| Hybrid organic-inorganic halide perovskites have witnessed a spectacular surge in scientific interest owing to the excellent power conversion efficiency?PCE?of perovskite solar cells?PSCs?,which stands at 23.3% nowadays,starting from 3.8% as reported by T.Miyasaka in 2009.Although the lead halide perovskite materials?CH3NH3PbI3?exhibit great optical and electrical properties,such as desirable band gap,high absorption coefficient,low exciton binding energy,ultralong charge carrier diffusion length,and unusual defect tolerance.Although great achievements for perovskites solar cells have been made in the last ten years,there are still some issues should be addressed when considering their future commercial applications.On one hand,CH3NH3PbI3 is relatively not stable because a decomposition reaction occurs drastically at a high temperature about 425 K or takes a slow process at room temperature,where CH3NH3PbI3 decomposes into a solid?PbI2?and two gases?CH3NH2 and HI?.On the other hand,environmental problems drag on the further development of halide perovskites solar cells based on CH3NH3PbI3 for the practical employment in daily life due to the heavy element,lead.It is imperative to find alternative absorber materials for CH3NH3PbI3 and to design a matching device structure.In the complicated material system,an effective method is to use computer simulation technology to summarize the existing material properties,and to screen the crystal library according to the property of the material,and to predict the performance of the device.Then,we can offers a guideline to the researchers.In this paper,perovskite solar cells are used as research objects,and a large number of theoretical studies are carried out using computer technology,including optical properties,electrical properties,stability,defect properties and simulated device performance.The main contents are as follows:1.A series of halide perovskites ABX3,where A can be CH3NH3,CH?NH2?2,Cs,or Rb;B and X belong to the same main group of lead and iodine in the periodic table of the elements respectively?B = Pb,Sn,Ge;X = I,Br,Cl,F?,are investigated by using DFT calculations,accompanying with Shockley-Queisser Maximum Solar Cell Efficiency?S-Q?and Spectroscopic Limited Maximum Efficiency?SLME?mathematical models.A systematical investigation of the optical and electrical properties indicated that the electronic structure of germanium perovskites bears a close similarity to that of lead perovskites with a small energy difference between the nonbonding orbital and antibonding orbital,but with a large energy difference comparing with that of tin perovskites?0.61.7 e V for Cs Ge I3 at Z point of the Brillouin zone,0.71.4 e V for CH3NH3PbI3 and 1.42.2 e V for CH3NH3SnI3 at R point of the Brillouin zone?.The germanium perovskites possess as high absorption coefficient around solar spectrum as lead perovskites,while tin perovskites only have a low absorption coefficient,which makes the short-circuit current of CsGeI3 and CH3NH3PbI3(0.029 Acm-2 and 0.027 Acm-2,simulated by SLME with a 200 nm absorber under AM1.5G)higher than that of CH3NH3SnI3(0.025 Acm-2)although the bandgap of Cs Ge I3 and CH3NH3PbI3?1.51 e V and 1.55 e V?are larger than that of CH3NH3 Sn I3?1.21 e V?.The effective mass of electrons and holes are approximate for germanium perovskites and lead perovskites?0.14:0.19 for Cs Ge I3 and 0.12:0.12 for CH3NH3PbI3?,indicating a balanced electrons and holes transport,whereas the electrons transport is much slower than the holes transport because tin perovskites due to the effective mass of electron is much larger than that of the holes?0.17:0.04 for CH3NH3 Sn I3?.As a result,the PCE of Cs Ge I3?27.9%?and CH3NH3PbI3?26.7%?is higher than that of CH3NH3 Sn I3?19.9%?.2.A series of 0D,1D,2D and 3D hybrid lead iodide perovskite homologous semiconductors is selected from the Cambridge Crystallographic Data Centre?CCDC?,and all of them are the most typical and common ones,such as PbI6?CH3NH3?4?H2O?2?FOL?with 0D structure,Pb I3?C4H4N2?CH3?BAT?,C9N2H17Pb I3?NUG?,CH3?CH2?2NH=C?CH3?2PbI3?QEX?and?NH2?2CIPbI5?YUV?with 1D structure,NH3?CH2?6NH3PbI4?WOG?,NH3?CH2?12NH3PbI4?AKO?,?NH3?NH2?2OH?2PbI4?NIC?and?CH3NH3??CH3?CH2?4NH3?2Pb2I7?UMU?with 2D structure and Cu?NH2CH2CH2NH2?2Pb2I6?XOC?and CH3NH3PbI3?3D-MAP?with 3D structure.The electrical property,optical property and stability of them have been investigated based on their electronic structures.The theoretical PCEs estimated by Spectroscopic Limited Maximum Efficiency?SLME?mathematical models were 1.9%10.9% for 1D perovskites homologous and 4.4%6.9% for 2D perovskites homologous according to their absorption spectra.However,the charge transport properties of 1D and 2D perovskites homologous display a big difference.The voltage current characteristic of polycrystalline model shows that 2D perovskites homologous maintain 43% of the electric conductivity of 3D-MAP,but the electric conductivity of 1D?both ? and ? type?perovskite homologous decreases to 0% of that of 3D-MAP.There are more charge transport paths in polycrystalline 2D perovskites homologous than those in polycrystalline 1D perovskites homologous.Our results provide reasonable explanations to the fact that 2D perovskites homologous are more easy to be fabricated as solar cells compared with 1D perovskites homologous under the traditional preparation processes?spin-coating?of PSCs.3.The electrical property and defect property of multilayer two dimensional?2D?Ruddlesden-Popper perovskites and the reference 3D perovskite have recently been researched by the density functional theory?DFT?.The calculation results indicated that there is a certain similarity between the properties of 2D perovskites and 3D perovskites,such as the composition of orbital,the distribution of transition energy levels,the distribution of formation energy of defects,the defect-tolerant property and the adjustability of semiconductor n-p type.In addition,2D perovskites have their own characteristics,as some outside electron donor defects or hole acceptor defects?Ii and VI?have deeper transition energy levels in 2D perovskites,which may reduce the carrier concentration.Our findings have great realistic significance to understand the defect property of 2D perovskite.4.An improved Spectroscopic Limited Practical Efficiency?SLPE?model was developed by replacing the two input parameters and equivalent ideal diode circuit of SLME.In SLPE,the light source spectrum AM1.5G has been replaced by the light source spectrum of solar simulator used in the devices property measurement,and the absorption spectrum estimated by DFT has been replaced by the experimental absorption spectrum measured by ellipsometry.A more complicated equivalent circuit with a diode and resistances has been utilized in SLPE and the introduced parameters can be worked out by fitting experimental I-V data.In order to check the rationality of SLPE,we fitted 5597 sets of I-V data for 26 different types of hybrid lead iodide perovskite devices we fabricated previously.Most of I-V data are in good agreement with fitting curves?r NEW>99%?,and then we analyzed the output parameters,current coefficient f1,voltage coefficient f2,diode ideality factor n,series resistance RS and shunt resistance RSH,for different types of device structures.We found RSH was close to the ideal value followed by f2,n and f1,but RS varied dramatically for our devices involved with the carrier mobility of hole transport layers?HTL?.For HTL with carrier mobility less than or equal to 10-5cm2V-1s-1, 10-4cm2V-1s-1 and 10-3cm2V-1s-1 respectively,Rs is larger than 103 ?,102-103 ? and 101-102 ?,which makes the devices loss more than 85%,20%-85% and less than 20% efficiency.Consequently,the work offers a guideline to the researchers for optimizing perovskite solar cells and ultimately approaching to the theoretical PCEs estimated by SLME. |
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