Al-Doped ZnO Thin Film: A New Transparent Conducting Layer for

Mar 26, 2010 - In this study, an aluminum-doped zinc oxide (AZO) layer was used as a transparent conducting oxide (TCO) layer in ZnO nanowire (NW)-bas...
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J. Phys. Chem. C 2010, 114, 7185–7189

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Al-Doped ZnO Thin Film: A New Transparent Conducting Layer for ZnO Nanowire-Based Dye-Sensitized Solar Cells Sung-Hae Lee, Se-Hoon Han, Hyun Suk Jung,* Hyunjung Shin, and Jagab Lee School of AdVanced Materials Engineering, Kookmin UniVersity, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Korea

Jun-Hong Noh, Sangwook Lee, and In-Sun Cho School of Materials Science and Engineering, Seoul National UniVersity, Shillim-dong, Seoul, 151-744, Korea

Jung-Kun Lee Department of Mechanical Engineering and Materials Science, Pittsburgh UniVersity, Pittsburgh, PennsylVania 15261

Jinyoung Kim Chemical and Biosciences Center, National Renewable Energy Laboratory, Golden, Colorado 80401

Hyunho Shin Department of Ceramic Engineering, Ganeneung-Wonju National UniVersity, Gangneung, 210-702, Korea ReceiVed: NoVember 9, 2009; ReVised Manuscript ReceiVed: March 8, 2010

In this study, an aluminum-doped zinc oxide (AZO) layer was used as a transparent conducting oxide (TCO) layer in ZnO nanowire (NW)-based dye-sensitized solar cells (DSSCs). The well aligned, single crystalline ZnO NW arrays that were grown on the AZO films exhibited a better DSSC performance (an increased photocurrent density and fill factor) than those grown on the fluorine doped tin oxide (FTO) films. The I-V characteristics and electrochemical impedance spectroscopy measurements for the ZnO NW arrays on the AZO and FTO films clearly showed that the superior DSSC performance was caused by the facilitated charge injection from the ZnO NW to AZO, resulting from the formation of an ohmic contact. This study demonstrates that the AZO films are more favorable for highly efficient ZnO NW-based photoenergy conversion devices. Introduction Dye-sensitized solar cells (DSSCs) have attracted attention because of their low fabrication costs and potential application in flexible devices.1,2 Usually, TiO2 nanoparticle films with transparent conducting oxide (TCO) layers are used as the photoelectrode in DSSCs because of their adequate surface area and chemical affinity for dye adsorption as well as their suitable energy band potential for charge transfer between the dye and electrolytes. However, the numerous grain boundaries between the TiO2 nanoparticles restrict fast electron transport, which is detrimental to carrying out a highly efficient energy conversion process. Recently, ZnO nanowire (NW) photoelectrodes have been exploited in order to improve the electron transfer by virtue of eliminating the grain boundaries.3 A single crystalline ZnO NW has the similar energy band position as TiO2, which makes it suitable for highly efficient photoelectrode materials. However, the energy conversion efficiency (η) of the ZnO NW-based DSSCs is inferior to the TiO2 nanoparticle-based DSSCs. Therefore, challenges still remain with ZnO NW-based DSSCs, and they must be further studied to improve their photovoltaic properties. Many reports have attempted to boost the efficiency of ZnO NW-based DSSCs.4-6 For example, Wu et al.7 recently reported that a mercurochrome (C20H8Br2HgNa2O) sensitizer is * To whom correspondence should be addressed. E-mail: hjung@ kookmin.ac.kr.

