Large scale GW-BSE calculations with N3 scaling: excitonic effects in dye sensitised solar cells

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Excitonic effects due to electron-hole coupling play a fundamental role in renormalising energy levels in dye sensitised and organic solar cells determining the driving force for electron extraction. We show that first-principles calculations based on many-body perturbation theory within the GW-BSE approach provide a quantitative picture of interfacial excited state energetics in organic dye-sensitized TiO2, delivering a general rule for evaluating relevant energy levels.To perform GW-BSE calculations in such large systems we introduce a new scheme based on maximally localized Wannier functions. With this method the overall scaling of GW-BSE calculations is reduced from O(N4) to O(N3).