NANO LETTERS
Binary Polyethylene Oxide/Titania Solid-State Redox Electrolyte for Highly Efficient Nanocrystalline TiO2 Photoelectrochemical Cells
2002 Vol. 2, No. 11 1259-1261
Thomas Stergiopoulos, Ioannis M. Arabatzis, Georgios Katsaros, and Polycarpos Falaras* Institute of Physical Chemistry, NCSR “Demokritos”, 153 10 Aghia ParaskeVi Attikis, Athens, Greece Received September 17, 2002; Revised Manuscript Received October 2, 2002
ABSTRACT Poly(ethylene oxide)/titania polymer electrolyte based photoelectrochemical cells have been fabricated with Ru(dcbpy)2(NCS)2 complex as the sensitizer and nanoporous TiO2 films as photoanodes. The introduction of the titania filler into the poly(ethylene oxide) matrix reduces the crystallinity of the polymer and enhances the mobility of the I-/I3- redox couple, resulting in outstanding overall conversion efficiency (4.2% under direct sunlight illumination) of the corresponding dye-sensitized nanocrystalline TiO2 solar cell, one of the best efficiencies reported to date for a solid-state device.
Dye-sensitized solar cells present conversion efficiencies as high as 10-12% and are considered as an alternative solution to conventional photovoltaic modules.1,2 Without neglecting the need of optimization of the nanostructured film electrode and the photosensitizer, the composition of a stable and efficient electrolyte remains one of the key challenges for practical applications. The problem is closely related to the solvent evaporation and leakage of the electrolyte, which directly affect the stability and long-term operation of the cell. In this regard, efforts have been made to replace the liquid electrolyte with room-temperature molten salts, inorganic p-type semiconductors, ionic conducting polymers, and organic hole transport materials.5,6 The solid photoelectrochemical cells present low conversion efficiencies compared with the liquid version because of the incomplete wetting of the photoelectrode by the electrolyte. In the cases of quasisolid cells, the low efficiency is mainly due to mass transport limitations of the ions.7 These cells also contain a significant quantity of volatile compounds, which at high temperatures results in high vapor pressure and serious sealing problems. To improve the mechanical, interfacial, and conductivity properties of the polymer electrolytes, in this paper we are concerned with an alternative approach, which consists of developing a solvent-free solid electrolyte by the addition of “solid plasticizers” (e.g., nanoscale inorganic fillers) to polymer electrolytes of the type PEO-XI-I2. For the solid * Corresponding author. E-mail:
[email protected]. Tel: +3010-6503644, Fax: +30-10-6511766. 10.1021/nl025798u CCC: $22.00 Published on Web 10/11/2002
© 2002 American Chemical Society
electrolyte preparation, to a dispersion of 0.0383 g TiO2 (Degussa P25) in acetonitrile (50 mL), the I-/I3- redox couple (0.1 g LiI and 0.019 g I2) was added. Then, the poly(ethylene oxide) (0.264 g PEO, MW ) 2 000 000) was slowly introduced and left under continuous stirring for 24 h. Finally the electrolyte was placed in the oven to evaporate the solvent. FTIR and Raman spectroscopies did not confirm the presence of acetonitrile in the electrolyte (absence of CtN vibrations). It has been demonstrated that the addition of inorganic fillers generally improves the transport properties, the resistance to crystallization, and the electrode-electrolyte stability.8 In parallel, the conductivity is enhanced (∼10-5 S cm-1) due to the enlargement of the amorphous phase in the polymer matrix.9,10 The filler particles, because of their large surface area, prevent the recrystallization and increase the amorphicity of the PEO, as it was confirmed by DSC measurements. The DSC trace, Figure 1, shows a small endotherm (∆Hm ) 18.3 J/g), which consists of two melting transitions. From the DSC thermograms the glass transition temperature (Tg), the melting temperatures (Tm1), (Tm2), and the melting enthalpy (∆Hm) were determined. Tm was taken as the peak of the melting endotherm and Tg as the inflection point.11 The results we obtained for the PEO-TiO2(I-/I3-) binary electrolyte are summarized in Table 1. The polymer chains stiffen and we note a significant increase of the Tg (at -33.96 °C). The increase of the glass transition temperature (the glass transition temperature for bare PEO was
Figure 1. DSC thermogram for the PEO-TiO2(I-/I3-) binary electrolyte. Table 1: Glass Transition Temperature (Tg), Melting Temperatures (Tm1, Tm2), Melting Enthalpy (∆Hm), and Crystallinity (Xc%) for PEO-TiO2(I-/I3-) Binary Electrolyte Tg (°C)
