- Email: [email protected]
Front side illuminated dye-sensitized solar cells using anodic TiO2 mesoporous layers grown on FTO-glass
Electrochemistry Communications 22 (2012) 157–161
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Front side illuminated dye-sensitized solar cells using anodic TiO2 mesoporous layers grown on FTO-glass Kiyoung Lee, Doohun Kim, Steffen Berger, Robin Kirchgeorg, Patrik Schmuki ⁎ Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremberg (FAU), Martensstrasse 7, D-91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 30 May 2012 Received in revised form 2 June 2012 Accepted 7 June 2012 Available online 15 June 2012 Keywords: TiO2 nanotube Mesoporous Anodic oxidation Dye-sensitized solar cells
a b s t r a c t In the present work, we investigate the performance of anodic, self-organized TiO2 mesoporous structures (TMSs) in dye-sensitized solar cells (DSSCs) under front-side illumination conditions. In order to fabricate structures usable in a front-side (FS) configuration, TMSs were formed on FTO glass by a full anodic conversion of evaporated Ti films on the glass substrate in a glycerol/K2HPO4 electrolyte at 180 °C. The resulting mesoporous layers can be assembled to FS dye sensitized solar cells which show a thickness of only 3 μm and a remarkable efficiency of 4.34%. This is significantly higher than, for example, anodic TiO2 nanotubes of the same length even after efficiency enhancing TiCl4 treatments (3.01%). The best efficiency value for TMS-based DSSCs that we currently reached is for a 7 μm thick TMS layer, this is ≈6.82% which is highly promising for further improvements. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Since Grätzel and O'Regan reported the principle of a TiO2 based dye-sensitized solar cell (DSSC) in 1991 [1], many research groups have extensively studied ways and means to enhance the efficiency of such cells. For the photoanode, the most widely used material is TiO2 in the form of a thin film (5–15 μm thickness), composed of compacted TiO2 nanoparticles that are coated with a monolayer of Ru based dye [2–5]. While the nanoparticle based electrode provides a large surface area (and thus a high specific dye-loading), it potentially provides drawbacks such as grain boundary losses (recombination) or hampered electron transfer (due to random walk effects) [6–8]. In order to overcome these effects, several approaches have been studied towards the use of one-dimensional nano-structures such as nanowires [9,10], nanorods [11,12], and nanotubes [8,13–15]. These one-dimensional structures have been considered to significantly improve the electron transport time and reduce recombination effects. On the other hand, these structures often suffer from an insufficient specific surface area and thus a limited dye-loading capability. An alternative to powders or tubes is represented by anodic TiO2 mesoporous structures (TMS) that can be fabricated in hot glycerol/ K2HPO4 electrolytes. These structures provide a much higher surface area than nanotubes, an ordered TiO2 network and (as they are “carved” from a compact TiO2 layer), a high interconnectivity of the porous structure. First experiments showed that efficiencies of 5.02% could be reached when the structures were applied to DSSCs [16,17]. In these
⁎ Corresponding author. Tel.: + 49 9131 852 7575; fax: + 49 9131 852 7582. E-mail address: [email protected] (P. Schmuki).
works, DSSCs were constructed from tubes on the metal substrate, i.e. allowing only the construction of back-side illuminated cells. This has the essential drawback of having light losses through the platinized counter electrode and light absorption in the iodine electrolyte which reduce the efficiency. In the present work, we prepare titania mesoporous structures (TMSs) on fluorine-doped tin oxide (FTO) coated glass and apply them in a front-side illuminated DSSC configuration. We compare the resulting efficiencies to the best established ordered nanostructures (TiO2 nanotube arrays) and show that indeed the TiO2 mesoporous layers represent a highly promising platform towards efficient DSSCs.
