Role of Transparent Electrodes for High Efficiency TiO2 Nanotube

Jan 16, 2014 - To fabricate transparent TiO2 nanotube electrodes, ... working electrodes as well as transparent platinized counter electrodes. The res...
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Role of Transparent Electrodes for High Efficiency TiO2 Nanotube Based Dye-Sensitized Solar Cells Kiyoung Lee, Robin Kirchgeorg, and Patrik Schmuki* Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany ABSTRACT: In the present work, we investigate the performance of anodic TiO2 nanotube layers in dye-sensitized solar cells under front- and back-side illumination configurations. To fabricate transparent TiO2 nanotube electrodes, we evaporated 5 μm thick metallic Ti layers on FTO glassthen the metal layers are completely anodized to form aligned nanotube layers. We compare different types of FTO glass (conductivity and transparency) and use them for working electrodes as well as transparent platinized counter electrodes. The results show that for TiO2 nanotube electrodes the resulting light conversion efficiency in DSSCs is highly affected by the type of glass used (efficiency rangedepending on configuration from 4.62% to 7.58%).

1. INTRODUCTION Since Grätzel et al. reported the first fully functional dyesensitized solar cell (DSSC) in 1991,1 the interest in this type of solar cell has been tremendous not only in science but also in technology. One of the key elements of the DSSCs is the photoanode that classically consists of a compacted TiO2 nanoparticle layer that is coated with an organic dye. The dye acts as a light absorber. Excited electrons from the dye lowest unoccupied molecular orbital (LUMO) are injected to the conduction band of the TiO2 scaffoldthis then serves merely as an electron transport medium to the back contact. One of the approaches to enhance the conversion efficiency of such DSSCs is using 1D nanostructures such as nanorods,2 nanowires,3,4 nanofibers,5 or nanotubes6−8 as an electron conductor instead of the nanoparticle layers. Such nanostructures are considered to increase the electron transport properties and reduce the recombination rate due to their directional electron transport pathway combined with improved (lower) recombination characteristics.9−12 Most recently, TiO2 nanotubes that can be formed on the Ti metal substrate by self-organizing electrochemistry have been considered widely for use in DSSCs.6,7 These tubes grow perpendicularly to the Ti metal substrate. In other words, such anodic layers on their metallic substrate only can be used directly, if a back-side illumination configuration is establishedthis means illuminating the tube layer through the transparent counter electrode. This carries the drawback that considerable light losses by absorption of the platinized counter electrode and in the electrolyte can be expected. To overcome this drawback, TiO2 nanotube layers can be transferred to transparent conductive glass or can be formed by complete anodization of thin Ti layers deposited on transparent conductive substrates.13−19 Generally, fluorine-doped tin © 2014 American Chemical Society

oxide (FTO) glass is widely used as a transparent conductive substrate. Interestingly, a lot of work focuses on the properties of the tube layers such as their geometry, crystal structures, etc. However, the effect of the characteristics of the FTO substrate in combination with TiO2 nanotubes and its effect on the conversion efficiency of DSSCs are hardly explored. In general, FTO substrates are characterized by the electric conductivity and optical transparency. These two factors affect each other inversely; i.e., a higher conductivity is combined with a lower transparency (this originates from the required doping concentration and the thickness of the FTO layer [the conductive part] on the glass). In the present work, we prepare TiO2 nanotube layers on different types of FTO glasses and apply them in DSSCs. We investigate the influence of the type of FTO when used as a counter and working electrode under front-side and back-side illuminated DSSC configurations.

2. EXPERIMENTAL SECTION Different types of FTO substrates (TCO22-7 (7 Ω/□) and TCO22-15 (15 Ω/□), Solaronix) were coated with 5 μm thick Ti layers using electron beam evaporation with a deposition rate of 0.6 nm/min at 5 × 10−7 to 2 × 10−6 mbar at the Fraunhofer Institute for Integrated Systems (Erlangen, Germany). The layers were completely anodized to nanotubes using a power source (LAB/SM 1300) in a two-electrode configuration with a counter electrode made of platinum gauze Special Issue: Michael Grätzel Festschrift Received: December 17, 2013 Revised: January 14, 2014 Published: January 16, 2014 16562

