Charge-transport in Organic Semiconductors: Probing High Mobility with Light
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Charge-transport in Organic Semiconductors: Probing High Mobility with Light Demetrio A da Silva Filho1, Pedro Henrique de Oliveira Neto1, M. Carmen Ruiz Delgado2, Juan T. Lopez Navarrete2 and Juan Casado2 1
Institute of Physics, University of Brasilia, Brasília, DF, 70919-970, Brazil.
2
Department of Physical Chemistry, University of Málaga, Málaga, Spain.
ABSTRACT Efficient charge transport is key to the operation of the various devices based on organic semiconductors, such as OLEDs, OPVs and OFETs. Both academia and industry are investing heavily in the development of new organic materials and processing techniques to improve device performance. An important parameter to tweak is the mobility of charge carriers. Triggered by an experimental result, here we investigate whether this parameter can be probed indirectly using UV-VIS spectroscopy. This would simplify the process of characterization and optimization of the mobility in amorphous molecular films, for example. INTRODUCTION Both experimental and theoretical results have shown that there is a clear relationship between mobility and molecular packing.1 In the case of single crystals, Sundar et al has shown that a simple rotation of the crystal with respect to the position of the electrodes was responsible for a reasonable change in the mobility of charge carriers in rubrene.2 Calculations latter clarified that the reason behind such difference was related to differences in molecular overlap along different crystalline directions.3 In the case of thin films, annealing and post processing conditions have shown to improve considerably device performance.4 Such improvements can be directly related to film morphology and again, at the molecular level, to the relative position of the molecules. Clark et al has used linear absorption spectroscopy in polythiophene films to obtain an overall description of the thin-film microstructure. A correlation between the free exciton bandwidth and the degree to which the mobility depends on the gate voltage was observed.5 Generally speaking, increase in crystalline quality is correlated to increase in mobility, thus processing parameters such as different solvents and annealing could be used together with a simple-to-use spectroscopic technique, such as linear absorption, to optimize the charge carrier mobility. A report on dicyanomethylene-substituted thienoquinoid oligomer (see Figure 1) has shown that this is indeed possible: a three-orders of magnitude increase in the electron mobility was obtained upon annealing of the spin-coated film.6 This increase in charge-carrier mobility was also followed by the appearance of an absorption peak, around 1000nm, not seeing in the solution UV-vis spectrum. Interestingly, the intensity of this peak was a maximum at the same annealing temperature that maximized the electron mobility.
Here we investigate this structure-property relationship by means of QuantumChemical/Molecular Dynamics calculations. We will simulate the annealing procedure using Molecular Dynamics and remove dimers
