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In this study, we explored the role of the intrinsic defects on the tuning thermoelectric (TE) performance of Al-doped ZnO (ZnO:Al) films. The intrinsic defects of the Zn interstitial (Zn i) and the oxygen vacancy (V O) were controlled by varying the Zn content by using the radio frequency atomic source vacuum evaporation method. In the Zn-rich case, the excessive Zn formed Zn i to provide most of the carriers. V O became the main origin of the carriers with a decreasing Zn content. When the Zn content was 55.2% and 52.5%, the segregation of Zn i decreased the carrier concentration and subsequently, enhanced the resistivity and the Seebeck coefficient. The ZnO:Al film with a Zn content of 52.5% had the highest power factor of 274.03 μW·m−1·K−2 at 819 K. In addition, the higher Zn i concentration had a more prominent effect on thermal conductivity. The thermal conductivity increased to 1.756 W·m−1·K−1 from 0.608 W·m−1·K−1 with the Zn content decreasing from 55.2% to 50.6%. The ZT value reached the highest of 0.121 when the film had the optimum Zn concentration of 51.5%. This study concluded that the tuning of the intrinsic defects was an effective method to enhance the TE performance. [ABSTRACT FROM AUTHOR]

Unstable nanosheet morphology of polycrystalline SnSe film becomes a limitation for its application in the field of waste heat recovery. This study dopes Co into the evaporated SnSe film to smooth the surface and to improve power factor. The results show that Co dopant makes the rough nanosheet change to surface densely packed structure. The dopant Co forms Co particles distributing around SnSe grain boundary, which suppress SnSe (011) plane growth and subsequent reduce nanosheet formation. In addition, Co dopant increases electric conductivity through increasing the carrier concentration, but the magnetism of Co particles decreases carrier mobility due to spin-charge coupling and also affects the Seebeck coefficient. The power factor thus is improved after doped Co. This study presents a method to fabricate polycrystalline SnSe film with smooth surface morphology and high power factor. [ABSTRACT FROM AUTHOR]

Joint improvement of the Seebeck coefficient S and conductivity σ is a pressing issue to overcome the limit of the figure of merit ( ZT ) enhancement of thermoelectric materials. This study presents a joint improvement in the ZnO:Al films by tuning the diffusion of Au layer via substrate temperature, using a radio frequency-assisted molecular beam vapor deposition method. The phenomenon occurs in the ZnO:Al films at the room temperature (RT) substrate deposited on a 10 nm Au layer (RT-ZnO:AuAl). The films have a hexagonal wurtzite structure with an in-plane (002) preferred orientation and smooth surface. The resistivity values of the films are at the same magnitude of the transparent conducting oxide (~10 −6  Ω·m). The carrier concentration reaches 2.60 × 10 21  cm −3 for the RT-ZnO:AuAl film. The S is about 29.5 μV/K at 240 °C. Joint improvement of conductivity and Seebeck coefficient makes the PF of the RT-ZnO:AuAl film reaches 3.58 × 10 −5  Wm −1  K −2 , which is two orders of magnitude larger than that of the RT-ZnO:Au film without joint improvement. The results indicate that the higher conductivity originates from the Au layer, in-plane (002) preferred orientation, and the easily excited carriers in the ZnO:Al due to the low binding energy of the Zn 2p electrons. The large S in the film is related to the high effective masses due to the impact of the diffusion of the Au layer on electronic interactions. [ABSTRACT FROM AUTHOR]

The effects of nanowire content on the thermoelectric properties of ZnO films were investigated. The nanowire content of ZnO films was tuned by thermal oxidation of evaporated Zn films. The results showed that hexagonal and polyhedral morphologies on the surface of Zn films can be used to tune the nanowire content of ZnO films. Hexagonal nanoplates with a diameter of 100–350 nm readily grew ZnO nanowires with c-axis preferential orientation. Conversely, it was difficult to grow nanowires on polyhedral nanoparticles with diameters of 500–750 nm because the meeting of ZnO (101) and (001) facets suppressed nanowire growth. Thermoelectric parameters were strongly affected by nanowire content. In particular, carrier concentration increased with nanowire content. Carrier mobility also increased with nanowire content because the nanowires behaved as channels for electronic migration. The band gap of the films narrowed with increasing nanowire content because the binding energy of O 1s electrons with oxygen vacancies decreased. The maximum power factor of the film with high nanowire content (8.80 μW/m K 2 at 530 K) was approximately 300% higher than that of the film with low nanowire content. [ABSTRACT FROM AUTHOR]

