Iodide Management And Fermi Level Tuning Of CH₃NH₃PbI₃ Polycrystalline Films For Perovskite Solar Cells

Perovskite（such as CH₃NH₃PbI₃）solar cells over the past years have attracted considerable attention because of the low-cost fabrication methods and their unique optoelectronic properties,such as high absorption coefficients,ambipolar charge transport and long charge carrier diffusion lengths.Generally,CH₃NH₃PbI₃-based solar cells operate as n-i-p junctions,with CH₃NH₃PbI₃ as the intrinsic（i）film.Moreover,CH₃NH₃PbI₃ with both p-type and n-type conductivity can be achieved by changing the rations of methylammonium halide（MAI）and lead iodide（PbI2）.There are different point defects in p-type and n-type conductivity CH₃NH₃PbI₃ films.Some theoretical calculations have shown that point defects of iodine vacancies（VI）are the most likely defects in CH₃NH₃PbI₃films because of their low formation energy,which may lead to CH₃NH₃PbI₃films with the I:Pb ratio less than stoichiometric 3:1.And we know that vacancy defects create shallow electronic levels,which reduces charge carrier diffusion lengths and charge-carrier lifetimes.And it can observe ion migration,which can corrode metal electrode,leading to reduce the power conversion efficiency（PCE）and the stability of CH₃NH₃PbI₃-based solar cells.So it is important to manage Vi and change the Fermi level of CH₃NH₃PbI₃.In this paper,we propose a controllable strategy to introduce molecular I2 into CH₃NH₃PbI₃,in which molecular I2 in anti-solvent diethyl ether is applied as the iodine source.We fabricate the I:Pb ratio in CH₃NH₃PbI₃ films with 2.90,partly filling Vi of CH₃NH₃PbI₃ films.After iodine manipulation treatment,steady-state and time-resolved photoluminescence measurements indicate that iodine-modified CH₃NH₃PbI₃ possesses reduced defect density and enhanced charge-carrier lifetimes;the best PCE of CH₃NH₃PbI₃-based solar cells is 17.7%;a 120 meV down-shift of Fermi level as compared to the pristine CH₃NH₃PbI₃ films.By management of Vi in CH₃NH₃PbI₃ films,we obtain CH₃NH₃PbI₃ films with less defect density,which can offer a method to fabricate the high efficient and stable perovskite solar cell;we achieve the Fermi level tuning of CH₃NH₃PbI₃ films and help the understanding of the electronic properties of perovskite,which can shed light on future rational design of more efficient perovskite solar cells and meet the needs of different applications,such as p-n or Schottky devices.