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Highly Transparent and Conductive ZnO: Al Thin Films from a Low Temperature Aqueous Solution Approach Harald Hagendorfer,* Karla Lienau, Shiro Nishiwaki, Carolin M. Fella, Lukas Kranz, Alexander R. Uhl, Dominik Jaeger, Li Luo, Christina Gretener, Stephan Buecheler, Yaroslav E. Romanyuk, and Ayodhya N. Tiwari* Transparent conductive oxides (TCO) are a unique class of materials exhibiting optical transparency combined with metallike electrical conductivity and thus are of utmost importance for the rapidly expanding fields of transparent electronics and sustainable energy generation.[1a-e] For most applications the standard compound is ITO exhibiting best optoelectronic performance amongst all TCO materials but to ensure a sustainable supply of such materials earth abundant and inexpensive alternatives such as aluminum doped zinc oxide (AZO) are crucial.[2] In fact this is also reflected in predicted markets of almost $1 Billion in 2016 for alternative TCOs.[3] Nowadays, plasma based (magnetron-sputtering),[4a] pulsed laser deposition (PLD),[4b] atomic layer deposition(ALD)[4c,d] or chemical vapor deposition (CVD)[4e] methods are employed on industrial scale to obtain high quality AZO thin films with resistivity of 10−3–10−4 Ω cm and visible transparency > 90%, although instrumental complexity poses high investment costs as well as limits scalability. In this respect low cost non-vacuum methods for AZO thin films are of immense interest. A variety of solution based approaches have been described but to obtain good optoelectronic properties comparable to vacuum deposition techniques, high temperature annealing (300–600 °C) preferably in vacuum (10−1–10−4 mbar), and for long time (60–90 min) are necessary. Some approaches employ flammable or toxic organic solvents (e.g. 2-methoxyethanol),[5a–c] and in the case of electrodeposition conductive substrates are inevitable.[5d] The most straightforward yet challenging approach Dr. H. Hagendorfer, K. Lienau, Dr. S. Nishiwaki, C. M. Fella, L. Kranz, Dr. A. R. Uhl, C. Gretener, Dr. S. Buecheler, Dr. Y. E. Romanyuk, Prof. A. N. Tiwari Empa – Swiss Federal Laboratories for Materials Science and Technology Laboratory for Thin Films and Photovoltaics Ueberlandstrasse 129, CH-8600 Duebendorf Tel.: +41 58 765 6119 E-mail: [email protected]; [email protected] Dr. D. Jaeger Empa – Swiss Federal Laboratories for Materials Science and Technology Laboratory for Nanoscale Materials Science Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland presently at OC Oerlikon Balzers AG, Systems, Iramali 18, FL-9496, Balzers, Liechtenstein. L. Luo ETH Zurich, Laboratory of Multifunctional Materials Department of Materials Wolfgang-Pauli-Strasse 10, 8093, Zurich, Switzerland
DOI: 10.1002/adma.201303186
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for deposition of ZnO thin films is aqueous solution growth[6] (e.g., chemical bath deposition or hydrothermal synthesis) on seeded substrates as illustrated in Figure 1a. The solution chemistry comprises a water soluble zinc salt and a complexant (usually ammonium salts, ethanolamine, or NH4OH) which is also used for adjusting the pH to a basic regime. The crystal growth is associated with the decreasing thermodynamic stability of the zinc complex leading to controlled supersaturation and the retrograde solubility of ZnO upon increased temperature.[7] Thus, phase pure ZnO thin films can be obtained already at temperatures of 60 °C. The use of water as solvent makes the method environmental friendly and due to the use of inexpensive chemicals – as mostly water soluble metal salts are employed – it can also be designated as low cost. A number of approaches have been reported to obtain intrinsic, undoped ZnO in the form of nanorod, nanoneedle, or nanopillar thin films[8a] using aqueous solution deposition, but only a few studies attempt to obtain conductive AZO thin films.