Improving the thermoelectric (TE) properties of typical p-type Bi 0.5 Sb 1.5 Te 3 film is crucial for power generation in film-based thermoelectric devices. The shape of the hexagonal structure in the Bi 0.5 Sb 1.5 Te 3 film can be controlled by adjusting the magnetron sputtering parameters. The results indicate that a high nucleation rate under high sputtering power promotes island growth with a preferred orientation of (015). Thermal energy influences the growth in the in-plane direction along the (00 l) orientation and induces the c-axis orientation in the out-of-plane direction. Consequently, a dense layered hexagonal structure with a high grain boundary density is formed, leading to increased defect density, carrier concentration, and phonon scattering. In this case, the power factor reaches 3149.01 μW·m-1·K-2 at 480 K and the ZT value is 1.14 at 480 K. These results indicate that the film with a dense layered hexagonal structure enhances the TE properties. [ABSTRACT FROM AUTHOR]

This paper studies the effects of high magnetic field (HMF) on the structure, optical and thermoelectric properties of the doped ZnO thin films. The results show that both Al dopant and application of HMF can affect the crystal structure, surface morphology, elemental distribution and so on. The particles of the thin films become small and regular by doping Al. The ZnO films oxidized from the Au/Zn bilayer have needle structure. The ZnO films oxidized from the Au/Zn-Al bilayer transform to spherical from hexagonal due to the application of HMF. The transmittance decreases with doping Al because of the opaque of Al element and decreases with the application of HMF due to the dense structure obtained under HMF. Electrical resistivity ( ρ ) of the ZnO films without Al decreases with increasing measurement temperature ( T ) and is about 1.5 × 10 −3 Ω·m at 210 °C. However, the ρ of the Al-doped ZnO films is less than 10 −5 Ω·m. The Seebeck coefficient ( S ) of the films oxidized from the Au/Zn-Al films reduces with increasing T . The S values oxidized under 0 T and 12 T conditions are 2.439 μV/K and −3.415 μV/K at 210 °C, respectively. Power factor reaches the maximum value (3.198 × 10 −4 W/m·K 2 ) at 210 °C for the film oxidized under 12 T condition. These results indicate that the Al dopant and the application of HMF can be used to control structure and thermoelectric properties of doped ZnO films. [ABSTRACT FROM AUTHOR]

Multi-nanocomposite is an effective method to achieve high thermoelectric (TE) performance by breaking the tradeoff among three TE parameters. This paper presents a comparative study of incorporated carbon nanotube (CNT)/Au in evaporated porous Al-doped ZnO (AZO) films. The effect of the nanocomposite interface on scattering of carriers and phonons is used to independently tune thermal conductivity and power factor in the multi-nanocomposite AZO films. This breaks the tradeoff and improves the figure of merit (ZT) of the film. Moreover, the temperature gradient retains in the out-of-plane of the films. These make the AZO film composited CNT and porosity have higher output power than that of the non-porous films. This provides a simple method for the low-dimension film to increase the TE properties through breaking the tradeoff and makes the TE materials suitable for efficient harvesting in low-grade waste heat. [Display omitted] • This study presents a simple method to prepare multi-nanocomposite film. • Multi-nanocomposite breaks the tradeoff among thermoelectric parameters. • Film with CNT and porosity has higher ZT value and output power. [ABSTRACT FROM AUTHOR]

