Study The Electronic Structure And Optical And Thermoelectric Properties Of Aliovalent Ion Doped TiO₂

Titanium dioxide (TiO2) has received much attention as an important material for photocatalysis and photoelectricity, which is relatively appropriate conduction band and valance band potential, wide in band gap, chemically stable, strongly reductive and oxidative, non-poisonous, and environmentally friendly. In recent years, the application of TiO2optical and thermoelectric properties became more and more due to its high dielectric properties and high Seebeck coefficient, TiO2become a new material in thermoelectric. It is know that the wide band gap of TiO2requiring ultraviolet irradiation limits the practical application. Therefore, increase TiO2’s light absorption region and enhance the photocatalystic active have became one of the hot spot problem.Base these problems, the paper used the theoretical and experiment method to study TiO2’s optical and thermoelectric properties. Firstly, study the crystal structure, electron structure, intrinsic point defects and doping properties et al some important problems by first principles calculation, on these studies of the metal and nonmetal codoped TiO2, we found that the metal and nonmetal codoped TiO2were not only broadened the visible light absorption region of TiO2, but also reduced the electrons and holes recombination ratio. Secondly, according to the result of theory, P-doped and V-doped and P/V codoped TiO2samples were prepared by a sol-gel technology, we found that doping can effectively enhance TiO2’s photoelectric and thermoelectric properties. The conclusions have outlined as follows:1. We have carried out the first-principles calculations for electric structure and optical properties and defect format energy of P-V-P/V-doped TiO2and analysed the reason metal and nonmetal doping, single element doping how to effect the TiO2’s photoelectric and thermoelectric properties. We put forward the co-doped synergistic effect theory.2. We have prepared the P-doped, V-doped, P/V co-doped TiO2’s simple on experiment, and studied their photoelectric and thermoelectric properties. The reasons showed P and V single doped TiO2is effective method to change TiO2’s band gap, single-element doping have two disadvantages:(i) single-element doped TiO2can produce a new recombination centers by introducing impurity ion in the band gap, which block the photocatalysis development,(ii) the impurity levels of non-metal doped TiO2locate at the top of the valence band, and they could be limited to decrease the band gap. codoping have a nanocompensated effect to overcome these fundamental limitations, and can also introduce the impurity-related states, consequently the band gap get narrowed without creating mid-gap states, which are necessary to maximize the photocatalytic performance of TiO2under the visible-light irradiation. It has been recognized that the codoped TiO2with both nonmetal-anion and metal-cation could reduce the recombination centers through passivating the defect bands and further preventing the formation of recombination centers.3. On the base of TiO2’s photoelectric properties, firstly, our calculation found that V and Al-doped TiO2can enhance Ti2’s thermoelectric properties by energy band and electronic transport theory. Secondly, we prepared Al-doped TiO2’s simples, and studied its thermoelectric properties. Our experiment result showed that Al-doped TiO2thin film simple shows a negative Seebeck coefficient and its value linearly decreases with increasing electrical conductivity, the Al doping can lead to phonon scattering at grain boundary interfaces and consequently decreases thermal conductivity. Therefore, the Al doping can essentially functions as a mechanism to separately adjust electrical and thermal conductivities in the Al-doped TiO2. With Al doping we obtained a maximal figure of merit (Z vale) to be1.30at475℃and0.48At23℃from the Al-doped TiO2at the doping concentration of3%, experiment result is consistent with theory result.4. On the base of previous work, we tentatively fabricate an ITO/LiF/P-doped TiO2/V-doped TiO2/Au thin film structure solar cell, used P and V doped TiO2respectively, and got the P type and N type TiO2thin film, V-doped TiO2as an active layer and N-doped TiO2as a transport layer. Their properties were characterized by the J-V curve and UV-visible spectrometry. Cell performances with different thickness of the TiO2thin film have been studied. The results show that its performance is best in these sample when thickness of the TiO2thin film are1.5μm, Jsc=8.32mA/cm2, Voc=0.54V, η＝1.41%. This kind of cell widen TiO2’s range of application.The innovative conclusions have outlined as follows:(1) we used the first-principle calculation and material preparation and experiment measurement methods can further understand these questions and quickly confirm the aim and mean of doping TiO2;(2) we found that alivalent ion doped TiO2can restrain the electron and hole composition, we get to the co-doped synergism effect theory, some TiO2’s powders and films samples were prepared by sol-gel mehod, these samples can be absorbed by the visible light at550nm;(3) we prepared the Al-doped TiO2simple, uncovered the reason of doping effect TiO2thermoelectric properties, and analysed their relationship of Seebeck coefficient, electric conductivity, thermal conductivity;(4) prepared an double deck film structure Solar Cell with plane P-N junction structure characteristic, the cell have a clear rectification characteristic and photovoltage and current.