Quantum Transport Properties Of Organic-inorganic Hybrid Halide Perovskites

The organic-inorganic hybrid halide perovskites(OIHHP)with the chemical formula ABX3(A=organic/inorganic monovalent cation,B=divalent metal cation,C=halide anion),are emerging as the most potential absorber materials for solar cells,due to the rapidly increasing power conversion efficiency and their unique properties,such as tunable bandgap,high absorption coefficient,shallow-level defects,electric-benign grain boundary,long lifetime and diffusion length of earriers.They have been attracted widely attention,especially after the report of solid-state OIHHP-based solar cells.At present,the ground state properties of OIHHP,such as structure,electrical,optical,and defects,have been studied theoretically using density functional theory,which provides a powerful guide for experimental research and understanding of the high performance of OIHHP-based solar cells.However,compared with the diversity and flexibility of OIHHP,the existing research results are still rare.The spin-orbit coupling(SOC)effect is closely related to heavy metal elements(such as Pb and Sn),and exists in the semiconductors containing heavy metal elements,and does not change with the computational methods.In the Pb-based OIHHP material,the SOC effect plays a very important role in the electrical properties,causing band degeneracy,and the bandgap reducing by about 1 eV At the same time,the photoelectric effect in OIHHP can efficiently convert light into electrical energy,making OIHHP a candidate for the third-generation solar cell.However,the analysis of SOC effect mostly focuses on the electrical properties,and the photovoltaic effect is mainly analyzed from the optical properties,which have been studied in experiments.In terms of materials science,the mechanisms of SOC and photoelectric effects in OIHHP devices is still poor understood.In this paper,the OIHHP-based model is constructed,and the influence of SOC and photoelectric effect of OIHHP-based device performance is calculated and analyzed from the feature of quantum transport.In the aspect of SOC effect,the band structure and density of states of MAPbI3 are firstly calculated,and the role of SOC effect in the electrical properties is verified.Secondly,the two-probe quantum transport model of Ag-MAPbI3-Ag is constructed.Under the normal operation of the device,the SOC effects in the equilibrium states and non-equilibrium states at different bias voltages are compared and analyzed.In the analysis of carriers’ transport,three polarization modes are considered:no spin polarization(NSP),noncolinear polarization excluding SOC(NCSP-SOC),and noncolinear polarization including SOC effect(NCSP+SOC),and the density of states,charge distribution,electron transmission coefficient and volt-ampere characteristics of Ag-MAPbI3-Ag system in equilibrium and non-equilibrium states are calculated to analyze the SOC effect in the non-equilibrium quantum transport characteristics of the MAPbI3 based device,using the non-equilibrium Green’s function method(NEGF-DFT)combined with density functional theory.The calculated J-V eurve shows that the current density of NCSP+SOC is about one order of magnitude higher than that of NSP,while NCSP-SOC and NSP have the same value.Moreover,the voltage drops along the transport direction of NCSP-SOC and NSP are completely coincident,and the difference between NCSP+SOC and NSP lies the central scattering region,which implies that the SOC effect changes the potential distribution of the device and causes an enhancement in the non-equilibrium quantum transport characteristics.By combining the density of state,charge density distribution and electron transmission coefficient of the system,it is known that the increase in current density of NCSP+SOC results from the increase of the non-equilibrium transmission coefficient in the bias window and the electron density in the conduction band contributed by the p-orbital of Pb.The above results indicate that the SOC effect is a critical factor affecting the non-equilibrium transport properties of MAPbb devices,and is essential for understanding the non-equilibrium quantum transport properties of OIHHP devices.In the research of photovoltaic effect,firstly,the band structure,density of states,dielectric function and absorption coefficient of MAPbI3 are calculated,and the high absorption coefficient of MAPbI3 is verified.Then,based on the above-mentioned Ag-MAPbI3-Ag model,we construct another two-probe quantum transport model of Au-MAPbI3-Au.After replacing the electrodes,the lattice matching rate between the electrode and MAPbI3 is greatly improved,and the charge accumulation at the interface between MAPbI3 and Au is alleviated.In this model,the standard AM1.5G solar spectrum is illuminated on MAPbI3 in the central scattering region,and the photocurrent is generated as a result of electron and hole transport along the z direction,which is affected by incident light(bias voltage,polarization direction,incident angle),MA cation orientation and medium thickness of the MAPbI3.When the MA cation is oriented along the transport direction,the short-circuit photocurrent density Jsc is 13.86 mA/cm2,comparable to previous experimental data.According to the J-V curve,the power conversion efficiency η of the device is about 13.18%,and the fill factor(FF)is about 0.26.Jsc still has a respectable value of 12.07 mA/cm2 even for a 90-degree incident angle relative to the transport direction.However,when the MA cation orientation is perpendicular to the transport direction,Jsc is almost negligible,and can be increased when a positive or negative bias voltage is applied.Moreover,by analyzing the photocurrent density as a function of incident angle at the equilibrium and non-equilibrium state,relatively large photocurrent is obtained when the incident angle is in the range of 0 to 30 degrees.In addition,the electron transmission coefficient and the density of states of the Au-MAPbI3-Au system are calculated,and the increase in the photocurrent may be attributed to an improvement in carries transport.Further,the preferred thickness of the MAPbI3-based solar cell is about 100-500 nm,using optical method through absorption coefficient and refractive index.The results show that the orientation of MA cations significantly affects the photovoltaic properties,which is the key to obtain high-performance of OIHHP-based solar cells,and provides powerful theoretical guide for the design of OIHHP-based solar cells and other optoelectronic devices.The above calculations reveal the important role of the SOC effect in the non-equilibrium transport properties of OIHHP-based devices containing heavy metal elements,and the photovoltaic effects closely related to photon energy,light polarization angle,incident angle,organic cation orientation,and thickness.To a certain extent,it fills the blank in the theoretical research of OIHHP-based devices,provides a guidance for the experimental design and insightful understanding of the high-performance of OIHHP-based devices,and lay a solid foundation for subsequent research on OIHHP-based devices(such as interface effects,ETM/HTM’s selection).