A Solution-Processable Approach For Absorber Layer Based On Pb And Bi Hybrid Perovskites And The Performance Of Solar Cells

Organic-inorganic hybrid Perovskite Solar Cells（PSCs）have attracted considerable attention due to its high efficiency photoelectric conversion efficiency（PCE）.Currently,the photoelectric conversion efficiency has reached 22.1%.In the course of the study,the researchers found that the morphology and properties of light absorption layer have significant effects on the photovoltaic performance of solar cells.The perovskite absorber layer can generate the electron-hole pairs when absorbing sunlight,and also can efficiently separate and transmit electron-hole pairs,and then the electrons and holes are transported to the electrodes,respectively.Therefore,high-quality perovskite light absorption layer plays a vital role in the device performance.Different preparation process,different device structure and post-treatment of solvent engineering,have an impact on the film quality of perovskite absorber layer,thereby affecting the performance of PSC.The main research and conclusions of this paper are as follows:1.Fabrication of perovskite absorber thin film with PbI₂ as raw material.A certain stoichiometric amount of PbI₂ and MAI were dissolved in DMF,and the precursor solution was spin-coated on cp-TiO₂ or mp-TiO₂.The samples were assembled into planar or mesoporous solar cells with carbon/gold as the counter electrode.The two-step spin-coating method was conducted as follow:the PbI₂/DMF solution was spin coated on cp-TiO₂ or mp-TiO₂ followed by soaking in the MAI/IPA solution to obtain CH₃NH₃PbI₃ film.The perovskite films prepared by one-step and two-step solution methods were both washed with chlorobenzene.The morphologies and properties of perovskite films were tested successively by XRD,UV-Vis and FE-SEM.After optimizing the fabrication process,the device based on the carbon as the electrode has obtained the best PCE of 8.3%,and the device based on the gold as the electrode has achieved the best PCE of 11.6%.2.Preparation of perovskite absorber thin film with Pb（Ac）₂ as raw material.A certain stoichiometric ratio of Pb（Ac）₂ and MAI were dissolved in DMF,and the precursor solution was spin-coated on cp-TiO₂ or mp-TiO₂.Then the devices were assembled into planar or mesoporous solar cells with gold used as the counter electrode.The two-step spin-coating method was conducted as follow:the Pb（Ac）₂/DMF solution was spin coated on cp-TiO₂ or mp-TiO₂ followed by soaking in the MAI/IPA solution to obtain CH₃NH₃PbI₃ film.The morphologies and properties of perovskite films were characterized by XRD,UV-Vis and FE-SEM.The device based on the gold as the electrode has obtained the best efficiency of15.04%.3.Preparation of non-toxic perovskite light-absorbing layer film with BiI₃ as raw material.A certain stoichiometric BiI₃ and MAI were dissolved in DMF,and the precursor was spin-coated on cp-TiO₂ or mp-TiO₂,then the devices were assembled into planar or mesoporous solar cells with the carbon as the counter electrode.The two-step spin coating method that spin the BiI₃/DMF solution and then soak in the MAI/IPA solution to obtain（CH₃NH₃）₃Bi₂I₉ film.The morphology and properties of Bi-based perovskite films were tested by XRD,UV-Vis,FE-SEM and AFM,respectively.The photovoltaic performance of the devices based on different MBI films and device architecture was tested under AM 1.5G irradiation(100 mW cm^-2).Compared with MAPbI₃-based devices,MBI exhibited a superior stability in ambient atmosphere.The mesoporous structure devices（1-mp and 2-mp）presented higher PCEs at around 0.05%compared with the planar structure devices（1-cp and2-cp）.Notably,the 1-mp devices exhibited the best PCE of 0.054%,which was attributed to the high open-circuit voltage(V_oc)of 895 mV.The planar structure device（1-cp）fabricated by the 1-S method exhibited a higher PCE at 0.039%compared with that（2-cp）fabricated by the2-S method.In addition,the PSC device with a hole conductor-free layer and a carbon counter electrode exhibited a relatively low fill factor（FF）,which can be attributed to the weakened attachment of the MBI layer to the carbon counter electrode.We further assessed the air stability of the MBI film based on the cp-TiO₂ or mp-TiO₂ layer through XRD measurements.On continuous exposure to ambient air for 201 days,the 1-mp device exhibited 63.0%of its initial PCE,which was attributed to the drop of J_sc from 0.255 to 0.161 mA cm^-2.Therefore,all results indicate that these PSCs exhibited excellent long-term stability.