JOURNAL OF RARE EARTHS, Vol. 29, No. 8, Aug. 2011, P. 783 Foundation item: Project supported by the Polish Ministry of Science and Higher Education (N N204 023538) Corresponding author: Maciej Zalas (E-mail: maciej.zalas@amu.edu.pl; Tel.: +48618291002) DOI: 10.1016/S1002-0721(10)60542-X Increase in efficiency of dye-sensitized solar cells by porous TiO 2 layer modification with gadolinium-containing thin layer Maciej Zalas 1 , Mariusz Walkowiak 2 , Grzegorz Schroeder 1 (1. Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 PoznaĔ, Poland; 2. Institute of Non-Ferrous Metals, Branch in PoznaĔ, Central Laboratory of Batteries and Cells, Forteczna 12, 61-362 PoznaĔ, Poland) Received 27 August 2010; revised 9 June 2011 Abstract: Modified with gadolinium-containing layer, nanoporous titania electrode and its application in dye-sensitized solar cells were reported. The electrode prepared was characterized with UV-Vis and X-ray diffraction (XRD) techniques. The amount of gadolinium was measured with inductively coupled plasma-optical emission spectrometry (ICP-OES) experiments. The modified electrode showed reduced N3 dye adsorption ability, but increased light conversion efficiency in comparison with the non-modified electrode. The overall conversion efficiencies, determined under 400 W/m 2 irradiation with tungsten- halogen lamp at room temperature, were 0.55% for non-modified and 0.74% for modified electrodes. Keywords: dye-sensitized solar cell; surface modification of TiO 2 ; Gd-doped TiO 2 ; rare earths Fossil fuels make 80% of energy carriers consumed over the world. Strictly limited resources of fossil fuels, combined with rapid increase in the world energy demand and com- bustion gases emission coupled with greenhouse effect have stimulated development of the green technologies of energy production. The solar to electric power conversion is the most promising alternative to fossil fuels use [1] . Conventional solar cells are built of ultra pure very expen- sive silicon materials, obtained by high energy consuming methods with the use of very toxic chemicals. For the above reasons silicon solar cells have not become very popular in spite of their high energy conversion efficiency (between 15% and 20%) [1,2] . In 1991, Grätzel et al. [3] first presented an innovative solar cell based on dye-sensitized colloidal TiO 2 . The cells of this type can work with efficiencies comparable to those of the silicon ones, but they can be constructed with materials of lower purity and being environmental-friendly. Since that publication of the dye-sensitized solar cells (DSSCs) have become a low-cost alternative for conven- tional silicon constructions [1–6] . The typical dye-sensitized solar cell consists of the fol- lowing parts [1–6] : - Photoactive electrode - transparent conducting oxide glass (TCO) coated with porous semiconducting oxide layer, usu- ally TiO 2 ; - Visible light absorbing dye adsorbed on porous oxide layer; - Electrolyte - iodide/triiodide redox couple solution; - Counter electrode - TCO coated with thin platinum layer. The excited electrons, from the visible light illuminated dye, are injected into the conduction band of the oxide mate- rial making a porous electrode and migrate to the TCO sur- face. From the TCO surface the electrons are transferred to the counter electrode through the external circuit, and reduce the oxidized form of the electrolyte, which is then able to regenerate oxidized molecules of the dye by donation of electrons, simultaneously the oxidized form of the electro- lyte is reproduced. The above-described process continues as long as illumination is applied or all parts of the cell are de- activated. The attention of the researches is focused on enhancement of performance of DSSCs in four areas [7] : - Development of sensitizing materials (dyes); - New anodic materials; - Use of various electrolytes; - Modification of anodic materials. The latter modification is caused by decrease in the cell efficiency because of the recombination of the electrons (in- jected into the conducting band of the TiO 2 layer) with the oxidized dye and/or the oxidized form of the redox couple of the electrolyte. Modification of TiO 2 electrodes by its cover- ing with a thin layer of an insulator or other semiconductor can generate an energetic barrier for the electron back- transfer and improve the photoelectrical properties of DSSCs [7–10] . If the modifying semiconductor has a slightly more negative potential of the conduction band level than TiO 2 , but less negative than the excited state of the sensitiz- ing dye, the photoinduced electrons can still be easily in- jected into the TiO 2 conduction band through the external modifying semiconductor layer. After the injection process, the energy of the electrons present in TiO 2 conduction band corresponds to that of the bandgap of the external semicon-