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Facile Preparation of Highly Conductive Metal Oxides by Selfcombustion for Solution-processed Thermoelectric Generators Young Hun Kang, Kwang-Suk Jang, Changjin Lee, and Song Yun Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10187 • Publication Date (Web): 09 Feb 2016 Downloaded from http://pubs.acs.org on February 9, 2016

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ACS Applied Materials & Interfaces

Facile Preparation of Highly Conductive Metal Oxides by Self-combustion for Solution-processed Thermoelectric Generators

Young Hun Kang, Kwang-Suk Jang, Changjin Lee, and Song Yun Cho*

Division of Advanced Materials, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea, E-mail: [email protected]

KEYWORDS: n-type thermoelectrics, self-combustion, indium zinc oxide, solution process, thermoelectric generator

ABSTRACT Highly conductive indium zinc oxide (IZO) thin films were successfully fabricated via a selfcombustion reaction for application in solution-processed thermoelectric devices. Self-combustion efficiently facilitates the conversion of soluble precursors into metal oxides by lowering the required annealing temperature of oxide films, which leads to considerable enhancement of the electrical conductivity of IZO thin films. Such enhanced electrical conductivity induced by exothermic heat from a combustion reaction consequently yields high performance IZO thermoelectric films. In addition, the effect of the composition ratio of In to Zn precursors on the electrical and thermoelectric properties of the IZO thin films was investigated. IZO thin films with a composition ratio of In:Zn = 6:2 at the low annealing temperature of 350 °C showed an enhanced electrical conductivity, Seebeck coefficient, and power factor of 327 S cm-1, 50.6 µV K-1, and 83.8 µW m-1 K-2, respectively. Moreover, the IZO thin film prepared at an even lower temperature of 300 °C retained a large power factor of 78.7 µW m-1 K-2 with an electrical conductivity of 168 S cm-1. Using the combustive IZO precursor, a thermoelectric generator consisting of 15 legs was fabricated by a printing process. The thermoelectric array generated a thermoelectric voltage of 4.95 mV at a low temperature difference (5 °C). We suggest that the highly conductive IZO thin films by self-combustion may be utilized for fabricating 1

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n-type flexible printed thermoelectric devices.

INTRODUCTION Recently, materials utilizing renewable and sustainable energy sources, such as solar cells, piezoelectrics, and thermoelectrics (TEs), have attracted extensive interest in a wide variety of applications. TE materials can directly convert thermal energy into electricity, with the potential to continuously convert body heat into electrical power.1,2 TEs utilize differences in entropy as driving forces to diffuse charge carriers between the high- and low-temperature surfaces of a device.3 The performance of TEs can be characterized by the dimensionless figure of merit ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ (κl + κe) is the thermal conductivity from the lattice (κl) and electronic contributions (κe), and T is the absolute temperature.4 For practical use in TE applications, TE materials should exhibit large Seebeck coefficients, high electrical conductivities, and low thermal conductivities. However, it is difficult to manipulate all parameters in a material to achieve the optimal ZT, since a carrier concentration-dependent trade-off point between σ and S exists and κe is proportional to σ according to the Wiedemann-Franz law.5 Conventional inorganic TE materials with high ZT values include metallic elements such as Te, Pb, Bi, Se, and Sb, which are toxic, scarce, and expensive.6-8 They also impose design limitations on flexible TE devices by the brittle nature and required high processing temperatures of the metals. However, metal oxide semiconductors are regarded as promising candidates for inclusion in flexible TE devices, as they possess good chemical stability, easy control of electron and phonon transport properties, and accessible synthetic methods. Furthermore, the TE performance of metal oxide semiconductors can be effectively improved by forming the materials into thin films because of the small heat capacity and fast heat transfer intrinsic to this geometry. These thin films can be easily adapted for use in planar-type TE devices. Recently, n-type oxide TE materials with power factors of 100–400 µW m-1 K-2 at the temperature of 300 K, based on materials such as indium zinc oxide (IZO), AZO, ZTO, GZO, and In2O3, have been explored as p-type partners in TE devices.9-14 Although these 2


