Synthesis and characterization of Nb, F-codoped titania nanoparticles for dye-sensitized solar cells

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, F-codoped TiO2 (NFT) nanoparticles are prepared via hydrothermal processes using Nb2O5 and hydrofluoric acid as doping source. Nb and F distribute homogeneously in the NFT nanoparticles as shown in scanning transmission electron microscopy elemental mappings. The codoping of Nb and F improves the crystallinity of TiO2 significantly and increases the Ti31 concentration, which results in the enhancement of electron injection and in the increase of the charge-transfer ability in dye-sensitized solar cells. The relative energy conversion efficiency can be 66.1% higher than that of the cell, based on pure TiO2, when the Nb:F:Ti molar ratio is about 0.03:0.15:0.97.

I. INTRODUCTION

Since dye-sensitized solar cells (DSSCs) based on the photosensitization of titania nanoparticles were first designed by Oregan and Graetzel,1 the concept has become one of the most promising alternative candidates to conventional photovoltaic cells.2 Research on the anode materials, such as TiO2, ZnO, and Nb2O5, is one of the most valued fields.3–6 Among these wide-band oxides, nanostructured porous TiO2 polycrystalline films have been the focus of a great deal of research,7 due to their low cost and acceptable energy conversion efficiency. Various methods are used to modify the TiO2, and doping has drawn much attention due to its convenience and performance diversification. TiO2 used as the anode material in DSSCs, in general, should be an n-type semiconductor. The n-type TiO2 can be obtained in various ways, either by Ti cation substitution with nonisovalent elements (Nb, Sb, V, Mo, and Ni)8,9 or by O2 anion replacement with extrinsic impurities (F, N, and S),10 as well as by chemical reduction (oxygen vacancies, Ti interstitials).11 Nb-doped TiO2 is obtained via the replacement of tetravalent Ti with pentavalent Nb during formation of Ti31 species, which contribute to a higher conductivity.12–14 Nb-doped TiO2 has been reported to hold promising applications in the DSSCs.13,15 However, the mechanism and structural characteristics of the effects caused by ion doping are still controversial. In addition, expensive niobium chloride or niobium alkoxide is used as the Nb-doping source in most preparations of Nb-doped TiO2 semiconductor, and these preparation techniques a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.379 230

J. Mater. Res., Vol. 29, No. 2, Jan 28, 2014

http://journals.cambridge.org

Downloaded: 14 Mar 2015

require numerous tedious steps,16 which limit the development of this material to some extent. F-doped TiO2, which is realized via the substitution of O2 anion by single-charged F anion, has also attracted close attention, since nonmetal-doped TiO2 is less expensive and more thermally stable.17 Most of the previous studies focused on its photocatalytic performance, where the existence of a certain amount of the Ti31 states in F-doped TiO2 played an important role, such as the reduction of the electron and hole recombination rate and the enhancement of the photocatalytic activity