more suitable than the Ru(dcbpy)2(NCS)2 (N719) dye for the ZnO NW-based DSSCs, which demonstrated that new materials containing sensitizers, electrolytes, and TCOs should be exploited to improve the energy conversion efficiencies. TCO layers extract photogenerated electrons from nanostructured semiconductors and transfer them to external circuits. Therefore, these layers are actively being researched in TiO2 nanoparticle-based DSSCs because their low charge collecting efficiency, may deteriorate the performance of solar cells.8,9 Usually, fluorine-doped tinoxide (FTO) is considered a promising TCO material in TiO2 nanoparticle-based DSSCs because of its good thermal stability, after an annealing process for binder burn-out and particle interconnection. Additionally, the majority of the research regarding ZnO NW-based DSSCs has used FTOs as the TCO materials.3,7,10,11 However, the utilization of FTO may not be the best method for improving the cell performance. For example, the small difference in the work function between ZnO (5.1-5.3 eV)12-14 and FTO (4.9 eV)15,16 does not provide sufficient driving force for the charge injection from the ZnO NWs to FTO, which implies that new TCO materials must be exploited in ZnO NW-based DSSCs. In this present study, an aluminum-doped ZnO (AZO) TCO material, with a reported work function of 3.7-4.6 eV17,18 was employed in a ZnO NW-based DSSC to increase the potential for charge extraction from the ZnO NW to AZO. The crystal qualities of the ZnO NWs grown on the AZO thin films and their DSSC performance were characterized and compared to

10.1021/jp1008412  2010 American Chemical Society Published on Web 03/26/2010

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Figure 1. Cross-sectional SEM images of the ZnO NW arrays, which were repeatedly grown on the AZO films: (a) 3, (b) 10, and (c) 15 times.

ZnO NW DSSCs with FTO materials. The ZnO NW DSSCs with the AZO thin layers exhibited an improved energy conversion efficiency which was ascribed to the enhanced electron transport between the ZnO NW arrays and the AZO thin films. Experimental Section Deposition of Al-Doped ZnO Thin Films. The AZO films were grown on fused silica substrates from 6 at. % Al-doped ZnO targets using the pulsed laser deposition (PLD) technique at 400 °C and an oxygen pressure of 1.0 × 10-1 Torr. The deposition was carried out using a KrF (248 nm) excimer laser with a pulse energy density of 2 J/cm2 and a repetition rate of 5 Hz. The resulting thickness of the AZO film was approximately 1 µm. Preparation of ZnO Nanowire Arrays. The ZnO nanocrystals were prepared through the hydrolysis of zinc acetate dehydrate and lithium hydroxide monohydrate and used as seed layers using a previously described experimental procedure.19 The ZnO seed solution was spun on both commercial FTO (Pilkington, approximately 700 nm in thickness) and AZO thin films. Then the deposited seed layers were annealed at 340 °C in air for 5 min. The ZnO NW arrays were grown on the ZnO nanocrystals-seeded FTO and AZO substrates using a chemical bath deposition method in an aqueous solution of zinc nitrate hexahydrate, hexamethylenetetramine, and polyethyleneimine at 95 °C.3 The ZnO NW arrays were repeatedly introduced into fresh solution baths to increase the length of the NW. Then the arrays were rinsed with deionized water and annealed at 350 °C for 30 min to remove any residual organic materials and to enhance the electrical contact between the ZnO NW arrays and the TCO layers. Photovoltaic Measurement. The ZnO NW photoelectrodes were dipped in a solution of N719 dye [SOLARONIX, Switzerland, dissolved in ethanol] at 50 °C for 2 h. Then the dye-adsorbed electrode was assembled with a Pt counterelectrode to form a sandwich-type dye-sensitized solar cell. A drop of the electrolyte solution (Iodolyte AG-50, SOLARONIX, Switzerland) was infiltrated between the two electrodes of the cell. The photovoltaic properties and electrochemical impedances of the fabricated solar cells were measured under illumination of an air mass 1.5 (ORIEL 91193 1000 W xenon lamp; intensity: 100 mW/cm2) with the aid of a potentiostat (CHI 608C, CH Instruments). The impedance of the cells was also measured under 100 mW/cm2 illumination and a reverse biased open circuit voltage using the same potentiostat. Characterization. The morphologies of the deposited materials were examined using field-emission scanning electron microscopy (FESEM). The crystal structure and morphology of the synthesized ZnO NW arrays were investigated using high resolution transmission electron microscopy (HRTEM). The electrical properties of the AZO and FTO layers were measured using the van der Pauw method (HL5500PC, BIO-RAD, UK) with a 4 point probe (SR1000N, AIT, Korea). The optical