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oxide materials show high TE performances, fabricating such materials remains a challenge due to the requirement of high-temperature annealing and difficult vacuum-based techniques. These techniques, such as spark plasma sintering, pulsed laser deposition, chemical vapor deposition, and radio frequency magnetron sputtering, often necessitate costly equipment and large vacuum facilities.15-17 On the other hand, oxide materials consisting of diverse compositions are facilely produced from oxide precursors through solution processes, which can be easily adapted to large-area and low-cost depositions. In solution-based methods for high TE performance with oxide materials, the selection of organic ligands is a crucial factor in obtaining highly conductive oxide films. Because the incomplete conversion of soluble precursors into metal oxides can negatively impact electrical characteristics such as electrical conductivity, carrier mobility, and carrier concentration by remaining organic impurities.1,18 This makes it quite difficult to obtain highly conductive behavior from solutionprocessed oxide films. Although high temperature annealing has been utilized to alleviate such problems and is considered an integral part of the process, electrical conductivity even above 450 °C annealing is not sufficient for TE applications. In the present work, we investigated the electrical and TE properties of IZO thin films prepared via a self-combustion reaction for solution-processed TE devices. Our work is mainly focused on the development of conductive metal oxide with higher electrical conductivities and less loss of the Seebeck coefficient for the high TE performance. To achieve this goal, highly conductive metal oxide thin films were fabricated at low annealing temperature by assistance of the exothermic heat from the spontaneous combustion of specific IZO precursors. We also attempted to tune the TE parameters of the product, such as electrical conductivity and Seebeck coefficient, through the stoichiometric control of In and Zn levels in the films. By utilizing a self-combustion reaction induced by two metal precursors containing fuel and oxidizer ligands, metal-oxygen-metal (M-O-M) lattices can be easily formed with facile removal of the remaining organic ligands, resulting in a highly conductive metal oxide. We believe that the facile formation of M-O-M linkages from metal oxide precursors at low annealing temperatures can enhance the electrical conductivity and consequent TE properties of oxide thin films. 3



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EXPERIMENTAL SECTION Preparation of Conductive IZO Thin Films. Zinc acetylacetonate hydrate (Zn(C5H7O2)2·xH2O) and indium nitrate hydrate (In(NO3)3·xH2O) purchased from Sigma-Aldrich were used for the preparation of IZO precursors. To synthesize a homogeneous IZO precursor, both meatal precursors were dissolved in anhydrous 2-methoxyethanol (Aldrich), and the mixture solution was agitated for 12 h at room temperature. All precursors were prepared at a fixed molar concentration of 0.3 M. The optimal composition suitable for the desired thermoelectric performance was investigated by changing the molar ratio of In precursor to Zn precursor over a range of 2:6 to 6:2. Spin-coating was conducted for the deposition of IZO precursors on the glass substrate. Prior to deposition, the glass substrates were sequentially cleaned using ultrasonic baths of detergent solution, acetone and hot isopropyl alcohol. The IZO precursor solution was spin-coated at 3000 rpm for 30 s, and the spin-coated IZO thin films were pre-baked on a hot plate at 100 °C for 10 min. The annealing process was carried out at either 300 or 350 °C for 1 h in air using a hot plate. After annealing, the fabricated IZO film was treated with Ar plasma at a power of 80 W and flow rate of 100 sccm for 10 min to clean the surface of the thin film. Characterization of Conductive IZO Thin Films. To investigate the thermal behavior of the IZO thin films, thermogravimetric analysis (TGA) and differential thermal analysis (DTA, SDT 2060, TA Instruments, USA) were performed at a heating rate of 10 °C·min–1 under ambient conditions. For the sample preparation of TGA and DTA, IZO precursor solutions were dried at 50 °C in a vacuum oven for 24 h. All IZO thin films annealed at 350 °C were analyzed using X-ray diffraction (XRD, DMAX-RC, Rigaku, Japan). A thin film diffractometer was used for the XRD measurements, in which the angle between the samples and the X-ray beam was fixed to 3°, with a source power of 60 mA at 40 kV. Surface morphology and roughness in an area of 1 µm2 of the IZO thin films were characterized using atomic force microscopy (AFM, NanoScope, Veeco, USA) in noncontact mode. To determine the thickness of IZO thin film, cross-sectional imaging was utilized via ultra-highresolution field emission scanning electron microscopy (UHR FE-SEM, S-5500, Hitachi, Japan). The 4


