Ordered and axially aligned poly(3-hexylthiophene) (P3HT) polymer/ZnO nanowire photovoltaic cells are an attractive alternative materials to those used in conventional solar cells. We use first principles electronic structure methods to study the P3HT/ZnO interface where chemically modified P3HT side chains are cross-linked to the surface of a ZnO nanowire by forming covalent bonds. Our work focuses on both the binding morphology as a function of nanowire diameter (i.e. curvature effects) as well as the electronic alignment of the polymer and ZnO band across the interface as a function of the interfacial geometry and chemistry. Morphologically, we find that the energetic competition between binding, curvature effects, and strain on the polymer backbone can induce a transition in P3HT orientation as a function of the ZnO nanowire diameter. Since such predictions require modeling of systems much larger than can be presently treated ab initio, so we show how the classic Frenkel-Kontorova model is of great aid in studying and understanding this and other similar systems. Electronically, we show using standard DFT as well as hybrid methods that obtaining a desirable staggered band alignment is possible via control of the passivation of the anchoring atom(s) and/or ZnO surface.
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