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Light-induced transverse thermoelectric effect has been investigated in c-axis tilted Bi2Sr2Co2Oy thin films coated with a single-wall carbon nanotubes light absorption layer. Open-circuit voltage signals were detected when the sample surface was irradiated by different lasers with wavelengths ranging from ultraviolet to near-infrared and the voltage sensitivity was enhanced as a result of the increased light absorption at the carbon nanotubes layer. Moreover, the enhancement degree was found to be dependent on the laser wavelength as well as the absorption coating size. This work opens up new strategy toward the practical applications of layered cobaltites in photo-thermo-electric conversion devices.
©2013 Optical Society of America
1. Introduction
Thermoelectric technology is the direct energy conversion from heat into electricity and has attracted much attention as a renewable energy solution. Conventional thermoelectric devices are based on the longitudinal thermoelectric effects in which the electrical and thermal currents are in the same direction. Using the light to heat thermoelectric devices is an alternative way to fully utilize solar energy [1–4]. However, the conventional thermoelectric devices are complicated in structure and restricted in planar shape, they are difficult to scale-up and integrated with the solar panels. Transverse thermoelectric devices, which are designed according to an unconventional thermoelectric effect in which heat and electric energy flow through a material perpendicular to each other, can overcome these problems and therefore have attracted increasing attentions to researchers [5–7]. In addition to power generators, the transverse thermoelectric effect also has great potential applications in broad-band light detectors. This effect emerges uniquely in tilted structures with anisotropic Seebeck coefficient. As shown in Fig. 1(a), when a c-axis tilted film is heated by light on the surface, a temperature difference is set up along the film thickness direction, and results in a transverse voltage signal expressed by:
Where ΔT is the temperature difference between the surface and bottom of the film, which is generated by heating the film surface due to the absorption of the incident light radiation; ΔS = Sab-Sc is the difference of the Seebeck coefficients of the crystalline ab plane and along the c axis of the film; θ is the tilted angle between c axis and the normal of the film surface; d is the thickness of the tilted film and l is the light irradiation length [8].
Fig. 1 (a) Schematic illustration of the LITT effect measurements; (b) SEM surface image of a 50 μm-thick SWNTs layer on the BSCO thin film; (c) HRTEM image and (d) the corresponding SEAD pattern of the BSCO/LaAlO3 cross section.
Layered cobaltites, mainly including MxCoO2(M = Na or Ca), Ca3Co4O9 and Bi2Sr2Co2Oy, have attracted great attention as promising thermoelectric materials due to their good thermoelectric performance as well as the good thermal stability, lack of sensitivity to the air and non-toxicity [9–11]. The crystal structure of these cobaltites is composed two different layers, with an alternative stacking of the conductive CoO2 layer and the insulating Nax (Cax or Srx), Ca2CoO3 or Bi2Sr2O4 layer along the c-axis [9]. This layered structure results in a large anisotropy of the Seebeck coefficient with ΔS in the order of tens of μVK−1 at room temperature, indicating that layered cobaltites are also promising candidates for transverse thermoelectric devices. Unfortunately, recent experimental results obtained from the light-induced transverse thermoelectric (LITT) effect in these layered cobaltites revealed that the conversion efficiency from light to electric voltage was not yet sufficient for practical applications in power generators or light detectors based on the LITT effect [12–16]. Very recently, Takahashi et al. demonstrated that adding an additional Au-black light absorption layer can greatly improve the photo-thermal-electric conversion efficiency of the LITT effect in tilted CaxCoO2 thin films, which provided valuable insights on the improvement in the performance of these new type devices [16]. However, the fibrous structure of Au-black is very delicate and prone to aging due to the collapse of this structure, particularly as a result of heating and physical contact. Carbon nanotubes are known to be efficient light absorbers with good mechanical properties as well as good physical and chemical stability [17–19]. In particular, large surface area and high thermal conductivity enable the light-generated heat in carbon nanotube coatings rapidly transfer to the underlying materials [17]. In this paper, for the first time, we presented the application of single-wall carbon nanotubes (SWNTs) in the LITT effect of tilted Bi2Sr2Co2Oy films as the light absorber. A significant enhanced voltage signal was observed when the sample surface was irradiated by different lasers with wavelength ranging from ultraviolet (UV) to near-infrared (NIR). Moreover, the magnitude of the enhancement was found to be dependent on the laser wavelength as well as the SWNTs coating size. We believe that our present study takes great strides in practical applications of tilted structures in photo-thermo-electric conversion devices based on the LITT effect.
2. Experimental details
Pristine carbon nanotubes are usually difficult to disperse into a liquid medium owing to their high surface hydrophobicity. To accelerate the dispersion effect and help to keep the carbon nanotubes well separated, the surfactant Polyvinylpyrrolidone (PVP) is used in this work to act as the dispersing agent. The solution, composed of SWNTs (50 mg, DaojinNanotechnologies), PVP (50 mg, Alfa Aesar) and alcohol (1 ml, 99.99%), were ultrasonically mixed for about 20 min to assist the dispersion process.
