Structural and Optical Properties of Mixed Diindenoperylene− Perfluoropentacene Thin Films
نویسندگان
چکیده
Mixed films of perfluorinated pentacene and diindenoperylene are studied for their structural and optical properties on SiO2 and glass substrates. Grazing incidence Xray diffraction shows that the compounds mix on the molecular level. Two structures are identified. Their out-ofplane lattice spacing is determined by X-ray reflectivity, and their growth morphology is characterized using atomic force microscopy. It is found that one structure consists of molecules in a standing-up and the other in a lying-down orientation. The uniaxial dielectric function of mixed films is determined with spectroscopic ellipsometry, and the in-plane optical extinction coefficient is measured by real-time differential reflectance spectroscopy during growth. The question of intermolecular coupling between perfluorinated pentacene and diindenoperylene is discussed. ■ INTRODUCTION Binary mixtures of organic materials in thin films have attracted significant attention, inter alia because of their extensive use as donor−acceptor bulk heterojunctions in organic photovoltaic devices or as highly doped conductive layers in organic electronics. A key question for the optical and structural properties of a mixture of two organic compounds is whether the materials are phase-separating or -mixing on the molecular level. Complete or partial mixing of two species may be possible if a mixed crystal exists that is energetically more favorable than two pure crystals. Of importance in this regard is the isostructural compatibility of both compounds, which is in many cases a prerequisite for efficient mixing. An ordered molecular mixture (molecular complex) may be formed due to strong interaction between certain parts of the mixed compounds. In this context, by molecular complex we mean an ordered, mixed crystal that is built up by sufficiently strong interaction (e.g. electrostatic or hydrogen bond), which does not necessarily involve a significant charge transfer in the ground state between the compounds involved. One such interaction forming molecular complexes is, for example, the arene/perfluoroarene interaction. In single crystals, the interaction between arenes and perfluorinated arenes leads to pronounced face-to-face stacking, which was also described as dimerization in the crystal. Therefore, these complexes exhibit a simple molar mixing ratio, mostly 1:1 or 2:1. In contrast to bulk single crystal growth, in which the minimization of lattice energy is the most relevant driving force, structure formation of thin films is often far from thermal equilibrium. For the full description of thin film structure formation, one also has to account for kinetic growth effects. The mixing behavior in such multicomponent organic thin films is as yet not well understood, and in fact, even the growth and structure of single-component thin films are already nontrivial. Recently, the formation of an equimolar molecular complex of pentacene (PEN) and perfluoropentacene (PFP) upon coevaporation were reported in thin films, which exhibits interesting optical and electrical characteristics. The formation of this molecular complex of PEN and PFP is also facilitated by the steric compatibility of both compounds. The present paper is dedicated to a detailed analysis of the optical and structural characteristics of PFP/DIP mixed thin films. Diindenoperylene (DIP) is a promising candidate for use in organic photovoltaics (OPV) because of its high structural order and its ability to function as an efficient electron donor for OPV applications. The combination of PFP and DIP might be of potential interest for photovoltaics, since it would absorb light in a wide frequency range, and the electronic energy levels might enable efficient exciton diffusion. Received: December 12, 2011 Revised: March 21, 2012 Published: April 26, 2012 Article
منابع مشابه
Structural, Electrical and Optical Properties of Molybdenum Oxide Thin Films Prepared by Post-annealing of Mo Thin Films
Molybdenum thin films with 50 and 150 nm thicknesses were deposited on silicon substrates, using DC magnetron sputtering system, then post-annealed at different temperatures (200, 325, 450, 575 and 700°C) with flow oxygen at 200 sccm (standard Cubic centimeter per minute). The crystallographic structure of the films was obtained by means of x-ray diffraction (XRD) analysis. An atomic force micr...
متن کاملStructural, Electrical and Optical Properties of Molybdenum Oxide Thin Films Prepared by Post-annealing of Mo Thin Films
Molybdenum thin films with 50 and 150 nm thicknesses were deposited on silicon substrates, using DC magnetron sputtering system, then post-annealed at different temperatures (200, 325, 450, 575 and 700°C) with flow oxygen at 200 sccm (standard Cubic centimeter per minute). The crystallographic structure of the films was obtained by means of x-ray diffraction (XRD) analysis. An atomic force micr...
متن کاملThe effect of Ga-doping on the structural and optical properties of ZnO thin films prepared by spray pyrolysis
In this research, zinc oxide thin films with gallium impurity have been deposited using the spray pyrolysis technique. The structural and optical properties of these films are investigated as a function of gallium doping concentrations. The ZnO and ZnO:Ga films grown at a substrate temperature of 350 ºC with gallium doping concentrations from 1.0 to 5.0.%. The XRD analysis indicated that ZnO f...
متن کاملInfluence of Co and Fe substitution on optical and structural properties of zinc oxide thin films
Zn0.97TM0.03O (TM = Co, Fe) thin films were deposited onto glass substrates by the sol–gel method and the effects of transition metals substitution on structural and optical properties of ZnO films were investigated. The X-ray diffraction patterns revealed that the films have wurtzite structure. Optical transmittance of the films was recorded in the range of 200 -800 nm wave length and the band...
متن کاملEffects of Annealing and Thickness on the Structural and Optical Properties of Crystalline ZnS Thin Films Prepared by PVD Method
Zinc Sulfide (ZnS) thin films were deposited on glass substrates at the pressure of 10-6 mbar by thermal resistor evaporation technique. The effects of annealing on the structural, optical properties of ZnS films were studied. Crystalline ZnS films have been analyzed by X-ray diffraction. Only cubic phase with the preferred (111) plane was found in ZnS films. Optical characteristics were studie...
متن کامل