Research On The Design,fabrication,and Performance Of Solar Thermal Conversion And Solar-thermal-electric Conversion Devices

Solar selective absorption films have important applications in various light-to-heat conversion systems.For different solar-to-heat conversion systems,the operating temperature and sun light concentration ratio are always different.Therefore,it is necessary to design the solar thermal conversion film structure aiming at particular operating temperature and solar concentration ratio.Solar thermal conversion films are often adopted in photo-thermal-electric conversion devices because they can absorb a large amount of solar radiation while maintaining the low thermal radiation.The traditional vertical solar thermoelectric conversion device always uses bulk thermoelectric materials,with its size being too large to be used as the portable and miniaturied divice.Nevertheless,with the development of Io T（internet of things）,miniturized and portable power supply devices are needed.Thin-film solar thermoelectric conversion device with the film thermoelectric materials and high efficient solar-thermal conversiom film can be used for self-powered miniturized device.By combining with the existing photolithography process and microelectronics process,the small sized solar-thermal-electric conversion device has been widely used in various applications such as wireless sensors,wearable electronic devices and etc.However,usually carbon balck or black paint were used for the solar absorber,which deteriorate the overall performance of the device due to its lower solar-thermal conversion efficiency of the proposed absorber.In consideration of the above problems,this thesis mainly carried out the following research work:（1）Considering the difference between the optical constants of the ultrathin metal film and the bulk film,the optical constants obtained by ellipsometry and fitting were proposed as the modeling parameters of multilayer films.The results showed that the optical constants of the ultrathin metal film and the bulk film were quite different.（2）A genetic algorithm was used to write a solar thermal conversion film design program.The optical constants measured in advance were introduced into the optimization program.A solar selective absorber was successfully fabricated by the program for the working temperature at600 K and solar concentration of 1 Sun（1000W/m²）.The film structure is as follows:Si O₂（61.9 nm）/Cr（3.7 nm）/Si O₂（61.9 nm）/Cr（24.8 nm）/Si O₂（7.2 nm）/Cu（>100 nm）.The experimental results shows that there is a good agreement between the numerical simulations and the experimental results.The sample shows that the reflectance in the wavelength range of 300 to 1200nm is less than 5%,while it increases rapidly in the long wavelength range,which means the high solar absorptivity and great spectral selectivity of the fabricated sample,verifing the feasibility of the program.It also demonstrates the significance of considering the optical constants difference between the ultrathin metal film and the bulk film in the simulations.（3）A thin-film photo-thermal-electric conversion device was prepared by the mask method.Genetic algorithm was used to obtain the solar photo-thermal conversion film for the working conditions of 300 K,1Sun and 600 K,and 1 Sun respectively.Simultaneously,a black paint thin film device and a device without solar-to-heat conversion film were also prepared.The temperature picture of different solar absorbing films on a hot plate with the temperature of100°C and 200°C respectively,were recorded by an infrared thermal imaging camera.The results show that absorber designed aiming to work 600 K and 1 Sun exhibits the lowest heat radiation,while the black paint film has the highest thermal emission.P-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.7Se_0.3 were used as thermoelectric materials and formed a tandem structure of12 pairs.The output performance of four kinds of devices were measured at room temperature and the concentration ratio of 1 Sun under the solar simulator.The results show that the device with solar absorber designed for the working condition of 300 K and 1 Sun exhibits the largest output voltage of about 1.3 m V.The above results illustrate the necessity of designing solar photothermal conversion films corresponding to the specific working conditions of different solar absorber when fabricating thin-film photo-thermal-electric conversion devices.However,due to the poor performance of used thermoelectric materials and large series resistance due to the use of a series thermoelectric pair structure,the output power of the device is still too low.Further research is needed.