Al-Doped ZnO Transparent

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A Newly Designed Nb-Doped TiO2/Al-Doped ZnO Transparent Conducting Oxide Multilayer for Electrochemical Photoenergy Conversion Devices Jun Hong Noh,† Hyun Soo Han,† Sangwook Lee,‡ Dong Hoe Kim,† Jong Hun Park,† Sangbaek Park,† Jin Young Kim,§ Hyun Suk Jung,*,| and Kug Sun Hong*,†,‡ WCU Hybrid Materials Program, Department of Materials Science & Engineering, Seoul National UniVersity, Seoul 151-742, Korea, Research Institute of AdVance Materials, Seoul National UniVersity, Seoul 151-742, Korea, Chemical and Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401, and School of AdVanced Materials Engineering, Kookmin UniVersity, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Korea ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: July 13, 2010

We present dye-sensitized solar cells (DSSCs) employing a thermally and chemically stable Nb-doped TiO2 (NTO)/Al-doped ZnO (AZO) multilayer transparent conducting oxide (TCO) thin film. The NTO overlayer was found to block oxygen diffusion into AZO during the air-annealing process for the fabrication process of the DSSCs, thereby exhibiting good thermal stability in electrical conductivity of the multilayer TCO. Moreover, the NTO overlayer suppressed the formation of Zn2+-dye aggregates at the surface of the AZO. The DSSC employing this multilayer TCO showed a photon to electron conversion efficiency of 3.8% compared to 1.9% for the cell employing the AZO single layer. The optical transmittance and charge transport properties that were measured using electrochemical impedance spectroscopy demonstrate that NTO/AZO is a promising TCO for large scale DSSCs. Introduction Dye-sensitized solar cells (DSSCs) have attracted interest as an alternative to conventional solar cells because of their low toxicity and production cost.1,2 The photoelectrode is one of the key components in DSSCs, because light harvesting, charge generation, and charge transport take place in it.3-6 The photoelectrode is composed of a nanoporous oxide thick film and a transparent conducting oxide (TCO) thin film. The nanoporous TiO2 network thick film has been studied extensively for DSSCs in the past. Most of the research has been focused on controlling the size, shape, and surface state of the TiO2 nanoparticles to improve the adsorption of the dye molecules, injection of the excited electrons from the dye into the conduction band of the TiO2, and transport of the injected charge carrier within the TiO2 network thick film.7-14 However, although the TCO is a fairly important component at which the light transmittance, charge collection, and charge extraction to the external circuit take place, it has not been actively researched, because most researchers selected only F-doped SnO2 (FTO) as the TCO for DSSCs. The reason for this is that the thermal stability of the optical and electrical properties of TCOs is critical for efficient light harvesting and charge extraction, since the air/oxygen-annealing process has to be performed to remove the organics in the TiO2 film and to make electron paths between the TiO2 nanoparticles.15 FTO has been utilized in DSSCs, due to its outstanding thermal stability. However, it has inferior conductivity and optical transmittance * To whom correspondence should be addressed, [email protected] and [email protected]. † WCU Hybrid Materials Program, Department of Materials Science & Engineering, Seoul National University. ‡ Research Institute of Advance Materials, Seoul National University. § Chemical and Biosciences Center, National Renewable Energy Laboratory. | School of Advanced Materials Engineering, Kookmin University.

compared to other TCO materials, including ITO (Sn:In2O3), ATO (Sb:SnO2), and AZO (Al:ZnO).16 Since these TCOs possess superior optical and electrical properties and are facile to produce on a large scale, it is necessary to improve their thermal stability in order to enhance the energy conversion efficiency and reduce the cost of the DSSCs. We previously reported multilayered Nb:TiO2 (NTO)/Sn: In2O3 (ITO) as a new TCO material in DSSCs.17 We found that the NTO overlayer conserved the high conductivity of ITO by preventing the penetration of oxygen during the annealing process and that it removed the electrical potential barrier between TiO2 and ITO, thereby facilitating charge extraction. However, the high material cost of ITO still restricts its use in commercial DSSCs. Al-doped ZnO (AZO) has been conceived as an alternative to ITO, because of its comparable electrical conductivity and optical transmittance, as well as its low material cost.18 Despite its economic advantages, AZO has not been greatly utilized in DSSCs because of its intrinsic thermal and chemical instability. According to our previous report, the generation of oxygen point defects such as oxygen interstitials, deteriorated the electrical conductivity after the air annealing process.19 Moreover, the Zn2+-dye aggregates that are formed during the dye loading process constitute another disadvantage of AZO film.20,21 Therefore, the thermal and chemical instability issues need to be solved before AZO films can be utilized in DSSCs. In the present study, we overcoated NTO onto AZO film to improve its thermal and chemical stability and checked the feasibility of using it as a TCO in DSSCs. The NTO/AZO film showed enhanced electrical conductivity after air annealing and suppressed Zn2+-dye aggregate formation behavior. The electron conversion efficiency of the DSSC employing the NTO/ AZO multilayer was twice that of the AZO-based DSSC. The performance of the DSSCs containing the NTO/AZO layers was

