Dye-sensitized solar cells containing plasma jet deposited hierarchically nanostructured TiO 2 thin photoanodes† Vanira Trifiletti, a Riccardo Ruffo, a Christian Turrini, b Dario Tassetti, b Rosaria Brescia, d Fabio Di Fonzo, c Claudia Riccardi * b and Alessandro Abbotto * a The investigation of dye-sensitized solar cells (DSCs) using a very thin (0.8 to 4.7 mm) transparent photoanode based on a hierarchically nanostructured TiO 2 film prepared via Plasma Assisted Supersonic Jet Deposition (PA-SJD), in combination with the DSC benchmark photosensitizer N719, is presented. The cell photovoltages (nearly 0.8 V) and the amount of adsorbed dye and photocurrent densities, normalized over the film thickness, are higher than those of DSCs prepared with thicker (9 mm) conventional screen-printed TiO 2 films. A record efficiency value of 5.7% (standard AM 1.5G solar irradiation) for very thin nanostructured titania photoanodes is obtained with a 2 mm PA-SJD TiO 2 film. Electrochemical impedance spectroscopy shows that PA-SJD devices are endowed with higher charge recombination resistance (lower recombination losses) and electron lifetimes compared to conventional nanocrystalline films. 1 Introduction The current energy crisis and its damaging environmental effects have accelerated the study of efficient exploitation of renewable energy sources, making this sector one of the most strategic scientic challenges of this century. In this context, cheap and stable innovative photovoltaic devices are demanded to represent one of the leading technologies for a possible more sustainable future. 1,2 Nano-crystalline dye-sensitized solar cells (DSCs) have proven to be a promising candidate for low-cost and efficient photovoltaic devices. 3,4 One of the key components of a DSC is the photoanode based on a wide band-gap semiconductor thin lm coated on a uorine doped tin oxide (FTO) glass, which is responsible for electron transport and collection following the photosensitizer excitation upon light harvesting. Although a number of metal oxides have been tested as n-type semiconductors, titanium oxide (TiO 2 ), to which the photosensitizer molecules are chemically graed, has given so far the best results because of its large surface area, good light absorption, and high photo- electrical response as a porous photo-electrode. 3 Gr¨ atzel and co-workers have optimized the device fabrica- tion 5 using a colloidal paste of TiO 2 nanoparticles 6 which is screen-printed on the FTO glass via a multilayer deposition process until the desired lm thickness has been reached 7 fol- lowed by sintering to ca. 500  C. The active layer of the photo- anode is composed, typically, of a 12 mm thick lm of transparent 10–20 nm TiO 2 nanoparticles covered by a 4 mm thick lm of larger (200–400 nm) particles which scatter photons back into the transparent lm. 8 Following light absorption, the excited dye injects electrons into the TiO 2 . The injected electrons diffuse through the titanium dioxide to be collected at the front-side transparent FTO electrode, whereas the oxidized dye is quickly reduced to its ground state by a redox shuttle, usually I 3  /I  , dissolved in an electrolyte solution. Diffusion of the oxidized form of the redox mediator to the counter-electrode closes the circuit. This procedure is now currently used in the vast majority of DSC studies. In a benchmark DSC conguration, light is absorbed by a ruthenium based dye, bis(tetrabutylammonium) cis-dithiocya- nato-bis(2,2 0 -bipyridine-4-COOH-4 0 -COO  )Ru(II) (N719), graed to the TiO 2 nanoparticle surface via its carboxylate moieties, 9 affording a validated power conversion efficiency (PCE) of 11.2% under standard AM 1.5G full sunlight (1000 W m 2 ). 10 Unfortunately, the use of a TiO 2 photoanode prepared from a colloidal paste likely hampers the improvement of DSC effi- ciencies because of the presence of numerous grain boundaries, which signicantly reduces electron mobility. In addition, the presence of a basically disordered network impedes an optimal a Department of Materials Science and Milano-Bicocca Solar Energy Research Center – MIB-Solar, University of Milano-Bicocca, INSTM unit, Via Cozzi 53, I-20125, Milano, Italy. E-mail: alessandro.abbotto@unimib.it b Department of Physics and Plasma Prometeo Centre, University of Milano-Bicocca, Piazza della Scienza 3, I-20126, Milano, Italy. E-mail: claudia.riccardi@unimib.it c Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70/3, 20133 Milano, Italy d Department of Nanochemistry, Istituto Italiano di Tecnologia, Via Morego 30, 16163, Genova, Italy † Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ta11485f Cite this: DOI: 10.1039/c3ta11485f Received 14th April 2013 Accepted 19th July 2013 DOI: 10.1039/c3ta11485f www.rsc.org/MaterialsA This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A Journal of Materials Chemistry A PAPER Published on 22 July 2013. Downloaded by University of Milan - Bicocca on 21/08/2013 10:41:28. View Article Online View Journal