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Figure 1. (a) Transparency of FTO glass depends on conductivity. (b) Current density transients during formation of nanotubes by anodization. (c)−(f) Cross-sectional SEM images of an evaporated Ti metal layer on an FTO substrate of (c) 7 Ω/□ and (d) 15 Ω/□ and (e)−(f) TiO2 nanotubes formed on FTO substrate of (e) 7 Ω/□ and (f) 15 Ω/□, respectively. The anodization was carried out at 60 V in 0.15 M NH4F/3 vol % H2O in ethylene glycol. (g) X-ray diffraction patterns of nanotube layers after annealing at 500 °C for 15 min. The peaks are annotated as Anatase (A), Rutile (R), and Substrate (S).

in an electrolyte of 0.15 M NH4F/3 vol % H2O in ethylene glycol at 60 V. After the anodization process, the samples were washed in ethanol and then dried in a nitrogen stream. To crystallize some of the samples they were annealed at 500 °C for 15 min in air using a Rapid Thermal Annealer (Jipelec

JetFirst100). For morphological characterization, emission scanning electron microscope (FE-SEM, SEM FE 4800) was used. X-ray diffraction analysis (XRD) was performed X’pert Philips MPD with a Panalytical X’celerator 16563

a field Hitachi with an detector

dx.doi.org/10.1021/jp412351g | J. Phys. Chem. C 2014, 118, 16562−16566

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Figure 2. (a) Schematic image of front-side and back-side illuminated dye-sensitized solar cells that are fabricated with TiO2 nanotubes. (b),(c) J−V characteristics for DSSCs with TiO2 nanostructure under (b) front-side and (c) back-side illumination. The tables give extracted photovoltaic characteristics of the dye-sensitized TiO2 layers. Jsc = short-circuit current density, Voc = open-circuit voltage, FF = fill factor, η = efficiency.

using graphite monochromatized Cu Kα radiation (wavelength 1.54056 Å). For dye sensitization, samples were immersed in a 300 μM Ru-based dye (cis-bis(isothiocyanato) bis(2,2-bipyridyl 4,4dicarboxylato) ruthenium(II) bistetrabutylammonium (D-719, Eversolar, Taiwan)) solution in a mixture of acetonitrile and tert-butyl alcohol (volume ratio: 1:1) for 1 day at 40 °C. After dye sensitization, the samples were rinsed with acetonitrile to remove nonchemisorbed dye. For the platinized counter electrode, two small holes were drilled in the FTO glass using a diamond-tipped drill. After drilling, the glass was cleaned by sonication in acetone and rinsed with ethanol. After then, the conductive side of the glass was coated with an atomic level sized platinum layer by spreading a minimal amount of solution of 5 mM H2PtCl6 dissolved in isopropyl alcohol. The platinum-coated FTO glass was dried in air and heated at 400 °C for 30 min for achieving the platinization.

For the cell fabrication, the sensitized samples were sandwiched together with a Pt-coated fluorine-doped glass counter electrode using a polymer adhesive spacer (SX1170-25, commercial name: Surlyn; Solaronix). The plastic is a gasket with a window between the anode and the counter electrode, and then, a commercial electrolyte (Iolilyt SB-163, Iolitec, Germany) was injected through a hole into the space between the sandwiched cells. The current−voltage characteristics of the cells were measured under simulated AM 1.5 conditions provided by a solar simulator (300 W Xe with optical filter, Solarlight) and applying an external bias to the cell while measuring the generated photocurrent with a Keithley model 2420 digital source meter. Intensity modulated photovoltage and photocurrent spectroscopy (IMVS and IMPS) measurements were carried out using modulated light (10% modulation depth) from a high power green LED (λ = 530 nm). The modulation frequency was controlled by a frequency response analyzer 16564

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Figure 3. (a) Intensity modulated photocurrent spectra (IMPS) and (b) intensity modulated photovoltage spectra (IMVS) characterization for dyesensitized solar cells (Figure 2(a)) with a different FTO substrate.

