The improvement of light scattering of dye-sensitized solar cells aided by a new dandelion-like TiO 2 nanostructures P.S. Musavi Gharavi, M.R. Mohammadi n Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, Tehran, Iran article info Article history: Received 3 December 2014 Received in revised form 27 December 2014 Accepted 29 January 2015 Keywords: Dandelion-like TiO 2 nanostructures Scattering layer Hydrothermal process Dye-sensitized solar cells. abstract We present a new dandelion-like TiO 2 spheres by a modified hydrothermal method for manufacture of dye-sensitized solar cells (DSSCs). This construct is composed of numerous nanowires for employment as the scattering layer of DSSCs. Such morphology is produced by nucleation-growth-assembly mechanism. The size of the dandelion-like spheres and their morphology can be tailored by controlling the processing parameters of the modified hydrothermal process. TiO 2 nanoparticles with narrow size distribution are also synthesized by hydrothermal route for the active layer of DSSCs. The nanoparticles show pure anatase phase with average size of 40 nm, whereas the dandelion-like TiO 2 spheres are pure rutile structure with average diameter in the range 5–9 μm. A systematic study is performed in order to improve the photovoltaic performance of the DSSCs with different arrangement modes. First, the monolayer DSSCs with various thicknesses are prepared using pure nanoparticles. The highest power conversion efficiency (PCE) of 7.4% is obtained for the monolayer cell with thickness of 31 μm. Second, the double layer DSSCs containing an under-layer and an over-layer are reported. The double layer DSSC made of nanoparticles as the under-layer (with thickness around 31 μm) and mixtures of nanoparticles and dandelion-like spheres as the over-layer (as light scattering layer with thickness around 8 μm) shows the highest PCE of 8.3%. The improvement of cell efficiency is explained by triple function mechanism including significant increase in light scattering, dye sensitization and photo-generated charge carriers. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide (TiO 2 ), a semiconductor with three main alotro- pies including rutile, anatase and brookite, exhibits a wide range of potential technological applications such as photocatalysis of pollu- tants [1], gas sensors [2], energy-storage devices [3], self-cleaning coatings of windows and tile [4] and transparent conducting electro- des for dye-sensitized solar cells (DSSCs) [5]. In addition, not only it has numerous applications in industry, but also encompasses a great number of advantages same as being inexpensive and safe and compatibility with the body and environment. The main stream of the research on DSSCs has been focused on development of materials which would enhance the conversion efficiency, simplify the production of the cell and assure their long- lifetime. In order to reach high conversion efficiencies, it is important to increase the electron injection and optical absorption. Since the electrode has huge surface area per projected area, solar cells made of dye-adsorbed nanoporous TiO 2 can drastically increase effective light absorption. Light scattering is already a well-known approach for boosting the optical absorption of photoelectrode in the conventional DSSCs [6]. In order to improve light scattering and dye loading simultaneously, double-layered films containing TiO 2 nanoparticles as the under-layer and TiO 2 nanowires [7], corn-like nanowires [8], nanospindles [9], nanotubes [10], nanofibers [11], nanorod-aggregates [12], hollow sphere particles [13] and multiwalled carbon nanotube– TiO 2 nanocomposite [14] as the over-layer has been proposed. Many efforts have been aimed to prepare TiO 2 nanostructures with various morphologies, including nanotubes [15], nanorods [16], three- dimensional (3D) hierarchical structures with hollow spheres [17], nanoflowers [18] and dandelion-like TiO 2 structure [19]. However, these morphologies have been employed for other applications rather than DSSCs. It seems that the dandelion-like rutile TiO 2 is an attractive morphology for the DSSC applications due to its facile electron transport and enhanced visible light scattering and absorption. On the other hand, rutile phase impresses an immense influence on scattering layer of DSSC due to its higher refractive index compare with the other phases of titania. Bai et al. [19] synthesized dandelion- like rutile TiO 2 structure containing several nanorods by the hydro- thermal method using TiCl 3 as the main starting material as well as HCl, NaCl and NaOH. Such structure was employed for degradation of Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2015.01.035 0927-0248/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ98 21 6616 5211; fax: þ98 21 6600 5717. E-mail address: mohammadi@sharif.edu (M.R. Mohammadi). Solar Energy Materials & Solar Cells 137 (2015) 113–123 Downloaded from http://www.elearnica.ir