[8b,c] However, achieving a compact, dense, and highly conductive ( 11). Addition of 1 mmol/L Al salt to the solution results in a reduction of the growth rate by 50% (from 50 nm min−1 for undoped ZnO films to 25 nm min−1). Films with resistivity not lower than 3–5 × 10−2 Ω cm and carrier densities of 7–9 × 1019 cm3 are obtained. Addition of higher concentrations of aluminum salt, as necessary for increasing the carrier density and lowering the resistivity, results in a strongly constrained growth of ZnO (>1 mmol L−1) or complete change of morphology (>5 mmol L−1) to flower or sheet-like ZnO structures (Figure 1b). Thus, we conclude that a continuous addition of Al in quantities low enough to prevent disturbed growth (300 °C), or in reactive atmospheres (N2/H2) leads to increased concentration of oxygen vacancies and lower density of electron traps at the grain boundaries. Indeed, vacuum annealing at temperatures of 300–500 °C (10−1 mbar, 60 minutes and rapid cooling to room temperature) results in a massive decrease of sheet resistance from initially 200 kΩ/sq to 100 – 300 Ω/sq for the solution deposited AZO thin films. Since such high temperatures are not in accordance with the ambitious goal to develop a low temperature process, another strategy inspired by atmospheric chemistry[17] was applied for removing absorbed oxygen species (Figure 2a) to increase carrier mobility. XPS spectra before and after UV treatment (Figure 2b) reveals only a slight difference in the Zn2p3/2 signal. In contrast the O1s signal is affected dramatically from exposure to UV radiation where a shift from binding energies assigned to hydroxides to lower energies
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O 1s before UV
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Figure 2. Influence of the UV treatment on opto-electronic properties of AZO. a) Mechanism of UV-annealing on absorbed oxygen species at AZO grain boundaries. b) XPS spectra of the AZO film before and after UV annealing and c) influence on UV/Vis transmission and band gap energy. d) Time dependence of sheet resistance employing UV treatment (80 °C, 70 mW/cm2) and thermal annealing at the same temperature (80 °C) and 500 °C, respectively.
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Figure 3. Applications of low temperature aqueous solution deposited AZO thin films. a) High quality AZO thin film on glass to demonstrate the optical transmission (40 ± 5 Ω/sq). b) Corresponding SEM cross section image. c) SEM cross section image, d) cell parameters, e) quantum efficiency-, and f) J(V) curve for a CIGS solar cell with a low temperature aqueous solution prepared AZO window layer. g) Photograph of an AZO film deposited on a PET film with h) corresponding SEM cross section images and i. UV/Vis transmission spectra.
is obtained without exceeding the temperature of 85 °C at any process step. Following the successful development of the aqueous solution deposition, the aim is to demonstrate the applicability of our approach to temperature sensitive substrates. The utilization of the aqueous solution grown AZO as front electrical contact in a Cu(In,Ga)Se2 (CIGS) thin film solar cell is an excellent application for this low temperature process since the underlying p-CIGS/n-CdS heterojunction can rapidly degrade when heated above 150 °C.[20] In Figure 3c–f a cross section SEM image, J(V)- and quantum efficiency curves are presented for a CIGS solar cell (see supporting information section II and III) incorporating a low temperature aqueous solution deposited AZO window layer and featuring a solar cell efficiency of 14.7%. The current density of 33.4 ± 0.5 mA cm−2 and series resistance of 1.1 ± 0.1 Ω cm2 are comparable to the optimized reference cell with a high quality sputtered AZO thin film (34.1 ± 0.6 mA cm−2, 1 ± 0.1 Ω cm2), thus demonstrating the excellent optoelectronic properties achieved for the aqueous solution deposited window layer. To further corroborate the feasibility of low temperature processing, we deposited AZO layers on PET films, for example to be used as a high quality window layer for flexible non-vacuum deposited organic photovoltaics. In Figure 3g–i a photograph, a SEM cross section image of the AZO thin film on PET, and a corresponding transmission spectrum are presented. The film had a sheet resistance as low as 70 Ω/sq and transmission above 85% in the visible range.
