Optik 125 (2014) 495–498 Contents lists available at ScienceDirect Optik jou rn al homepage: www.elsevier.de/ijleo Direct synthesis of natural dye mixed titanium dioxide nano particles by sol–gel method for dye sensitized solar cell applications S. Ananth, T. Arumanayagam, P. Vivek, P. Murugakoothan ∗ MRDL, PG and Research Department of Physics, Pachaiyappa’s College, Chennai 600 030, India a r t i c l e i n f o Article history: Received 27 February 2013 Accepted 1 July 2013 Keywords: Sol–gel Natural dye TiO2 nano particle DSSC a b s t r a c t Titanium dioxide (TiO 2 ) is an important photo anode material for solar energy conversion in dye sensitized solar cells (DSSC). Among the three polymorphs of TiO 2 namely, anatase, rutile and brookite, the anatase phase is having better photo catalytic activity. The anatase phase TiO 2 nano particles were synthesized by sol–gel method. A natural dye was mixed during the synthesis of TiO 2 nano particles itself to obtain dyed titanium dioxide nano particles which can be used as photo anode for DSSC. The juice extracted from beetroot which is rich in betanin was used as a natural dye sensitizer. The direct mixing of natural dye sensitizer during synthesis has yielded TiO 2 nano particles with uniform adsorption of dye and the photo anode material (TiO 2 + dye sensitizer) for the fabrication of DSSC was ready in a single step. The dye mixing during the synthesis of TiO 2 nano particles results very low dye aggregation. The pure and natural dye mixed TiO 2 nano particles were subjected structural, optical, morphological and compositional studies. DSSC was fabricated using this dye mixed TiO 2 nano particles into a thin film on fluorine-doped tin oxide (FTO) glass plate by ‘doctor-blade’ technique. The betanin sensitized DSSC showed a solar light energy to electron conversion efficiency of 0.69%. © 2013 Elsevier GmbH. All rights reserved. 1. Introduction The ever increasing demand for energy requires our quick action to utilize the renewable energy sources effectively. Solar cells are used to convert light photons from solar energy into elec- trons. Among the various solar cells, dye sensitized solar cell is an attempt to reproduce nature’s photosynthesis process [1]. The four major parts of DSSC are the photo electrode, the dye, the elec- trolyte and the counter electrode. Each part has its significant role in light to electron conversion efficiency (). In DSSC, the pho- tons are converted into electric current by charge injection from excited dye molecules into the conduction band of a wide band gap semiconductor [2]. The absorption of light by the metal oxide semiconductor is less due to its wide band gap and it affects the charge creation also. Hence, a sensitizer is needed and the light is absorbed by the sensitizer (dye) which is available on the surface of the metal oxide semiconductor. Charge separation takes place at the interface due to the photo-induced electron injection from dye to the conduction band of the semiconductor. This is in con- trast to the conventional silicon based solar cells, where the metal oxide semiconductor performs both the task of light absorption and charge separation [3]. The dye molecule was regenerated by a redox (for example I + /I 3 - ) system, which itself is regenerated at ∗ Corresponding author. Tel.: +91 9444 447 586. E-mail address: murugakoothan03@yahoo.co.in (P. Murugakoothan). the counter electrode by electrons passing through the load [4]. The metal oxide semiconductors that can be used as photo anode are TiO 2 , ZnO, MnO 2 , etc. The most common semiconductor used in DSSC is TiO 2 (anatase) due to its superior properties, like non- toxic, inexpensive, good chemical stability, bio compatibility, etc. [5,6]. The photo catalytic activity of TiO 2 depends on its crystalline nature, defects on the surface, photon absorption ability, particle size and surface area [7]. The sensitizer can be a natural dye or a chemical one. The chemical dyes are toxic and very expensive. The natural dyes are very cheap and available in plenty in the form of plant parts, like flowers, seeds, barks, leaves, stem, etc. By keeping the environment factor as first priority than efficiency, we use nat- ural dye as sensitizer and tried to improve the efficiency of DSSC. Unlike the conventional silicon based solar cells, DSSC does not need high purity and other requirements. So, it can be fabricated at a comparatively very low cost. Even then, the key parameters have to be optimized for getting maximum efficiency [8]. The sol–gel method is a versatile technique used for the syn- thesis of TiO 2 in anatase phase. In sol–gel process, a colloidal suspension or a sol is formed from the hydrolysis and poly- merization reactions of the precursors [9]. The hydrolysis and condensation take place immediately when the alkoxide precursor reacts in the presence of water leading to the formation of larger and inhomogeneous product [10]. Complete polymerization and loss of solvent leads to the transition from the liquid sol into a solid gel phase. After drying, uniform nano particles were obtained. In the present study, pure TiO 2 nano particles were synthesized using 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.07.018