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This paper distinguished hydrogen roles to improve electron mobility and carrier concentration in ZnO and Al doped ZnO sputtered films. By combining experimental evidences and theoretical results, we find out that hydrogen located at oxygen vacancy sites (HO) is the main factor gives rise to increase simultaneously mobility and carrier concentration which has not been mentioned before. Introducing appropriate hydrogen content during sputtering not only results in crystalline relaxation but also supports doping Al into ZnO, increasing carrier concentration and electron mobility in the film. First principles calculations confirmed hydrogen substitutional stability for oxygen vacancy, significantly reducing electron conductivity effective mass and hence increasing electron mobility. In particular, 0.8% hydrogen partial pressure ratio achieved 61 cm2V−1s−1 maximum electron mobility, optical transmittance above 82% in visible and near-infrared regions, and 2 × 1020 cm−3 carrier concentrations for H Al co-doped ZnO film. These values approach ideal electrical and optical properties for transparent conducting oxide films. The presence of one maximum electron mobility was attributed to competition between increasing mobility due to restoring effective electron mass and hydrogen passivation of native defects, and decreased electron mobility due to electron-phonon scattering.

Conductivity σ and thermal conductivity κ are directly related to carrier concentration while Seebeck coefficient S is inversely proportional to carrier concentration. Therefore, improving thermoelectric (TE) performance is challenging. Here, the first-time analysis of secondary phase-controlled TE performance in terms of density-of-state effective mass m d ∗ , weighted mobility μw and quality factor B is discussed in ZnO system. The results show that the secondary spinel phase Ga2O3(ZnO)9 not only impacts on κ but also on σ and S at high temperature, while the effect of carrier concentration seem to be dominant at low temperature. For the high-spinel-segregation sample, a compensation of dopant atoms from the spinel to substitutional sites in the ZnO matrix at high temperature leads to a low decreased rate of temperature-dependent m d ∗ . The compensation process also induces a band sharpening, a small μw reduction, and a large B enhancement. As a result, In and Ga co-doped ZnO bulk with the highest spinel segregation achieves the greatest PF improvement by 112.8%, owing to enhanced Seebeck coefficient by 110% as compared to the good Zn-substitution sample.

Structure modification has been found to tune significantly the transparent-conducting performance, especially mobility and conductivity of hydrogenated Ga-doped ZnO (HGZO) films. The strong correlation between film thickness and mobility of the films is revealed. The mobility increases quickly with increasing the thickness from 350 to 900 nm, and then tends to be saturated at further thicknesses. A higher mobility than 50 cm2/Vs can be achieved, which is an extra-high value for polycrystalline ZnO films deposited by using the sputtering technique. The thickness-dependent mobility originates from scatterings on grain boundaries and dislocation-induced defects controlled by thin-film growth. Based on the Volmer-Weber model, an expansion model is built up to describe the thickness-dependent crystal growth of the HGZO films, especially at the thick films. As a result, the 800 nm-thick HGZO film obtains the highest performance with high mobility of 51.5 cm2/Vs, low resistivity of 5.3 × 10−4 Ωcm, and good transmittance of 83.3 %.

Introduction: Zinc oxide (ZnO) is well-known as a promising thermoelectric material owing to its safety, inexpensiveness, and thermal stability. This research provides an overview of thermoelectric potentials, including structure, electrical conductivity, Seebeck coefficient, and power factor of pure ZnO semiconductor synthesized in bulk and thin-film forms. Methods: The ZnO bulk was synthesized by solid-state reaction at high temperature, while the thin film was prepared by d.c. magnetron sputtering technique. The temperature-dependent thermoelectric properties of all the samples were measured by the Seebeck LSR-3 system. The crystallographic and surface morphological information of the samples were obtained by using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), respectively. Results: The XRD results confirm that both the bulk and thin-film have polycrystalline structure and characteristics of hexagonal-wurtzite ZnO. Through the FESEM observation, the bulk is well densified under high-temperature condition, while the thin-film achieve good orientation and close-packed grains. At 573 K, the obtained thermoelectric properties (electrical conductivity, Seebeck coefficient, and power factor) are respectively 352.4 S/cm, -89.5 µV/K and 282.5 µW/mK2 for the ZnO bulk; and 289 S/cm, -113.8 µV/K and 374.3 µW/mK2 for the ZnO film. Conclusion: The comparative study shows the good thermoelectric potential of ZnO material in both forms of bulk and thin film. Among them, the thin film has better results, especially in the Seebeck coefficient and power factor than one.