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surface chemical states of the IZO thin films were analyzed using an X-ray photoelectron spectrometer (XPS, K-Alpha, Thermo Fisher Scientific, USA) with an Al Kα excitation source. Peak locations were defined using the C 1s electron peak at 284.5 eV as a standard. The electrical conductivity was measured via the four-point probe method with the combination of a Keithley 220 current source and a Keithley 195A digital multimeter. The Seebeck coefficient was measured by using the sample with Ag electrodes in a home-built setup. Two Ag electrodes, 2 mm in width, were separated by a distance of 15 mm. The temperature gradient between the electrodes was varied from 1 to 10 °C. The setup consisted of two Peltier devices to control the surface temperature of both electrode parts, which independently functioned as hot and cold parts. For measurement, the combination of a Keithley 2460 source meter, a Keithley 2700 multimeter, a Keithley 2182A nanovoltmeter, and a Keithley 6485 picoammeter was used. Fabrication of Thermoelectric Generators. A TE generator (TEG) was fabricated on a glass substrate using the combustive IZO precursor with a In/Zn composition ratio of 6:2. To fabricate the 15 TE legs, the combustive IZO precursor was sprayed onto the substrate using a shadow mask (dimensions of each leg: 2 mm × 15 mm, gap between two legs: 2 mm). The sprayed TE arrays were then annealed at 350 °C for 1 h on a hot plate in ambient air. After being annealed, the fabricated TE arrays were treated with an Ar plasma at a power of 80 W and flow rate of 100 sccm for 10 min, in order to clean the surface of the TE device. To connect the spray-printed TE legs, metal electrodes were dispensed through a 200 µm nozzle (SHOMASTER 200 DS-S, Musashi Engineering Inc., Japan) and connected using a conductive silver paste (NB05, Soulbrain, Korea). The printed TEG was then annealed again at 150 °C on a hot plate for 30 min, in order to improve the electrical conductivity of the Ag electrodes and to reduce the contact resistance between the Ag electrodes and the TE elements.

RESULTS AND DISCUSSIONS We previously reported solution-processable metal oxide semiconductors, created using a combustion process, for use in field effect transistors (FET).19 In this previous report, semiconductive oxides were 5



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successfully prepared at low annealing temperatures using combustive exothermic energy. Contrary to our previous work, which focused on the semiconductive properties of oxide materials, the conductive properties of combustive IZO precursors induced by different In:Zn ratios and annealing temperatures are emphasized in this study. Furthermore, such conductive properties should be controlled and optimized from the perspective of Seebeck effects in order to obtain optimal TE properties. From the perspective of our present study, self-combustion behaviors specified for the highly conductive IZO thin films were systemically analyzed, as highly conductive oxides are essential for TE applications compared to FET applications. To investigate how spontaneous exothermic reactions between the precursors containing fuel and oxidizer ligands, and the different ratios of In to Zn, influence the combustive properties, TGA and DTA were conducted as shown in Figure 1. The intense exothermic peaks in the range of 130–240 °C, which are attributed to the combustion reaction, are observed in all IZO precursors with a corresponding dramatic loss in mass in the TGA thermogram. In addition, a second set of small and broad exothermic peaks related to a small mass loss in the range of 300– 400 °C is attributed to the dehydroxylation, densification, and complete decomposition of organic ligands, such as anhydrous nitrate. Although the exothermic peaks are clearly observed in all IZO precursors, the different ratio of In to Zn has a significant influence on the decomposition temperature of the IZO precursors. This indicates that the optimal conditions for preparing highly conductive IZO thin films can be obtained by controlling the relative amount of fuel combined with Zn compared to the oxidizer combined with In. According to the previous study, the oxidizing species such as NH4NO3 and HNO3 formed by nitrate ions intensely accelerated the decomposition process by producing CO2, decreasing annealing temperatures as low as 130 °C.20 For this reason, the decomposition and condensation temperatures of the IZO precursor with the composition ratio of In:Zn = 6:2 are the lowest of the tested precursors, which may considerably affect the improvement of the electrical conductivity and the TE performance of the IZO thin film. However, in the case of the IZO precursors consisting excess amounts of one precursor compared to the other (In:Zn = 7:1 or In:Zn = 1:7), temperatures above 400 °C are required for the complete conversion of the precursors (Supporting Information Figure S1). More importantly, 6


