The Role of Disorder in the Charge Dissociation and Recombination Processes of Organic Photovoltaic Materials
Organic Photovoltaic devices (OPVs) are becoming adequately cost and energy efficient to be considered a good investment and it is, therefore, especially important to have a concrete understanding of their operation. In this work, we use a fully quantum mechanical model of the electronic states of a bulk-heterojunction interface to investigate how presence of disorder in OPVs affects the recombination of triplet charge-transfer states and influences the free energy of an electron as it separates away from the interface into the free carrier phase. Our model simplifies complicated molecular structures to a lattice system, while taking into account electron and hole Coulombic and exchange interactions, lattice vibrations, and electron-phonon couplings. In addition, we examine the role of band-width and interfacial driving forces in determining the dissociation free energy of an electron/hole pair. With proper statistical treatment of the CT energies we recover experimentally observed ''hot'' and "cold'' exciton dissociation pathways. We also recover experimental values for the open-circuit voltage for the systems with 50-100 meV of energy disorder. Our model combines quantum and statistical treatments of a system with the large number of parameters and all possible electron-hole configurations to give results that provide a unifying picture linking various proposed mechanisms for charge separation. With many different theoretical tools and protocols available, our model stands out as combines scalability of molecular dynamics simulations with quantum mechanical treatment of electronics states.
Organic photovoltaics, Exciton, Disorder, Lattice Model, Entropy
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