Hybrid donor/acceptor interfaces have acquired a primary role in many areas of science and technology, ranging from photovoltaics and photocatalysis to electronics and sensing. A fundamental property of such interfaces is the alignment of the quasiparticle energy levels between the donor and acceptor materials. The energy-level alignment underpins a variety of complex phenomena such as charge-transfer doping, carrier injection and recombination, and exciton dissociation dynamics. In the context of solar energy technology the level alignment determines the ability of the interface to transfer energy from the donor to the acceptor by exchanging photoexcited charges. While the physics of the energy-level alignment at conventional semiconductor heterojunctions is currently well established, little is known about hybrid interfaces involving metal oxides and soft materials such as polymers and light-harvesting complexes. In this talk, by reviewing some prototypical case studies from our recent activity, I will try to provide a critical assessment of where we stand in the calculation of the energy-level alignment at semiconductor/polymer and semiconductor /molecule interfaces. In particular I will touch upon the interfaces between the polymer P3HT and the semiconductors ZnO, GaAs, as well as the interfaces between TiO2 and molecular chromophores and semiconductor sensitizers (N3, Sb2S3). I will discuss how we can use first-principles calculations in combination with photoelectron spectroscopy and scanning tunneling microscopy in order to improve our understanding of the atomistic structure and the electronic properties of such interfaces; and how to use first-principles calculations for performing some rudimentary computational design. Finally I will briefly comment on the potential importance of including quantum nuclear effects in future calculations of level alignments, and the necessity of studying complete spectral lineshapes as opposed to discrete energy levels. • C. E. Patrick and F. Giustino, O-1s core-level shifts at the anatase TiO2(101)/N3 photovoltaic interface: signature of H-bonded supramolecular assembly. Phys. Rev. B. 84, 085330 (2010). • C. E. Patrick and F. Giustino, Structural and electronic properties of semiconductor-sensitized solar cells interfaces. Adv. Funct. Mater. 21, 4663 (2011). • C. E. Patrick and F. Giustino, GW quasiparticle band gaps of anatase TiO2 starting from DFT+U. J. Phys.: Condens. Matter. 24, 202201 (2012). • K. Noori and F. Giustino, Ideal energy-level alignment at the ZnO-P3HT photovoltaic interface, Adv. Func. Mater. 22, 5089 (2012). • M. R. Filip, C. E. Patrick, and F. Giustino, GW quasiparticle band structures of stibnite, antimonselite, bismuthinite, and guanajuatite Phys. Rev. B 87, 205125 (2013). • C. E. Patrick and F. Giustino, Quantitative analysis of valence photoemission spectra and quasiparticle excitations at chromophore-semiconductor interfaces, Phys. Rev. Lett. 109, 116801 (2012). • C. K. Yong, K. Noori, Q. Gao, H. J. Joyce, H. H. Tan, C. Jagadish, F. Giustino, M. B. Johnston, and L. M. Herz, Strong carrier lifetime enhancement in GaAs nanowires coated with semiconducting polymer, Nano Letters 12, 6293 (2012). • C. E. Patrick and F. Giustino, Quantum nuclear dynamics in the photophysics of diamondoids, Nat. Commun. 4, 2006 (2013).
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