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such difference in annealing temperature by different combustive effect has a significant influence on the electrical conductivities of the IZO thin films, as summarized in Table 1. This effect will be further discussed with the XPS analysis. From these results, we can conclude that using moderately higher amounts of oxidizer than fuel can be advantageous in realizing conductive IZO thin films by efficiently lowering the annealing temperature . The surface chemical state of the IZO thin films depending on the composition ratio of In to Zn precursors was confirmed by XPS analysis, as shown in Figure 2. O 1s peaks were deconvoluted using Gaussian functions with three components centered at 529.5 ± 0.2 eV, 531.2 ± 0.2 eV, and 532 ± 0.2 eV, which were assigned to oxygen bound to metal, oxygen vacancy, and hydroxyl groups, respectively.19, 21–23 The intensities of three components in the O 1s peaks were critically influenced by the In to Zn ratios. For a quantitative comparison of the O compositions in each film, the atomic percentages were calculated based on the ratio of area integration of each O 1s peak to that of the total O 1s peak, as shown in Figure 2 (f). With the increase of the In:Zn ratio from 2:6 to 6:2, the atomic percentage of O in M-O-M bonds increases from 33 to 44% and that of O in M-OH bonds decreases from 21 to 11%. This reveals that M-OH bonds are more efficiently converted into M-O-M bonds at a lower decomposition temperature by heat from the combustion reaction. M-O-M bonds play an important role as the conducting pathway for electrons because the s-state of In and Zn minimizes the conduction band in the IZO thin film. Therefore, creation of M-O-M bonds with the removal of hydroxyl groups is critical for the realization of improved conductive properties in metal oxide films.24, 25 The formation of the M-O-M frameworks in our study was determined by the decomposition temperature, which is affected by the relative ratio between the fuel and oxidizer as confirmed by thermal and XPS analyses. This is because the decomposition temperature has a considerable effect on the hydrolysis, condensation, and densification reactions in the sol-gel based system. This result tells us that the combustion reaction depending on the In:Zn ratios is a significant factor for the conductive nature and resultant TE properties of IZO thin films. The XRD patterns in Figure 3, obtained from a thin-film diffraction method, show that the variation in the crystallinity of the IZO thin films depends on the composition ratio of In to Zn precursors. The 7



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XRD patterns of all IZO thin films exhibit only broad hump shapes regardless of the composition of In and Zn, lacking characteristics peaks corresponding to In2O3, ZnO, and IZO, which confirms that all IZO thin films are amorphously structured. To investigate the surface roughness and thickness for the In:Zn-dependent IZO thin films, AFM and UHR FE-SEM were performed as shown in Figure 4. The composition ratio of In to Zn slightly affects the surface morphology of the IZO thin films. IZO thin films with higher amounts of In show uniformly distributed and densely packed fiber-like textures, while those with lower In amounts feature agglomerates of small nanocrystallites on the surface. The root mean square (RMS) roughness value increases from 0.82 to 1.68 nm with the increase of Zn content, except for the composition ratio of In:Zn = 3:5. Small nanocrystallites with higher amounts of Zn can be developed during the condensation of the precursor and become embedded over the area of the amorphous IZO film structure, as confirmed by the XRD analysis. The film thickness of all IZO films were measured to be approximately 15 nm, which is confirmed by the cross-sectional SEM image of IZO thin films shown in Figure 4(f). The electrical and TE performances of the IZO thin films fabricated from the different ratios of In to Zn precursors at the annealing temperature of 350 °C were investigated to establish the optimal composition that produced the maximum power factor value, as shown in Figure 5 (a). The power factors, calculated from the electrical conductivity and the Seebeck coefficient, of films with various In to Zn composition ratios are found to range from 27.8–83.8 µW m-1 K-2, and the maximum power factor is observed in the IZO thin film with a composition ratio of In:Zn = 6:2, as summarized in Table 1. The Seebeck coefficient of all IZO thin films is negative, indicating that the dominant charge carriers within all films are electrons, which is consistent with the electrical behavior obtained by the Hall measurement. With the increase of In content, the electrical conductivity is observed to increase from 31.7 to 327 S cm-1, while the Seebeck coefficient decreases from 93.7 to 50.7 µV K-1. In cases where the Zn content is higher than the In, the electrical conductivity of the IZO thin film is remarkably degenerated, because the decreased In content reduces the number of charge carriers. To verify the interrelation between the electrical conductivity and Seebeck coefficient, the Hall measurement was performed at room temperature and the results are presented in Figure 5 (b). As the 8