2. Experimental part Titanium layers, 1.2 μm, 2.5 μm, 3.1 μm and 7.2 μm thick, were electron beam evaporated on FTO glass (TCO22-15, Solaronix). To form TiO2 mesoporous structures (TMSs) anodization was carried out in 10 wt.% K2HPO4 (Sigma-Aldrich) in anhydrous glycerol (99.8% purity, b1% H2O, Fluka) at 1 V at 150–180 °C. Prior to anodization, the electrolyte was held at 200 °C for 4 h to reduce the water content. For comparison we grew nanotubes from Ti films on FTO using anodization in ethylene glycol containing 0.15 M NH4F and 3 vol.% of additional H2O at 60 V. To fully crystallize the material, all samples were annealed at 450 °C for 1 h in air with a heating and cooling rate of 30 °C/min using a Rapid Thermal Annealer (Jipelec JetFirst100). The anodized TMS samples were immersed in 30 wt.% H2O2 at 40 °C for 1 h for subsequent etching. After treatment, the samples were rinsed with DI water and dried in nitrogen. Additionally, the
1388-2481/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.06.005
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K. Lee et al. / Electrochemistry Communications 22 (2012) 157–161
K. Lee et al. / Electrochemistry Communications 22 (2012) 157–161
samples were dried at 120 °C for 30 min to remove the remaining water from the layers. For morphological characterization of the samples, a field-emission scanning electron microscope Hitachi FE-SEM S4800 was used. The cross-section images were taken from cleaved layers. X-ray diffraction analysis (XRD) was performed with an X'pert Philips MPD with a Panalytical X'celerator detector using graphite monochromized Cu Kα radiation (wavelength 1.54056 Å). Dye-sensitized cell fabrication as well as I–V and dye desorption measurements were described in Refs. [16,17]. 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 (FRA, Zahner). The light intensity incident on the cell was measured using a calibrated Si photodiode. 3. Results and discussion Fig. 1(a)–(d) shows the scanning electron microscopy (SEM) images of the TiO2 mesoporous structures (TMSs) grown on FTO glass by total anodization of the evaporated Ti metal layer. For layers up to 3.1 μm thickness a regular mesoporous morphology is obtained over the entire layer that contains regular porosity, with typical TiO2 feature sizes in the range 5–10 nm and pores that are approximately 10 nm (Fig. 1(a)). The thickness of mesoporous layers corresponds to the evaporated metal layer (Fig. 1(b) and (g)). This is in contrast to other self-organizing anodization approaches that for example form TiO2 nanotube structures in fluoride ion containing electrolytes (Fig. 1(e) and (f)). Nanotube layers show an increase in thickness during anodization by a factor of 2.6. This effect was explained by a plastic flow mechanism during the growth of nanopores or nanotubes [18–20]. Fig. 1(h) shows the different current– time behavior during anodization of the metallic thin layers forming TiO2 mesoporous structures and nanotubes. For both morphologies, first a rapid drop (top layer passivation) [21] is observed followed by a constant current flow of 8–10 mA/cm2, until the entire layer is oxidized and a drop in the current takes place. A significant difference is that for TMS the current remains low and the resulting layer adheres well on the FTO (Fig. 1(h) inset) while for nanotubes once the etch front reaches the substrate, a current increase occurs that is combined with a lift-off of the tube layer [22]. Fig. 1(i) shows the X-ray diffraction patterns (XRD) of the as formed mesoporous structure and after annealing at 450 °C for 1 h in air. Clearly, the as-formed TiO2 mesoporous structures are amorphous but can be crystallized to anatase by the thermal treatment. In order to evaluate the performance of these structures in dyesensitized solar cells, DSSCs were assembled and I–V curves were measured. Fig. 2(a) and (b) shows a compilation of I–V curves for DSSCs that is illuminated through the Pt counter electrode (“backside illumination”, Fig. 2(a)) or through the glass bottom of the TiO2 nanostructures (“front-side illumination”, Fig. 2(b)). These results show that front-side illumination results in a considerably increased efficiency compared with the back-side configuration. Furthermore, increasing the thickness of the layer leads to an increase in short circuit current (Jsc, from 4.64 mA/cm 2 and 5.69 mA/cm 2 for 1.2 μm thick TMS layer to 7.59 mA/cm 2 and 9.70 mA/cm 2 for 3.1 μm TMS layer, respectively). This increase is mainly attributed to the higher surface area for dye adsorption. Even though the open circuit potential (Voc) and fill factor (FF) slightly drop with an increase in thickness, which may be ascribed to the higher series resistance for the thick
