Theoretical Research On The Photovoltaic Performance Of Organic-Inorganic Hybrid Perovskite

Owing to low cost,good stability and easy fabrication process,the perovskite solar cells should be able to compete with the conventional silicon-based solar cells,and have become a new direction for developing highly efficient solar cells.Organic-inorganic hybrid perovskite(such as CH3NH3PbI3 and CH3NH3PbI3-xClx)has been utilized as the light-absorbing materials in solar cells,due to excellent performance for light harvesting and carrier transport.The power conversion efficiency(PCE)of perovskite solar cells has rapidly increased from 3.8%to 23.3%.Despite the rapid progress in cell performance,the theoretical research of some puzzling phenomena,such as giant dielectric constant(GDC),anomalous hysteresis,has not yet been reported.Research suggests that the room-temperature CH3NH3PbI3 has no typical crystal structure,due to the geometrical arrangement of CH3NH3+ being even unknown in detail.Thus the periodic boundary condition(PBC)in density functional theory(DFT)calculations is not applicable to disordered CH3NH3+ at room temperature.In this paper,the characteristics of CH3NH3+in CH3NH3PbI3 are systematically studied,and the scientific theoretical explanation for GDC and anomalous hysteresis loop of hybrid perovskite is given.The effects of perovskite materials,transport materials and device structures on the performance of solar cells were calculated by numerical simulation and optimization.The main contents and findings of this paper are summarized as follows:1.According to the dielectric measurement of CH3NH3PbI3,CH3NH3+ decoupled with inorganic framework is activated and disordered at room temperature,which had been put forward after theoretic analysis.For no electron injection case,the dielectric constant of CH3NH3PbI3 at room temperature under low frequency has no voltage amplitude and frequency dependence.For electron injection case,it is found that the dielectric constants in a structure of Au/CH3NH3PbI3/PEDOT:PSS/ITO depend on the applied voltage amplitude and frequency.2.Base on the first-principle calculations,the calculation indicates that the strong polarization of CH3NH3PbI3 induced by electron injection is mainly caused by the relative displacement between positive charge of Au and negative charge of[PbI3]-,and the polarization field has gradient characteristics.The calculation results show that the larger amplitude of applied voltage,it takes shorter time for electron transfer from CH3NH3 to CH3NH3+,and the longer duration of applied voltage,the more electrons are injected into the CH3NH3PbI3,and the more layers of CH3NH3 are formed.The strong polarization of CH3NH3PbI3 induced by the electron hopping from CH3NH3 to CH3NH3+ is completely different from that of traditional ferroelectric material,and may be the origin of GDC and anomalous hysteresis.3.From first-principle calculations,the CH3NH3+ properties at TiO2/CH3NH3PbI3 heterojunction are systemistically investigated.The theoretical calculation shows that the TiO2/CH3NH3PbI3 heterojunction has three potential advantages.Firstly,the channel separation of carrier transport and diffusion electrons can reduce the interference between organic and inorganic moieties and improve the efficiency of carrier transport.Secondly,the CH3NH3 orientational order induced by strong polarization maintains the strength of built-in electric field and the sustained carrier separation.Lastly,due to no hole consumption in the process of forming built-in electric field,the CH3NH3PbI3 at the heterojunction provides more free carriers than traditional p-n junction.These results can explain the high efficiency for CH3NH3PbI3 solar cells.4.Three types of architectures for perovskite solar cells are simulated with the AMPS-1D software.The simulation demonstrates that the HTM-free FTO/ZnO/CH3NH3PbX3(X = I,Br)/Au solar cells attain a high PCE of 24.1%.Simulation results for the FTO/ZnO/CH3NH3PbX3(X = I,Br)/CuSCN/Au solar cells reveal that:(ⅰ)Adjusting the thickness of perovskite material can optimize the device performance,and an optimal thickness 150 nm for CH3NH3PbI3 solar cell is close to the electron difusion length 129 nm.(ⅱ)Reducing the defect density in perovskite material and interface trap density can effectively enhance the performance of solar cells.It was found that there is a synchronous changing relationship between lifetime of minority carrier and performance of solar cells when the defect density decreases.(ⅲ)Further improvements of carrier mobility in electron and hole transport material cannot improve the PCE of solar cell when the hole mobility in CuSCN reaches 0.01 cm2·V-1·s-1 and electron mobility in ZnO arrives 0.1 cm2·V-x·s-1.Surprisely,we predict that a maximum PCE of 26.60%for MAPbI3 solar cell and a maximum open-circuit voltage(VOC)of 1.83 V for CH3NH3PbBr3 solar cell are arrived by optimizing parameters.Compared with the two devices,CH3NH3PbI3 and CH3NH3PbBr3 solar cells have higher PCE and VOC,respectively.The FTO/ZnO/CH3NH3PbI3/CH3NH3PbBr3/CuSCN/Au solar cell with double light absorbers was first designed and simulated,and exhibits an extraordinary characteristic of tunable PCE and VOC by changing the thicknesses of CH3NH3PbI3 and CH3NH3PbBr3.The simulation results shows the effects of architectures and material properties on the cell performanc,and can provide some guidance for an optimal design of high-performance and low-cost perovskite solar cells.