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In content increases from In:Zn = 2:6 to 6:2, the average electron concentration increases by approximately seven times, from -4.92 × 1019 to -34.4 × 1019 cm-3. Electron mobility also increases by approximately five times, from 2.7 to 10.8 cm2 V-1 s-1, which contributes to the great improvement of the electrical conductivity in the IZO thin film with In:Zn = 6:2. The extensive overlap between neighboring s orbitals of In3+, generated by the excess In in the IZO structure, which are larger than those of Zn2+, may improve the electrical conductivity of IZO thin films by providing more conducting pathways for electrons.26 Therefore, IZO thin films with higher In content should possess higher electrical conductivity than IZO thin films having higher Zn content. It is essential to compare the electrical conductivity of the combustive IZO precursors with that of noncombustive precursors to exclude this effect of excess In and isolate the combustive effect. When the IZO films were prepared from indium nitrate and zinc nitrate or indium acetylacetonate and zinc acetylacetonate with In:Zn = 6:2, thus excluding either the fuel or the oxidizer component, the enhancement of the electrical conductivity and resulting TE properties in the produced film was limited, as shown in Table S2. From the results, we confirm that the particularly enhanced electrical performance of films from combustive precursors is mainly attributed to the complete conversion into metal oxide by lowering the decomposition temperature due to the combustion reaction, not simply by the higher In ratio. According to the Mott equation,1

S=

+

()

(1)

()),

where is the specific heat ( =

is the carrier concentration, "() is the

energy-correlated carrier mobility, #$ is the Boltzmann constant, and () is the density of states, respectively, the carrier concentration and Seebeck coefficient are inversely proportionate, while the electron mobility and Seebeck coefficient are directly proportionate. Therefore, the increase in n by increasing the In content in the IZO thin films may be a principle factor for the reduction in the 9



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Seebeck coefficient, because the effect of the measured n is more profound than that of the electron mobility. Despite the reduced Seebeck coefficient, however, the IZO thin film with higher In content provides a larger power factor because of the dramatic improvement in the electrical conductivity of the film. In particular, when the In:Zn ratio is 6:2, the power factor reaches the maximum value of 83.8 µW m-1 K-2 with a high electrical conductivity of 327 S cm-1. Even at the lower annealing temperature of 300 °C, the IZO thin film shows a large power factor of 78.7 µW m-1 K-2 and an electrical conductivity of 168 S cm-1, as shown in Supporting Information Figure S4. A planar-type TEG consisting of 15 legs in a row was fabricated on a glass substrate. The IZO patterns, which had a width of 2 mm and length of 15 mm, were spray printed using a shadow mask and connected with silver electrodes fabricated using a dispenser, as shown in Figure 6 (a). To evaluate the total TE output voltage of the fabricated TEG, its open-circuit voltage (Voc) was measured as a function of the temperature difference (∆T) for different numbers of legs, as shown in Figure 6 (b). As the number of TE legs was increased from 1 to 15, the output voltage increased gradually from 0.2 to 4.95 mV (at ∆T = 5 °C). The Seebeck voltage, as calculated from the Voc and ∆T values, was approximately 60 ± 5 µV K-1; this corresponded to that of the TE unit device on the IZO thin film. Figure 6 (c) shows the power output of the fabricated TEG as a function of the output voltage and output current at ∆T = 10 °C. The measured output voltage was 10.96 mV. However, the output power was relatively low at 0.1 nW; this was attributable to the coarse surface of the IZO thin film, which was formed by the spray-printing process, and the high contact resistance of the electrical interconnections between the surface of the IZO thin film and the silver electrodes. The performance of the TEG needs to be improved through process optimization and the introduction of compatible ptype TE materials. However, based on these results for solution-processed n-type TE materials, it should be possible to fabricate a variety of solution-processed TE devices using polymer-based p-type materials.

CONCLUSIONS 10


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We successfully fabricated highly conductive IZO thin films with excellent TE properties by employing soluble metal oxide precursors containing metal-coordinated fuel and oxidizer ligands. In addition, the influence of the spontaneous combustion reaction on the TE properties of IZO thin films, depending on the precursor composition and annealing temperature, was investigated. TGA and DTA experiments confirmed that a strong exothermic reaction lowered the annealing temperature, which is essential for the formation of conductive metal oxides. Strong exothermic heat from the combustion reaction efficiently supported the easy formation of M-O-M frameworks with the reduction in M–OH bonds. Furthermore, the precursor composition had a great influence on the electrical conductivity and Seebeck voltage of the IZO thin films. The IZO thin film with higher In content exhibited a larger power factor with a higher electrical conductivity, even though the Seebeck coefficient was reduced. A considerably improved electrical conductivity of 327 S cm-1 and a power factor of 83.8 µW m-1 K-2 were achieved in the IZO thin film with a composition ratio of In:Zn = 6:2 at the low annealing temperature of 350 °C. The printed TE array generated a stable TE voltage of more than 4.95 mV at a low temperature difference (5 °C). These results suggest that the facile preparation of highly conductive IZO thin films with high TE performances by the self-combustion reaction is a promising method for the practical application of n-type flexible printed thermoelectric devices.