159
layers [23], the overall efficiency considerably increases up to 4.34% for the front-side illuminated structures. In comparison to TiO2 nanotube structures with a similar thickness of ≈3.1 μm yield under front-side conditions ≈2.89%, clearly the TMS layer performs better. The higher efficiency may mostly be attributed to a higher specific dye loading of the TMS layers (see tables in Fig. 2). Nevertheless, also electron transport and recombination are different in the two structures. IMPS/IMVS measurements (Fig. 2(c)–(d)) reveal that the electron transport time constant (τc) through the mesoporous structures is approximately 5 times faster than through the nanotube structure (Fig. 2(c)). Even though recombination time constant (τr) for the mesoporous structure is also higher for the mesoporous structures than for the nanotubes (Fig. 2(d)), the average value of overall charge collection efficiency (ηcc=1−τc /τr), shows that ηcc for the mesoporous structures is ca. 2 times higher than that for the nanotubes. Overall, it may be concluded that the enhanced solar cell conversion efficiency of the mesoporous structure can be attributed to higher specific dye loading as well as enhanced electron collection efficiency. In order to overcome one drawback of tubes, we increase the specific surface area of the nanotubes by a well established TiCl4 hydrolysis treatment [24,25]. This leads to a uniform TiO2 nanoparticle decoration of the tube walls (shown in Fig. 2(e) and (f)). While indeed the treatment leads to the expected increase in efficiency, it is interesting to note that the conversion efficiency is more enhanced for back-side illumination (from 2.34% to 2.89%) than for front-side illumination (from 2.89% to 3.01%). Nevertheless, in comparison with the mesoporous structure, the decorated nanotube layers still show a lower solar cell efficiency. Finally we would like to point out that, at present, the efficiency limit for TMS layers seems to be the reliable production of thicker Ti metal layers on FTO that also can be fully converted to a TMS layer (Fig. 2(g)). In Fig. 2(h), we show the results for our currently thickest (7.2 μm) fully converted TMS layer where an efficiency of 6.82% could be reached under front side illumination conditions. If one further considers that neither anodization of thin layers nor post formation etching is fully optimized at present, the results of this work indicate the high potential of these layers if technological fabrication limits, mainly the production of thick defect free Ti films on FTO, can be overcome. 4. Conclusions Highly mesoporous titania structures can be formed on FTO glass by completely converting Ti metal layers deposited on glass using anodization in a K2HPO4/hot glycerol electrolyte. These layers can be used to construct front-side illuminated DSSCs which show a high solar light conversion efficiency, and in particular a higher efficiency than comparable nanotube structures. These higher values can be attributed to a higher specific surface area and a faster electron transport in the TMS structure. Currently the record efficiency for these layers is at 6.82% – limited by a reliable processing of thicker films – but it seems reasonable to assume that this value can further be improved in upcoming more detailed (parameter-optimizing) work. Acknowledgments The authors would like to acknowledge DFG and the Erlangen DFG Cluster of Excellence (EAM) for financial support.
Fig. 1. (a)–(d): Cross-sectional SEM images of TiO2 mesoporous structures (TMS) formed on FTO glass by complete anodization of a Ti thin film. (b)–(d) TiO2 mesoporous structures with different thicknesses. (e) and (f): Reference TiO2 nanotubes on FTO glass formed by anodization in fluoride ion containing electrolyte. TiO2 mesoporous structures (b) and nanotubes (e) formed by anodization of (g) evaporated Ti metal layer on FTO glass. (h) Typical current density transient during anodizing of formation of 1.2 μm TiO2 mesoporous structure on FTO glass formed at 1 V in 10 wt.% K2HPO4/glycerol at 180 °C (b) and formation of nanotubes on FTO glass in ethylene glycol containing 0.15 M NH4F and 3 vol.% of additional H2O at 60 V (e). Inset of panel (h) shows the border of FTO layer and mesoporous structures. (i) XRD spectra of 1.2 μm thick TiO2 mesoporous structures (TMS) before, and after annealing at 450 °C for 1 h. The peaks are annotated as anatase (A) and FTO substrate (S).
