The Control Of Trap States In SnO₂ Electron Transport Layer For Perovskite Solar Cells

As one of most researched electron transport layer?ETL?materials,SnO₂ has many merites.However,suffered for the problems such as low actual electron mobility and serious energy level offset,so the power conversion efficiency?PCE?of perovskite solar cell?PSC?hasn't gotten a breaking improvement yet.Essentially,these questions mainly stem from the too high trap states density on the surface of SnO₂.To deal with this key question,the trap states inner SnO₂ ETLs are controlled innovatively,and two deepgoing researches are shown as followed.First one,considering the nonstoichiometric character of real SnO₂,the components of SnO_x ETL are successfully tuned by controlling the annealing atmosphere.The density of oxygen vacancy in SnO_x is decreased with the rising of oxygen/nitrogen ratio.The x of SnO_x is 1.84 after annealed in pure oxygen folw,while just 1.64 in pure nitrogen flow,determined by X-ray photoelectron spectra.Along with the increase of x,the electron mobility of SnO_x is increased,while the energy offset is narrowed,thus the short circuit current density(J_sc),open circuit voltage(V_oc),fill factor?FF?as well as PCE of the PSCs are increased.The champion PSC is based on SnO_x ETL that annealed in pure oxygen,with a PCE of20.46%.Second one,the photoelectric properties of SnO₂ ETL is improved by the adopting of phosphoric acid,thus elevate the performance of PSC.The phosphorus is mainly localized at the crystal boundaries of SnO₂ nanocrystals in the form of polyphosphate after annealed.The polyphosphate is bonded with the tin ions on the surface of SnO₂,as the Sn dangling bonds are partially eliminated,thus the electron mobility and tansmmitance of SnO₂ thin film along with the stability of perovskite are improved.The augment of PSC performance mainly come from the increase of J_sc and FF,which stem from the passivation of SnO₂ boundaries by polyphosphate.For the PSC,the optimal concentration of phosphoric acid in SnO₂ precuser is 7.4 at%and the champion PSC has a PCE 21.02%.In summary,this thesis not only contributes to the research of PSCs in the deriction of controlling trap states inner SnO₂ ETLs,but also provides enlightenments for the similar researches in the field of photoelectric devices.