Tm1 (°C)
Tm2 (°C)
∆Hm (J/g)
Xc (%)
-33.96
17.7
42.98
18.23
14.8
-58.79 °C) shows that the new polymer electrolyte has incorporated a significant quantity of the available inorganic oxide filler (TiO2). The system shows a slight increase in the melting enthalpy (∆Hm), which has no important consequence in the crystallinity. It has been proposed that the Lewis base interactions between the surface groups of the filler and the electrolyte ionic species can lower the ionic coupling and in that way support the dissolution of the salts.12 The large quantity of the filler could increase the dissolution of the LiI salt, by affecting the crystallinity of our system, which is now mainly amorphous. Poly(ethylene oxide) presents a crystallized network of regular spherulites developing spirals and branches of welldistributed surface contours.13,14 The introduction of the inorganic filler into the polymer matrix produces dramatic morphological changes to the host polymer structure. The AFM top-view image of the binary electrolyte, Figure 2, reveals that, in the presence of the titania filler, the crystallinity of the poly(ethylene oxide) decreases considerably, in excellent agreement with the DSC results. The AFM analysis shows the existence of two distinct phases: the first one corresponds mainly to the original polymer matrix, it is crystalline and very compact; the second is an amorphous area consisting of polymer subunits held together in a parallel orientation and forming straight long chains, along which TiO2 spherical particles of about 20-25 nm in diameter are distributed. The polymer chains separated by the titania particles are arranged in a three-dimensional, mechanically stable network that creates free space and voids into which the iodide/triiodide anions can easily migrate. Opaque, rough, high surface area, nanostructured TiO2 thin films were prepared by the doctor-blade technique2,15 on TEC 15 (Hartford Glass) conductive glass substrates. The TiO2 thin films were sensitized with the dye Ru(dcbpy)2(NCS)2 1260
Figure 2. Top view AFM picture of the binary poly(ethylene oxide)/titania electrolyte.
Figure 3. Photocurrent action spectrum of TiO2 (P25 Degussa)/ Ru535 dye sensitized solar cell using the PEO/TiO2(I-/I3-) binary poly(ethylene oxide)/titania electrolyte.
(Ru 535, purchased from Solaronix) and incorporated in a sandwich type solar cell employing the PEO/TiO2(I-/I3-) binary polymer electrolyte and using a platinized TEC 8 counter electrode. With this cell, induced photon-to-current efficiency (IPCE) values as high as 60% were obtained in a large part of the visible region, Figure 3. The photocurrent action spectrum matches well the absorption characteristics of the N3 dye2 and goes to zero at long wavelengths. Figure 4 exhibits the current-voltage characteristics of such a typical dye-sensitized solar cell measured at a direct sunlight irradiance of 65.6 mW cm-2. The current voltage curve shows a short circuit current density (Jsc) of 7.2 mA/ cm-2 and an open-circuit voltage (Voc) of 0.664 V. The fill factor (FF) and the overall energy conversion efficiency (η) are 0.58 and 4.2% respectively. The above-mentioned overall conversion efficiency value is among the highest reported in the literature for an all-solid-state titania photoelectrochemical cell, and between the highest even in quasi-solid systems.16,17 Nano Lett., Vol. 2, No. 11, 2002
Acknowledgment. Thanks must be addressed to Degussa AG Frankfurt-Germany and Delis AE Athens-Greece for generously providing the TiO2 Degussa P25 powder. Financial support from NCSR “Demokritos” and GSRT-Greece is greatly acknowledged. References
Figure 4. Photocurrent-voltage (I-V curve) characteristics of the TiO2 (P25 Degussa)/Ru535 dye sensitized solar cell using the PEO/TiO2(I-/I3-) binary poly(ethylene oxide)/titania electrolyte. Conditions: temperature, 20 °C; area, 0.25 cm2; solar irradiance, 65.6 mW cm-2; Voc ) 0.664 V; Isc ) 1.8 mA (Jsc ) 7.2 mA cm-2); FF ) 0.58, η ) 4.2%.
We conclude that upon that addition of titania filler, the redox couple mobility between the poly(ethylene oxide) chains increases. The polymer “filling” process has a general character and can be extended to many different inorganic materials (ZrO2, ZnO, MoO3, Al2O3, clays). Optimization of the performance of the corresponding solar cells in terms of the polymer molecular weight and the nature and particle size distribution of the filler is expected to increase the conversion efficiency to the level of systems using a liquid electrolyte.
Nano Lett., Vol. 2, No. 11, 2002
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