ASSOCIATED CONTENT Supporting Information Thermal behaviors of the IZO precursors with In:Zn = 7:1 and 1:7 and the non-combustive IZO precursors. XPS spectra and TE properties of the IZO thin films annealed at 300 °C. TE properties of the non-combustive IZO thin films.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] 11



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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by a grant from the KRICT Core Project (KK 1507-C06) and the R&D Convergence Program of the National Research Council of Science and Technology of the Republic of Korea.

REFERENCES (1) Xiong, X.; Tian, R.; Lin, X.; Chu, D.; Li, S. Thermoelectric Properties of Sol-Gel Derived Lanthanum Titanate Ceramics. RSC Adv. 2015, 5, 14735-14739. (2) Wu, C.-F.; Wei, T. R.; Li, J. -F. Electrical and Thermal Transport Properties of Pb1-xSnxSe Solid Solution Thermoelectric Materials. Phys. Chem. Chem. Phys. 2015, 17, 13006-13012. (3) Liu, Q.; Hu, D.; Wang, H.; Stanford, M.; Wang, H.; Hu, B. Surface Polarization Enhanced Seebeck Effects in Vertical Multi-Layer Metal-Polymer-Metal Thin Film Devices. Phys. Chem. Chem. Phys. 2014, 16, 22201-22206. (4) Hong, C. T.; Yoo, Y.; Kang, Y. H.; Ryu, J.; Cho, S. Y.; Jang, K. -S. Effect of Film Thickness and Crystallinity on the Thermoelectric Properties of Doped P3HT Films. RSC Adv. 2015, 5, 11385-11391. (5) Casian, A. Violation of the Wiedemann-Franz Law in Quasi-One-Dimensional Organic Crystals. Phys. Rev. B. 2010, 81, 155415. (6) Guo, Q.; Chan, M.; Kuropatwa, B. A.; Kleinke, H. Enhanced Thermoelectric Properties of Variants of Tl9SbTe6 and Tl9BiTe6. Chem. Mater. 2013, 25, 4097-4104. (7) Bjerg, L.; Madsen, G. K. H.; Lversen, B. B. Enhanced Thermoelectric Properties in Zinc Antimonides. Chem. Mater. 2011, 23, 3907-3914. (8) Wang, H.; Hwang, J.; Snedaker, M. L.; Kim, I.; Kang, C.; Kim, J.; Stucky, G, D.; Bowers, J.; Kim, W. High Temperature Performance of a Heterogeneous PbTe Nanocomposite. Chem. Mater. 2015, 27, 12


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


944-949. (9) Orikasa, Y.; Hayashi, N.; Muranaka, S. Effect of Oxygen Gas Pressure on Structural, Electrical, and Thermoelectric Properties of (ZnO)3In2O3 Thin Films Deposited by RF Magnetron Sputtering. J. Appl. Phys. 2008, 103, 113703. (10) Jood, R.; Mehta, R. J.; Zhang, Y.; Peleckis, G.; Wang, X.; Siegel, R. W.; Borca-Tasciuc, T.; Dou, S. H.; Ramanath, G. Al-Doped Zinc Oxide Nanocomposite with Enhanced Thermoelectric Properties. Nano Lett. 2011, 11, 4337-4342. (11) Ardynian, M.; Moeini, M.; Azimi Juybari, H. Thermoelectric and Photoconductivity Properties of Zinc Oxide-Tin Oxide Binary System Prepared by Spray Pyrolysis. Thin Solid Films 2014, 552, 39-45. (12) Liu, Y.; Liu, D. B.; Yu, M.; Lin, Y.-H.; Nan, C.-W. Enhanced Thermoelectric Properties of GaDoped In2O3 Ceramics via Synergistic Band Gap Engineering and Phonon Suppression. Phys. Chem. Chem. Phys. 2015, 17, 11229. (13) Barasheed, A. Z.; Sarath Kumar, S. R.; Alshareef, H. N. Temperature Dependent Thermoelectric Properties of Chemically Derived Gallium Zinc Oxide Thin Films. J. Mater. Chem. C. 2013, 1, 41224127. (14) Lan, J.-L.; Liu, Y.; Lin, Y-H.; Nan, C.-W.; Cai, Q.; Yang, X. Enhanced Thermoelectric Performance of In2O3-Based Ceramics via Nanostructuring and Point Defect Engineering. Sci. Rep. 2015, 5, 7783. (15) Inoue, Y.; Okamoto, M.; Kawahara, T.; Okamoto, Y.; Mormoto, J. Thermoelectric Properties of Amorphous Zinc Oxide Thin Films fabricated by Pulsed Laser Deposition. Mater. Trans. 2005, 46, 1470-1475. (16) Gautam, D.; Engenhorst, M.; Schilling, C.; Schierning, G.; Schmechel, R.; Winterer, M. Thermoelectric Properties of Pulsed Current Sintered Nanocrystalline Al-doped ZnO by Chemical Vapour Synthesis. J. Mater. Chem. A. 2015, 3, 189-197. (17) Finefrock, S. W.; Zhang, G.; Bahk, J. – H.; Fang, H.; Yang, H.; Shakouri, A.; Wu Y. Structure and Thermoelectric Properties of Spark Plasma Sintered Ultrathin PbTe Nanowires. Nano Lett. 2014, 14, 3466-3473. 13