transmittance and reflectance of the ZnO NW arrays on the AZO and FTO films were measured using a UV-vis spectrophotometer (Perkins-Elmer, USA) equipped with an integrating sphere accessory. The dye molecules were desorbed from the photoelectrodes through soaking in alkaline alcoholic solutions. The optical absorption of the dye solutions was also characterized using the UV-vis spectrophotometer to compare the degree of adsorbed dye molecules. Results and Discussion Growth of ZnO Nanowire Arrays on AZO Films. Figure 1a-c shows the change in the morphologies of the ZnO NW arrays on the AZO films as a function of the growth time. The aspect ratio (length/diameter) of the ZnO NW arrays was approximately 17 and was not significantly changed with respect to the growth time indicating that the lateral and vertical growth rate of the ZnO NW were almost identical, which was consistent with previously reported results.20 The diameter of the ZnO NW increased with the growth time, which led to the increased areal density of the ZnO NW arrays on the substrate. In Figure 2a, the TEM bright field image of the individual NW, which was removed from the 10 times grown array, indicates that the ZnO NW possesses a single-crystalline structure because of the absence of grain boundaries. The HRTEM image of the NW shows that the NW was well crystallized and grew in the [001] direction (Figure 2b). Given that the bright spots are observed in the selected area diffraction (SAD) pattern (Figure 2c), the ZnO NW possesses a single crystal structure. Figure 2d is the X-ray diffraction θ-2θ scan of the 10 times-grown ZnO NW array on the AZO film. The ZnO (002) peak intensity was more than 300 times larger than other ZnO peaks, indicating the formation of highly crystalline ZnO with a strongly preferred (002) orientation. The TEM and XRD analysis demonstrates that well-aligned ZnO NW arrays with a high crystal quality were successfully grown on the AZO films. DSSC Performance of ZnO Nanowire Arrays Employing AZO Films. The DSSC performance of the ZnO NW arrays grown on the AZO films were characterized and compared to the FTO-based ZnO NW DSSC. Figure 3 shows the photocurrent density vs the voltage curves of the DSSCs containing the 10 times-grown ZnO NW arrays on the AZO and FTO thin films, respectively. Although the morphologies of the ZnO NW arrays grown on both the AZO and FTO films are similar (the inset of Figure 3), the AZO-based DSSC exhibits a better energy conversion efficiency than the FTO-based one. In Table 1, the fill factor (FF) of the AZO-based photoelectrode is 42.4%, which is larger than the FTO-based one (37.8%). Remarkably, the short circuit current density, Jsc, of the AZO-based photoelectrode significantly increased from 2.0 to 2.5 mA/cm2, corresponding to a 25% improvement. The increased FF and Jsc led to the superior DSSC performance of the AZO based ZnO NW photoelectrode.

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Figure 2. (a) TEM micrograph, (b) HRTEM images, (c) SAD pattern, and (d) X-ray diffraction pattern of the ZnO NW array grown on the AZO film.

Figure 3. J-V curves for the DSSCs containing the ZnO NW arrays grown on the AZO (black line) and FTO films (red line). The crosssectional SEM images of the ZnO NW arrays grown on the AZO and FTO films are presented in the inset.

Figure 4. UV-vis absorption spectra of dye solution that was desorbed from the ZnO NW arrays grown on the AZO (black line) and FTO (red line) films.

