Mixed Cation Strategy And Surface Treatment For Highly Efficient And Stable Perovskite Solar Cells

In perovskite solar cells,the absorber thin film layer plays a vital role in enhancing the overall device’s performance.However,there is a high probability that defects can quickly emerge in this layer.In order to reduce these shallow defects in perovskite thin films,the mixed cation is an efficient practice to create a smooth and defect-free film with higher crystallinity.For example,perovskite solar cells based on mixed cation structure can have better structural stability,which results in high open circuit voltage(VOC)and power conversion efficiency(PCE).Moreover,the surface morphology of the perovskite layer is associated with the local current,i.e.,the degradation of the Jscis linked with the surface morphology.Therefore,a smooth and dense film with higher crystallinity can cause enhancement in Jsc.Another approach to reducing these defects,which primarily exist on the surface of an absorber layer,is to use passivation treatment directly on the perovskite absorber layer.These methods provide beneficial approaches to enhance the quality of the perovskite absorber layer,which in return offers high-performance perovskite solar cells.In this thesis,we upgraded the performance of n-i-p planar perovskite solar cells by utilizing mixed cation perovskite and surface treatment to reduce surface defects along with non-radiative recombination,respectively.The mechanisms of device performance improvement were evaluated in detail.1.Mixed cations-based perovskite absorber layer was employed to achieve highly efficient perovskite solar cells with higher stability.A precisely small amount of CH(NH2)2,(FA)cations were introduced in methylammonium lead iodide(MAPbI3).A remarkably high PCE of 22.01%was successfully achieved.Analytically,in order to fabricate high-efficiency perovskite solar cells,it is essential to have compact,smooth perovskite film with high crystallinity.Keeping that in mind,a precisely small amount of FA cation has been incorporated in the most commonly used MAPbI3 perovskite to make FAxMA1-xPbI3.This incorporation of small ratios of FA cation not only reduces the risk of having a yellow phased FAI but also benefits from having more structural and thermal stability.Moreover,the small incorporation of FAI into MAI cations extends the absorption onset towards the larger wavelength,reducing the bandgap compared to standard MAPbI3.This combination increases the Jsc.This achievement establishes that the small amount of FA cations is enough to induce higher crystallization into standard MAI-based perovskite solar cells,and it allows utilizing black-phase of FAI cations with less trace of yellow phase and helps to shift enough the bandgap towards the shorter value.We also found that introducing a small amount of FA into MAI within the gaps of the PbI6 octahedra stabilized the MAPbI3 structure into a "quasi-cubic" phase at room temperature.As a result,the stability of the perovskite solar enhanced remarkably.Additionally,higher charge extraction and transportation were permitted with the most negligible loss induced by recombination processes because of the dense and smooth surface qualities of FA0.1MA0.9PbI3 films with large grain size and better crystallinity.Therefore,JSC and VOC were significantly improved.The reduced bandgap and the enhanced absorption in FA0.1MA0.9PbI3 thin film-based perovskite solar cells also partially contributed to the increased JSC.As a result,the champion PSC based on FA0.1MA0.9PbI3 exhibits an outstanding high efficiency of 22.02%.Thus,this work provides a simple approach to fabricating high-quality perovskite films;consequently,highly efficient perovskite solar cells with improved stability can achieve.Organic halide PEACl was employed as surface passivation treatment directly on the top of the perovskite absorber layer.The efficient Phenethylammonium chloride(PEACl)treatment reduces halide deficiency at the perovskite surface and reduces surface defects and non-radiative recombination.Typically,the surface of a perovskite layer is where defects are most easily formed.These defects lead to non-radiative recombination and relative energy losses due to this recombination.Besides,the difference in electrical potentials and ion migration rates between grain boundaries and bulks would damage electrical properties that finally result in the noticeable unsatisfactory performance of the overall Perovskite solar device.Therefore,the passivation of the surface defects is ultimately the most crucial task in any solar cell.PEACl for post-treatment of mixed perovskites MA1-xFAxPbI3 suppresses the surface defects of perovskite poly-crystalline films to fabricate efficient solar cells.Small radii(1.67?)Cl-ions have proved to be a practical approach for high-performance perovskite solar cells due to their higher diffusivity of filling out the traps state that exists on the surface or throughout perovskite thin film.In addition,we employ PEA that performs a spacer and effectively reduces non-radiative recombination.After PEACl surface passivation treatment,the effective filling of the halide vacancies dominant on the surface causes a substantial decrease in deep or shallow trap states.Accordingly,the PEACl treatment device harvests the highest PCE of 21.49%with a higher open-circuit voltage(VOC)of 1.15 V,92%of the Shockley-Queisser limit VOC(1.25 V).The remarkable device performance can be explained by efficiently reducing unwanted non-radiative recombination and related energy losses.Furthermore,there is a tremendous increase in FF after PEACl treatment.Besides,after PEACl treatment,the stability was improved significantly compared to the standard perovskite device.There are 43 figures,8 tables,and 241 references in this thesis.