Publication date: 1st April 2013
Due to their unique features, semiconductor quantum dots (QDs) are often presented as the ultimate frontier as sensitisers for photoelectrochemical solar cells.[1],[2] Up to now, the most interesting results in terms of device performances have been obtained by using polidisperse, in situ generated QDs by means of successive ionic layer absorption and reaction (SILAR) technique.[3],[4] This approach allows obtaining naked QDs directly grown on the porous structure of the metal oxide based photoanodes, thus guaranteeing an intimate contact between the two interfaces. Moreover, the deposition of networks of QDs presenting absorption features able to collect a wider region of the solar spectrum is easily possible.[5]
This study is focused on the application of spray deposition (SD) to the SILAR technique to generate CdS and PbS QDs on solar cell photoanodes. Spray techniques are well known for being powerful and flexible tools for the deposition of a large variety of materials, allowing a fine modulation of the material aspect according to the working parameters. A systematic investigation of QD deposition by usually adopted immersion SILAR and SD-SILAR demonstrate that the use of the latter approach systematically results in higher amount of QDs deposited on TiO2 scaffold, together with smaller nanocrystals (Figure 1, left).
It is worth noting, moreover, that a reduced amount of chemicals is needed for the preparation of QDs and no wastes are produced, thus significantly decreasing the environmental impact of the procedure. SD provides for a highly homogeneous coverage of the TiO2 photoanodes for the whole depth of the substrate (Figure 1, right). Evaluation of the performances of the quantum dot-sensitized solar cells indicates that devices prepared via SD-SILAR present improved and very reproducible functional properties, especially related to photoconversion efficency and generated photocurrent density, both of them being almost two-fold the corresponding prepared by immersion SILAR.
Figure 1. (a) Comparison between the calculated sizes of CdS QDs deposited at different temperatures by SD-SILAR(hollow markers) and immersion SILAR (full markers). (b): Cross section SEM images and related EDX elemental mapping of TiO2 nanoparticulate film on FTO glass sensitized with CdS or PbS QDs via spray deposition of immersion SILAR.(c)Cell parameters (top to bottom: PCE, Voc, Jsc) according to the number of SD-SILAR cycles.
Rühle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized Solar Cells. Chem. Phys. Chem. 2010, 11, 2290-2304. Shabaev, A.; Efros, A. L.; Nozik A. J. Multiexciton Generation by a Single Photon in Nanocrystals. Nano Lett. 2006, 6, 2856-2863 Lee Y-L.; Lo Y-S. Highly Efficient Quantum-Dot-Sensitized Solar Cell Based on Co-Sensitization of CdS/CdSe. Adv. Func. Mat. 2009, 19, 604-609 Lee H.; Wang M.; Chen P.; Gamelin D.R.; Zakeeruddin S.M.; Grätzel M.; Nazeeruddin Md. K. Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process. Nano Lett. 2009, 9, 4221-4227 Braga A.; Gimenez S.; Concina I.; Vomiero A.; Mora-Seró I. Panchromatic Sensitized Solar Cells Based on Metal Sulfide Quantum Dots Grown Directly on Nanostructured TiO2 Electrodes J. Phys. Chem. Lett. 2011, 2, 454-460
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