Adv. Mater. 2014, 26, 632–636
In conclusion, a new path is demonstrated to produce highly conductive and transparent thin films using a low cost, low temperature, aqueous solution based procedure. Different morphologies from AZO nanorods towards densely packed films are accomplished by altering the solution chemistry. Using metallic Al as a continuous and controllable doping source allows the fabrication of AZO thin films with tunable doping gradients and doping concentrations. Excellent optoelectronic properties are achieved at process temperatures not exceeding 85 °C, which is a substantial improvement compared to previously reported non-vacuum methods. The aqueous solution deposition combined with the low temperature UV annealing makes the method useable for any substrate (also non-conductive ones) and is expected to be a huge step towards lowering the equipment cost and complexity for fabrication of AZO based TCOs. The understanding of how AZO growth is governed by the Al dopant and how it can be controlled will be helpful to extend this concept to other doped ZnO systems (e.g., Ga, B, or Cl doping) deposited with aqueous solution growth. Furthermore, the investigated solution chemistry may potentially be transferred to other deposition techniques such as spray deposition for roll-to-roll production. This opens new opportunities for non-vacuum deposition of doped ZnO based thin films on temperature sensitive, flexible and non-conductive substrates such as polymers for photonics, photovoltaics, and various consumer opto-electronic applications.
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Experimental Section Seed Layer: The seed layer used for systematic investigations of the CBD deposition was a 60–80 nm intrinsic ZnO layer deposited using RF-magnetron sputtering at room temperature (Ar/O2 gas, at 150 W power). For complete non-vacuum and low temperature processing the ZnO seed layer was prepared by spin- or dip coating of a AZO nanoparticle dispersion (10 mg mL−1) in ethanol (3000 rpm, 30 s, 2x) and drying on a hotplate at 80 °C for 2 min resulting in 60–80 nm thick layers. The AZO nanoparticles were synthesized by a microwaveassisted non-aqueous sol-gel method.[11] Details about nanoparticle characterization are given in the supplementary information (section I). Aqueous Solution Deposition: The precursor solution was prepared as follows: ammonium citrate (1 mmol L−1, Sigma Aldrich, p.a. grade) and ZnO powder (10 g L−1, Sigma Aldrich, p.a. grade) were dissolved and suspended in 18 MΩ cm deionised water, respectively. NH4OH (28 wt%) was added until the pH reached a value > 11. The solution was stirred overnight and filtered (1 μm glass fiber filters) into PE bottles and stored at room temperature until usage. The dissolved Zinc concentration in the solution was 36 ± 4 mmol/L. Deposition took place at a temperature of 85 °C for 30 to 120 minutes, depending on the desired film thickness. Aluminum metal in the form of a 200 μm uncoated Al foil was immersed before or during deposition depending on the desired doping profile. After deposition the substrate was washed with deionised water and dried in a N2 stream. For deposition on flexible substrates the PET films were Ar-plasma cleaned and fixed in a holder for seeding and processing. Deposition took place under the same conditions as reported for glass substrates with a somewhat lower deposition temperature of 80 °C. UV Treatment: UV radiation exposure of washed and dried substrates was performed using a 250W UV-handheld apparatus (Hoenle UV technology, 5 cm distance to substrate, 70 mW cm−2 UVA) with an iron doped Hg lamp. The optimum annealing time was found to be less than 10 minutes. The AZO thin films were annealed directly after deposition, washing and drying with N2 without any thermal pre-annealing. The substrate surface temperature was determined with a thermocouple and did not exceed 80 °C at any time.