Tin-based Oxide Electron Transport Materials In Mesoscopic Perovskite Solar Cells

The power conversion efficiency(PCE)of Perovskite solar cells(PSCs)based on the titanium dioxide(TiO2)electron transport layer has been improved to 24.2%in a decade.The mostly conventional configuration of PSCs is n-i-p stack,which includes electron transport layer(ETL)and hole transport layer(HTL)to extract electrons and holes.Generally,the carrier mobility and extraction efficiency of HTL or ETL play a decisive role on PSCs performance.In recent years,Han Hongwei group has developed a triple mesoporous perovskite solar cells(TMPSCs)based on mesoporous TiO2,zirconia(ZrO2)and porous carbon electrodes,which abandons the expensive HTL and gold counter electrode,but retains the essential electron transport layer.Among the PSCs’ structures,TiO2 is the most commonly used as ETL,but it exhibits low electron mobility and photocatalytic activity,which causes the decomposition of perovskite and seriously affects long-term stability.The wide band gap semiconductor tin-oxide(SnO2)shows a faster electron mobility than that of TiO2,suitable band structure and non-photocatalytic activity,making it a conceptually candidate as an electron transport material for PSCs.The optical and electronic properties of ternary metal oxides can be modified by adjusting the stoichiometric ratio of components or elements doping.The zinc stannate(Zn2SnO4)and barium stannate(BaSnO3)express faster electron mobility and suitable band structure,which are coincident with the conditions of ETL.In this thesis,we focused on fabricating the electron transport materials to replace TiO2 in TMPSCs.The Nb-doped SnO2 was introduced to enhance the performance of TMPSCs,and the ternary metal oxides Zn2SnO4 and BaSnO3 were firstly fabricated as mesoporous layer in this structure.The mainly research works are summarized as follows:(1)The TMPSCs based on the SnO2 dense layer and the carbon counter electrode were prepared.The cells were assembled by SnO2 compact layer,TiO2 mesoporous layer,ZrO2 mesoporous layer and carbon counter electrode from bottom to top.The SnO2 compact layer was fabricated by spin-coating method.The mesoporous layer was prepared through screen print method,such as m-TiO2,m-ZrO2 and m-C.The perovskite,MAPbI3,was infiltrated into the mesoporous TiO2/ZrO2/C triple scaffold by drop-casting a precursor solution that contained PbI2,MAI in γ-butyrolactone(GBL).The visible light transmittance of FTO that covered with a SnO2 compact layer was measured by Ultraviolet-Visible Spectrophotometer(UV-Vis).In addition,the crystal type and surface morphology of SnO2 compact layer were examined by X-ray Diffraction(XRD)and Field-emission Scanning Electron Microscope(FESEM).The homogeneous SnO2 compact layer was assembled into TMPSCs,achieving the PCE of 11.7%.(2)The niobium(Nb)doped SnO2 compact layer was prepared by spin-coating the precursor solution that contained Nb5+.The effect of SnO2 compact layer on the performance of TMPSCs under different Nb doping concentrations was investigated.The films characteristic under different concentrations were measured by XRD,SEM and UV-vis.Furthermore,the surface roughness and 3D morphology were measured by Atomic Force Microscopy(AFM).Finally,the distribution of elements of Sn,O and Nb were obtained by Energy Dispersive X-ray spectroscopy(EDS).The results showed that the open circuit voltage and PCE of TMPSCs under 2%Nb doping condition were significantly enhanced to 0.92 V and 13.5%,which increased by 15.4%compared with the undoped group.The short circuit current and fill factor are 24.13 mA/cm2 and 0.609,respectively.(3)The ternary metal oxide of Zn2SnO4 and BaSnO3 nanoparticles were successfully synthesized by hydrothermal method and coprecipitation method.In this experiment,we investigated that the pH caused the phase separation of Zn2SnO4 in the hydrothermal process.The Zn2SnO4 and BaSnO3 paste were obtained by ball milling.Then,the TMPSCs were assembled by screen-printing in which the TiO2 mesoporous ETL was replaced with Zn2SnO4 and BaSnO3 paste,respectively.The TMPSCs based on BaSnO3 ELT achieved the PCE exceeding 5%,and open circuit voltage reached 0.872 V;but the performance of cells based on Zn2SnO4 ETL showed lower PCE and open circuit voltage as 4.07%and 0.84 V.hese two ternary metal oxides exhibited a large application prospect with the simple synthesis method,the high yielding and the excellent electronic properties.