10.1021/jp104247t  2010 American Chemical Society Published on Web 07/27/2010

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Noh et al. were measured using a potentiostat (model CHI 608C, CH Instruments, USA) and a solar simulator (AM 1.5, 100 mW/ cm2) (model PEC-L11, Pecell, Japan). Results and Discussion Figure 1 shows the cross-sectional TEM image and selected area diffraction (SAD) pattern of the NTO/AZO multilayer thin film grown on the fused silica substrate. The image demonstrates that the multilayer film contains an AZO layer covered by an NTO overlayer and that it does not contains any pores or voids. Moreover, the explicit SAD pattern is indicative of the high crystallinity of the AZO layer. However, an NTO pattern similar to the crystalline TiO2 pattern was not observed, which implies that the as-deposited NTO layer is amorphous. The NTO layer was crystallized into anatase phase after air annealing at 450 °C. The electrical properties of the as-grown AZO, air-annealed AZO, and air-annealed NTO/AZO films are summarized in Table 1. The as-grown AZO film exhibits a low sheet resistance of 6.8 Ω/0 because of its high carrier concentration and mobility. As expected, however, the sheet resistance dramatically increased to 98 Ω/0 after air annealing at 450 °C. Since the deteriorated sheet resistance leads to the retardation of charge extraction and the increase of the series resistance of the DSSC, the AZO film itself is not suitable for achieving high efficiency DSSCs. In contrast, the sheet resistance of NTO/AZO was decreased to 3.7 Ω/0, thus making it favorable for efficient charge transfer in DSSCs. The chemical instability of ZnO-based films also restricts their potential utilization as TCO materials in DSSCs. ZnO materials easily react with acidic dye molecules such as N3 and N719 leading to the formation of Zn2+-dye aggregates, which deteriorate the photovoltaic characteristics of the ZnO-based DSSC.21 The isoelectric point (IEP) for ZnO is approximately 9, which is higher than the corresponding value of 5 for TiO2. This makes the ZnO surface unstable to dye molecules. Al-doped ZnO may also form Zn2+-dye aggregates, which degrade the photovoltaic performance. Figure 2 shows the optical transmittance, while pictures of AZO and NTO/AZO before and after dye loading are presented in the inset. Each film was immersed in N719 dye solution for 2 h at 50 °C. As shown in Figure 2a, the AZO film is colored in red and the resultant transmittance is significantly reduced, which is related to the formation of Zn2+dye aggregates. The transmittance of NTO/AZO was not changed after immersing it in the dye solution, which demonstrates that the NTO overlayer prevents the reaction between AZO and the acidic dye molecules. Therefore, the NTO overlayer can maintain the transmittance of the AZO film as well as preventing its chemical reaction with the dye molecules. Transmittance of the TCO in contact with the electrolyte is more important than that of the TCO itself while DSSC is working. We found that transmittance of NTO/AZO in contact with the electrolyte is also higher than that of AZO in the range of 450 and 550 nm. (Transmittance spectra are presented in the Supporting Information.) Figure 3 shows the J-V characteristics of the DSSCs containing the AZO and NTO/AZO films. The properties of

Figure 1. Cross-sectional TEM image of the NTO/AZO multilayer structure on fused silica substrate. Inserted image shows SAD pattern of the NTO/AZO film.

