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Title: Multi-scale Modelling of Organic Photovoltaics System P3HT:PCBM
Keywords: Organic photovoltaics, molecular dynamics, Monte Carlo, morphology, charge transport, P3HT:PCBM
Issue Date: 28-Apr-2014
Source: TO TRAN THINH (2014-04-28). Multi-scale Modelling of Organic Photovoltaics System P3HT:PCBM. ScholarBank@NUS Repository.
Abstract: Understanding of active layer morphology evolution and device physics in poly(3-hexyl thiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) Organic Photovoltaics (OPV) is crucial towards the improvement of device performance. To this end, Molecular Dynamics (MD) and ab-initio methods are used in conjunction with one another. By applying coarse-graining method, MD calculation speed was reduced by more than 200 times. Using the coarse-grained forcefield, P3HT:PCBM interface was studied at different P3HT orientations. This leads to the conclusion that phase separation in P3HT:PCBM bulk heterojunction is carried out via P3HT nucleation crystallization at the interface followed by diffusion of PCBM out of crystalline P3HT-rich region. To further improve the simulation efficiency, P3HT:PCBM interfacial energy profile was employed for Monte Carlo simulations. Analysis of the resultant domains sizes, volume of percolating domains and P3HT:PCBM interfacial areas suggests that 1:1 blend ratio is most optimal. Pre-grown crystal was found to speed up domains formation in the early stage. This means that sufficient pattern of P3HT crystals grown before thermal treatment could help influence the resultant active layer morphology. To corroborate the active layer morphology and device performance, a charge transport model based on Huckel method and Marcus theory was developed incorporating morphological information obtained from MD simulations. Light absorption and dark J-V calculations suggested that the corresponding processes are associated with intra-chain and inter-chain transport respectively. Optimal P3HT inter-chain coupling energy was found to be intermediary between crystalline and disordered systems. Furthermore, the maximisation of the closest distance between phenyl group on PCBM and thiophene group on P3HT were also shown to lead to larger interfacial HOMO/LUMO mismatch and consequently lower leakage energy under illuminated conditions. The approach and methodology shown in this work is general and applicable to similar systems. Since the study presented is based on multi-scale theoretical approach with minimal needs for experimental input, the framework can be used guide the molecular design of active layer materials relevant for high performance OPVs.
Appears in Collections:Ph.D Theses (Open)

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