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Fig. 2. (a)–(b) I–V characteristics for dye-sensitized TiO2 nanostructure layers under (a) back-side illumination and (b) front-side illumination. (c) IMPS and (d) IMVS characterization for dye-sensitized TiO2 nanostructure layers under front-side illumination. (e) and (f) shows the cross sectional SEM images of TiCl4 treated TiO2 nanotubes. Inset shows top view of layer (e). (g) The cross sectional SEM images of 7.2 μm thick TiO2 mesoporous layers. (h) I–V characteristics for dye-sensitized layer (g) under front-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.
K. Lee et al. / Electrochemistry Communications 22 (2012) 157–161
References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11] [12] [13]
B. O'Regan, M. Grätzel, Nature 353 (1991) 737. M. Grätzel, Nature 414 (2001) 338. M. Grätze, Comments on Inorganic Chemistry 12 (1991) 93. M.K. Nazeeruddin, Q. Wang, L. Cevey, V. Aranyos, P. Liska, E. Figgemeier, C. Klein, N. Hirata, S. Koops, S.A. Haque, J.R. Durrant, A. Hagfeldt, A.B.P. Lever, M. Grätzel, Inorganic Chemistry 45 (2006) 787. S.A. Haque, E. Palomares, B.M. Cho, A.N.M. Green, N. Hirata, D.R. Klug, J.R. Durrant, Journal of the American Chemical Society 127 (2005) 3456. J.R. Jennings, A. Ghicov, L.M. Peter, P. Schmuki, A.B. Walker, Journal of the American Chemical Society 130 (2008) 13364. M. Grätzel, Inorganic Chemistry 44 (2005) 6841. K. Zhu, T.B. Vinzant, N.R. Neale, A.J. Frank, Nano Letters 7 (2007) 3739. Q. Shen, T. Sato, M. Hashimoto, C. Chen, T. Toyoda, Thin Solid Films 499 (2006) 299. M. Adachi, Y. Murata, J. Takao, J. Jiu, M. Sakamoto, F. Wang, Journal of the American Chemical Society 126 (2004) 14943. S.H. Kang, S.-H. Choi, M.-S. Kang, J.-Y. Kim, H.-S. Kim, T. Hyeon, Y.-E. Sung, Advanced Materials 20 (2008) 54. S. Pavasupree, S. Ngamsinlapasathian, Y. Suzuki, S. Yoshikawa, Journal of Nanoscience and Nanotechnology 6 (2006) 3685. A. Ghicov, S. Albu, R. Hahn, D. Kim, T. Stergiopoulos, J. Kunze, C.-A. Schiller, P. Falaras, P. Schmuki, Chemistry, an Asian Journal 4 (2009) 520.
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[14] P. Roy, D. Kim, K. Lee, E. Spiecker, P. Schmuki, Nanoscale 2 (2010) 45. [15] J.M. Macak, H. Tsuchiya, A. Ghicov, P. Schmuki, Electrochemistry Communications 7 (2005) 1133. [16] D. Kim, K. Lee, P. Roy, B.I. Birajdar, E. Spiecker, P. Schmuki, Angewandte Chemie International Edition 48 (2009) 9326. [17] K. Lee, D. Kim, P. Schmuki, Chemical Communications 47 (2011) 5789. [18] K.R. Hebert, J.E. Houser, Journal of the Electrochemical Society 156 (2009) C275. [19] K.R. Hebert, S.P. Albu, I. Paramasivam, P. Schmuki, Nature Materials 11 (2012) 162. [20] S. Berger, R. Hahn, P. Roy, P. Schmuki, Physica Status Solidi B Basic Research 247 (2010) 2424. [21] P. Roy, S. Berger, P. Schmuki, Angewandte Chemie International Edition 50 (2011) 2904. [22] S. Berger, A. Ghicov, Y.-C. Nah, P. Schmuki, Langmuir 25 (2009) 127. [23] D. Kuang, S. Ito, B. Wenger, C. Klein, J.-E. Moser, R. Humphry-Baker, S.M. Zakeeruddin, M. Grätzel, Journal of the American Chemical Society 128 (2006) 4146. [24] P. Roy, D. Kim, I. Paramasivam, P. Schmuki, Electrochemistry Communications 11 (2009) 1001. [25] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, Journal of the American Chemical Society 115 (1993) 6382.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Front side illuminated dye-sensitized solar cells using anodic TiO2 mesoporous layers grown on FTO-glass Kiyoung Lee, Doohun Kim, Steffen Berger, Robin Kirchgeorg, Patrik Schmuki ⁎ Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremberg (FAU), Martensstrasse 7, D-91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 30 May 2012 Received in revised form 2 June 2012 Accepted 7 June 2012 Available online 15 June 2012 Keywords: TiO2 nanotube Mesoporous Anodic oxidation Dye-sensitized solar cells