discussed in terms of their optical characteristics and charge transport properties. Experiments A 680 nm thick Al-doped ZnO thin film was deposited on a fused silica substrate at 200 °C with a working pressure of 5 mTorr and Ar flow rate of 20 sccm using rf-magnetron sputter equipment from a 3 atom % Al-doped ZnO ceramic target. Nbdoped (6 atom %) TiO2 (NTO) layers were deposited on the sputtered AZO films or fused silica substrates by PLD at room temperature and an oxygen pressure of 5.0 × 10-4 Torr using a KrF (248 nm) excimer laser with a pulse energy density of 2 J/cm2 and a repetition rate of 5 Hz. The as-deposited NTO films were annealed for 1 h under an air atmosphere at a temperature of 450 °C. For the fabrication of the DSSC, a TiO2 nanocrystalline electrode was prepared on AZO and multilayered NTO/AZO films via the screen printing method using a paste consisting of TiO2 nanoparticles with a thickness and area of 10 µm and 5 × 5 mm, respectively. The paste was prepared by mixing some organics and TiO2 nanoparticles with a size of 10-15 nm using a three-roll mill. The as-prepared thick films were annealed at 450 °C for 1 h for the removal of the organics and formation of necking between the particles. The prepared TiO2 electrodes were immersed in a solution of ruthenium dye (ruthenium (2,2′bipyridyl-4,4′-dicarboxylate)2(NCS)2 (SOLARONIX) and ethanol for 2 h at 50 °C. The final cells were obtained by the assembly of the dye-absorbed TiO2 electrode, counter electrode, and Pt-sputtered FTO/glass, and the infiltration of triiodide electrode solution (SOLARONIX). High-resolution transmission electron microscopy (HRTEM, model JEM-3000F, JEOL, Japan) was used to obtain a crosssectional view of the films. The electrical properties of the films were measured using the van der Pauw method (HL5500PC, BIO-RAD, UK) and by a four point probe (SR1000N, AIT, Korea). The optical transmittance was measured by a UV-vis spectrophotometer (model Lambda 35, PerkinElmer, USA). The photovoltaic properties and electrochemical impedance spectra TABLE 1: Electrical Properties of AZO and NTO/AZO Films

as-grown AZO air-annealed AZO air-annealed NTO/AZO

sheet resistance (Ω/0)

resistivity (Ω cm)

6.8 98 3.7

5.2 × 10-4 6.8 × 10-2 3.8 × 10-

carrier concentration (cm-3) 5.1 × 1020 1.5 × 1019 5.6 × 1020

Hall mobility (cm2/(V s)) 24 6.0 33

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Figure 4. Nyquist plots for NTO/ITO DSSC and NTO/AZO DSSC.

TABLE 3: Series Resistances (Rseries) and Fill Factors (FF) of the 70, 120, and 220 nm Thick NTO Overlayer Coated AZO DSSCs 70 nm NTO 120 nm NTO 220 nm NTO

Figure 2. Transmittance of the AZO and the NTO/AZO films before and after soaking in N710 dye solution for 2 h in an oven at 50 °C and their pictures.

Figure 3. J-V characteristics of AZO and 120 nm thick NTO/AZObased DSSCs.

TABLE 2: Cell Parameters of AZO DSSC and NTO/AZO DSSC AZO/FS NTO/AZO/FS

JSC (mA/cm2)

VOC (V)

FF (%)

η (%)

4.66 8.52

0.73 0.74

57 60

1.9 3.8

the DSSCs are summarized in Table 2. The photon to electron conversion efficiency (PCE) of the NTO/AZO-based DSSC of 3.8% is twice that of the AZO-based one (1.9%). The high

Rseries (Ω)

FF (%)

260 145 104

42 60 70

short circuit current (JSC) of 8.52 mA/cm2 for the NTO/AZObased DSSC results in a better energy conversion efficiency, which is attributed to its high transmittance in the range of 400 and 600 nm, as well as its low sheet resistance after the annealing process. Although the NTO/AZO-based DSSC showed a higher energy conversion efficiency, the fill factor (FF) was not significantly improved; rather it was lower than that of the conventional F-doped SnO2-based DSSCs.8 As mentioned above, the sheet resistance of NTO/AZO was 3.7 Ω/0, which is much lower than that of FTO, viz., 10 Ω/0. This indicates that the sheet resistance does not significantly impact the FF of the NTO/AZO-based DSSC. In our previous report, the DSSC using the NTO/ITO multilayer TCO film with a sheet resistance of 8.5 Ω/0 showed a high FF of 76%.17 The NTO layer inhibited the formation of a Schottky barrier between TiO2 and ITO, thereby causing the NTO/ ITO-based DSSC to have a high FF. (The J-V curves of the NTO/ITO and NTO/AZO DSSCs are presented in the Supporting Information). In order to understand the origin of the different FFs of the NTO/ITO- and NTO/AZO-based DSSCs, the electrochemical impedance spectroscopy analysis of each DSSC was performed. Figure 4 shows the Nyquist plots of the NTO/ ITO and NTO/AZO DSSCs under open circuit conditions with light illumination. One can see a fairly large arc in the case of the NTO/AZO-based DSSC in the frequency regime of 103-105 Hz, which is assigned to the impedance at the conducting layer/TiO2 interface.22,23 This large impedance retards the charge transport between the TiO2 and NTO/AZO film, leading to the deterioration of the FF. The series resistances of the cells, Rseries ) (dV/dI)J)0, and FF as a function of the thickness of the NTO overlayer were measured to investigate the origin of the retarded charge transport in the NTO/AZO-based photoelectrode, and the results are summarized in Table 3. (The J-V curves of each DSSC are available in the Supporting Information.) Rseries remarkably decreases with increasing thickness of the NTO overlayer and, consequently, the FF increases, which indicates that the charge transport properties of NTO/AZO are improved by increasing the thickness of the NTO overlayer. The large Rseries of the NTO/

