Dependence of energy conversion efficiency of dye-sensitized solar cells on the annealing temperature of TiO 2 nanoparticles Masood Hamadanian a,b , Vahid Jabbari a,n , Afshar Gravand a a Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran b Department of Physical Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran article info Available online 1 February 2012 Keywords: Nanoparticle Titanium dioxide Porous structure DSSCs Annealing temperature abstract Dye-sensitized solar cells (DSSCs) were fabricated from porous electrodes derived from sol–gel-synthesized (SGS) nanoparticles (NPs) of TiO 2 . Current–voltage measurements were performed to investigate performance characteristics of electrodes derived from SGS-NPs of TiO 2 annealed at different temperatures. Experimental results indicate that the effects of bulk traps and surface states within TiO 2 films on recombination of photo-injected electrons in DSSCs depend upon annealing temperature of SGS-TiO 2 NPs. Moreover, electrodes fabricated from SGS-TiO 2 showed higher photoelectric conversion efficiency than nonporous commercial (P25) TiO 2 NPs. Porous structures within SGS-TiO 2 NPs are of great benefit to sensitizer dye adsorption, and consequently to improvement of photo-electrochemical properties of DSSCs. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction The development of clean alternatives to current power generation methods is imperative due to the increasing global demand for energy sources and heightened environ- mental awareness concerning climate change. Although solid-state photovoltaic devices promise to offer a solution to these problems, current silicon-based photovoltaic tech- nologies are hindered by processing-based pollution and high production cost. As a novel renewable and clean solar- to-electricity conversion system, dye-sensitized solar cells (DSSCs) are considered viable substitutes for conventional silicon-based photovoltaic cells since the pioneering work reported by Gr¨ atzel [1], and subsequent attention due to their low cost and simple fabrication processes [2]. Generally, DSSCs are mainly composed of three parts: dye-sensitized semiconductor nanocrystalline film photo- anode, redox couple (usually I 3 /I  ) in organic solvent(s), and platinized transparent conducting oxide (TCO) glass as counter electrode. Among those, photoanode consists of porous semoconductor nanocrystalline film adsorbed by dye molecules (such as a ruthenium complex), which can absorb the light energy especially visible light of sunshine [3,4]. Dye molecules move to the excited states when DSSCs is exposed to light irradiation with suitable energy, and excited-state electrons are injected quickly into the conduction band of semoconductor; these injected electrons are collected through conducting sub- strate of the photoanode. Dye molecules can be regener- ated by iodide ions in the electrolyte, and then resulting triiodide ions can accept electrons from the platinized TCO counter electrode to fulfill a complete current cycle in DSSCs [5] (Fig. 1). TiO 2 is one of the most common materials that can be employed to make porous film electrode of DSSCs. Mor- phology and particle size of TiO 2 play critical roles in photoelectric conversion efficiency of DSSCs [6–13]. For example, TiO 2 materials with different morphologies, such as nanoparticles [8], nanotubes [9–11], nanowires [12], and nanofibers [13], have been applied to fabricate the porous film electrodes. Nakade and his co-workers Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/mssp Materials Science in Semiconductor Processing 1369-8001/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2011.12.004 n Corresponding author. Tel.: þ98 361 5912382; fax: þ98 361 5552930. E-mail address: vahid_jabbari.azeri@yahoo.com (V. Jabbari). Materials Science in Semiconductor Processing 15 (2012) 371–379