| Development and utilization of renewable energy are effective ways to solve environmental pollutions and energy depletion problems facing human beings.As a main photoanode material for dye-sensitized solar cells(DSSC),the wide bandgap of titanium dioxide(TiO2)greatly limits the photoelectric conversion efficiency of DSSC.By inducing the generation of oxygen vacancies on the surface of metal oxides,laser surface modification technology can improve its energy band structure,thereby improving its absorption and charge transporting ability.In order to investigate the thermal interaction mechanism during the modification process,a 3D heat transfer model was established in COMSOL Multiphysics and MATLAB.A rough3 D surface,as the surface of TiO2 film,constructed by MATLAB was parameterized according to sample's roughness and set as sample's upper surface.And the absorption and reflection of light on sample's rough surface can be shown accurately.Meanwhile,the effective thermal conductivity(expressed as k)of TiO2 was calculated via simulation and experiment in order to characterize its heat conduct ability in air accurately.The temperature field of TiO2 irradiated by single laser pulse was calculated taking the convection and radiation losses into consideration to obtain the temperature curve of sample's lower surface.Combining the experiment and simulation results,the value of k is determined to be 1.6499 W/(m?K).Then the value was used to simulate the transient temperature distribution of TiO2 film irradiated by continuous laser,and the effects of different laser parameters are calculated based on the simulation.The simulation results show that the transient temperature field of TiO2 film irradiated by continuous laser is related to laser's power,spot radius,irradiation time.And the maximum of average surface temperature of the film meets the linear relation with laser power,and the Exp Assoc nonlinear relation with laser spot radius.The mechanism of high power laser modify TiO2 film was given macroscopically.The effects of oxygen vacancy defects in TiO2 was studied on Material Studio.The supercell models of pure and defected anatase TiO2 were constructed and the stablest defect structure was confirmed.Then,the Local-Density Approximation(LDA)and General Gradient Approximate(GGA)methods were used to geometrically optimize the supercell.The models were geometrically optimized by LDA method,then the electrical and optical properties were modified by GGA method.Comparing the band structure and optical properties,the modification mechanism of TiO2 was illustrated intuitively.It is illustrated in this investigation that a new donor energy level in the energy level structure of TiO2 was generated by oxygen vacancy defect,which reduces its band gap from 2.2148 e V to 1.823 e V,and increases its effective mass of electron at the bottom of conduction band from 0.190m0 to 0.288m0.Besides,the optical absorption curve is red-shifted and the light absorption threshold is increased by6.25%.The mechanism of high power laser modify TiO2 film was given microscopiclly.Finally,directed by the simulation and calculation results,the experiment of laser modify TiO2 film was carried out.The laser modification system was built based on CO2 laser.The morphology,crystalline phase,optical absorption curve,band gap and resistivity of TiO2 film with different values of laser's parameters,such as power,radius and time,were investigated,and the effect of modification was analyzed via simulation and calculation results discussed above.The experimental results show that the application of laser surface modification technology on TiO2 film causes following results: TiO2 film's optical absorption curve is redshifted,the light absorption threshold is increased by 13.5%,the band gap is reduced by 18.2%(2.75 e V)and the film resistivity is reduced.In general,the optical and electrical properties of TiO2 film were significantly improved. |
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