Figure 1(a) presents the schematic illustration of the LITT effect measurements for the SWNTs/BSCO structure. A circle shaped SWNTs light absorption layer (~50 μm in thickness) was spin-coated on the tilted BSCO film by using the above suspension and its diameter can be adjusted by a metal mask. As seen in the Fig. 1(b), the SWNTs layer is composed of a large number of intersecting SWNTs with random orientation. The large surface area and the coarse surface structure of the SWNTs networks are favorable for increasing the light absorption and improving the photo-thermal conversion ability. The underlying c-axis tilted BSCO thin film (~100 nm in thickness) was grown on LaAlO3 (001) single crystal substrate by using the chemical solution deposition method. Details about the film fabrication and properties characterization were described in Ref. [20]. Figure 1(c) shows the cross-sectional high-resolution transmission electron microscopy (HRTEM) image of the tilted BSCO film on LaAlO3 substrate. The corresponding selected area electron diffraction (SAED) pattern, shown in Fig. 1(d), suggests the epitaxial growth of BSCO film on LaAlO3 and the tilting angle is about 10°.
For the LITT measurements, two indium electrodes with the diameter of 0.5 mm were symmetrically deposited on the film surface along the tilted direction and they were separated by 6 mm. To prevent the generation of any electric contact effect, the electrodes were always kept in the dark. Three continuous-wave (cw) lasers (365, 532 and 971 nm) were used as the light sources. The incidence direction of the laser beam was directly perpendicular to the sample surface. The laser spot was located at the centre position between the two electrodes and its diameter was about 2 mm. The induced transverse voltage signals were recorded using a 2400 Keithley source meter.
3. Results and discussion
Figures 2(a)-2(c) present the light-induced open circuit voltage signals of the c-axis tilted BSCO films with and without the SWNTs light absorption layer (~3 mm in diameter) under the illuminations of 365, 532 and 971 nm cw lasers with a power of 50 mW. Under the laser irradiation, the temperature difference between the film surface and bottom (∆T) is rapidly developed and a voltage signal is thus generated due to the transverse thermoelectric effect. The induced voltage amplitude (Vp) is improved after coating the SWNTs light absorption layer, and the improvement R, defined as the ratio of Vp values with and without SWNTs layer, increases with the increase of wavelength. Figure 2(d) shows the variation of Vp value with the laser power on the surface for these two samples under the 532 nm irradiation. For each power value, the Vp value of the SWNTs-coated BSCO film appears larger than that of the bare BSCO film, which further demonstrates an enhanced voltage sensitivity has been obtained by coating a layer of SWNTs light absorber. In addition, Vp increases linearly with P when the laser power P is below the destruction limit of the sample. Such characteristic is a signature of the transverse thermoelectric effect since the temperature difference ΔT in Eq. (1) is directly proportional to the amount of absorbed laser power of the sample. Table 1 lists Vp and R values of BSCO films under different laser irradiations with the power of 50 mW. The longer of the incident laser wavelength is used, the smaller Vp value is observed in the bare BSCO films. The Vp value is improved and becomes almost the same after coating the SWNTs light absorption layer on the BSCO film surface. Moreover, the improvement in Vp values increases with increasing the wavelength of the incident laser. Especially, Vp value obtained in the SWNTs-coated BSCO film is almost 5 times larger than that in bare BSCO film when irradiated by a NIR laser, and it still has great potentials to be further improved through optimizing the SWNTs light absorption layer by adjusting its lateral size, thickness and morphology.
Fig. 2 Light-induced voltage signals of the bare BSCO film and the SWNTs-coated BSCO film under the cw laser illuminations of (a) 365, (b) 532 and (c) 971 nm with the power of 50 mW. (d) Variation of Vp with the laser power on the sample surface for these two samples.
Table 1. Vp values of the bare and SWNTs-coated BSCO films under the illumination of different lasers as well as the improvement (R) in voltage sensitivity after coating the SWNTs light absorber
To explain the dependence of Vp value on the wavelength of the incident laser and the absorption layer, we examined the optical absorption spectrum of these two samples at the wavelength range of 250-2000 nm using a Hitachi U-4100 spectrophometer. As can be seen in Fig. 3, in the laser wavelength ranges (300-1000 nm) used in this work, the absorptivity of the bare BSCO film decreases remarkably with increasing the wavelength while that of the SWNTs-coated film keeps a constant of approximately 0.98. The smaller VP in bare BSCO is suggested to be related to its lower absorption (i.e. higher transmittance or reflection) for the incident light, which indicates that only part of the laser power is effectively used for heating the bare BSCO film. Furthermore, the lower absorption of the bare BSCO for the incident light make the light penetrate further into the film from its surface and lead to a more uniform heating of the BSCO films along its thickness direction. This will eventually result in a smaller ∆T and thus a smaller Vp. After coating a SWNTs absorption layer on the BSCO film, the total light absorption of the coated film increases and more laser powers can be used for heating the film surface. An enhanced Vp is therefore observed in the coated film when compared with that of the bare BSCO film. Moreover, the same absorption of the coated film for the laser irradiations with different wavelengths leads to an almost same ∆T and therefore a similar Vp value.