a b s t r a c t In the present work, we investigate the performance of anodic, self-organized TiO2 mesoporous structures (TMSs) in dye-sensitized solar cells (DSSCs) under front-side illumination conditions. In order to fabricate structures usable in a front-side (FS) configuration, TMSs were formed on FTO glass by a full anodic conversion of evaporated Ti films on the glass substrate in a glycerol/K2HPO4 electrolyte at 180 °C. The resulting mesoporous layers can be assembled to FS dye sensitized solar cells which show a thickness of only 3 μm and a remarkable efficiency of 4.34%. This is significantly higher than, for example, anodic TiO2 nanotubes of the same length even after efficiency enhancing TiCl4 treatments (3.01%). The best efficiency value for TMS-based DSSCs that we currently reached is for a 7 μm thick TMS layer, this is ≈6.82% which is highly promising for further improvements. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Since Grätzel and O'Regan reported the principle of a TiO2 based dye-sensitized solar cell (DSSC) in 1991 [1], many research groups have extensively studied ways and means to enhance the efficiency of such cells. For the photoanode, the most widely used material is TiO2 in the form of a thin film (5–15 μm thickness), composed of compacted TiO2 nanoparticles that are coated with a monolayer of Ru based dye [2–5]. While the nanoparticle based electrode provides a large surface area (and thus a high specific dye-loading), it potentially provides drawbacks such as grain boundary losses (recombination) or hampered electron transfer (due to random walk effects) [6–8]. In order to overcome these effects, several approaches have been studied towards the use of one-dimensional nano-structures such as nanowires [9,10], nanorods [11,12], and nanotubes [8,13–15]. These one-dimensional structures have been considered to significantly improve the electron transport time and reduce recombination effects. On the other hand, these structures often suffer from an insufficient specific surface area and thus a limited dye-loading capability. An alternative to powders or tubes is represented by anodic TiO2 mesoporous structures (TMS) that can be fabricated in hot glycerol/ K2HPO4 electrolytes. These structures provide a much higher surface area than nanotubes, an ordered TiO2 network and (as they are “carved” from a compact TiO2 layer), a high interconnectivity of the porous structure. First experiments showed that efficiencies of 5.02% could be reached when the structures were applied to DSSCs [16,17]. In these
⁎ Corresponding author. Tel.: + 49 9131 852 7575; fax: + 49 9131 852 7582. E-mail address: [email protected] (P. Schmuki).
works, DSSCs were constructed from tubes on the metal substrate, i.e. allowing only the construction of back-side illuminated cells. This has the essential drawback of having light losses through the platinized counter electrode and light absorption in the iodine electrolyte which reduce the efficiency. In the present work, we prepare titania mesoporous structures (TMSs) on fluorine-doped tin oxide (FTO) coated glass and apply them in a front-side illuminated DSSC configuration. We compare the resulting efficiencies to the best established ordered nanostructures (TiO2 nanotube arrays) and show that indeed the TiO2 mesoporous layers represent a highly promising platform towards efficient DSSCs.
