Inkjet-printed Flexible Ag₂Se Based Thermoelectric Material And Device

The development of wearable and implantable devices has presented a new challenge to power generating devices which should meet the reqirements of high reliability,environmental friendliness and excellent flexibility.In this context,flexible thermoelectric（FTE）materials and devices,which can achieve power output by using the temperature difference between human skins and ambient environment,are believed to play an important role in the area of flexible electronics.However,the performances of most FTE materials and devices are too low to be practical,which encourages people to do more research on this topic.For FTE materials,organic ones have good flexibility but poor performance.The performances of inorganic semiconductor can be very high,but their rigidity limits the application for FTE materials and device.On the other hand,FTE devices are always fabricated by vapor phase deposition,spin-coating and vacuum-assisted filtration,but these methods are not compatible with the integration of devices.Therefore,it is necessary to focus on the material improvement and device fabrication to optimize their output performance and mechanical stability.Silver selenide（Ag₂Se）is one of the materials which have the highest thermoelectric performance at room temperature.Although Ag₂Se is considered as a rigid material,recent works have proved that Ag₂Se can achieve flexibility by loading on flexible substrates through thin-film forming process.Meanwhile,printing（especially inkjet printing）processes have been widely used to fabricate flexible electronics due to the unique advantages in terms of patterning,high resolution and low cost.These provide a new path to prepare FTE materials and devices.Here,this thesis regards the combination of Ag₂Se and inkjet printing process as a main line with the systematical research on synthesis,ink formulation,printing process and device designing.Finally,high-performance Ag₂Se-based FTE materials and devices are successfully prepared.Main works are as follows:（1）Ag₂Se powders were synthesized by various wet chemical methods,and those who had small size were dispersed in different solvents to form stable inks.The jetting parameters were further regulated to make a smooth jetting of ink droplets.We found that the Ag₂Se particles which prepared by solvothermal method were only 100-200 nm in size,and showed good dispersion and stability in ethanol,which proved that the Ag₂Se-based ink had been formed.Furthermore,Ag₂Se-based ink can be jetted by setting the preset DI-water waveform and an 18-V jetting voltage without producing satellite droplets.（2）Based on results from the last chapter,Ag₂Se thin films were prepared on the flexible polyimide substrates by inkjet printing.The crystalline phase,morphology and TE performance of Ag₂Se films were systematically investigated by adjusting the concentration of ink,droplet spacing and number of printed layers.We found that Ag₂Se film can have power factor as high as420μW m^-1 K^-2 at room temperature with a 5-mg m L^-1 ink,20-μm droplet spacing and a 40-layer setting.The TE performance and carrier transport characteristic of non-stoichiometric materials(Ag_2+xSe)were further studied,and the power factor of Ag_2.2Se could reach 665μW m^-1 K^-2 at 360K.（3）Based on the results from the last chapter,fully inkjet-printed FTE devices were designed and fabricated.By the theoretical finite element analysis and fabrication process optimization,the influence of structure design on the devices’output performance was studied.As a result,the structure-optimized Ag₂Se-based FTE device could achieve an output power of 386n W,and the power density of it was far higher than other inkjet-printed FTE devices.