17) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

Performance enhancement of diindenoperylene based organic photovoltaic cells by nanocolumn arrays

Crystalline and uniform nanocolumns of the organic semiconductor diindenoperylene DIP were fabricated by glancing angle deposition and employed in organic photovoltaic cells OPVCs forming an interdigitated donor acceptor heterojunction, with fullerene as electron acceptor. In comparison to reference bilayer devices the nanocolumn based solar cells exhibit increased power conversion efficiency. Based on a comprehensive structural and morphological analysis, we identify three advantages of the interdigitated nanocolumn structures i The active donor acceptor interface area, crucial for exciton dissociation, is increased and the column diameter is in the range of the exciton diffusion length. ii The molecular orientation of DIP is such in the nanocolumns that light absorption is enhanced. iii The ubiquitous presence of vertical interfaces throughout nanocolumnbased devices is further beneficial to light absorption, as it fully compensates wavelength dependent interference effects within the device structure. This work shows how the benefits of nanocolumns can go beyond simple interface area enlargement to improve the efficiency of OPVC

Structural Substituent Effect in the Excitation Energy of a Chromophore: Quantitative Determination and Application to S-Nitrosothiols

A methodology for the prediction of excitation energies for substituted chromophores on the basis of ground state structures has been developed. The formalism introduces the concept of ?structural substituent excitation energy effect? for the rational prediction and quantification of the substituent effect in the excitation energy of a chromophore to an excited electronic state. This effect quantifies exclusively the excitation energy variation due to the structural changes of the chromophore induced by the substituent. Therefore, excitation bathochromic and hypsochromic shifts of substituted chromophores can be predicted on the basis of known ground and excited potential energy surfaces of a reference unsubstituted chromophore, together with the ground state minimum energy structure of the substituted chromophore. This formalism can be applied if the chemical substitution does not affect the nature of the electronic excitation, where the substituent effect can be understood as a force acting on the chromophore and provoking a structural change on it. The developed formalism provides a useful tool for quantitative and qualitative determination of the excitation energy of substituted chromophores and also for the analysis and determination of the structural changes affecting this energy. The proposed methodology has been applied to the prediction of the excitation energy to the first bright state of several S-nitrosothiols using the potential energy surfaces of methyl-S-nitrosothiol as a reference unsubstituted chromophore.Ministerio de Ciencia e InnovaciónUniversidad de Alcal