Bi 2 Te 3 is a successful and commercial thermoelectric material. However, Bi 2 Te 3 is easily generates point defects, which limits its thermoelectric (TE) properties. This study presents a novel method to tune point defects by using short-range atomic diffusion via pulsed electric field (PEF) treatment in C-doped Bi 2 Te 3 films. The C atoms migrate from the interstitial position in the Bi layer (C i) and the Van der Waals gap (C vdw) to the Bi vacancy after the PEF treatment. For this case, it increases the content of the Te–C bonds for the PEF films, resulting in increased electron concentration. And the PEF can eliminate amorphous regions of the C-doped Bi 2 Te 3 films. In addition, the shorter bond length of the Te–C improves the compressive stress of the film and increases the effective mass of carriers. Thus, the conductivity and the Seebeck coefficient are improved simultaneously. In this study, the power factor was 2450.32 μW m−1·K−2 at 513 K after the PEF treatment. Therefore, the PEF is an effective method to improve TE properties by short-range atomic diffusion. [ABSTRACT FROM AUTHOR]

• Angle of grain boundaries converts to low from high by electrostatic force. • Low energy of low-angle grain boundary scatters carrier but not phonon. • Pulse current treatment provides a method to get high power factor and power density. Current applications of thermoelectric (TE) materials are limited by their low conversion efficiency. Interfacial engineering is a common way to improve TE properties. This study employed a pulsed electric field to tune the grain interfacial structure of Bi 2 Te 3 films and thereby improve their power factor. The high-angle grain boundaries of the as-deposited films were transformed into low-angle grain boundaries (LAGBs) by the electrostatic force of a high pulse current density. The low interfacial energy of LAGBs maintains high carrier mobility while their reduced defects result in a low carrier concentration. These effects improve the Seebeck coefficient and conductivity simultaneously, with the power factor reaching 5394.77 μW·m−1·K−2 at 513 K. In addition, the compression stress generated by LAGBs enhances phonon scattering, creating a film with the highest power density yet reported (1.31 W·m−2). This provides a method for using PEF to improve the performance of TE materials. [ABSTRACT FROM AUTHOR]

Abstract Al-doped ZnO films (Zn 0.94 Al 0.06 O x) can be fabricated by thermal oxidation of evaporated metallic Zn-Al film. Oxygen defects were strongly related to oxygen pressure at a given temperature (500 °C) and duration (3 h). Under oxygen pressure of 0.021 MPa (O 2 -0.021 MPa), the main component was the Zn phase. Zn 0.94 Al 0.06 O x films were formed under oxygen pressure of 0.1 MPa and 0.12 MPa, with the conductivity and Seebeck coefficient improving simultaneously with increased oxygen pressure. The flat particles of the O 2 -0.12 MPa film weakened the electron scattering, leading to increased conductivity. Furthermore, the decrease in oxygen vacancies caused by the higher oxygen pressure increased the Seebeck coefficient. As a result, the power factor of the O 2 -0.12 MPa film was about 30 times greater than that of the O 2 -0.1 MPa film, characterized by an increasing content of singly ionised oxygen. Highlights • Carrier concentration was changed by oxygen pressure in thermal oxidation. • Power factor was affected by content of single ionised oxygen. • Conductivity and Seebeck coefficient increased simultaneously with oxygen pressure. [ABSTRACT FROM AUTHOR]

Sb-doped ZnO thermoelectric films with microporous structures are fabricated by oxidizing evaporated Zn-Sb thin films in a leaf-like surface. High magnetic field (HMF) and Sb are employed to tune the formation of nanowires and nanorods in the microporous films and conduction type. Nanowire is formed in the film with Sb content of 3.0% and nanorod is formed with 4.6% Sb with the absence of HMF. P-type ZnO films with a wuterzite are formed. The resistivity of the films decreases by two orders of magnitude by increasing Sb content. The resistivity of films decreases 45% and 80% by forming nanowires and nanorods, respectively. The power factor of the nanorod structures increases by two orders of magnitude by comparison with others and reaches to 52.6 μW/m K 2 . This indicates that the nanorod structures with a higher Sb content are easy to obtain stable p -type semiconductor with a higher power factor. [ABSTRACT FROM AUTHOR]

