Study On Doping Modification Of Electron Transport Layer Of Perovskite Solar Cells

As a dye-sensitized solar cell,perovskite solar cell uses a perovskite material with excellent light absorption capability as a light absorber,and performs photovoltaic effects and generate current together with materials such as an electron transport layer and a hole transport layer.The electron transport layer and the hole transport layer respectively function to collect electrons and holes generated by the perovskite material excited.At present,the commonly used electron transport layer material is anatase titanium dioxide TiO₂,but TiO₂has many problems,such as large band gap,low visible light transmittance,low electron mobility,etc.,resulting in its applications have been greatly limited on perovskite solar cells.So many scholars began to modify the TiO₂ by doping or composite methods to obtain better optoelectronic properties,and realize energy level matching optimization with other materials.In this paper,the density functional theory of first principle is used to study the binding energy,defect formation energy,photoelectric properties and differential charge density of intrinsic TiO₂ and different concentrations of Nb and Ta doped TiO₂.Firstly,the analysis of binding energy and defect formation energy indicates that both doping types can be stably existed,which confirms the feasibility of the study,and Nb doping defects are easier to form.The electronic structure analysis found that the band gap of different concentrations of Nb and Ta doped TiO₂ changed little compared with the intrinsic TiO₂.From the analysis of density of states,it was found that the Fermi levels of both doping types have entered the conduction band,which an N-type degenerate semiconductor is formed.Optical properties analysis showed that the visible light absorption and reflectivity of Nb-doped TiO₂ with the same doping concentration were lower,indicating that Nb doped TiO₂ is more suit as a transparent electrode material.The differential charge density shows that after single doping of TiO₂,the Nb atom can provide more free electrons than the Ta atom.Finally,the analysis of electrical parameters shows that the same doping concentration of Nb doped TiO₂ can obtain higher electron mobility and conductivity,which proves that Nb is more suit as N-type doping target of TiO₂.The binding energy,defect formation energy and photoelectric properties of Nb/N co-doped TiO₂ with different concentrations were also studied.Firstly,the binding energy and defect formation energy of the co-doped system were calculated and analyzed,it was found that defects were more easily formed in the oxygen-rich environment,and all the co-doped systems could be stably existed.Then the electronic structure and optical properties of the co-doped system were calculated and analyzed,it was found that the N element has a significant effect on reducing the band gap,and the increase of Nb concentration also causes the Fermi level of the co-doped system to gradually enter the conduction band and forms an N-type semiconductor with a higher electron concentration,and the transmittance is improved,but the increase of N concentration is disadvantageous for the co-doped system to form an N-type semiconductor.Combined with the comparative analysis of electron mobility and conductivity,it was found that a co-doped system with a Nb doping concentration of 6.3 at.%and N doping concentration of 1.6 at.%can obtain better photoelectric properties.Finally,the energy level arrangement between the intrinsic TiO₂,Nb doped TiO₂,Nb/N co-doped TiO₂,conductive glass FTO,perovskite layer and the hole transport layer were theoretically analyzed by calculating the work functions of intrinsic TiO₂,each doping system and perovskite CH₃NH₃PbI₃.It is found that the content of Nb has a certain regulation effect on the work function and energy level of TiO₂.Meanwhile,the conduction band energy level and work function of the single-doped system and the co-doped system are both reduced,and the barrier between the conduction band and the Fermi level of the FTO is lowered,which makes it possible to serve as a buffer layer between electron transport layer and FTO in order to reduce the recombination of carriers at the interface,increases the extraction efficiency of the carriers,and ultimately improves the photoelectric conversion efficiency of the perovskite solar cells.