In this work, several X-ray and nuclear analysis techniques were used to examine ZnO materials co-doped with In and Ga, or IGZO materials. X-ray diffraction analysis, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy revealed multi-scale defects in the materials. A nanoscale secondary Ga2Zn9O12 spinel phase, mesoscale grain boundaries, and atomic-scale lattice defects were detected. The lattice defects included oxygen vacancies, zinc vacancies, and complex defects. Positron annihilation spectroscopy and Doppler broadening spectroscopy provided evidence of interactions between charge carriers and defects sites, which explained the low thermal conductivities of the IGZO materials (κtotal ≈ 3.9 W/mK) at 773 K. This combination of X-ray and nuclear analytical techniques can be viewed as a novel approach for investigating the thermoelectric properties of materials with complex crystal structures that contain atomic-scale voids, nanoscale secondary phases, and mesoscale grain boundaries.

Introduction: Magnesium tin silicide (MgSiSn) is known as a good-thermoelectric-performance, safe and cost-efficient alloy material. The goal of this work is to design a magnetron co-sputtering configuration for depositing alloy thin films from three independent metal targets including magnesium (Mg), silicon (Si) and tin (Sn). Methods: By this solution, the elemental composition of the MgSiSn thin films can be effectively controlled through changing sputtering power of the individual magnetron. The actual values of elemental composition in the as-deposited films were verified by using energy-dispersive X-ray spectroscopy. The as-deposited thin films were investigated carefully by using the X-ray diffraction to recognize crystalline structure characteristics. Most importantly, typically thermoelectric parameters including Seebeck coefficient, electrical conductivity and power factor were indicated as functions of temperature. Results: The XRD analysis exhibits cubic anti-fluorite-type structure characteristic of the MgSiSn films; however, the presence of segregated Mg phase is still observed. The testing results for the representative MgSiSn thin film with good adherence show the power factor of PF ~15×10-3 W/mK2, as a result of Seebeck coefficient of S ~132 µV/K and electrical conductivity of σ ~9000 S/cm, at 373 K. At higher temperature than 473 K, the semiconducting behavior of the films tends to transform from p-type to n-type. Conclusion: The three-target co-sputtering configuration shows the possibility of successfully preparing alloy MgSiSn thin films with good adherence on Si substrate. Furthermore, the testing result suggests that the as-deposited MgSiSn thin films have some potential thermoelectric characteristics, which can be improved more significantly.

Introduction: ZnO is a potential semiconductor material for thermoelectric (TE) applications at high temperature. In this study, doping Bi atoms aims to improve the power factor of ZnO ceramic. Based on the analyses of structural and TE properties, the enhancement in Seebeck coefficient proves the benefit of Bi doping. Methods: The ZnO and Bi-doped ZnO (BZO) ceramics were fabricated by solid-state reaction at 1400oC in air. The Seebeck LSR-3 system measured both samples’ TE properties, with a combined Hall effect at room temperature. The crystalline structure was analyzed through the X-ray diffraction (XRD) technique. Results: The XRD patterns show all samples having polycrystalline hexagonal wurtzite structure. Although the crystallinity is enhanced, Bi doping reduces significantly carrier concentration from 3.4×1019 to 1.1×1019 cm-3 at room temperature. As a result at 500oC, the electrical conductivity, Seebeck coefficient, and power factor are 126 Scm-1, -99 µVK-1, and 124 µWm-1K-2 for pure ZnO; and 76 Scm-1, -142 µVK-1, and 165 µWm-1K-2 for BZO ceramic, respectively. Conclusion: The power factor of Bi-doped ZnO is improved by 32% as compared to pure ZnO at 500oC. The raise of Seebeck coefficient is the main reason for the growth of power factor.