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Figure 5. Approximate band diagrams at interface between AZO and NTO after air annealing.

AZO DSSC with the thin NTO overlayer may be ascribed to a potential barrier between NTO and AZO formed by the generation of oxygen interstitials in the AZO underlayer. We previously reported that oxygen interstitials formed in epitaxial AZO thin films after annealing at 450 °C, reducing the carrier concentration.19 The presence of oxygen interstitials in the AZO film can be confirmed by the existence of a peculiar orange emission in its photoluminescence spectrum.24,25 In the PL spectrum, the air-annealed NTO/AZO film showed green (2.3 eV) and orange (1.9 eV) emissions. (The PL spectra are available in the Supporting Information.) The green emission is associated with the typical intrinsic defects such as oxygen vacancies and/or zinc interstitials in ZnO. Especially, the orange emission that was evolved after the annealing process indicates that the oxygen penetrated into the NTO overlayer, consequently forming oxygen interstitials in the AZO underlayer. We believe that the oxygen interstitials exist on the top of the AZO layer (AZO:Oi layer). The oxygen interstitials reduce the carrier concentration, thereby lowering the Fermi level.19 Therefore, the AZO:Oi layer might be electrically nondegenerate. Figure 5 shows the schematic band diagrams of the airannealed NTO/AZO containing oxygen interstitials. The Fermi levels of the degenerate NTO and AZO, and the nondegenerate AZO:Oi layer are predicted on the basis of the electron affinity (χ) data of TiO2 (4.0 eV) and ZnO (4.0 eV).26,27 As shown in the band diagrams, an electrical potential barrier (Φ) could be formed due to the presence of the nondegenerate AZO:Oi layer. This barrier blocks the carrier transfer and increases the Rseries value of the NTO/ AZO DSSC, which leads to the deterioration of the FF in the cells. In the case of the thicker NTO-covered AZO layer, the oxygen diffusion is significantly retarded and the formation of the AZO:Oi layer is suppressed, which enhances the FF of the DSSCs. This prediction is in accordance with the experimental data shown in Table 3, showing that the FF of the DSSCs is remarkably improved by increasing the thickness of the NTO overlayer. However, a thicker NTO layer may not be favorable for the optical transmittance properties, which demonstrates that more studies on improving the optical properties are needed before the commercialization of NTO/AZO multilayers in photoelectrochemical devices including DSSCs and water-splitting cells. Conclusion The multilayered Nb-doped TiO2/Al-doped ZnO (NTO/ AZO) films, fabricated by rf magnetron sputtering and pulsed laser deposition, showed good electrical and chemical stabilities for use as transparent conducting oxide (TCO) layers in dye-sensitized solar cells (DSSCs). The NTO overlayer was

