www.afm-journal.de FULL PAPER © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2160 www.MaterialsViews.com wileyonlinelibrary.com Adv. Funct. Mater. 2012, 22, 2160–2166 Giulia Grancini, R. Sai Santosh Kumar, Agnese Abrusci, Hin-Lap Yip, Chang-Zhi Li, Alex-K. Y. Jen, Guglielmo Lanzani, and Henry J. Snaith* 1. Introduction Hybrid solar cell architecture, based on semi-conducting polymers infiltrated into mesostructured metal oxide elec- trodes, [1–17] combines the high electron mobility, high electron affinity, non-toxicity, low-cost and phys- ical and chemical stability of the metal oxide nanoparticles, with the good light absorbing and hole transporting proper- ties of semiconducting polymers. [1–9] The metal oxide pre-ordered geometry ensures a high interfacial area [8] leading to well distributed exciton quenching sites, and a guaranteed bi-continuous network of pure phases providing effective charge transport paths. Importantly, metal oxides such as TiO 2 have extremely high dielec- tric constants, promising to overcome charge separation issues associated with Coulombically bound electron-hole pairs forming at all organic donor/acceptor interfaces. [18–20] Despite these “theoretical advantages” over all organic devices, to date these hybrid systems have under- performed with respect to competing concepts. The major limitation in these hybrid metal oxide/polymer solar cells has been previously thought to be related to ineffective pore infiltration of the semiconducting polymer into the mesopo- rous metal oxide electrode. [9,10] However, recent work has clearly demonstrated that this is not typically the case, and for instance poly(3-hexylthiophene) (P3HT) can infiltrate and col- lect charge effectively from mesoporous TiO 2 films as thick as 7 µm. [21] This therefore leaves inefficient photoinduced electron transfer at the polymer/TiO 2 interface [3,4] to be the most likely performance limiting mechanism. This could be related to the quality of the interface, [12,16] or due to the specific orientation of the polymer and electronic coupling between the excited state energy levels in the polymer and the conduction band states in the oxide. A further limitation in hybrid solar cells is due to the narrow photoresponse limited to light absorbed in the visible spectral region from the polymer, related to the polymers previ- ously employed with ∼1.9 eV band gaps. [9,11] Solid-state dye-sensitized and extremely thin absorber solar cells have performed significantly better than polymer absorber based hybrid solar cells. [22,23] Recent reports of solid-state dye- sensitized solar cells, employing the light absorbing polymer P3HT as the hole conductor, have indeed delivered efficiencies over 3%. [9,11,21] However, for these systems, almost all the photo- current originates from light absorbed in the dye absorber layer, and is not delivered from the polymer hole conductor. [21,24] An Boosting Infrared Light Harvesting by Molecular Functionalization of Metal Oxide/Polymer Interfaces in Efficient Hybrid Solar Cells Hybrid solar cells based on light absorbing semiconducting polymers infiltrated in nanocrystalline TiO 2 electrodes, have emerged as an attrac- tive concept, combining benefits of both low material and processing costs with well controlled nano-scale morphology. However, after over ten years of research effort, power conversion efficiencies remain around 0.5%. Here, a spectroscopic and device based investigation is presented, which leads to a new optimization route where by functionalization of the TiO 2 surface with a molecular electron acceptor promotes photoinduced electron transfer from a low-band gap polymer(poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4- b0]dithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)] (PCPDTBT) to the metal oxide. This boosts the infrared response and the power conversion efficiency to over 1%. As a further step, by “co-functionalizing” the TiO 2 surface with the electron acceptor and an organic dye-sensitizer, panchromatic spectral photoresponse is achieved in the visible to near-IR region. This novel archi- tecture at the heterojunction opens new material design possibilities and represents an exciting route forward for hybrid photovoltaics. DOI: 10.1002/adfm.201102360 G. Grancini, Dr. A. Abrusci, Dr. H. J. Snaith Oxford University Department of Physics Clarendon Laboratory, Parks Road, Oxford, OX13PU, UK E-mail: h.snaith1@physics.ox.ac.uk G. Grancini, Prof. G. Lanzani Dipartimento di Fisica Politecnico di Milano P.zza L. da Vinci 32, 20133 Milano, Italy Dr. R. Sai Santosh Kumar, Prof. G. Lanzani Center for Nano Science and Technology @Polimi Istituto Italiano di Tecnologia Via Pascoli 70/3 20133 Milano, Italy H.-L. Yip, Dr. C.-Z. Li, Prof. A.-K. Y. Jen Department of Materials Science and Engineering and Institute of Advanced Materials and Technology University of Washington Seattle, WA 98195, USA