Defect And Photoelectric Properties Of Lead Halide Perovskites

Perovskite materials have high defect tolerance,easy fabrication,and adjustable bandgap,which makes them quickly become a hot topic in the field of photoelectric semiconductors.Recently,perovskite solar cells rapidly developed into photovoltaic stars,identifying as the most possible to achieve large-scale market-oriented thin-film solar cells.Taking advantage of the adjustable bandgap of perovskite,the author prepared perovskite solar cells with different bandgaps and obtained highly efficient photovoltaic devices.Besides,the author deeply studied the influence of the dopant,interfacial modification,and bulk passivation on the perovskite photovoltaic property,revealing the relationship between the internal defect type of the perovskite semiconductor and device stability.The physical properties of metallic lead in perovskite films were systematically studied,and the origin and influence of metallic lead were illustrated.Our research focuses on the improvement the perovskite photovoltaic performance,and the main research results are summed as follows:（1）We employed potassium as a dopant into a 1.70 e V wide-bandgap FA_0.8Cs_0.2Pb(I_0.7Br_0.3)₃ perovskite film.Then,a phase-stable wide-bandgap perovskite photovoltaic device was fabricated.Benefiting from the strong bonding energy of halide-potassium,the potassium effectively suppresses the ion migration in the mixed halide perovskite absorber.Furthermore,potassium suppresses the phase segregation phenomenon in the wide bandgap perovskite,which enhances the phase stability of perovskite.Besides,potassium also reduces the defect state density,prolongs the charge carrier lifetime,and enhances the crystallization of wide-bandgap perovskite.As a result,we achieved a power conversion efficiency（PCE）of 18.3%in inverted structure and the devices exhibit excellent photostability.（2）Utilizing the strong electron-withdrawing ability of the nitrile group,we use a nitrile-rich molecule of F4-TCNQ as a passivation material to treat with a 1.67 e V FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃ wide-bandgap perovskite material.F4-TCNQ molecule would react well with the FA⁺cation in perovskite precursor,which further enhances the stability of FA⁺cation in perovskite film.We measured the F4-TCNQ passivated wide-bandgap perovskite film,which exhibited a low defect state density,improved activation energy for ion migration,and reduced nonradiative recombination.We achieved a PCE of 20%in the inverted structure and the device maintains 88%initial efficiency after 840 hours of light irradiation.（3）Metallic lead（Pb⁰）in the perovskite film is the main reason that led to the device performance decrease.We employed a series of characterizations for revealing the relationship among perovskite performance,lead iodide,and Pb⁰.We found that Pb⁰ stems from the decomposition of excess lead iodide in perovskite materials under light or X-ray irradiation.In addition,we identified that the Pb⁰ may neither come from the perovskite precursor nor the annealing process for perovskite growth.It may have been caused by the decomposition of excess lead iodide existing in the perovskite by X-ray radiation during the XPS measurement.We also intentionally introduce the Pb⁰ impurities into the perovskite films,where we found that Pb⁰inhibits the crystallization of perovskite,induces related deep energy level defects,and lowers the activation energy of the trap.And Pb⁰ also obviously reduces the photovoltaic performance of perovskite solar cells.Meanwhile,we propose that using some two-dimensional material can decrease the excess lead iodide in perovskite film and thus enhance the stability of devices.（4）We employ a one-step chemical reaction that anchors chlorine onto the surface of tin dioxide quantum dots（Sn O₂ QDs）.Chlorine-anchored Sn O₂ QDs not only enhance the binding energy between perovskite and electron transfer layer but also promote the extraction of photogenerated carriers.Furthermore,interfacial chlorine raises the Fermi energy level and improves the carrier mobility of the Sn O₂ electron transport layer which all are beneficial for obtaining highly efficient perovskite solar cells.As a result,we utilize chlorine anchored Sn O₂ electron transport layer to fabricate a 1.58 e V bandgap Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃ perovskite solar cells,which achieve a high open-circuit voltage of 1.195 V and a PCE of 20%.