Carrier Transport And Lead Replacment In Organometal Halide Perovskite Solar Cell Materials

Organometal halide perovskite materials have shown great potentials in solar cells applications due to their unique optical and electronic properties and low cost with simple synthetic methods.However,it is still necessary to optimize the perovskite solar cells in both of long term stability and environmental friendliness,to design new high-efficiency and stable organometal halide perovskite materials for the mass commercial applications.In this paper,we mainly focused on three aspects of the perovskite materials: carriers transport mechanism,lead replacement and stability.By using the first-principles calculation,we firstly calculated the electronic structure of the MAPb I3 perovskite,studied the internal mechanism of carrier transport in the MAPb I3 perovskite,and tried to find out the direct influence of Cl doping to the carrier transport in MAPb I3 perovskite.Then we compared the optical and electrical properties of perovskite materials with different metal atoms in them and discussed the fleaxible solutions for lead replacement.Then,we improved the oxidation resistance of a typical lead-free perovskite MASnI3 by introducing the concept of “superatom” clusters into it.At last,we further expanded the application range of “superatom” clusters to the carbon-based hydrogen storage materials.The main contents and results are as follows:?1?By using first-principles calculations,we studied the internal mechanism of carriers transport in MAPb I3 perovskite and the influence of Cl doping to it.It shows that Cl doping could enhance the electron localization around it to decrease the carrier mobility;and it also increase the lattice inconformity in MAPb I3-xClx perovskite and therefore the lattice scattering effects to the carrier transport.The lattice of MAPbI3-xClx perovskite will be compressed along with the increasing of the Cl doping concentration.This could suppress the orientation disorder of MA molecule and increase the lattice polarization to improve the carrier mobility;meanwhile,it will also introduce a small splitting around the band extremes and turn the MAPb I3-xClx perovskite to an indirect bandgap semiconductor,and therefore decrease the carrier recombination rate.Because of the joint effect of all these mechanisms,the carrier lifetime and diffusion lengths in MAPb I3 perovskite can be improved by Cl doping.?2?We investigated four types of MABI₃?B = Pb,Sn,Ge and Sr?perovskite materials and attempt to understand the structural and electronic influences of the metal atoms on the properties of perovskite in order to find a solution for Pb replacement.It is found that,?a?the ionic radius should be close to Pb2+ to keep the Goldschmidt factor of the perovskite lattice around 0.92.Otherwise,it will cause a lattice distortion and increase the anti-orbital interaction between B2+ and I-,which will increase the bandgap and electron localization,and therefore lower the optical absorption and carrier transport performance.?b?A metal ion with a full d shell could be helpful for better optical and electrical properties.Therefore,the solutions to the Pb replacement might be metal or metallic clusters that have an ionic radius close to Pb2+ and outer ns2?n-1?d10 electrons configuration on the metal ions.It seems that Sn2+ may be a good choice for Pb2+ ion replacement.Therefore,much work need to be done in the long-term stablitly optimization of MASnI3 perovskite.?3?In order to improve the oxidation resistance of lead-free MASnI3 perovskite for a better long-term stability,we introduced the concept of “superatom” to the perovskite materials,and proposed the new idea which is to using the high-electron-affinity “superhalogen” BH4/AlH4 to seize the “excessive” electrons on Sn2+,and therefore to improve the oxidation resistance of Sn2+ in the MASnI₃ perovskite.It is shown that,the oxidation resistance of reactant SnX2 can be enhanced along with the increasing of electron affinity of X ion.The new superhalogen perovskite materials MASnI2BH4 and MASn I2AlH4 also show good stability and better oxidation resistance than MASnI3 perovskite.Meantime,high carrier mobility is still remained for these superhalide perovskites with a slight decrease in optical absorption strength.By carefully controlling the superhalide concentration in perovskites,a superhalide perovskite with high performance and reproducibility can be expected.?4?As a part of this paper,we expanded the application of “superatom” clusters to the hydrogen storage field,and proposed a new idea to enhance the H2 binding affinity for carbon-based hydrogen sorbents C60 by coating with superalkali Li2 F cluster.It is found that,because of the low ionization potential of Li2 F cluster,there will be a large electron redistribution between hosts and H2 molecule which enhances the electrostatic and orbital interaction between Li2 F cluster and H2 molecule and therefore improve the hydrogen storage capacity.Eventually the gravimetric hydrogen capacity of C60?Li2F?12 cluster can go up to 10.86 wt% with an average binding energy of 0.12 eV/H2,much close to the room-temperature hydrogen storage strardands 2017 made by U.S.DOE Office.This research provide a new way to enhance the interaction between hydrogen and hosts,and could be beneficial for the development of carbon-based hydrogen storage materials.