Zn 4 Sb 3 has brought into sharp focus by its low cost and excellent thermoelectric (TE) properties in the medium temperature range. Zn atoms can diffuse in the Zn 4 Sb 3 matrix under the treatment of temperature or current. This provides a way to control its TE performance. In this study, the TE properties of the Zn 4 Sb 3 films were improved by the Zn atomic diffusion under the treatment of periodic pulsed electric field. The Zn atomic diffusion is caused by the electrostatic interaction of the pulsed electric field and the diffusion mode is related to the pulse period. When the pulse period is smaller than 10 s, the interstitial Zn atom diffuses first. The Zn vacancy increases, leading to the increase of the carrier concentration and the decrease of the resistivity. When the pulse period increases to 100 s, more Zn(1) atoms diffuse through the interstitial position, resulting in a small amount of ZnSb phase. Thereby in the Zn 4 Sb 3 films, the carrier concentration decreases and the Seebeck coefficient increases. This kind of the synergistic effect decreases the resistivity and increases the Seebeck coefficient. The power factor of the Zn 4 Sb 3 films treated with a pulse period of 100 s is more than 4500 μW m−1 K−2 in the range of 373 K–573 K. This study presents a new method to control atomic diffusion by periodic pulsed electric field, and the TE properties of the Zn 4 Sb 3 films are improved. • Pulsed electric field can control the diffusion of Zn atoms in Zn–Sb films. • The diffusion of interstitial Zn i atoms increased the carrier concentration in films. • The diffusion of Zn(1) atoms increased with the increase of pulse period. • The diffusion of Zn(1) atoms caused a small amount of the ZnSb phase. • The power factor of the film treated with the pulse period is 4901.31 μW m−1 K−2. [ABSTRACT FROM AUTHOR]

Cu 2 Se is one kind of popular thermoelectric materials due to its performance of the phonon glass-electron crystal. However, the poor thermal stability limits its application. Regulating the distribution of the Cu+ by elemental dopant is a general method to solve the stability and to improve the thermoelectric performance. In this study, the Ag-doped β-Cu 2−x Se films were prepared by vacuum thermal evaporation method. The Cu+ distribution on the octahedral 32(f) sites in the Cu 2 Se crystal is tuned by the Ag content. It found that when the Ag content is 1.0%, the Cu+ and Ag+ contents in the 32(f) sites reach the maximum. In this case, the power factor of the film is 1750 μW·m−1·K−2 at 530 K. As the Ag content rises, the second phase of CuAgSe appears. This results in the decrease of the power factor. It shows that controlling the distribution of the Cu+ by the Ag dopant is conducive to improve the thermoelectric properties of the β-Cu 2−x Se films. • Occupancy increase of Cu+ and Ag+ in 32(f) sites decreases resistivity. • CuAgSe second phase increases carrier concentration and effective mass. • Power factor of Ag-1.0% film is maximum and 300% higher than that of Ag-0.0% film. [ABSTRACT FROM AUTHOR]

• Different dimensions of carbon have different effects on thermoelectric performance. • The lower the dimension of carbon, the higher the carrier concentration of the film. • The CNT composite makes ZnO: Al films have the best performance. The low-dimensional carbon composite is expected to improve electrical conductivity and reduce the thermal conductivity of thermoelectric (TE) materials, which is considered an effective method to improve the TE properties of materials. However, the different dimensions of carbon have various effects on the properties of TE film composite. This study investigated the effects of zero-dimensional fullerene, one-dimensional carbon nanotubes (CNT), and two-dimensional graphene on the TE properties of vacuum evaporated ZnO: Al films. The results demonstrated that these three types of composites influenced the carrier concentration and mobility of the films, which in turn affected the TE properties and output power. The lower the dimension of carbon, the higher the carrier concentration of the film. The CNT composite did not affect the microstructure and preferred orientation. However, it increased the particle size and surface roughness. The interface between CNT and ZnO: Al provided more carriers and improved effective mass, thereby increasing the Seebeck coefficient. In addition, the CNT can be used as a carrier transport channel to improve carrier mobility because of its unique tubular structure. Therefore, the CNT composited ZnO: Al films demonstrated the best TE performance and output power. Its power factor reached 136.5 μWm−1K−2 at room temperature. [ABSTRACT FROM AUTHOR]