2. Experimental part Titanium layers, 1.2 μm, 2.5 μm, 3.1 μm and 7.2 μm thick, were electron beam evaporated on FTO glass (TCO22-15, Solaronix). To form TiO2 mesoporous structures (TMSs) anodization was carried out in 10 wt.% K2HPO4 (Sigma-Aldrich) in anhydrous glycerol (99.8% purity, b1% H2O, Fluka) at 1 V at 150–180 °C. Prior to anodization, the electrolyte was held at 200 °C for 4 h to reduce the water content. For comparison we grew nanotubes from Ti films on FTO using anodization in ethylene glycol containing 0.15 M NH4F and 3 vol.% of additional H2O at 60 V. To fully crystallize the material, all samples were annealed at 450 °C for 1 h in air with a heating and cooling rate of 30 °C/min using a Rapid Thermal Annealer (Jipelec JetFirst100). The anodized TMS samples were immersed in 30 wt.% H2O2 at 40 °C for 1 h for subsequent etching. After treatment, the samples were rinsed with DI water and dried in nitrogen. Additionally, the
1388-2481/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.06.005
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K. Lee et al. / Electrochemistry Communications 22 (2012) 157–161
samples were dried at 120 °C for 30 min to remove the remaining water from the layers. For morphological characterization of the samples, a field-emission scanning electron microscope Hitachi FE-SEM S4800 was used. The cross-section images were taken from cleaved layers. X-ray diffraction analysis (XRD) was performed with an X'pert Philips MPD with a Panalytical X'celerator detector using graphite monochromized Cu Kα radiation (wavelength 1.54056 Å). Dye-sensitized cell fabrication as well as I–V and dye desorption measurements were described in Refs. [16,17]. 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 (FRA, Zahner). The light intensity incident on the cell was measured using a calibrated Si photodiode. 3. Results and discussion Fig. 1(a)–(d) shows the scanning electron microscopy (SEM) images of the TiO2 mesoporous structures (TMSs) grown on FTO glass by total anodization of the evaporated Ti metal layer. For layers up to 3.1 μm thickness a regular mesoporous morphology is obtained over the entire layer that contains regular porosity, with typical TiO2 feature sizes in the range 5–10 nm and pores that are approximately 10 nm (Fig. 1(a)). The thickness of mesoporous layers corresponds to the evaporated metal layer (Fig. 1(b) and (g)). This is in contrast to other self-organizing anodization approaches that for example form TiO2 nanotube structures in fluoride ion containing electrolytes (Fig. 1(e) and (f)). Nanotube layers show an increase in thickness during anodization by a factor of 2.6. This effect was explained by a plastic flow mechanism during the growth of nanopores or nanotubes [18–20]. Fig. 1(h) shows the different current– time behavior during anodization of the metallic thin layers forming TiO2 mesoporous structures and nanotubes. For both morphologies, first a rapid drop (top layer passivation) [21] is observed followed by a constant current flow of 8–10 mA/cm2, until the entire layer is oxidized and a drop in the current takes place. A significant difference is that for TMS the current remains low and the resulting layer adheres well on the FTO (Fig. 1(h) inset) while for nanotubes once the etch front reaches the substrate, a current increase occurs that is combined with a lift-off of the tube layer [22]. Fig. 1(i) shows the X-ray diffraction patterns (XRD) of the as formed mesoporous structure and after annealing at 450 °C for 1 h in air. Clearly, the as-formed TiO2 mesoporous structures are amorphous but can be crystallized to anatase by the thermal treatment. In order to evaluate the performance of these structures in dyesensitized solar cells, DSSCs were assembled and I–V curves were measured. Fig. 2(a) and (b) shows a compilation of I–V curves for DSSCs that is illuminated through the Pt counter electrode (“backside illumination”, Fig. 2(a)) or through the glass bottom of the TiO2 nanostructures (“front-side illumination”, Fig. 2(b)). These results show that front-side illumination results in a considerably increased efficiency compared with the back-side configuration. Furthermore, increasing the thickness of the layer leads to an increase in short circuit current (Jsc, from 4.64 mA/cm 2 and 5.69 mA/cm 2 for 1.2 μm thick TMS layer to 7.59 mA/cm 2 and 9.70 mA/cm 2 for 3.1 μm TMS layer, respectively). This increase is mainly attributed to the higher surface area for dye adsorption. Even though the open circuit potential (Voc) and fill factor (FF) slightly drop with an increase in thickness, which may be ascribed to the higher series resistance for the thick
159