TABLE 1: Cell Parameters of the DSSCs Containing ZnO NW Arrays Grown on the AZO and FTO Films

differences between each sample (Supporting Information). Hence, the optical properties of the AZO- and FTO-based ZnO NW photoelectrodes are not responsible for the superior performance of the AZO-based ZnO NW DSSC. The dye-adsorptive characteristics of both the AZO- and FTObased photoelectrodes were measured. Figure 4 shows the UV-vis absorption spectra of the solutions with the dissolved dye molecules from each photoelectrode.21 The absorbance peak at 510 nm, originating from the dye molecules, is larger for the AZO-based photoelectrode than for the FTO-based one. The calculated dye loading of the AZO-based photoelectrode is 6.2 × 10-8 mol/cm2, 17% larger than the FTO-based one (5.3 × 10-8 mol/cm2). This data indicates that the ZnO NW arrays grown on AZO possess a higher surface area than that on FTO although the each ZnO NW array shows similar morphologies (the inset of Figure 3). The surface areas of each

sample

Voc (V)

Jsc (mA/cm2)

FF (%)

η (%)

AZO-based ZnO NRs FTO-based ZnO NRs

0.55 0.55

2.5 2.0

42.4 37.8

0.6 0.4

Several factors were investigated in order to determine the origin of the improved DSSC performance for the AZO-based photoelectrode. The electrical resistance of TCO films could impact the FF and Jsc of the DSSC. However, the AZO film exhibited a sheet resistance of 12.2 Ω/0, which was higher than FTO (10.0 Ω/0). Thus, the higher resistance of AZO did not cause the superior performance of the AZO-based DSSC. The optical transmittance and reflectance of both the AZO- and FTObased ZnO NW photoelectrodes were measured with no apparent

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Figure 5. I-V characteristics of the ZnO NW arrays grown on the AZO (black line) and FTO (red line) films. The schematic diagram for I-V measurements is presented in the inset.

array may be different because controlling the precise surface area is not so simple. The 17% increase in the dye adsorption alone cannot solely explain the enhanced photocurrent density of the AZO-based photoelectrode, which increased by 28%. Since the work functions of both the AZO and FTO films have been reported as respectively 3.7-4.6 and 4.9 eV, the charge injection from the ZnO NWs into each film can be altered.15-18 The I-V characteristics of the ZnO NWs arrays grown on each film were measured and plotted in Figure 5. The schematic structure of the measured samples is illustrated in the inset of Figure 5. The ZnO NWs array grown on the AZO film exhibits an ohmic contact behavior in contrast to the Schottky contact behavior of the FTO based ZnO NWs array. The reported work function of ZnO was 5.1-5.3 eV.12-14 Therefore, the ohmic contact with the AZO film was formed because the work function of ZnO was larger than AZO. Also, the similar chemical compositions of ZnO and AZO were favorable for the formation of a strong chemical bond at the ZnO/AZO interface, which facilitated the charge transfer from the ZnO NW to the AZO film. In contrast, the surface states at the ZnO/FTO interface, which were associated with the weak chemical bond between ZnO and FTO, acted as electron traps, forming a high potential barrier that induced the Schottky contact behavior.22-25 Zhao et al.26 also suggested that the surface state at the ZnO NW/ITO interface was generated by the weak chemical bonds. The suppressed dark current at positive bias for ZnO NW array grown on AZO is consistent with the better charge transfer between ZnO and AZO film (Supporting Information). The electrochemical impedance spectroscopy (EIS) measurements were performed to verify the influence of the electrical contact behavior between TCO and ZnO in DSSCs. Figure 6a shows the Nyquist plots of the DSSCs containing the AZOand FTO-based ZnO NW photoelectrodes. The arcs observed in the frequency regime of 102-104 (ω1 or ω2) are associated with the resistances at the conducting layer/ZnO (ω1) or Pt/ electrolyte interfaces (ω2).27-29 Also, the arc may be related to the resistance between nanocrystals for the case of TiO2 nanocrystals-based DSSCs.27 However, the single crystalline ZnO NW does not have grain boundaries. Therefore, the semicircles are related to the charge transfer at the FTO/ZnO or AZO/ZnO interfaces because the same Pt layers were used as the counter electrode in both cases. The semicircle of the

Figure 6. (a) Nyquist plots and (b) fill factors vs growth time for the DSSCs containing the ZnO NW arrays grown on the AZO (black square symbol) and FTO films (red circle symbol).