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The research leading to these results received partly funding from the European Community Seventh Framework program (FP7/2007–2013) under agreement No 284486. The authors are greatly thankful for technical assistance of Till Coester and Jachen Daescher provided constantly throughout the conducted work. Dr. Stephan Hug and Thomas Ruettimann from the Department Water Resources and Drinking Water, EAWAG, Swiss Federal Institute of Aquatic Science and Technology are thanked for providing access to ICPMS instrumentation. Thanks to Dr. Timothy Ashworth (NanoScan AG, Switzerland) and Dr. Ulrich Mueller for the support with AFM and XPS measurements, respectively. Dr. Erik Lewin is thanked for providing access and introduction to the XRD equipment. B. Bissig, P. Fuchs, F. Pianezzi, J. Perrenoud, T. Jaeger, M. Werner, D. Keller and A. Norris are appreciated for helpful discussions and M. Rawlence and N. Murray are thanked for their support in correcting the manuscript. Received: July 11, 2013 Revised: August 26, 2013 Published online: October 22, 2013
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[1] a) D. S. Ginley, H. Hosono, D. C. Paine, Handbook of Transparent Conductors, Springer, New York, Heidelberg, Dordrecht, London 1st Ed, 2010; b) H. V. Liu, V. Avrutin, N. Izyumskaya, U. Özgür, H. Morkoç, Superlattices Microstruct 2010, 48, 458; c) T. Minami, Semicond. Sci. Technol. 2005, 20, 35; d) C. G. Granqvist, Sol. Energy Mater. Sol. Cells 2012, 99, 1; e) C. G. Granqvist, Sol. Energy Mater. Sol. Cells 2007, 91, 1529. [2] K. Ellmer, Nat. Photonics 2012, 6, 809. [3] NanoMarkets, Report# Nano-425, Emerging markets for non-ITO transparent conductive oxides Market study TCOs, 2011, published September 26. [4] a) F. Ruske, A. Pflug, V. Sittinger, W. Werner, B. Szyszka, D. J. Christie, Thin Solid Films 2008, 516, 4472; b) H. Aguraa, A. Suzukia, T. Matsushitaa, T. Aokia, M. Okudab, Thin Solid Films 2003, 445, 263; c) J. Meyer, P. Görrn, S. Hamwi, H.-H. Johannes, T. Riedl, W. Kowalsky, Appl. Phys. Lett. 2008, 93, 073308; d) H. Cheun, C. Fuentes-Hernandez, J. Shim, Y. Fang, Y. Cai, H. Li, A. K. Sigdel, J. Meyer, J. Maibach, A. Dindar, Y. Zhou, J. J. Berry, J. L. Bredas, A. Kahn, K. H. Sandhage, B. Kippelen, Adv. Funct. Mater. 2012, 22, 1531; e) A. Illiberi, P. J. P. M. Simons, B. Kniknie, J. van Deelen, M. Theelen, J. Cryst. Growth 2012, 347, 56. [5] a) R. M. Pasquarelli, D. M. Ginley, D. O’Hayre, Chem. Soc. Rev. 2011, 40, 5406; b) A. Hardy, M. K. Van Bael, Nat. Mater. 2011, 10, 340; c) T. Stubhan, I. Litzov, N. Li, M. Salinas, M. Steidl, G. Sauer, K. Forberich, G. J. Matt, M. Halik, C. J. Brabec, J. Mater. Chem. A 2013, 1, 6004; d) D. Lincot, MRS Bull. 2010, 35, 778. [6] G. Hodes, Phys. Chem. Chem. Phys. 2007, 9, 2181. [7] J. J. Richardson, F. F. Lange, Cryst. Growth Des. 2009, 9, 2570. [8] a) R. T. Tian, J. A. Voigt, J. Liu, B. McKenzie, M. J. McDermott, Nat. Mater., 2003, 2, 821; b) M. Miyake, H. Fukui, T., Hirato, Phys. Status Solidi A 2012, 209, 945; c) R. Chandramohan, T. A. Vijayana, S. Arumugamb, H. B. Ramalingamb, V. Dhanasekaranc, J. Mater. Sci. Eng. B 2011, 176, 152. [9] a) L. E. Greene, M. Law, D. H. Tan, M. Montano, J. Goldberger, Nano Lett. 2005, 5, 1231; b) M. Kokotov, G. J. Hodes, J. Mater. Chem. 2009, 19, 3847. [10] A. Zainelabdin, S. Zaman, G. Amin, O. Nur, M. Willander, Cryst. Growth Des. 2010, 10, 3250. [11] L. Luo, M. D. Rossell, D. Xie, R. Erni, M. Niederberger, ACS Sustainable Chem. Eng. 2013, 1, 152. [12] J. T. Chen, J. Wang, R. F. Zhuo, D. Yan, J. J. Feng, Appl.Surf. Sci. 2009, 255, 3959. [13] J. Joo, B. Y. Chow, M. Prakash, E. S. Boyden, J. M. Jacobson, Nat. Mater. 2011, 10, 596. [14] A. F. Mayadas, M. Shatzkes, Phys. Rev. B 1970, 1, 1382. [15] R. Martins, E. Fortunato, P. Nunes, I. Ferreira, A. Marques, Appl. Phys. 2004, 96, 1398. [16] M. Zhu, H. Huang, J. Gong, C. Sun, X. Jiang, J Appl. Phys. 2007, 102, 043106–6. [17] J. H. Seinfeld, S. N. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change John Wiley and Sons, New York, 2nd Ed, 2006. [18] J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy Perkin-Elmer Corp., Physical Electronics Division, 1st Ed, 1979. [19] F. J. Pern, B. To, S. H. Glick ,R. Sundaramoorthy, C. DeHart, S. Glynn, C. Perkins, L. Mansfield, T. Gessert, Proc. SPIE 2010, 7773, 77730R-1. [20] J. F. Guillemoles, L. Kronik, D. Chaen, U. Rau, A. Jasanek, H. W. Schock, J. Phys. Chem. B 2000, 104, 4849.