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(18) Zhang, L.; Tosho, T.; Okinaka, N.; Akiyama, T. Thermoelectric Properties of Solution Combustion Synthesize Al-doped ZnO. Mater. Trans. 2008, 49, 2868-2874. (19) Kang, Y. H.; Jeong, S.; Ko, J. M.; Lee, J.-Y.; Choi, Y.; Lee, C.; Cho, S. Y. Two-component Solution Processing of Oxide Semiconductors for Thin-Film Transistors via Self-Combustion Reaction. J. Mater. Chem. C. 2014, 2, 4247-4256. (20) Hennek, J. W.; Kim, M. G.; Kanatzidis, M. G.; Facchetti, A.; Marks, T. J. Exploratory Combustion Synthesis: Amorphous Indium Yttrium Oxide for Thin-Film Transistor. J. Am. Chem. Soc. 2012, 134, 9593-9596. (21) Rim, Y. S.; Jeong, W. H.; Kim, D. L.; Lim, H. S.; Kim, K. M.; Kim, H. J. Simultaneous Modification of Pyrolysis and Densification for Low-Temperature Solution-Processed Flexible Oxide Thin-Film Transistors. J. Mater. Chem. 2012, 22, 12491-12497. (22) Jun, T.; Jung, Y.; Song, K.; Moon, J. Influences of pH and Ligand Type on the Performance of Inorganic Aqueous Precursor-Derived ZnO Thin Film Transistors. ACS Appl. Mater. Interfaces. 2011, 3, 774-781. (23) Kang, T. S.; Kim, T. Y.; Lee, G. M.; Sohn, H. C.; Hong, J. P. Highly Stable Solution-Processed ZnO Thin Film Transistors Prepared via a Simple Al Evaporation Process. J. Mater. Chem. C. 2014, 2, 1390-1395. (24) Jeong, S.; Ha, Y. G.; Moon, J.; Facchetti, A.; Marks, T. J. Role of Gallium Doping in Dramatically Lowering Amorphous-Oxide Processing Temperatures for Solution-Derived Indium Zinc Oxide Thin-Film Transistors. Adv. Mater. 2010, 22, 1346-1350. (25) Litton, C. W.; Reynolds, D. C.; Collins, T. C. Zinc Oxide Materials for Electronic and Optoelectronic Device Application. Wiley, 2011, pp 117-119. (26) Koo, C. Y.; Song, K.; Jun, T.; Kim, D.; Jeong, Y.; Kim, S. H.; Ha, J.; Moon, J. Low Temperature Solution-Processed InZnO Thin-Film Transistors. J. Electrochem. Soc. 2010, 157, J111-J115.

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Page 14 of 22

Page 15 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Illustration of Thermoelectric IZO Film Formation through Self–Combustion Reaction

15



(b) In:Zn = 3:5

100

80 60 40 20 10

(c) Weight loss (%)

In:Zn = 2:6

100

Weight loss (%)

60 40 20

0 200

400

600

0

800

200

400

600

20

5

0

200

400

600

800

Temperature (oC)

(e)

In:Zn = 5:3

Weight loss (%)

Weight loss (%)

40

-5

800

Temperature (oC)

(d)

In:Zn = 6:2

100

80 60 40 20

80 60 40 20 10

-1

-1

DTA (µ V mg )

10

DTA (µ V mg )

60

-1

5

Temperature (oC)

100

80

10

-1

-1

DTA (µ V mg )

10

5

In:Zn = 4:4

100

80

DTA (µ V mg )

Weight loss (%)

(a)

DTA (µ V mg )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 22

5

0 200

400

600

800

5

0 200

Temperature (oC)

400

600

800

Temperature (oC)

Figure 1. Thermal behaviors of the IZO precursors varying with ratio of In to Zn precursors: In:Zn = (a) 2:6, (b) 3:5, (c) 4:4, (d) 5:3, and (e) 6:2.

