Study Of Defect Passivation And Thin Film Modification In Inverted Perovskite Solar Cells Using Ionic Liquids

Developing high-efficiency photoelectric conversion devices is an important way out to face the energy challenges.The organometallic halide-based perovskite solar cell has the advantages of high efficiency,low cost,and easy to prepare,which have been extensively investigated in the past few years.In addition,perovskite materials with very high absorption coefficient can cover wide range of absorption by preparing thin film layer.Moreover,the perovskite materials are capable of matching the electron transport layer and hole transport material as a result of its adjustable bandgap.Currently,most of the perovskite solar cells with PCE over 23%are prepared in NIP configuration,which needs intricate processes of fabrication,annealing in high temperature and doping with metal.It is not suitable to deposit in a flexible substrate.Furthermore,a solar cell with NIP configuration is more likely to suffer from hysteresis and it hinders practical applications.Whereas PIN configuration,which is regarded as a promising structure,can suppress the hysteresis and be used in flexible devices.However,defects in perovskite solar cells have negative impact on achieving its high efficiency and stability.Therefore,the most significant process of perovskite devices is to fabricate the absorb layers with less defects and the cost-effective hole transport materials.This paper is going to study a typical solar cell with PIN configuration.We lay more emphasis on how ionic liquid affects the passivation process and photoelectric properties and finding some feasible strategies.The research can be divided into the following three aspects:1.To begin with,the experiment is carried out in a typical solar cell device with PIN configuration(ITO/PEDOT:PSS/MAPb I_3-xCl_x/PCBM/BCP/Ag).We introduce different kind of detailed analysis by optimizing the preparation method,characterization instrument and data analysis.FESEM,ultraviolet-visible light spectroscopy,XRD and other common characterizations were used to characterize the perovskite film.More than 15%of PCE fabricated from this perovskite film was obtained.we found that the prepared MAPb I_3-xCl_x showed a band gap of 1.603 e V obtained from the ultraviolet-visible light spectrum test.This band gap ensures that the light-absorbing layer covers most of the solar spectrum.However,according to the J-V curve,the short-circuit current density of this PIN perovskite device is far lower than the theoretically calculated value of the same perovskite material.Furthermore,the time-resolved photoluminescence spectrum shows that the lifetime of the perovskite film is just 38.8 ns.And in electrochemical impedance spectroscopy,the recombination impedance is also relatively low than other references.Based on the above experimental characterization,we can conclude that the light-absorbing layer of the PIN type perovskite solar cell selected in this experiment still has defects.There is no deny that these vacancies and defects are detrimental to the device performance due to undesirable charge recombination and then performance deterioration.2.Use ionic liquids to passivate defects in the light-absorbing layer of PIN perovskite solar cells.During perovskite film preparation,migration of the halide ions leads to form the defects in the thin film,which is crucial to achieve a high-quality film.In general,the introduction of an additive is an effective approach to control the film morphology and to reduce the defect density.In this study,the representative and simplest ionic liquid,1-methyl-3-propylimidazolium bromide(MPIB),was selected as an additive due to its extremely high conductivity and lone-pair electrons in its molecular structure.The power conversion efficiency(PCE)was improved from 15.9%of the pristine device to 18.2%after adding the additive into the perovskite film.The scanning electron microscopy,X-ray photoelectron spectroscopy,Fourier transform infrared spectroscopy were employed to investigate the doping effects,two contributions of the MPIB additive are addressed:(1)the primary one is the passivation of the uncoordinated Pb²⁺to reduce the defects in the perovskite film by the lone-pair electrons in its cationic group,and(2)the secondary one is beneficial to promote crystal growth to improve the film quality.Hence,doping perovskite film by MPIB provides an easy approach to achieve a high-performance perovskite solar cell via passivation of the uncoordinated Pb²⁺in the perovskite film by the lone-pair electrons in the cationic group of the ionic liquid.3.Preparation and performance optimization of PIN perovskite solar cells based on NiO_x as the hole transport material.Nickel oxide is an excellent hole transport layer which is suitable for solar cell device with PIN configuration.Pure stoichiometric NiO_xis an excellent insulator with a room temperature electrical conductivity lower than 10^–11 S cm^-1.Its work function is well matched with the MAPb I_3-xCl_x that we choose to study,and wide bandgaps ranging from 3.6 to 4.0 e V,enabling it to be highly transparent in the near ultraviolet and visible range.However,intrinsic nickel oxide has poor conductivity.To prepare high conductive NiO_x films,it is necessary to strictly control the annealing temperature,precursor concentration and other factors to adjust the amount of oxygen and form Ni vacancies for hole transporting.In this chapter,we employed two methods to prepare NiO_x films by optimizing their process parameters,and a PCE of 14%was obtained.Among them,we make it clear that the main reason for the low efficiency of undoped nickel oxide is the low fill factor,which is due to the low conductivity of undoped nickel oxide.Therefore,ionic liquids can serve to increase the fill factor of NiO_x-based PIN perovskite solar cells as their excellent electrical conductivity,thereby improving the PCE in low-cost perovskite solar cells.After introducing the ionic liquid,AFM,SEM and contact angle test showed the reduction of roughness made by NiO_x-AMIC,and its uniformity was better.Ultimately,the fill factor of the device increased from 69%to74%,and the highest PCE reached at 15.67%.All devices stored under air condition showed good stability.Although the performance of Ni-based solar cells in this chapter was worse than polymer-based solar cells,introducing ionic liquid additives as an effective method provides a new pathway to promote the photoelectronic properties of nickel-based perovskite solar cells.