This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 3987–3995 3987 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 3987–3995 Organic photovoltaic devices with colloidal TiO 2 nanorods as key functional componentsw Anna Loiudice, ab Aurora Rizzo,* bc Luisa De Marco, b Maria R. Belviso, d Gianvito Caputo, c P. Davide Cozzoli ac and Giuseppe Gigli abc Received 12th December 2011, Accepted 12th December 2011 DOI: 10.1039/c2cp23971j We report on a novel approach to integrate colloidal anatase TiO 2 nanorods as key functional components into polymer bulk heterojunction (BHJ) photovoltaic devices by means of mild, all-solution-based processing techniques. The successful integration of colloidal nanoparticles in organic solar cells relies on the ability to remove the long chain insulating ligands, which indeed severely reduces the charge transport. To this aim we have exploited the concomitant mechanisms of UV-light-driven photocatalytic removal of adsorbed capping ligands and hydrophilicization of TiO 2 surfaces in both solid-state and liquid-phase conditions. We have demonstrated the successful integration of the UV-irradiated films and colloidal solutions of TiO 2 nanorods in inverted and conventional solar cell geometries, respectively. The inverted devices show a power conversion efficiency of 2.3% that is a ca. three times improvement over their corresponding cell counterparts incorporating untreated TiO 2 , demonstrating the excellent electron-collecting property of the UV-irradiated TiO 2 films. The integration of UV-treated TiO 2 solutions in conventional devices results in doubled power conversion efficiency for the thinner active layer and in maximum power conversion efficiency of 2.8% for 110 nm thick devices. In addition, we have demonstrated, with the support of device characterizations and optical simulations, that the TiO 2 nanocrystal buffer layer acts both as electron-transporting/hole-blocking material and optical spacer. Introduction In the past decade organic bulk heterojunction photovoltaic (OPV) cells fabricated from polymers 1 and small molecules 2 have attracted significant interest in academic and industrial communities because of their technological potential for the realization of low-cost, printable, lightweight, large-area and flexible devices. 3,4 To improve the performance of organic photovoltaic cells, disparate strategies have been pursued, including the development of new low-band-gap materials capable of harvesting a broader range of solar wavelengths, 5,6 advanced methods to control the morphology of active film layers as a means of improving charge transport and reducing recombination, 7 and the construction of novel device architectures for tandem 8,9 and inverted cells. 10 In addition, the optimization of charge collection efficiency and interfacial stability at the organic layer/electrode junction is considered to be critically important. 11 One of the main strategies to enhance the power conversion efficiency (PCE) is to engineer the electrode interface by the insertion of a buffer layer between the active layer and either the anode or the cathode. 12 For this purpose, various materials have been explored, such as metals (Ca, Ba), 13,14 salts (LiF, Cs 2 CO 3 ), 15 organic materials and transition-metal oxides (Ti, Cr, Zn, Ni, oxides). 16–21 Among them titanium oxides indeed hold great promise for the fabrication of different device structures, thanks to their unique electronic and optical properties. In particular crystalline nanostructured TiO 2 synthesized by established synthetic routes 22,23 has been commonly used as n-type material for photovoltaic applications, in dye-sensitized solar cells (DSSC) 24,25 and polymer/TiO 2 hybrid solar cells. 26,27 Recently, solution-processed nonstoichiometric titanium oxide (TiO X ) has been exploited as an optical spacer, an oxygen barrier and an electron-transporting/ hole-blocking (ETL/HBL) layer in the fabrication of conventional and inverted organic solar cell geometries. 20,28–30 The preparation of such material involves the sol–gel reaction of metal alkoxide a Dipartimento di Ingegneria dell’Innovazione, Universita ` del Salento, via per Arnesano, I-73100 Lecce, Italy b CBN - Center for Biomolecular Nanotechnologies, Italian Institute of Technology, Energy Platform, Via Barsanti sn, 73010 Arnesano (Lecce), Italy c NNL CNR-Istituto di Nanoscienze, c/o Distretto Tecnologico, Via per Arnesano Km 5, 73100 Lecce, Italy. E-mail: aurora.rizzo@nano.cnr.it; Fax: +39-0832-298237; Tel: +39-0832-298211 d Scuola Superiore ISUFI, Universita ` del Salento, via per Arnesano, I-73100 Lecce, Italy w Electronic supplementary information (ESI) available. See DOI: 10.1039/c2cp23971j PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by Universita del Salento on 09 March 2012 Published on 10 February 2012 on http://pubs.rsc.org | doi:10.1039/C2CP23971J View Online / Journal Homepage / Table of Contents for this issue