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Highly Transparent and Conductive ZnO: Al Thin Films from a Low Temperature Aqueous Solution Approach Harald Hagendorfer,* Karla Lienau, Shiro Nishiwaki, Carolin M. Fella, Lukas Kranz, Alexander R. Uhl, Dominik Jaeger, Li Luo, Christina Gretener, Stephan Buecheler, Yaroslav E. Romanyuk, and Ayodhya N. Tiwari* Transparent conductive oxides (TCO) are a unique class of materials exhibiting optical transparency combined with metallike electrical conductivity and thus are of utmost importance for the rapidly expanding fields of transparent electronics and sustainable energy generation.[1a-e] For most applications the standard compound is ITO exhibiting best optoelectronic performance amongst all TCO materials but to ensure a sustainable supply of such materials earth abundant and inexpensive alternatives such as aluminum doped zinc oxide (AZO) are crucial.[2] In fact this is also reflected in predicted markets of almost $1 Billion in 2016 for alternative TCOs.[3] Nowadays, plasma based (magnetron-sputtering),[4a] pulsed laser deposition (PLD),[4b] atomic layer deposition(ALD)[4c,d] or chemical vapor deposition (CVD)[4e] methods are employed on industrial scale to obtain high quality AZO thin films with resistivity of 10−3–10−4 Ω cm and visible transparency > 90%, although instrumental complexity poses high investment costs as well as limits scalability. In this respect low cost non-vacuum methods for AZO thin films are of immense interest. A variety of solution based approaches have been described but to obtain good optoelectronic properties comparable to vacuum deposition techniques, high temperature annealing (300–600 °C) preferably in vacuum (10−1–10−4 mbar), and for long time (60–90 min) are necessary. Some approaches employ flammable or toxic organic solvents (e.g. 2-methoxyethanol),[5a–c] and in the case of electrodeposition conductive substrates are inevitable.[5d] The most straightforward yet challenging approach Dr. H. Hagendorfer, K. Lienau, Dr. S. Nishiwaki, C. M. Fella, L. Kranz, Dr. A. R. Uhl, C. Gretener, Dr. S. Buecheler, Dr. Y. E. Romanyuk, Prof. A. N. Tiwari Empa – Swiss Federal Laboratories for Materials Science and Technology Laboratory for Thin Films and Photovoltaics Ueberlandstrasse 129, CH-8600 Duebendorf Tel.: +41 58 765 6119 E-mail: [email protected]; [email protected] Dr. D. Jaeger Empa – Swiss Federal Laboratories for Materials Science and Technology Laboratory for Nanoscale Materials Science Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland presently at OC Oerlikon Balzers AG, Systems, Iramali 18, FL-9496, Balzers, Liechtenstein. L. Luo ETH Zurich, Laboratory of Multifunctional Materials Department of Materials Wolfgang-Pauli-Strasse 10, 8093, Zurich, Switzerland
DOI: 10.1002/adma.201303186
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for deposition of ZnO thin films is aqueous solution growth[6] (e.g., chemical bath deposition or hydrothermal synthesis) on seeded substrates as illustrated in Figure 1a. The solution chemistry comprises a water soluble zinc salt and a complexant (usually ammonium salts, ethanolamine, or NH4OH) which is also used for adjusting the pH to a basic regime. The crystal growth is associated with the decreasing thermodynamic stability of the zinc complex leading to controlled supersaturation and the retrograde solubility of ZnO upon increased temperature.[7] Thus, phase pure ZnO thin films can be obtained already at temperatures of 60 °C. The use of water as solvent makes the method environmental friendly and due to the use of inexpensive chemicals – as mostly water soluble metal salts are employed – it can also be designated as low cost. A number of approaches have been reported to obtain intrinsic, undoped ZnO in the form of nanorod, nanoneedle, or nanopillar thin films[8a] using aqueous solution deposition, but only a few studies attempt to obtain conductive AZO thin films.