Optimization of a Quasi-Solid-State Dye-Sensitized Photoelectrochemical Solar Cell Employing a Ureasil/Sulfolane Gel Electrolyte** By Elias Stathatos, Panagiotis Lianos,* Angela Surca Vuk, and Boris Orel 1. Introduction Dye-sensitized photoelectrochemical solar cells (DSPECs) are considered as the most promising low-cost option for the photovoltaic conversion of solar energy. Since the original model cell reported in 1991 by O'Regan and Grätzel, [1] an ever-growing interest has been demonstrated by scientists of different disciplines so that a vast knowledge on these systems has already been accumulated. As a consequence, a more or less standard configuration of DSPECs has been established, as described in the multilayer structure appearing in Figure 1A. The functional components are sandwiched between two fluorine-doped tin oxide electrodes (FTO, SnO 2 :F). A thin mesoporous TiO 2 film is deposited on the negative electrode. A dye-sensitizer is adsorbed and covalently bound on TiO 2 nanoparticles, typically cis-di(thiocyanato)-N,N-bis(2,2¢-bipyri- dyl-4,4¢-dicarboxylic acid)ruthenium(II) dihydrate [RuL 2 (NC- S) 2 ]´2H 2 O. [2] The counter electrode consists of FTO with ther- mally deposited Pt. Finally, the space between the TiO 2 /dye surface and counter electrode is filled with an electrolyte that typically contains the I 3 ± /I ± redox couple, that makes a conveni- ent combination with TiO 2 and [RuL 2 (NCS) 2 ]´2H 2 O. [3,4] The overall efficiency of a DSPEC employing a liquid electrolyte is around 10 %, [2] that is, better than commercial photovoltaic cells based on amorphous silicon. However, some concern has been expressed recently as to the long-term stability of this type of DSPEC, owing to long-term sealing problems or sol- vent endurance against photochemical degradation. For this reason, an effort has been made to substitute liquid-electrolyte cells by quasi-solid-state constructions. One such approach has been adopted by us, by proposing nanocomposite organic/inor- ganic gels, deposited by sol±gel chemistry, as hosts of the I 3 ± /I ± redox couple. [5±7] These materials gave a substantial improve- ment of the concept of the dye-sensitized solid-state solar cell (DSSC). DSSCs employing a gel electrolyte have a lower over- all efficiency than liquid-electrolyte DSPECs, due to restric- tions in ionic mobility. However, the advantages they offer compensate for the lower ionic conductivity. In this respect, and in order to obtain an appreciable ionic conductivity, it is necessary that the nanocomposite gel provides a percolating ion-conducting organic subphase and this is facilitated when a solvent is incorporated into the gel. The I 3 ± /I ± redox couple is usually obtained by co-dissolving an iodide salt with iodine. A rather limited number of organic solvents containing polar functional chemical groups (for example, propylene carbonate, acetonitrile, etc.) can simultaneously dissolve an alkaline io- dide salt and iodine. However, no solvent can be equally good for both substances, and for this reason crystallization, for ex- ample, of KI, has been a major source of deterioration of the cell. One approach to solve this problem is to use ionic liquids, for example alkylimidazolium iodides, [8,9] which have low melt- ing points and are liquids at normal temperatures. This approach has been very successful. However, it is not unique. We have recently studied combinations of nanocomposite gels with different functional solvents and searched for the ªidealº solvent for the KI/I 2 couple, which would be immune to the above problems and offer DSSCs with maximum efficiency and stability. We have thus found very encouraging results by combining ureasil gels with sulfolane. Cells made in this man- ner are stable and efficient, and are even more efficient when working at elevated temperatures. 2. Results and Discussion The cells have the standard configuration shown in Fig- ure 1A. The I 3 ± /I ± gel electrolyte was synthesized using sol±gel chemistry and a ureasil precursor composed of a short poly- Adv. Funct. Mater. 2004, 14, No. 1, January DOI: 10.1002/adfm.200304479  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 45 ± [*] Prof. P. Lianos, Dr. E. Stathatos Engineering Science Department, University of Patras GR-26500 Patras (Greece) E-mail: lianos@upatras.gr Prof. B. Orel, Dr. A. Surca Vuk National Institute of Chemistry Hajdrihova 19, SLO-1001 Ljubljana (Slovenia) [**] This work was supported by the following research grants: ªK.KAPA- HEODXPHRº of the University of Patras and the Greece±Slovenia Bilateral R&T Cooperation Program. A quasi-solid-state, dye-sensitized photoelectrochemical solar cell employing a gel electrolyte obtained by sol±gel chemistry is described. The gel electrolyte is based on a ureasil precursor (i.e., a poly(propylene oxide) oligomer end-capped by triethoxysi- lane groups through urea bridges) and sulfolane and it incorporates the I 3 ± /I ± redox couple. It is shown that the combination of these two reagents prevents crystallization of KI, thus ensuring a long life for the cell and a satisfactory overall efficiency that surpasses 5 %. Cell efficiency increases with temperature. Optimization of gel-electrolyte performance has been obtained by studying mobility with fluorescence-quenching techniques complemented by direct-current conductivity measurements. FULL PAPER