| Three-dimensional（3D）organic-inorganic halide perovskites（OIHPs）have attracted tremendous interest in photovoltaic applications owing to their exceptional optoelectronic properties.Recently,the recorded highest power conversion efficiency（PCE）of single-junction 3D OIHPs solar cells is up to 25.5%,which is quite competitive with traditional solar cells like Si（26.1%）and Ga As（27.8%）.However,the stability is an essential issue limiting the commercialization of 3D OIHPs.Take 3D FAPb I3 as an example,its photoactive blackα-（cubic）phase is synthesized at high temperature（T=150-185℃）,but it will spontaneously convert into the nonperovskite yellowδ-（hexagonal）phase during the cooling process,making it unstable at the room temperature.To enhance the stability of OIHPs,degrading the dimension is one of the effective and attractive strategies.The well-designed 2D OIHPs present fascinating performances better than their 3D structures,such as enhanced stability,tunable bandgap,higher carrier mobility,longer carrier lifetime,and superior optical absorption.On the other hand,electron spin,as a possible information carrier,is considered to be the most promising candidate for future information technology.The Rashba spin splitting owing to spin-orbit coupling（SOC）effect was first observed by Bychkov and Rashba in 1984,and after that,extensive research has been carried out to find a way to control electron spin using（large）Rashba effect in real materials.Especially,in OIHPs,the Rashba effect can suppress the electron-hole recombination,which leads to long carrier lifetime and diffusion length,and thus accounts for enhanced photovoltaic efficiency in these materials.However,there are still debates about the detailed mechanism of the Rashba effect,especially in the low-dimensional limit.It matters to study the photoelectric properties of 2D OIHPs,which directly determines their photovoltaic performance when they are made into absorption devices of solar cells.Among the photoelectric properties of perovskites,the carrier effective mass and mobility determine the transportation of carriers from the absorption layer to the transportation layer and their collection by electrodes.The absorption coefficients of perovskites determine their performance as a sunlight absorber.Therefore,the transportation and optical properties of perovskites have a vital impact on the PCE of perovskite solar cells.In this work,we carry out a systematically theoretical study on 2D Ruddlesden-Popper（RP）phase ofα-FABX3 perovskites,in a form of FAn+1BnX3n+1,from first-principle using density functional theory（DFT）calculations.Our study reveals the physics behind the experimental observations that the stability of 2D FAX-terminatedα-FABX3 perovskites is enhanced as decreasing the number of layers.Furthermore,the Rashba spin splitting of 2Dα-FABX3 perovskites presents a significant modulation with the number of layers,with large Rashba parameters in a wide range of 0.54-1.58 e V·（?）.Additionally,our results show that the few-layer 2Dα-FABX3（1-4 layers）perovskites have a widely tunable range of bandgaps,from 0.99 to 3.03 e V,and thus quite promising in the application of both high-efficiency single-junction and tandem solar cells.Moreover,2Dα-FASn Br3（3 layer）andα-FAPb I3（1-3 layer）perovskite possess high carrier mobility(10~2～10~3 cm~2V-1s-1)and quite large absorption coefficient in the visible region(>10~5cm-1),which means they are excellent absorber for perovskite solar cells.Our studies show that 2Dα-FABX3 perovskites have better stability,more excellent optical properties and wider application prospects,compared with their 3D structure. |