[8b,c] However, achieving a compact, dense, and highly conductive ( 11). Addition of 1 mmol/L Al salt to the solution results in a reduction of the growth rate by 50% (from 50 nm min−1 for undoped ZnO films to 25 nm min−1). Films with resistivity not lower than 3–5 × 10−2 Ω cm and carrier densities of 7–9 × 1019 cm3 are obtained. Addition of higher concentrations of aluminum salt, as necessary for increasing the carrier density and lowering the resistivity, results in a strongly constrained growth of ZnO (>1 mmol L−1) or complete change of morphology (>5 mmol L−1) to flower or sheet-like ZnO structures (Figure 1b). Thus, we conclude that a continuous addition of Al in quantities low enough to prevent disturbed growth (300 °C), or in reactive atmospheres (N2/H2) leads to increased concentration of oxygen vacancies and lower density of electron traps at the grain boundaries. Indeed, vacuum annealing at temperatures of 300–500 °C (10−1 mbar, 60 minutes and rapid cooling to room temperature) results in a massive decrease of sheet resistance from initially 200 kΩ/sq to 100 – 300 Ω/sq for the solution deposited AZO thin films. Since such high temperatures are not in accordance with the ambitious goal to develop a low temperature process, another strategy inspired by atmospheric chemistry[17] was applied for removing absorbed oxygen species (Figure 2a) to increase carrier mobility. XPS spectra before and after UV treatment (Figure 2b) reveals only a slight difference in the Zn2p3/2 signal. In contrast the O1s signal is affected dramatically from exposure to UV radiation where a shift from binding energies assigned to hydroxides to lower energies
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O 1s before UV
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Figure 2. Influence of the UV treatment on opto-electronic properties of AZO. a) Mechanism of UV-annealing on absorbed oxygen species at AZO grain boundaries. b) XPS spectra of the AZO film before and after UV annealing and c) influence on UV/Vis transmission and band gap energy. d) Time dependence of sheet resistance employing UV treatment (80 °C, 70 mW/cm2) and thermal annealing at the same temperature (80 °C) and 500 °C, respectively.
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a
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20 0
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Figure 3. Applications of low temperature aqueous solution deposited AZO thin films. a) High quality AZO thin film on glass to demonstrate the optical transmission (40 ± 5 Ω/sq). b) Corresponding SEM cross section image. c) SEM cross section image, d) cell parameters, e) quantum efficiency-, and f) J(V) curve for a CIGS solar cell with a low temperature aqueous solution prepared AZO window layer. g) Photograph of an AZO film deposited on a PET film with h) corresponding SEM cross section images and i. UV/Vis transmission spectra.
is obtained without exceeding the temperature of 85 °C at any process step. Following the successful development of the aqueous solution deposition, the aim is to demonstrate the applicability of our approach to temperature sensitive substrates. The utilization of the aqueous solution grown AZO as front electrical contact in a Cu(In,Ga)Se2 (CIGS) thin film solar cell is an excellent application for this low temperature process since the underlying p-CIGS/n-CdS heterojunction can rapidly degrade when heated above 150 °C.[20] In Figure 3c–f a cross section SEM image, J(V)- and quantum efficiency curves are presented for a CIGS solar cell (see supporting information section II and III) incorporating a low temperature aqueous solution deposited AZO window layer and featuring a solar cell efficiency of 14.7%. The current density of 33.4 ± 0.5 mA cm−2 and series resistance of 1.1 ± 0.1 Ω cm2 are comparable to the optimized reference cell with a high quality sputtered AZO thin film (34.1 ± 0.6 mA cm−2, 1 ± 0.1 Ω cm2), thus demonstrating the excellent optoelectronic properties achieved for the aqueous solution deposited window layer. To further corroborate the feasibility of low temperature processing, we deposited AZO layers on PET films, for example to be used as a high quality window layer for flexible non-vacuum deposited organic photovoltaics. In Figure 3g–i a photograph, a SEM cross section image of the AZO thin film on PET, and a corresponding transmission spectrum are presented. The film had a sheet resistance as low as 70 Ω/sq and transmission above 85% in the visible range.
