The Influence of Morphology on the Charge Transport in Two-Phase Disordered Organic Systems
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The Influence of Morphology on the Charge Transport in Two-Phase Disordered Organic Systems Cristiano F. Woellner1, Leonardo D. Machado1, Pedro A. S. Autreto1, José A. Freire2 and Douglas S. Galvão1 1 Applied Physics Department, State University of Campinas, 13083-970 Campinas, São Paulo, Brazil. 2 Physics Department, Federal University of Paraná, 81531-990, Curitiba, Paraná, Brazil
ABSTRACT In this work we use a three-dimensional Pauli master equation to investigate the charge carrier mobility of a two-phase system, which can mimic donor-acceptor and amorphouscrystalline bulk heterojunctions. Our approach can be separated into two parts: the morphology generation and the charge transport modeling in the generated blend. The morphology part is based on a Monte Carlo simulation of binary mixtures (donor/acceptor). The second part is carried out by numerically solving the steady-state Pauli master equation. By taking the energetic disorder of each phase, their energy offset and domain morphology into consideration, we show that the carrier mobility can have a significant different behavior when compared to a one-phase system. When the energy offset is non-zero, we show that the mobility electric field dependence switches from negative to positive at a threshold field proportional to the energy offset. Additionally, the influence of morphology, through the domain size and the interfacial roughness parameters, on the transport was also investigated. INTRODUCTION High performance conjugated polymers have gained significant attention in recent years due to their low-cost processing and high ductility, fundamental features for applications in flexible electronics. These features provide them with a significant competitive advantages over other technologies based on crystalline semiconductors [1]. However, despite their great potential for applications, weak device performance is still a limiting factor and many fundamental questions regarding the origin of these problems remain unsolved. Thus, a better understanding of charge transport in these materials is necessary in order to overcome these difficulties. With the advent of solar cells based on bulk heterojunctions (BHJs) [2,3] the need for models that describe the charge transport in such systems increased. In 2005 Watkins et al. [4] proposed an effective model based on the dynamical Monte Carlo method to generate the morphologies of binary mixtures, and in 2010, using a proper combination of Pasveer [5] and Watkins approaches, Koster [6] showed for the first time that the mobility in donor-acceptor blends could exhibit a negative electric field dependence. Despite these advances, all models so far for charge transport in BHJs explicitly assumed that the transport occurs exclusively in one phase (donor or acceptor), independently of the difference in energy between the electronic states of the two phases or on the applied electric field value [7].
THEORY In this work we investigated the charge carrier mobility of a two-phase system using an approach that can be separ
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