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Structural and optical characterization of electrodeposited CdSe in mesoporous anatase TiO2 for regenerative quantum-dot-sensitized solar cells

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 23 (2012) 395401 (11pp)

doi:10.1088/0957-4484/23/39/395401

Structural and optical characterization of electrodeposited CdSe in mesoporous anatase TiO2 for regenerative quantum-dot-sensitized solar cells Frédéric Sauvage1,2 , Carine Davoisne1 , Laetitia Philippe3 and Jamil Elias3 1 Laboratoire de Réactivité et Chimie des Solides, Université de Picardie Jules Verne, CNRS UMR7314, 33 rue Saint-Leu, F-80039 Amiens Cedex, France 2 Laboratoire de Photonique et Interfaces, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 6, CH-1015, Lausanne, Switzerland 3 Laboratory for Mechanics of Materials and Nanostructures EMPA—Materials Science and Technology, Feuerwerkstrasse 39, 3602 Thun, Switzerland

E-mail: [email protected]

Received 30 May 2012, in final form 31 July 2012 Published 12 September 2012 Online at stacks.iop.org/Nano/23/395401 Abstract We investigated CdSe-sensitized TiO2 solar cells by means of electrodeposition under galvanostatic control. The electrodeposition of CdSe within the mesoporous film of TiO2 gives rise to a uniform, thickness controlled, conformal layer of nanostructured CdSe particles intimately wrapping the anatase TiO2 nanoparticles. This technique has the advantage of providing not only a fast method for sensitization (24 h) owing to the

lack of a driving force for chemisorption. As a consequence, these photo-active films exhibit both lower optical density and lower photovoltaic performances in comparison to the direct methods of sensitization. In this work, we report on the use of the electrodeposition technique to introduce CdSe as a light absorber into a screen-printed mesoporous structure of TiO2 . The major advantage of this technique, which has already been used for extremely thin absorber (ETA) technology with either CdS or CdSe in combination with ZnO nanostructures [53–58], comes from its fast and single step process of sensitization (i.e. 98%), sodium sulfite 0.4 M (Na2 SO3 Fluka, purity > 98%) and selenium 0.2 M (Se, Riedel de Haën ∼99%). Selenium was dissolved in 0.4 M Na2 SO3 aqueous solution at 60 ◦ C and stirred for 5 h. After preparation, the solution can be used for few weeks. The pH was adjusted to 8.5 with diluted acetic acid solution (∼3%). The electrodeposition was carried out galvanostatically at room temperature in a two-electrode electrochemical cell. The working electrode was a TiO2 /ITO cathode and the counter electrode was a Pt spiral wire. The applied current density was −3 mA cm−2 and the charge density was varied between 30 and 300 mC cm−2 . 3.3. Material characterization The structural characteristics of TiO2 and CdSe were analysed by x-ray diffraction using a Bruker D8 Advance diffractometer in a (θ–2θ ) configuration with Cu Kα1 ˚ The thickness of the printed radiation (λ = 1.54 056 A). semi-conductor films were measured using a KLA Tencor alpha-step 500 surface profiler. The optical properties of TiO2 and CdSe/TiO2 were characterized by an Ocean Optics

3.5. Photovoltaic measurements A 450 W xenon light source (Oriel, USA) was used to provide an incident irradiance of 100 mW cm−2 at the surface of the solar cells. The spectral output of the lamp was filtered using Schott K113 Tempax sunlight filter glass (Präzisions Glas & Optik GmbH, Germany), which enables transmission of light 9

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acknowledged for fruitful discussions. EJ acknowledges the Bundesamt für Energie (BFE) project, Switzerland, for partially financing this work.

from 350 to 750 nm and hence reduces the light mismatch between real solar illumination and the simulated one to less than 2%. Light intensities were regulated with wire mesh attenuators. The (J–V) measurements were performed using a Keithley model 2400 digital source meter (Keithley, USA) by applying independently an external voltage to the cell and by measuring the photo-generated current out from the cell. Incident photon-to-current conversion measurements were realized using a 300 W xenon light source (ILC Technology, USA). A Gemini-180 double monochromator (Jobin Yvon Ltd (UK)) was used to select and vary the irradiation wavelength to the cell. The monochromatic incident light was passed through a chopper running at 1 Hz frequency and the on/off ratio was measured by an operational amplifier. This was superimposed on a white light bias corresponding to 10 mW cm−2 intensity.

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4. Conclusions To conclude, we have successfully introduced another direct method to sensitize TiO2 mesoporous films using quantum dots. The strong points of the electrodeposition technique are its rapid sensitization process, excellent control of QD loading together with particle size and control on the morphology depending upon the deposition conditions. Finally, it is a versatile technique that affords an easily scaling process. In this work, we have shown that using galvanostatic deposition, by applying −3 mA cm−2 current density, we achieved a homogeneous and conformal deposition of CdSe within the mesopores of randomly distributed nanoparticles of TiO2 . The films obtained are composed of the hexagonal polymorph. The growth of CdSe on TiO2 is found to proceed in a hierarchical way, starting from a TiO2 /CdSe core–shell structure followed by an increased CdSe particle size. The films obtained are strongly opaque to light and the preliminary power conversion efficiency is 0.8% under A.M. 1.5 G–100 mW cm−2 illumination conditions (Voc = 485 mV, Jsc = 4.26 mA cm−2 , ff = 0.37) in association with a new regenerative redox couple based on cobalt polypyridil complex. Although this performance remains modest in comparison to the best performing CdSe-based solar cells, we strongly believe that the performance of these devices could be much improved and would likely attain or overtake the actual state of the art QD-solar cells after optimizing the host TiO2 mesostructure as well as introducing point defects in TiO2 to promote faster charge injection and greater charge collection efficiency. A detailed study on how to produce a TiO2 film containing higher porosity and larger pore size, thus allowing incorporation of a thicker CdSe quantum dot layer, while displaying optimized intra-band acceptor energies to increase the APCE is currently underway.

Acknowledgments FS acknowledges Professor Michael Graetzel for having measured these cells in his laboratory and Dr Shaik Zakeeruddin for the preparation of the electrolyte. Dr Mohammad K Nazeeruddin and Pascal Comte are also 10

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