this work, impacts of the thickness on electron mobility of Ga and H2 co-doped ZnO (HGZO) thin films were investigated. The HGZO films were prepared on glass substrate by using magnetron sputtering from ceramic Ga-doped ZnO (GZO) target in the gas mixture of argon and hydrogen. Based on the Hall measurement, the mobility enhanced fastly from 44.6 to 53.4cm2/Vs with the increasing thickness from 350 to 900 nm, then tends to be saturated at ~55cm2/Vs with further thickness. Most of the films achieve the mobility of >50cm2/Vs, which is very high value for sputtered TCOs thin films. The thicknessdependent mobility is explained in term of grain boundary scattering. The improvement of crystalline quality reduced grain boundary scattering, which lead to the fast increase in mobility of the films with 350–900nm in thickness. When the thickness increased more than 900nm, however, the appearance of many defects increased scattering centers and saturates the mobility. Furthermore, the results showed the HGZO films with optimum thickness of 800nm obtained low resistivity (5.3 10-4􀁛cm), high average transmittance (83.3%) in the wide wavelength range of 400–1100nm, and the highest figure of merit (10.3 103􀁛-1cm-1) corresponding to high mobility (51.1cm2/Vs).

The transparent-conducting performance is estimated through figure-of-merit (FOM) value. To improve poor FOM value of pure ZnO thin films, boron (

In this paper, thermoelectric performance of the ZnO thin films were investigated via doping (Gallium - Ga and Indium - In) and post-thermal treatment. Various kinds of defects, such as zinc (Zn) vacancies, Ga substitute for Zn, zinc interstitials, oxygen vacancies, oxygen interstitials, oxygen antisite, were induced in the ZnO thin films by doping with Ga, In and post-thermal treatment. Annealed thin films have better thermoelectric performance than unannealed thin films. The figure of merit ZT values at 300 °C were 0.114 and 0.186 for the single Ga-doped ZnO (GZO@500) and dually In, Ga doped ZnO (IGZO@500) thin films annealed at 500 °C, respectively. Higher concentration of zinc interstitial led the GZO@500 thin film to have larger power factor and larger total thermal conductivity, resulting in lower ZT than the IGZO@500 thin film. To the best of our knowledge, the ZT value of 0.186 at 300 °C obtained from our IGZO@500 thin films is the highest ever reported for n-type ZnO based thin films.

Mg-doped CuCrO2 material has emerged as a p-type semiconductor, especially, in the field of thermoelectric materials because of its lowest resistivity in delafossite material family. In this work, CuCr0.95Mg0.05O2 bulk materials were prepared in a series of sintering temperatures including 1000, 1200, and 1400°C using solid-state reaction method. From XRD results, all samples exhibit the significant existence of delafossite phase CuCrO2, and a small portion belongs to the spinel phase MgCr2O4. An interesting finding in this study is that the Vickers hardness is mainly governed by the crystallite size of (110) plane. Besides, the change in the crystallite size of (110) plane also indirectly affects the carrier concentration of the compounds with increasing sintering temperature. The highest power factor (PF) was reached for the sample prepared at the sintering temperature 1400°C with the value of 193 μW/mK2 along with the Seebeck coefficient value of 600 μV/K measured at 400°C.

In this work, role of phase composition and interstitial oxygen on the thermoelectric properties of CuCr0.85Mg0.15O2 materials prepared at different sintering temperatures (1273–1673 K) for short time (3 h) are reported. Interestingly, the samples sintered at 1273 K and 1673 K had the same Seebeck coefficient, despite the 1273 K sample having significantly lower conductivity, which are discussed via phase composition and interstitial oxygen induced during the CuCrO2 phase formation and its related defect (VCu, VCr, …). Besides, high conductivity of the 1673 K sample originated from the low percentage of CuO phase and the appearance of Cu2O phase. Since the 1273 K and 1673 K sintered samples have almost the same percentage of delafossite and spinel phases so the 1673 K sample still remain its high Seebeck coefficient. We suggest some reaction chemical equations for elucidating the roles of interstitial oxygen on the phase composition and thermoelectric properties CuCr0.85Mg0.15O2 delafossite material.

