The Studies On Optical Properties Of Doped ZnO And Dye-sensitized Solar Cells

Znic oxide (ZnO),a direct band-gapâ…¡-â…¥semiconductor with energy gap of about3.37 eV at room temperature,has a large exciton binding energy of 60 meV.Based onthese properties,ZnO has attracted much attention on light emitting diode (LED),highefficient phosphors,semiconductor laser and UV detectors.Although the opticalproperties of ZnO have been studied by many researches,the mechanism to thephotoluminescence is still in debate.Otherwise,the three basic light emissions arenecessary for the white light emission LED,thus,many studies have been preformed bydoping,especially rare earth elements doping.Until now,the three basic light emissionsemitting from ZnO have not been achieved by doping only one element.Moreover,theenergy transfer mechanism in rare earth doped ZnO is still unclear.In this thesis,the mainconclusions on ZnO are as following:(1) ZnO and Zn_1xFe_xO films were prepared by sol-gel method and their opticalproperties were studied in detail.The results showed that the films are compact andconsisted of particles.UV-Vis shows their high transmission.The crystal quality becomesworse and the optical energy band gap has a blue shift with the increase of Fe content.Moreover,photoluminescence shows a regular change:the ZnO film has a green lightemission; when x=0.03 or 0.051,samples show green and orange emissions; whenx=0.072 or 0.094,sample show blue light emission.The mechanism of light emission isstudied in detail.The regular change of the optical properties is really important for ZnOin its application.(2) ZnO and ZnO:Eu nanospheres were synthesized by self-assembly method.Growth mechanism and photoluminescence of these nanospheres were studied.With helpof scanning electron microscope (SEM) and selected area electron diffraction (SAED),apossible growth mechanism for the two-step self-assembly ZnO nanospheres is proposed.To investigate the origin of light emissions,ZnO nanospheres were annealed in differentatmospheres including air,oxygen,argon and forming gas.It shows that the green andorange-red emission is attributed to the oxygen vacancy and surface states.ZnO:Eunanaospheres were also prepared.It was shown that Eu ions were located at different sites(substitutional site,interstitial site and location on the surface of the nanoparticles).Apossible energy transfer process was described.In addition,a nearly white emission which was visible by naked eyes was achieved by introducing Eu into the ZnO nanospheres.Allthese results are much useful to the growth control,doping and its application in photoniccrystal.The study on photoluminescence of ZnO:Eu is very important to explore efficientZnO based phosphors.The solar energy is abundant and infinite,and it is one of the best ways to solve theenergy crisis.As one kind of new solar cells,dye-sensitized solar cells (DSSC) haveattracted more attention due to their low-cost and simple technique recently.In this thesis,the main conclusions on DSSC are as following:(1) ZnO nanospheres and nanoparticles have been synthesized by two-stepself-assembly and sol-gel calcination method,respectively.When they were applied inDSSC,The overall light to electricity conversion efficiency (η) of cells consisting of ZnOnanospheres was significantly enhanced due to light scattering compared with the ZnOnanoparticles based DSSC.After annealing the film in oxygen ambience and placing aZnO blocking layer on the FTO conducting substrate,the fill factor (FF) and open-circuitvoltage (V_oc) were considerably improved andηincreased.All the results showed that thesignificant improvement of the cell performance might be due to the suppression of therecombination and the decrease in the resistances existing in the cell.All these results maybe important to study ZnO based DSSC.(2) Nanoparticle TiO₂ films were fabricated using electrostatic spray coating (ESC)method and MEA was introduced for the first time.Compared with the usual preparationmethod of electrode,ESC is simple,low cost and well repeatability.The transparency ofthe samples and the adhesion between TiO₂ film and the substrate were greatly improvedas MEA was introduced into the conventional ESC.In addition,the size of TiO₂ particlescould be easily controlled by adjusting the proportion of MEA.Moreover,by analyzingthe photoluminescence results,the relationship between V_oc of cells and defects in anodeswere studied in details.All results could be significant to understand the micro-progress incells and provide guide for large-area preparation of DSSC electrodes in industrialmanufacture.