Impedance spectroscopy study of solid-state dye-sensitized solar cells with varying Spiro-OMeTAD concentration
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1211-R06-03
Impedance spectroscopy study of solid-state dye-sensitized solar cells with varying Spiro-OMeTAD concentration Márcio S. Góes,1 Francisco Fabregat-Santiago,2 Paulo R. Bueno,1 Juan Bisquert2 Departamento de Físico-Química, Instituto de Química de Araraquara, Universidade Estadual Paulista, R. Prof. Francisco Degni s/n, 14800-900 Araraquara SP, Brazil. 2 Photovoltaic and Optoelectronic Devices Group, Departament de Física, Universitat Jaume I, Av. Sos Baynat, s/n, 12071 Castelló, Spain. 1
ABSTRACT This work reports on the changes of performance of solid-state cells dye-sensitized solar cells with the variation of concentration of spiro-OMeTAD between 5% and 25% in the fabrication of the cell. Variations of charge recombination and capacitance correlate with the improvement of current-potential characteristics at increasing spiro-OMeTAD content, which is explained by reduction of transport resistance for hole transport, the increase of charge separation in the dye molecules, and importantly, with the increase of the β-factor in the recombination resistance, that causes a reduction of the diode ideality factor. INTRODUCTION There has been a great interest recently in the study and improvement of solid dyesensitized solar cells using spiro-OMeTAD hole conductor [1, 2]. It has been observed that filling the pores of nanoporous TiO2 with the hole conductor poses some difficulties, and this limits the practical thickness of the active film, and consequently, the total light absorption and the photocurrent. On another hand, this kind of cells is rapidly increasing efficiency which has allowed a quantitative study of the electronic aspects of the device and the main photovoltaic parameters using impedance spectroscopy (IS) [3, 4]. While electronic aspects of the higher performance DSC using liquid electrolytes are well known [5, 6], the operation of the solid hole transport material (HTM) in the DSC requires further investigation [7]. Therefore we decided to study a set of solid DSCs where the OMETAD content in the TiO2 pores was progressively increased from only 5% to 25% and here we report the main results. EXPERIMENT The solid-state DSCs were prepared according to a previous report [2]. The fluorine doped tin oxide (FTO) coated glass sheets (15 Ω/□, Hartford glass) were etched with zinc powder and HCl (1N) to give the required electrode pattern. The sheets were subsequently cleaned with soap (2% helmanex in water), distilled water, acetone, methanol and finally treated under oxygen plasma for 10 min to remove any organic residues. The FTO sheets were then coated with a compact layer of TiO2 (100 nm) by spray pyrolysis deposition at 450º C using air as the carrier gas. A Dyesol TiO2 nanoparticle paste was doctor bladed onto the compact TiO2 to give dry film thickness between 1.6 and 1.8 µm, governed by the concentration of the paste. These sheets were then slowly heated to 500 ºC (ramped over 30 min) and baked at this temperature for 30 min under an oxygen flow. After cooling, the sheets were cut into slides o
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