Research On Structure Design Of Perovskite Homojunction Solar Cells

Perovskite solar cells have developed rapidly in recent years and are on the verge of commercialization.The common perovskite p-i-n structure cell is limited by the problem of carrier recombination.Perovskite homojunction solar cells introduce p-n junction by the defect characteristics of the perovskite,which has the advantage of promoting carrier separation and oriented transport.In this paper,two researches have been carried out on the structure design of perovskite p-n homojunction.One is to introduce exogenous doping of metal ions into the preparation process of perovskite p-n homojunction for adjusting the conductivity type.The second is to find the parameter optimization strategy of n-type layer and p-type layer for improving perovskite homojunction performance.Based on these two aspects of work,the main research results are as follows:(1)The exogenous doping of Bi is used to achieve n-type conductivity of perovskite films and prepare perovskite p-n homojunction solar cells.MAPbI3 films with different doping proportions of Bi were prepared by a one-step spin-coating method.Bi presents the Bi3+valence state,and the replacement of Bi3+to Pb2+will provide more valence electrons.Kelvin force microscope(KPFM)shows that the surface potentials of 5 mol%/15 mol%Bi-doped films were-0.275 V/-0.296 V,compared to that of 0.091 V for p-type films.At the same time,valence band spectrum of X-ray photoelectron spectroscopy(XPS)shows that the Fermi level of 10 mol%Bi-doped film is 1.067 eV away from the valence band,which exceeds 0.790 eV of band gap midpoint.These verify the n-type conductivity of Bi-doped MAPbI3.However,more than 15 mol%Bi-doped MAPbI3 will have the problem of p-type compensation intrinsic defects(including deep-level defects),which weakens n-type conductivity.Finally,combining the n-type Bi exogenous doping and the p-type self-doping,perovskite p-n homojunction solar cells were prepared.Owing to the built-in electric field,the cell efficiency is 18.52%when doped with 10 mol%Bi,while that of the control p-i-n cell is 17.26%.(2)Combining theoretical analysis with wxAMPS simulation,a coordinated adjustment strategy for doping concentration and thickness of the perovskite homojunction is proposed and demonstrated,with the rules for selecting the upper and lower limits of thickness given.The calculation formula of space charge region shows that the doping concentration of perovskite homojunction restricts the selection of its thickness.There are two types of depletion regions coming from the p-n homojunction and the induction of two carrier transport layers,xn/xp and xext-n/xext-p,respectively.The lower limit for coordinated adjustment thickness adopts xn+xext-n and xp+xext-p,to ensure the integrality of space charge regions.The upper limit of coordinated adjustment thickness is min {carrier diffusion length,perovskite absorption depth}.Finally,wxAMPS software was used to simulate three types of device with the doping concentration of 1 ×1018 cm-3 and the thickness much lower than/equal to/exceeding the lower limit,whose efficiencies are 11.30%,17.28%,and 15.02%,respectively.The distribution of electric field and carrier verified the validity of the rules for selecting the upper and lower limits of thickness in the coordinated adjustment strategy.Related research of Bi doping proves that both exogenous doping and self-doping can realize the preparation of perovskite p-n homojunction solar cells.In the future,more kinds of exogenous dopants can be used to prepare perovskite p-n homojunction,expanding the development direction.Meanwhile,the coordinated adjustment strategy for perovskite p-n homojunction utilizes the relationship between the parameters of perovskite layer,instead of optimizing different parameters in isolation,which is more instructive for improving the preparation process of perovskite homojunction cells.