Stability Of Two-Dimensional Ruddlesden-Popper Perovskite Solar Cells

Organic–inorganic metal halide perovskites are focus of intensive studies in recent years owing to their excellent properties,such as long electron-hole diffusion length,large absorption coefficient,low density of deep electronic trap states,tunable bandgap and room temperature processability.Hence,they hold great promise for the highly efficient devices at low fabrication cost.The efficiency of laboratory-based inorganic-organic perovskite solar cells?PSCs?currently exceeds 24%,endowing them the most promising candidates for next-generation solar cell technology.However,the instability of PSCs due to moisture,light,and heat is still a major limitation towards their commercialization.Recently,two-dimensional Ruddlesden-Popper perovskites?2D-RPPs?have attracted attention due to their superior moisture stability.However,the efficiency of 2D-RPPs solar cells is still far from that of traditional three-dimensional perovskite cells.Therefore,improving the efficiency of the 2D-RPPs soalr cells under the premise of maintaining excellent stability properties is the key problem.In this paper,we mainly focus on the research of 2D-RP perovskites film and device,and devote ourselves to the preparation of high efficiency and stable perovskite solar cells.we first studied the film properties and device properties of the 2D-RPPs based on different spacer cation groups,and selected cyclohexane methylamine?CMA?2D-RPPs as the main research object through comparison.A series of new type 2D-RPPs were prepared and their optical properties and morphology were characterized.Then,we systematically optimized the preparation process of the film,including the change of different n values,the optimization of the solvent of the precursor,the optimization of the time droping antisolvent,the optimization of the film thickness,and the doping of bromine.Unlike previously reported RPPs,based on femtosecond transient spectrum,it is found that the deposited films of?CMA?₂?MA?_n-1Pb_nI_3n+1?MA is CH₃NH₃⁺,n=1,2,3,…?exhibit multiple phases with reverse-graded quantum well?QW?distribution;small n?n=2?RPPs are located at the surface and large n?n?10?RPPs at the bottom.It forms a type-II band alignment between different phases,which favors self-driven charge transport,and the natural structure of graded QWs expand the range of photon collection.Attributed to these properties,we achieved the best efficiency of 15.05%,with high open-circuit voltage(V_oc)of 1.10 V for our first-generation solar cell containing?CMA?₂?MA?₈Pb₉I₂₈.To the best of our knowledge,the PCE?15.05%?obtained for this CMA perovskite,thus far,is among the highest reported values for the MA-based regular 2D-RPP solar cells.The thin films of?CMA?₂?MA?_n-1Pb_nI_3n+1 showed considerable stability under moisture and thermal tests.The unencapsulated device shows significantly improved long-term stability,retaining approximately 95%of the initial efficiency after 5000 h exposure in the ambient conditions under 40%-70%RH.The superior stability of the CMA films and PSCs can be attributed to the hydrophobic property of the organic spacer,as well as the unique phase distribution of the CMA 2D-RPP series.The ligands of outer small n components create a stable barrier that protects water molecules from penetrating and attacking the vulnerable large n components,thus slowing down the degradation of perovskite lattice.Our findings provide inspiration for efficient and stable photovoltaic devices using CMA-based 2D perovskite materials,and would also be appealing for the other applications in optoelectronics.