Theoretical Study On The Photoelectric Properties And Interfacial Electron Transfer Behaviors Of ZnSe And CdSe Quantum Dots

The development of new energy resources is promoted by the demand for economic growth and the concern about supply chains of fossil fuel.Solar energy is regarded as a potential renewable energy because of its extensive distribution,inexhaustible supply and low cost.Solar cells,as the most promising photovoltaic devices at present,convert solar energy into electric energy for further utilization.Quantum dot sensitized solar cells（QDSSCs）have been developed rapidly in the past ten years due to their high theoretical photoelectric conversion efficiency,high stability and low cost.However,the photoelectric conversion efficiency of QDSSCs in reality is still lower than its theoretical limit because of their low light absorption efficiency and serious interfacial carrier recombination in QDSSCs.Therefore,boosting the light absorption efficiency of photoanode,promoting the interface transfer of photogenerated electrons and inhibiting electron–hole recombination can improve the performance of QDSSCs to some extent.In addition,the optical properties of semiconductor quantum dots（QDs）as sensitizers on the photoanode also have a great influence on the performance of QDSSCs.This dissertation first takes（Zn Se）N（N = 1–13）clusters as QDs model to study the effects of various factors,such as QDs size,electron transition energy and structural relaxation during excitation,on the radiative /non-radiative recombination process of QDs.Afterwards,the QDs–TiO₂ interface model was established by using（Cd Se）N（N = 6,13,19,25,34）clusters as QDs to study the effects of Mn doping and QDs size on light absorption,electron injection and electron–hole recombination,which can provide a theoretical basis for exploring high-efficiency QDSSCs for experiments.Five chapters are included in this dissertation:In Chapter 1,we briefly introduced the basic concept of QDs,including their physical effects,such as quantum confinement effect,multi-exciton effect and photoluminescence phenomenon.Secondly,the structural composition and working processes of QDSSCs were expounded.Thirdly,we outlined the charge transfer behaviors and their influencing factors in QDSSCs.Finally,the current research status of photoanodes for QDSSCs are introduced,and the main research contents of this dissertation are summarized.In Chapter 2,we focused on the theoretical fundamentals and the computational methods involved in this dissertation.Firstly,the physical model and the basic approximation of ab initio calculation are briefly described.Secondly,the concept,principle and recent development of density functional theory（DFT）are expounded.Thirdly we introduced the basic model of electron transfer rate,and gave a brief description of their advantages and disadvantages as well as their applicable scope.Finally,the computational methods of some required parameters in the electron transfer model and the computational softwares are introduced.In Chapter 3,the geometrical structures,absorption / emission spectra and photoluminescence quantum yields of（Zn Se）N（N = 3 – 13）cluster have been studied by means of density functional theory.Several factors,such as size,morphology,electron transition energy and structural relaxation during excitation,on the radiative and non-radiative recombination rates have been discussed in detail.The results show that the absorption spectra of Zn Se clusters have a trend of red-shift with the cluster size.Nevertheless,the emission spectra varies in a complicated way,resulting from the combined effect of size-dependent geometrical and electronic structures.The radiative and non-radiative recombination rates are related to the cluster size,but do not present a single size effect.Based on the Einstein spontaneous emission rate equation,we find that the structural rigidity is an important factor for influencing the radiative recombination rate by means of vertical emission energy.The stronger the rigidity,the smaller the structural change between the excited state and the ground state,and the larger the vertical emission energy.The vertical emission energy usually presents a positive correlation with the radiative recombination rate.The calculation of the non-radiative recombination rate shows that the reorganization energy has the greatest influence on the non-radiative recombination rate although there are other influencing factors such as the vertical emission energy and mean vibration energy.Since the reorganization energy itself is a physical quantity related to structural rigidity,that is,the stronger the rigidity is,the smaller the reorganization energy is,the structural rigidity is a key factor affecting the non-radiative recombination rate.Above results indicate that the photoluminescence quantum yield can be improved by increasing the structure rigidity in experiments by means of increased radiative recombination rate and decreased non-radiative recombination rate.In Chapter 4,the effect of Mn doping on the electron injection rate in QDSSCs is studied by first principles calculation.（Cd Se）13 and Cd12 Mn Se13 quantum dots are adsorbed on the TiO₂（101）surface as the calculation models.The results indicate that the absorption spectrum of Cd Se quantum dots has a redshift by Mn doping,that is,its absorption range was broadened.The electrostatic interaction between Mn-doped quantum dots and TiO₂ surface is stronger,and then the more charges flow from quantum dots to TiO₂.As a result,the Fermi energy level Ef of TiO₂ increases,which is conducive to the increase of open-circuit voltage.The electron injection rate in the Cd Se/TiO₂ interface increases by nearly 80% after Mn doping,mainly because the lowest unoccupied molecular orbital（LUMO）energy level of quantum dots decrease and then the energy level corresponds to a denser density of states of TiO₂ conduction band.Therefore,the number of effective channels for electron transfer in Mn-doped QDSSCs increase.In addition,Mn doping enhances the electron coupling between the LUMO of quantum dots and the conduction band of TiO₂,which further improves the probability of electron injection.Besides adjusting the composition of quantum dot,the structure and size can lead the LUMO change.Hence the electron injection rate can be effectively improved by adjusting structure and size of quantum dot to make their LUMO levels correspond to the denser conduction band levels of semiconductor oxides.In Chapter 5,the effects of the size and morphology of the quantum dots on the electron–hole recombination rate of the QDSSCs were investigated by using first-principles calculation.The（Cd Se）N（N = 6,13,19,25 and 34）quantum dots with different sizes are absorbed on the TiO₂（101）surface as a computational model.The results indicate that the electron–hole recombination rates decrease with quantum dot size for same morphology.The electron–hole recombination rates in hollow cage structure are lower than these in core-shell structure for same size.Since the energy gaps between the conduction band bottom（CBM）of TiO₂ and the highest occupied molecular orbital（HOMO）of quantum dots are usually greater than the recombination energy,the electron–hole recombination process generally occurs in the inversion region of Marcus theoretical model.By calculating the electronic structure,we found that the energy level differences between the CBM of TiO₂ and the HOMO of QDs vary in 0.1 e V with the size,so the electron–hole recombination rates are mainly affected by the electron coupling constant and the reorganization energy.As the size of Cd Se quantum dots increases,the reorganization energy decreases and then the electron transfer barrier increases,which suppresses the electron–hole recombination.Moreover,the electron coupling between the CBM of TiO₂ and the HOMO of quantum dots are gradually weakened by increasing size,which further reduces the probability of electron–hole recombination.Since both the electron injection rate and the electron–hole recombination rate decrease with quantum dot size,it is important to find an optimal quantum dot size to balance the competition between them.