Solar Energy Materials & Solar Cells 91 (2007) 1611–1617 Unsymmetrical alkoxy zinc phthalocyanine for sensitization of nanocrystalline TiO 2 films L. Giribabu a,Ã , Ch. Vijay Kumar a , V. Gopal Reddy a , P. Yella Reddy a , Ch. Srinivasa Rao a , Song-Rim Jang b , Jun-Ho Yum b , Md. K. Nazeeruddin b , M. Gra¨tzel b a Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500007, India b Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland Received 13 March 2007; received in revised form 1 May 2007; accepted 7 May 2007 Available online 27 June 2007 Abstract A new photosensitizer, unsymmetrical alkoxy zinc phthalocyanine based on ‘push–pull’ concept, has been synthesized and fully characterized by CHN, MALDI-TOF, UV–Vis, fluorescence spectroscopies and cyclic voltammetry. The new phthalocyanine photosensitizer has eight alkoxy and two carboxyl groups that act as electron releasing (push) and withdrawing (pull), respectively. Moreover, the alkoxy groups increase the solubility of the new photosensitizer in common organic solvents, and the two carboxyl groups serve to graft on to nanocrystalline TiO 2 . The new photosensitizer was tested in dye-sensitized solar cells and its performance was compared with PCH001. r 2007 Elsevier B.V. All rights reserved. Keywords: Dye-sensitized solar cells; Phthalocyanine; Alkoxy groups; Unsymmetrical; Redox electrolyte 1. Introduction Nanocrystalline dye-sensitized solar cells (DSSCs) are currently attracting widespread interest for the conversion of sunlight into electricity because of their low cost and high conversion efficiency [1,2]. In these cells, dye sensitizer is one of the key components for high power conversion efficiencies. The most successful charge-transfer sensitizers employed so far in such cells are bis(tetrabutylammonium)- cis-di(thiocyanato)-bis(4,4 0 -dicarboxy-2,2 0 -bipyridine)r- uthenium(II) (the N719 dye) and trithiocyanato 4,4 0 4 00 - tricaboxy-2,2 0 :6 0 ,2 00 -terpyridine ruthenium(II) (the black dye), produced solar-energy-to-electricity conversion effi- ciencies (Z) of up to 11% under AM 1.5 irradiation and stable operation for millions of turnovers [3,4]. In spite of this, the main drawbacks of these sensitizers are the lack of absorption in the red region of the visible spectrum and also relatively low molar extinction coefficient above 600 nm. In contrast, phthalocyanines possess intense absorption bands in the near-IR region and are known for their excellent chemical, light and thermal stability; and have appropriate redox properties for sensitization of wide-bandgap semiconductors, e.g., TiO 2 , rendering them attractive for DSSC applications [5]. Several groups have tested phthalocyanines as sensitizers for wide-bandgap oxide semiconductors, although they all have reported power conversion efficiencies around 1% until recently [6–10]. The low efficiency of cells incorporat- ing phthalocyanines appears to be due to aggregation, solubility and lack of directionality in the excited state. One of the essential requirements for the light-harvesting systems of a molecular/semiconductor junction is that the sensitizer possesses directionality of its electronic orbitals in the excited state. This directionality should be arranged to provide an efficient electron transfer from excited sensitizer to the TiO 2 conduction band by good electronic coupling between the lowest unoccupied molecular orbital (LUMO) of the sensitizer and Ti 3d orbital. ARTICLE IN PRESS www.elsevier.com/locate/solmat 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.05.004 Ã Corresponding author. Tel.: +91 40 27193186; fax: +91 40 27160921. E-mail address: giribabu@iict.res.in (L. Giribabu).