Noh et al. found to retard the degradation of the electrical conductivity during the air-annealing process, as well as to effectively prevent the formation of Zn2+-dye aggregates at the surface of AZO. The photon to electron conversion efficiency of the DSSC containing the NTO/AZO layer was 3.8%, which is 2 times higher than that of the AZO-based DSSC (1.9%). However, the NTO/AZO-based DSSC showed a low FF of 60%, because of the large impedance for charge transport at the NTO/AZO interface that was confirmed by electrochemical impedance spectroscopy. The FF of the NTO/AZO-based DSSCs significantly improved with increasing thickness of the NTO overlayer. This result demonstrated that the NTO overlayer blocked the formation of an AZO:Oi layer, which would retard the charge transport from the NTO to AZO layer. Further studies on improving the optical properties of the NTO overlayer will facilitate commercialization of the NTO/AZO TCO layers for photoelectrochemical devices such as DSSCs and water-splitting cells. Acknowledgment. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2009-0092779). This work was also supported by a grant from the Korea Science and Engineering Foundation (KOSEF) of the Korean Government (MEST) (R112005-048-00000-0, ERC CMPS, and R01-2008-20581-0). Supporting Information Available: Transmittance spectra of AZO and [NTO/AZO] structures, J-V curves of the NTO/ ITO and the NTO/AZO DSSCs, PL spectra of the NTO/AZO multilayer before and after air annealing, and J-V curves of DSSCs employing NTO/AZO multilayers as a function of thickness of NTO overlayer. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (2) Gratzel, M. J. Photochem. Photobiol., A 2004, 168, 235. (3) Fortunato, E.; Ginley, D.; Hosono, H.; Paine, D. C. MRS Bull. 2007, 32, 242. (4) Tennakone, K.; Sendadeera, G. K. R.; Perera, V. P. S.; Kottegoda, I. R. M.; De Silva, L. A. A. Chem. Mater. 1999, 11, 2474. (5) Liu, X.; Luo, Y.; Li, H.; Fan, Y.; Yu, Z.; Lin, Y.; Chen, L.; Meng, Q. Chem. Commun. 2007, 2847. (6) Varghese, O. K.; Paulose, M.; Grimes, C. A. Nat. Mater. 2009, 4, 592. (7) Thavasi, V.; Renugopalakrishnan, V.; Jose, R.; Ramakrishna, S. Mater. Sci. Eng., R 2009, 63, 81. (8) Lee, S.; Cho, I.-S.; Lee, J. H.; Kim, D. H.; Kim, D. W.; Kim, J. Y.; Shin, H.; Lee, J.-K.; Jung, H. S.; Park, N.-G.; Kim., K.; Ko, M. J.; Hong, K. S. Chem. Mater. 2010, 22, 1958. (9) Li, G.; Richter, C. P.; Milot, R. L.; Cai, L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W.; Batista, V. S. Dalton Trans. 2009, 10078. (10) Lu, X.; Mou, X.; Wu, J.; Zhang, D.; Zhang, L.; Huang, F.; Xu, F.; Huang, S. AdV. Funct. Mater. 2010, 20, 509. (11) Tan, B.; Wu, Y. J. Phys. Chem. B 2006, 110, 15932. (12) Katoh, R.; Huijser, A.; Hara, K.; Savenije, T. J.; Siebbeles, L. D. A. J. Phys. Chem. C 2007, 111, 10741. (13) Koo, H.-J.; Kim, Y. J.; Lee, Y. H.; Lee, W. I.; Kim, K.; Park, N.-G. AdV. Mater. 2008, 20, 195. (14) Wang, Z.-S.; Yanagida, M.; Sayama, K.; Sugihara, H. Chem. Mater. 2006, 18, 2912. (15) Kawashima, T.; Ezure, T.; Okada, K.; Matsui, H.; Goto, K.; Tanabe, N. J. Photochem. Photobiol., A 2004, 164, 199. (16) Rottkay, K. V.; Rubin, M. Mater. Res. Soc. Symp. Proc. 1996, 426, 449. (17) Noh, J. H.; Lee, S.; Kim, J. Y.; Lee, J.-K.; Han, H. S.; Cho, C. M.; Cho, I. S.; Jung, H. S.; Hong, K. S. J. Phys. Chem. C 2009, 113, 1083. (18) Minami, T. Thin Solid Films 2008, 516, 5822. (19) Noh, J. H.; Jung, H. S.; Lee, J.-K.; Kim, J. Y.; Cho, C. M.; An, J.-S.; Hong, K. S. J. Appl. Phys. 2008, 104, 073706. (20) Horiuchi, H.; Katoh, R.; Hara, K.; Yanagida, M.; Murata, S.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B 2003, 107, 2570.

Transparent Conducting Oxide Multilayer (21) Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Langmuir 2000, 16, 4688. (22) Hoshikawa, T.; Yamada, M.; Kikuchi, R.; Eguchi, K. J. Electrochem. Soc. 2005, 152, E68. (23) Van de Lagemaat, J.; Park, N.-G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044. (24) Kwok, W. M.; Leung, Y. H.; Djurisic, A. B.; Chan, W. K.; Phillips, D. L. Appl. Phys. Lett. 2005, 87, 093108.

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