Fig. 3 Absorption spectrums of the bare BSCO film and the SWNTs-coated BSCO film. The thickness of BSCO film and SWNTs layer is about 100 nm and 50 μm respectively.
To better understanding the above experimental observations presented in Figs. 2(a)-2(c) and Table 1, the heat diffusion process through the samples is discussed. According to the one-dimensional heat diffusion equation, the temperature rise of the film along its thickness direction (z-axis), when being irradiated by a laser from the front side, is given by [21, 22]:
Where T is temperature, t is time, αF is the absorption coefficient of the film for the incident light, P is the laser power density on the sample, DF, ρF and CF are the thermal diffusivity, density and heat capacity of the film, respectively (ρCD = κ, κ is the thermal conductivity). The above diffusion equation is subjected to the following initial and boundary conditions:Here we define the z position at the film surface and the film bottom as z = 0 and z = d (d is the film thickness), T0 is the initial temperature of the substrate surface. The steady-state ∆T between the film surface and bottom is then given by:Equation (4) indicates that ∆T will increase with increasing αF. From the optical absorption spectrum shown in Fig. 4, we notice that absorption of bare BSCO film for the incident light, αF, decreases with the increase of wavelength. Therefore, light irradiation with longer wavelength will generate a smaller temperature difference ∆T between the BSCO film surface and bottom, leading to a smaller voltage signal according to the transverse thermoelectric effect. When an additional absorption layer is added, the initial and boundary conditions have to be modified toHere we define the z position at the surface and bottom of SWNTs layer as zSWNTs = 0 and z = d* (d* is the thickness of the SWNTs layer). In this case, the steady-state ∆T between the film surface and bottom after coating a SWNTs absorption layer can be expressed asSince <<αFd under present conditions, the above equation can be approximatively written in the following formIt can be clearly seen that after coating a thick SWNTs layer with perfect absorption for the incident light, the temperature difference between the film surface and bottom ∆T, generated by absorption of the light, is almost same for different light irradiations, resulting in similar Vp values. In addition, ∆T calculated from Eq. (6) or Eq. (7) is larger than that calculated from Eq. (4), meaning that the temperature difference between the film surface and bottom will be increased when its surface being coated a perfect light absorption layer and thus a higher voltage will be induced due to the transverse thermoelectric effect.Fig. 4 Light-induced voltage signals of SWNTs-coated BSCO films with different coating diameter of 2, 3 and 4 mm. For comparation, a light-induced voltage signal of bare BSCO film is also presented. The laser spot and the laser power on the surface of all samples are 2 mm and 50 mW, respectively.
For a more detailed investigation of voltage signal dependence of the light absorption layer, the signal is recorded for three different coating areas when the sample surface is irradiated under the 532 nm cw laser, as shown in Fig. 4. It can be seen that the magnitude of the voltage signals reaches its maximum when the coating size of the light absorption layer equals to the laser spot diameter on the samples. Further increasing the coating size will lead to a decrease in ∆T due to the strong heat dissipation of the SWNTs light absorption layer resulting from its high thermal conductivity. It is worth to mention here that other features of the SWNTs absorption layer such as the thickness and the morphology (i.e. with aligned or random tubes) should also has a significant effect on the LITT signal, which will be investigated detailedly in our future work.
4. Conclusions
In conclusion, we investigated the light-induced transverse thermoelectric effect in the c-axis tilted BSCO thin films coated with a layer of SWNTs light absorber by using three cw lasers with wavelengths ranging from UV to NIR. An enhanced open circuit voltage signal was all detected in the films for these three irradiations due to the increased light absorption at the SWNTs layer. Moreover, the magnitude of the voltage signals was found to be dependent on the coating size of the SWNTs absorption layer and reach the maximum when the coating size of the light absorption layer equals to the laser spot diameter on the samples. These results suggest that the SWNTs can be used as a promising light absorber to improve the photo-thermal conversion efficiency and is very useful for fabrication of power generators or brand-band light detectors based on the transverse thermoelectric effect.
Acknowledgment
This work is supported by the National Basic Research Program of China under Grant No. 2011CB612305 and Nature Science Foundation for Distinguished Young Scholars of of Hebei Province, China under Grant No. 2013201249.
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