Phase Separated Morphology of P3HT:PCBM Bulk Heterojunction from Coarse-Grained Molecular Dynamics and Monte Carlo Simul
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Phase Separated Morphology of P3HT:PCBM Bulk Heterojunction from Coarse-Grained Molecular Dynamics and Monte Carlo Simulation Tran Thinh To1, Jing Han Yap2, Rayavarapu Prasada Rao2 and Stefan Adams1,2 1
Solar Energy Research Institute of Singapore (SERIS), 7 Engineering Drive 1, #E3A-06,
Singapore 117574 2
Department of Materials Science and Engineering, National University of Singapore, 5
Engineering Drive 2, #E2-05-22, Singapore 117579 ABSTRACT Morphology of the active layer in bulk heterojunction P3HT:PCBM organic solar cell was studied using Monte Carlo (MC) and coarse-grained dynamics simulations. While coarsegrained molecular dynamics allow us to quickly estimate the P3HT:PCBM interfacial energy of bilayer structure as a function of underlying layer thickness, bridging the dimension and time gap between dynamics simulations and experiment is computationally expensive and therefore not viable. Using MC technique with input from dynamics simulations allowed us to speed up the calculation and obtain final morphological information based on energetics and entropy, and at the same time retained the physics fidelity in-built in our validated coarse-grained model. The final structure gives phase separated domains with dimension of approximately 12 nm, on par with reported experimental result. The method can be applied to other organic photovoltaics systems to predict active layer morphology relevant for device performance or 3-dimensional device modelling at continuum level. INTRODUCTION Organic photovoltaics (OPV) have been receiving much research interest since its introduction[1] due to the prospect of low cost materials, solution processing techniques and flexibility.[2] Active layer in OPV devices often assume a bulk heterojunction morphology where a bi-continuous network of donor and acceptor phase separated domains are formed by thermal treatment of the corresponding blended mixture.[3, 4] Bulk heterojunction morphology greatly enhances interfacial areas between donor and acceptor; consequently, boost the exciton dissociations which take place mostly at the donor:acceptor interface.[5, 6] While the importance of active layer morphology in OPV has been discussed at length in available literature, the physics underlying the process is yet fully understood. To shed light on the mechanism of phase separation as well as solicit the active layer morphology in OPV after thermal treatment process, simulation approaches were employed most notably using coarse-grained dynamics.[7, 8] However, this method is computationally demanding and is often not possible to bridge the dimension and time scale of simulation to experiment. Furthermore, the employment of overly coarse forcefield could lead to inaccurate reproduction of phase separation process, i.e. neglecting the side chains in poly(3hexylthiophene) (P3HT) and fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM)
could affect the accuracy of donor:acceptor interfacial interaction where the orientation of the side chains could play a crucial role.[9] I
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