more important than that of the counter electrode and that light absorption is less important than substrate conductivity in this absorption range. Moreover, the enhancement is governed by the short-circuit current density (Jsc) (Voc and fill factors are virtually identical at ∼0.79 V and 50−60%, respectively). Jsc is closely related to the electron transport rate on the working electrode. In other words, a higher substrate conductivity enhances the electron transport even for the same TiO2 layer (same surface area and same morphology) and less light absorption. To evaluate the electron mobility in different types of DSSCs, intensity modulated photocurrent spectra (IMPS) and intensity modulated photovoltage spectra (IMVS) (Figure 3) were taken. The AC amplitude value was 10% of the stationary DC value. From the measurements it is apparent that electron recombination time constants (τr) are similar for all samples, but electron transport time constants (τt) are faster on high conductive substrates. Therefore, it can be concluded that the enhanced solar cell efficiency of a high conductive working electrode substrate in the front-side configuration can be attributed to the electron transport rate. It may also be noted that the efficiency of 7.58% for front-side illuminated DSSCs directly fabricated from an evaporated layer is the highest value so far reported.15

(FRA, Zahner). The light intensity incident on the cell was measured using a calibrated Si photodiode.

3. RESULTS AND DISCUSSION Figure 1(a) shows typical optical transmittance characteristics of the two glass substrates. It is evident that in the visible range the higher conductive FTO glass (>∼7 Ω/□) shows ca. 15% lower transparency than the lower conductivity FTO glass (>∼15 Ω/□). After Ti metal coating, TiO2 nanotubes were grown on the different substrates by complete anodization of the metal layer for the two types of samples. Figure 1(b) shows a typical current vs time behavior. For both layers, complete anodization is reached after 4200 s. As shown in Figure 1(c), (d), both layers show a similar evaporated metal texture. Also on both substrates the aligned nanotube morphologies are virtually the same, i.e., forming a 14−15 μm thick layer and ∼100 nm diameter TiO2 nanotubes, and the different conductivities of the substrate do not affect the growth of anodic TiO2 nanotube layers (Figure 1(e),(f)). All anodic TiO2 nanotubes were amorphous but could be crystallized by an additional heat treatment. Figure 1(g) shows X-ray diffraction (XRD) patterns of the tube layers after annealing at 500 °C for 15 min in air. By the heat treatment, the nanotube layers are mainly converted to the anatase phase with a small amount of rutile phase. To evaluate the conversion efficiency of DSSCs using these TiO2 nanotube layers, four different types of solar cells were fabricated with combinations of two different types of working electrodes and two different types of counter electrodes (Figure 2(b) and (c)). As shown in Figure 2(b) and (c), in both cases (front-side and back-side illumination) the conductivity of the electrode influences the resulting conversion efficiency. In comparison to the counter electrodes, in all cases the conversion efficiency of corresponding pairs is virtually the same (sample 1, 7.58%, and sample 2, 7.26%; sample 3, 6.23%, and sample 4, 6.43%; sample 5, 5.92%, and sample 6, 5.76%; and sample 7, 4.62%, and sample 8, 4.76%). In contrast, a higher conductivity on the substrate (working electrode) leads in all cases to a higher conversion efficiency (sample 1, 7.58%, and sample 3, 6.23%; sample 2, 7.26%, and sample 4, 6.43%; sample 5, 5.92%, and sample 7, 4.62%; and sample 6, 5.76%, and sample 8, 4.76%). From this result, it is clear that DSSCs on the higher conductivity working electrodes enhance the conversion efficiency by more than ∼20%. We can thus easily conclude that the conductivity of the working electrode is much

4. CONCLUSIONS In the present work, we investigate the influence of the glass substrate on the performance of a dye-sensitized solar cell fabricated from TiO2 nanotubes. The results show that with increasing the conductivity of the working electrode the photoconversion efficiency is significantly increased. However, the conductivity of the counter electrode substrate is not of major influence. Moreover, for the TiO2 nanotube based solar cells it can be concluded that the lower resistance of electrode is more important than losses of the light absorption in the working electrode, if a higher conductive glass is used.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49-9131-852-7575. Fax: +49-9131-852-7582. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 16565

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Front-Illuminated Dye-Sensitized Solar Cells. Energy Environ. Sci. 2011, 4, 3420−3425.

ACKNOWLEDGMENTS The authors would like to acknowledge DFG and the EAMDFG cluster of excellence for financial support.

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REFERENCES

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