We investigated the effect of single and multidopants on the thermoelectrical properties of host ZnO films. Incorporation of the single dopant Ga in the ZnO films improved the conductivity and mobility but lowered the Seebeck coefficient. Dual Ga- and In-doped ZnO thin films show slightly decreased electrical conductivity but improved Seebeck coefficient. The variation of thermoelectric properties is discussed in terms of film crystallinity, which is subject to the dopants' radius. Small amounts of In dopants with a large radius may introduce localized regions in the host film, affecting the thermoelectric properties. Consequently, a 1.5 times increase in power factor, three times reduction in thermal conductivity, and 5-fold enhancement in the figure of merit ZT have been achieved at 110 °C. The results also indicate that the balanced control of both electron and lattice thermal conductivities through dopant selection are necessary to attain low total thermal conductivity.

Introduction: Harvesting the waste heat emitted from the activities of humanity based on thermoelectric devices is an appropriate way to reduce the overconsumption of fossil fuel nowadays. Methods: In this work, CuCr0:85Mg0:15O2 compounds prepared by conventional solid-state reaction method were investigated to find out that the short sintering time is enough for thermoelectric applications, directly low the cost of the devices. Results and Conclusion: We find out that there is a significant change in the crystal structure, the chemical state, and thermoelectric properties along with the increase of the sintering time, but eventually, the dimensionless figure of merit ZT is almost constant regardless of the long or short sintering time which means that the increase of electrical conductivity will compromise the increase of thermal conductivity. The highest ZT value is 0.03 measured at 500 oC for both samples prepared at the sintering time of 3 and 12 hours.

In this work, we prepare H and Ga co-doped ZnO (HGZO) films by a sputtering method with over 80% transparency in 400–1100 nm wavelength range and a sheet resistance as low as 3.9 Ω/sq. Hydrogen is recognized as a key element to enhance transmittance and conductivity. Spectroscopic evidence has indicated that hydrogen assists in substituting Ga3+ for Zn2+ (GaZn) in ZnO lattice; it produces more effective GaZn donors and increases electron concentration. We propose for the first time based on interstitial and substitutional hydrogen configurations that the interactions of Zn atom with its neighboring hydrogen atoms can weaken Zn–O bonds in ZnO lattice, and thus facilitate the substitution of Ga3+ for Zn2+. Furthermore, hydrogen can improve the quality of crystalline grains by lowering point-defect density such as Ga, Zn interstitials, Zn, and O vacancies, which strongly enhance electron mobility in HGZO films. The highest mobility of 48.6 cm2/Vs was obtained in the best-performing film.

The thermoelectric performance of CuCr1-xMgxO2 materials in terms of multi-scale defects induced at various Mg dopant concentrations (x = 0–0.3) was thoroughly studied in this paper. At 748 K and for x = 0.05, 0.15, and 0.30, we report the following power factors and thermal conductivities: 175, 213, and 2.3 μW/m K2 and 7.85, 5.60, and 3.82 W/m K, respectively. In the low doping regime (x

The transparent-conducting performance is estimated through figure-of-merit (FOM) value. To improve poor FOM value of pure ZnO thin films, boron ( B ) as a donor impurity was doped into the films. Direct-current magnetron sputtering was used to prepare B -doped ZnO (BZO) thin films from sintered ZnO targets with variable B 2 O 3 content changing from 0 to 2 wt. %. The x ray diffraction analysis confirmed the preferably c -axis-oriented structure of hexagonal wurtzite ZnO host. The results also showed variation in the film structure versus the B 2 O 3 content through calculations of crystal size and residual stress. Depending on the B 2 O 3 content, a competition of interstitial and substitutional B 3 + ions induced more stress or relaxation in lattice structure of the films. At 1% B 2 O 3 , the BZO thin film had the best crystalline characterization with the lowest stress and large crystal size. In consequence, the BZO 1% film obtained the lowest resistivity of 2.7 × 1 0 − 3 Ω c m , average transmittance of 82.1%, and the best FOM value of 18.8 × 1 0 2 Ω − 1 c m − 1 . The transparent-conducting performance of the ZnO thin films deposited by direct-current (DC) magnetron sputtering was significantly enhanced through B doping. The good-performance BZO film at 1% B 2 O 3 is believed to be of use as electrodes in thin-film solar cells.