AZO-based solar cell is smaller than the FTO-based one, indicating that the AZO/ZnO junction better facilitates the injected electron transfer. Hence, the AZO film exhibited better charge collecting properties than FTO in the case of the ZnO NW photoelectrode systems. In Figure 6b, the higher fill factors for the AZO-based DSSCs (the photocurrent density-voltage curves, Jsc and efficiency of the AZO- and FTO-based DSSCs are plotted as a function of the ZnO NW growth time in Supporting Information), support the better electron transfer performance of the AZO films. These findings demonstrate that the AZO films are more suitable for achieving higher efficiencies in the ZnO NW-based photoenergy conversion devices such as DSSCs and photoelectrochemical (PEC) cells. In conclusion, ZnO nanowires (NWs) arrays were grown onto Al-doped ZnO (AZO) films, and their photovoltaic performances were characterized. The structural analysis showed that the ZnO NWs arrays were well-aligned on the AZO films and consisted of a single crystalline ZnO structure. The comparative study of the AZO- and FTO-based ZnO NW photoelectrodes revealed that the AZO films more beneficially facilitate the charge transfer from ZnO to AZO, using the IV characteristics and electrochemical impedance spectra of the ZnO NW arrays grown on the AZO and FTO films. These results demonstrate that the AZO films are promising TCO materials in ZnO NWbased photoenergy conversion devices including DSSCs and water-splitting photoelectrochemical (PEC) cells. Acknowledgment. This work was supported by a grant from the Korea Science and Engineering Foundation (KOSEF) of the Korean government (MEST) (R11-2005-048-00000-0, ERC, CMPS, and 2009-0065889). This work was also supported by

Al-Doped ZnO Thin Film the Nano R&D program through the National Research Foundation of Korea funded by MEST (2009-0082659) and the research program 2009 of Kookmin University in Korea. Supporting Information Available: The optical transmittance and reflectrance spectra of the ZnO NR arrays grown on the AZO and FTO films, the photocurrent density-voltage curves, and the Jsc and efficiency of the AZO and FTO based DSSCs as a function of the ZnO NW growth times. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Oregan, B.; Gra¨tzel, M. Nature 2004, 353, 737. (2) Jiang, C. Y.; Sun, X. W.; Tan, K. W.; Lo, G. Q.; Kyaw, A. K.; Kwong, D. L. Appl. Phys. Lett. 2008, 92, 143101. (3) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (4) Baxter, J. B.; Walker, A. M.; van Ommering, K.; Aydil, E. S. Nanotechnology 2006, 17, S304. (5) Cheng, A. J.; Tzeng, Y. H.; Zhou, Y.; Park, M.; Wu, T. H.; Shannon, C.; Wang, D.; Lee, W. W. Appl. Phy. Lett. 2008, 92, 092113. (6) Noh, J. H.; Lee, S. H.; Lee, S.; Jung, H. S. Electron. Mater. Lett. 2008, 4, 71. (7) Wu, J.-J.; Chen, G.-R.; Yang, H.-H.; Ku, C.-H.; Lai, J.-Y. Appl. Phys. Lett. 2007, 90, 213109. (8) Lee, S.; Noh, J. H.; Bae, S. T.; Cho, I. S.; Kim, J. Y.; Shin, H.; Lee, J. K.; Jung, H. S.; Hong, K. S. J. Phys. Chem. C 2009, 113, 7443. (9) 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, 7443. (10) Law, M.; Greene, L. E.; Radenovic, A.; Kuykendall, T.; Liphardt, J.; Yang, P. D. J. Phys. Chem. B 2006, 110, 22652.

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