16


Page 17 of 22

(b)

(a)

(c)

In:Zn = 5:3

In:Zn = 4:4

Intensitiy (arb. unit)



In:Zn = 6:2

526 528 530 532 534 536

526 528 530 532 534 536

526 528 530 532 534 536

Binding energy (eV)

Binding energy (eV)

Binding energy (eV)

(d)

(e)

(f) 50

In:Zn = 3:5

Atomic percent (%)

In:Zn = 2:6



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


526 528 530 532 534 536

526 528 530 532 534 536

Binding energy (eV)

Binding energy (eV)

40 M-O-M VO

30

M-OH

20 10

6:2

5:3

4:4

3:5

2:6

Ratio of In to Zn

Figure 2. XPS spectra of the IZO thin films prepared at the annealing temperature of 350 °C from different composition ratios of In to Zn precursors: In:Zn = (a) 6:2, (b) 5:3, (c) 4:4, (d) 3:5, and (e) 2:6. (f) Atomic percentage of O 1s peaks for M-O-M, VO, and M-OH according to composition ratio.

17



In:Zn = 2:6

Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

In:Zn = 3:5 In:Zn = 4:4 In:Zn = 5:3 In:Zn = 6:2 20

40

60

80

2θ (deg.) Figure 3. XRD patterns of the IZO thin films according to composition ratio of In to Zn.

(a)

(b)

(c)

1.10 nm

0.82 nm (d)

(e)

1.47 nm (f) Pt IZO Si wafer

1.06 nm

1.68 nm

Figure 4. AFM images of surface morphology, with RMS values, of the IZO thin films prepared from different composition ratios of In to Zn precursors: In:Zn = (a) 6:2, (b) 5:3, (c) 4:4, (d) 3:5, and (e) 2:6. (f) Cross-sectional SEM image of the IZO thin film with In:Zn = 6:2.

18


Page 19 of 22

(b)

500

100

-60

3:5

2:6

-1

Ratio of In to Zn

-30 15 -20

10

-10

5

5:3

6:2

4:4

3:5

-1

4:4

-2

5:3

-1

-20

6:2

20

20

-1

-40

100

40

-40

2

200

60

-1

Conductivity (S cm )

300

25

Mobility (cm V s )

-80

80

Power factor (µW m K )

-100 400

Carrier concentration (×1019cm-3)

(a) Seebeck coefficient (µV K )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2:6

Ratio of In to Zn

Figure 5. Thermoelectric properties, carrier concentration, and Hall mobility of the IZO thin films according to composition ratio of In to Zn: (a) electrical conductivity (black circles), Seebeck coefficient (red circles), and power factor (blue circles) and (b) carrier concentration (black circle) and Hall mobility (blue circles).

19



(a)

(b)

(c) 0.15

3

12

∆T = 10 °C

2 1

0.10

10 8 6

0.05

4 2

Output voltage (mV)

4

Device 1 Device 2 Device 3 Device 4 Device 5 Device 10 Device 15

Output power (nW)

5

∆ V (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

0 1

2

3

4

5

5

o

∆ T ( C)

10

15

20

25

30

Output current (nA)

Figure 6. (a) Image of the planar thermoelectric generator consisting of 15 legs fabricated on a glass substrate, (b) open-circuit thermoelectric voltage (Voc) vs. temperature difference (∆T) for different numbers of TE legs, and (c) power output curve of the thermoelectric generator for a temperature difference of 10 °C.

20


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Electrical and Thermoelectric Characteristics of the IZO Thin Films Annealed at 350 °C. molar composition ratio of In to Zn

conductivity (S cm-1)

Seebeck coefficient (µV K-1)

power factor

6:2

327 ± 15.2

50.6 ± 1.4

83.8 ± 3.8

5:3

240 ± 18.0

57.4 ± 1.3

79.0 ± 5.9

4:4

190 ± 14.7

61.4 ± 1.3

71.8 ± 5.5

3:5

77.1 ± 7.2

86.9 ± 2.7

58.2 ± 5.5

2:6

31.7 ± 3.4

93.7 ± 2.6

27.8 ± 3.0

21


-2

(µW m-1 K )


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic

22


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Facile Preparation of Highly Conductive Metal ... - ACS Publications

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