layers [23], the overall efficiency considerably increases up to 4.34% for the front-side illuminated structures. In comparison to TiO2 nanotube structures with a similar thickness of ≈3.1 μm yield under front-side conditions ≈2.89%, clearly the TMS layer performs better. The higher efficiency may mostly be attributed to a higher specific dye loading of the TMS layers (see tables in Fig. 2). Nevertheless, also electron transport and recombination are different in the two structures. IMPS/IMVS measurements (Fig. 2(c)–(d)) reveal that the electron transport time constant (τc) through the mesoporous structures is approximately 5 times faster than through the nanotube structure (Fig. 2(c)). Even though recombination time constant (τr) for the mesoporous structure is also higher for the mesoporous structures than for the nanotubes (Fig. 2(d)), the average value of overall charge collection efficiency (ηcc=1−τc /τr), shows that ηcc for the mesoporous structures is ca. 2 times higher than that for the nanotubes. Overall, it may be concluded that the enhanced solar cell conversion efficiency of the mesoporous structure can be attributed to higher specific dye loading as well as enhanced electron collection efficiency. In order to overcome one drawback of tubes, we increase the specific surface area of the nanotubes by a well established TiCl4 hydrolysis treatment [24,25]. This leads to a uniform TiO2 nanoparticle decoration of the tube walls (shown in Fig. 2(e) and (f)). While indeed the treatment leads to the expected increase in efficiency, it is interesting to note that the conversion efficiency is more enhanced for back-side illumination (from 2.34% to 2.89%) than for front-side illumination (from 2.89% to 3.01%). Nevertheless, in comparison with the mesoporous structure, the decorated nanotube layers still show a lower solar cell efficiency. Finally we would like to point out that, at present, the efficiency limit for TMS layers seems to be the reliable production of thicker Ti metal layers on FTO that also can be fully converted to a TMS layer (Fig. 2(g)). In Fig. 2(h), we show the results for our currently thickest (7.2 μm) fully converted TMS layer where an efficiency of 6.82% could be reached under front side illumination conditions. If one further considers that neither anodization of thin layers nor post formation etching is fully optimized at present, the results of this work indicate the high potential of these layers if technological fabrication limits, mainly the production of thick defect free Ti films on FTO, can be overcome. 4. Conclusions Highly mesoporous titania structures can be formed on FTO glass by completely converting Ti metal layers deposited on glass using anodization in a K2HPO4/hot glycerol electrolyte. These layers can be used to construct front-side illuminated DSSCs which show a high solar light conversion efficiency, and in particular a higher efficiency than comparable nanotube structures. These higher values can be attributed to a higher specific surface area and a faster electron transport in the TMS structure. Currently the record efficiency for these layers is at 6.82% – limited by a reliable processing of thicker films – but it seems reasonable to assume that this value can further be improved in upcoming more detailed (parameter-optimizing) work. Acknowledgments The authors would like to acknowledge DFG and the Erlangen DFG Cluster of Excellence (EAM) for financial support.
Fig. 1. (a)–(d): Cross-sectional SEM images of TiO2 mesoporous structures (TMS) formed on FTO glass by complete anodization of a Ti thin film. (b)–(d) TiO2 mesoporous structures with different thicknesses. (e) and (f): Reference TiO2 nanotubes on FTO glass formed by anodization in fluoride ion containing electrolyte. TiO2 mesoporous structures (b) and nanotubes (e) formed by anodization of (g) evaporated Ti metal layer on FTO glass. (h) Typical current density transient during anodizing of formation of 1.2 μm TiO2 mesoporous structure on FTO glass formed at 1 V in 10 wt.% K2HPO4/glycerol at 180 °C (b) and formation of nanotubes on FTO glass in ethylene glycol containing 0.15 M NH4F and 3 vol.% of additional H2O at 60 V (e). Inset of panel (h) shows the border of FTO layer and mesoporous structures. (i) XRD spectra of 1.2 μm thick TiO2 mesoporous structures (TMS) before, and after annealing at 450 °C for 1 h. The peaks are annotated as anatase (A) and FTO substrate (S).
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Fig. 2. (a)–(b) I–V characteristics for dye-sensitized TiO2 nanostructure layers under (a) back-side illumination and (b) front-side illumination. (c) IMPS and (d) IMVS characterization for dye-sensitized TiO2 nanostructure layers under front-side illumination. (e) and (f) shows the cross sectional SEM images of TiCl4 treated TiO2 nanotubes. Inset shows top view of layer (e). (g) The cross sectional SEM images of 7.2 μm thick TiO2 mesoporous layers. (h) I–V characteristics for dye-sensitized layer (g) under front-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.
K. Lee et al. / Electrochemistry Communications 22 (2012) 157–161
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