Preparation Of Perovskite Films With Two-Step Method And Applications In Low-Temperature Perovskite Solar Cells

In recent years,due to the energy and environmental issues,an increasing interest is looking for renewable energy to replace the traditional fossil energy.The low-cost and renewable features of solar cells have attracted a considerable attention.In the past five years,the power conversion efficiency?PCE?of perovskite solar cell?PSCs?has soared.PSCs have become a shining star in the field of the solar cells.Although the PCE of PSCs has been rising,there are still issues to be solved,e.g.,?1?It was found that the presence of remnant lead iodide crystals in CH₃NH₃ Pb I₃ can passivate the grain boundaries of the perovskite films,inhibiting the recombination of electron and holes.Two-step sequential deposition method has been extensively employed to prepare the CH₃NH₃ Pb I₃ active layer from the Pb I₂ precursor in PSCs.However,the variation of photovoltaic performance of PSCs made by this method was always attributed to different dipping times that induce complete/incomplete conversion of Pb I₂ into CH3NH3 Pb I₃.It is necessary to previsely control the passivative Pb I₂ in perovskite layer.?2?Inorganic metal oxide electron transport layers are widely used in PSCs.Most of the inorganic electron transport layers require high temperature annealing to exhibit high conductivity with less defects.However,this is a drwaback for fabricating of flexible and tandem solar cells.Therefore,preparing low-temperature electron transport layer for PSCs is one of the important issues.To solve the problems mentioned above,I provide two solutions in this thesis as follows:To solve the first issue,I employed solvent vapor annealing?SVA?method to prepare Pb I₂ crystallites with large grain size for preparation of high quality perovskite.With this method,the increased Pb I₂ dipping time in CH3NH3 I solution was found to reduce the photovoltaic performance of resulting PSCs without a significant change in Pb I₂/CH3NH3 Pb I₃ contents in the perovskite films.I attribute this abnormal reduction of photovoltaic performance to intercalation/deintercalation of Pb I₂ core with CH3NH3 Pb I₃ shell,which causes the doping effect on both Pb I₂ and CH3NH3 Pb I₃ crystal lattice and the formation of CH3NH3 Pb I₃ capping layer on the surface,as revealed by UV-Vis absorption,X-ray diffraction,FT-IR,and scanning electron microscope measurements.Based on our findings,a multi-step dipping-drying process was employed as an alternative method to improve the crystalline quality.The method achieved power conversion efficiency up to 11.4% for the compact-layer free PSC sharing a simple device structure of ITO/perovskite/spiro-OMe TAD/Ag.To solve the second issue,I develop a new strategy to prepare the Zn S/Zn O nanocomposites as low-temperature processed electron transport layers for fabricating PSCs.The surface morphology was improved by mixing Zn S and Zn O nanoparticles,by which the recombination at the interface between the electron transport layer and perovskite layer was suppressed.Moreover,the upper shift of Fermi level upon composition of the Zn O/Zn S film results in a better alignment of energy level,which facilitates cascade charge extraction and thus the improve the current density of PSCs.The photoluminescence spectroscopies measured in both steady and transient states were carried out to characterize the charge extraction at the interface between CH3NH3 Pb I₃ and the electron transport layers of either Zn O or Zn O/Zn S composite.Clearly,the composite of Zn O/Zn S significantly enhanced the photovoltaic performance of PSCs through increasing both the short-circuit current and open-circuit voltage.