Study On Electron Transporting Layer Of TiO₂-Intercalated Graphite Nanosheets In MA_xFA_1-xPbI₃ Perovskite Solar Cells

With the increasingly serious energy crisis and environmental pollution,solar energy as a green energy is deeply concerned.Perovskite solar cell(PSC)has developed rapidly among solar cells due to the advantages of solution processing and low manufacturing cost.The power conversion efficiency(PCE)has increased from initial 3.8%to 24.2%.It is deemed that the formation of a dense electron transporting layer(ETL)with titanium diisopropoxide bis(acetylacetonate)(TiAcAc)degraded at high temperature can improve the conductivity of PSC.CH(NH2)2I(FAI)and CH3NH3I(MAI)have strong absorption coefficient,long carrier diffusion length and tunable band gap in perovskite materials.Therefore,the double-layer dense ETLs are prepared on FTO by one-step spin-coating and chemical deposition from TiAcAc and TiCl4 solution respectively.Then,high quality MAxFA(1-x)Pbl3 perovskite films are prepared by solvent engineering and step-temperature annealing.At last,by spin-coating spiro-OMeTAD as a hole transporting layer and evaporating the Ag electrode,the PSC with excellent photovoltaic properties are prepared in air.The following conclusions can be obtained from the testing analysis of each functional layer and the overall PSC:1.The test analysis of the production from TiAcAc after heating treatment at 500?(TiO2-NG500?)shows that:TG,FTIR and Raman results prove that TiO2-NG500?contains anatase TiO2 and graphite.The images of HRTEM and SEM indicate that TiO2-NG500? has a sheet structure.And the 20 value of X-ray diffraction peak of(002)crystal plane in graphite is smaller because the intercalation of TiO2 nanoparticles increases the interplanar space between the graphite nanosheets,manifesting the nanosheet structure of TiO2 intercalated graphite.2.Comparing the photovoltaic property of MAPbI3 PSC with different dense ETL:The series resistance(Rs)of dense TiO2-NG500?-containing PSC(545.56 ohm)is smaller than the TiO2-containing PSC(900.55 ohm).The PSC containing double-layer dense ETLs of TiO2-NG500? and TiO2 possesses the minimum Rs of 158.11 ohm and the maximum PCE of 10.70%,which is attributed to three points:i.The presence of graphite in TiO2-NG500? strengthens the conductivity and current density;ii.The deposited TiO2 nanoparticles can be filled with TiO2-NG500? film defects and increase the thickness of the double-layer dense ETL which can avoid leakage current;iii.The TiO2 film can effectively extract photogenerated electrons of perovskite layer because its CBM(-3.9 eV)is consistent with MAPbI3 film.And the CBM of TiO2-NG500?(-4.0 eV)locates in TiO2 and FTO,which can reduce the electron transporting barrier.3.Comparison of XRD,FTIR and XPS results of MA,FA(1-x)Pbl3 films annealed at different temperatures:FAI easily forms 8-FAPbI3 inactive perovskite phase at low temperature.While,MAPbI3 decomposes and pure FAPbI3 forms a-FAPbI3 active perovskite phase at 140?.Annealing at 100?,the perovskite film has good compatibility with FA+ and MA+.The addition of FA+ induces the lattice expansion,and the shift of electron bonding energy.The MA0.7FA0.3PbI3 film has the lowest bonding energy.The large hydrogen bond density formed by NH…I among MA+,FA+and PbI6 octahedral sublattice can reduce the bonding energy.These results prove the formation of stable symmetrical active perovskite structure of the MA0.7FA0.3PbI3 film.4.Comparing the surface SEM and AFM images of MAPbI3,MA0.7FA0.3PbI3 and FAPbI3 films annealed at 100?,it can be seen that MA0.7FA0.3PbI3 film has uniform crystal size,good compactness and low roughness(Sa=20.0 nm).In the cross-sectional SEM images of the corresponding PSC,the functional layers in the MA0.7FA0.3PbI3 PSC show tight contact and few internal defects.5.Comparison of steady-state fluorescence emission spectra,UV-vis-NIR absorption spectra and UPS of MAxFA(1-x)PbI3 films annealed at 100?:The fluorescent quenching property and absorption intensity in the visible region light increase initially and decrease afterwards with the content increase of FA+.MA0.7FA0.3PbI3 film has the best fluorescent quenching property and its VBM is 0.05 eV higher than MAPbI3 film.The level distribution of corresponding PSC is more matched than MAPbI3 PSC.6.Comparing the test photovoltaic performance of MAxFA(1-x)Pbl3 PSC with double-layer dense ETLs:The photovoltaic parameters firstly increase and then decrease with the content increase of FA+.The MA0.7FA0.3PbI3 PSC has the best photovoltaic performance with the maximum PCE of 15.15%.Statistically,the PCE of 73.50%PSC exceeds 9%,indicating that the reproducible experimental results.