Adv. Mater. 2014, 26, 632–636
In conclusion, a new path is demonstrated to produce highly conductive and transparent thin films using a low cost, low temperature, aqueous solution based procedure. Different morphologies from AZO nanorods towards densely packed films are accomplished by altering the solution chemistry. Using metallic Al as a continuous and controllable doping source allows the fabrication of AZO thin films with tunable doping gradients and doping concentrations. Excellent optoelectronic properties are achieved at process temperatures not exceeding 85 °C, which is a substantial improvement compared to previously reported non-vacuum methods. The aqueous solution deposition combined with the low temperature UV annealing makes the method useable for any substrate (also non-conductive ones) and is expected to be a huge step towards lowering the equipment cost and complexity for fabrication of AZO based TCOs. The understanding of how AZO growth is governed by the Al dopant and how it can be controlled will be helpful to extend this concept to other doped ZnO systems (e.g., Ga, B, or Cl doping) deposited with aqueous solution growth. Furthermore, the investigated solution chemistry may potentially be transferred to other deposition techniques such as spray deposition for roll-to-roll production. This opens new opportunities for non-vacuum deposition of doped ZnO based thin films on temperature sensitive, flexible and non-conductive substrates such as polymers for photonics, photovoltaics, and various consumer opto-electronic applications.
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Experimental Section Seed Layer: The seed layer used for systematic investigations of the CBD deposition was a 60–80 nm intrinsic ZnO layer deposited using RF-magnetron sputtering at room temperature (Ar/O2 gas, at 150 W power). For complete non-vacuum and low temperature processing the ZnO seed layer was prepared by spin- or dip coating of a AZO nanoparticle dispersion (10 mg mL−1) in ethanol (3000 rpm, 30 s, 2x) and drying on a hotplate at 80 °C for 2 min resulting in 60–80 nm thick layers. The AZO nanoparticles were synthesized by a microwaveassisted non-aqueous sol-gel method.[11] Details about nanoparticle characterization are given in the supplementary information (section I). Aqueous Solution Deposition: The precursor solution was prepared as follows: ammonium citrate (1 mmol L−1, Sigma Aldrich, p.a. grade) and ZnO powder (10 g L−1, Sigma Aldrich, p.a. grade) were dissolved and suspended in 18 MΩ cm deionised water, respectively. NH4OH (28 wt%) was added until the pH reached a value > 11. The solution was stirred overnight and filtered (1 μm glass fiber filters) into PE bottles and stored at room temperature until usage. The dissolved Zinc concentration in the solution was 36 ± 4 mmol/L. Deposition took place at a temperature of 85 °C for 30 to 120 minutes, depending on the desired film thickness. Aluminum metal in the form of a 200 μm uncoated Al foil was immersed before or during deposition depending on the desired doping profile. After deposition the substrate was washed with deionised water and dried in a N2 stream. For deposition on flexible substrates the PET films were Ar-plasma cleaned and fixed in a holder for seeding and processing. Deposition took place under the same conditions as reported for glass substrates with a somewhat lower deposition temperature of 80 °C. UV Treatment: UV radiation exposure of washed and dried substrates was performed using a 250W UV-handheld apparatus (Hoenle UV technology, 5 cm distance to substrate, 70 mW cm−2 UVA) with an iron doped Hg lamp. The optimum annealing time was found to be less than 10 minutes. The AZO thin films were annealed directly after deposition, washing and drying with N2 without any thermal pre-annealing. The substrate surface temperature was determined with a thermocouple and did not exceed 80 °C at any time.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The research leading to these results received partly funding from the European Community Seventh Framework program (FP7/2007–2013) under agreement No 284486. The authors are greatly thankful for technical assistance of Till Coester and Jachen Daescher provided constantly throughout the conducted work. Dr. Stephan Hug and Thomas Ruettimann from the Department Water Resources and Drinking Water, EAWAG, Swiss Federal Institute of Aquatic Science and Technology are thanked for providing access to ICPMS instrumentation. Thanks to Dr. Timothy Ashworth (NanoScan AG, Switzerland) and Dr. Ulrich Mueller for the support with AFM and XPS measurements, respectively. Dr. Erik Lewin is thanked for providing access and introduction to the XRD equipment. B. Bissig, P. Fuchs, F. Pianezzi, J. Perrenoud, T. Jaeger, M. Werner, D. Keller and A. Norris are appreciated for helpful discussions and M. Rawlence and N. Murray are thanked for their support in correcting the manuscript. Received: July 11, 2013 Revised: August 26, 2013 Published online: October 22, 2013
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