Studies Of The Fabrication And Properties Of Formamidinium-based Layered Perovskite Films And Photovoltaic Devices

In 2014,solar cells based on layered perovskite were first reported and showed excellent ambient stability.In the next few years,the power conversion efficiency（PCE）of layered perovskite solar cells increased from 4.73%to 19.3%.It has been demonstrated that the formamidinium（FA）-based perovskite may be more suitable as the active layer of the solar cell than the methylammonium（MA）-based one for its better environmental stability and narrower bandgap.However,the formation energy of the layered perovskite increased as the number of inorganic layers（n）increases,which makes it difficult to form large n phase component in FA base perovskite.The lack of large n-phase not only lead to insufficient absorption of sunlight,but also result in poor charge transfer and energy transfer,both of that will produce poor performance of the solar cells.Now,FA-based layered perovskite has not been attracted the focus of researchers,but,according to the current reports on FA-based perovskite,we think it is necessary to study FA-based perovskite more comprehensively for developing layered perovskite devices with better performance.In this dissertation,properties of the films and devices based on Ruddlesden-Popper（RP）layered perovskite of the（BA）₂（FA）_n-1Pb_nI_3n+1 has been studied.The research contents and results are mainly as follows:（1）Optimization of crystallization of the（BA）₂（FA）₂Pb₃I₁₀ films.Thiourea（THA）was added to the precursor solution for fabrication of the films,the emission located at high energy in PL spectra of the films was significantly weakened,and the absorption spectrum show a slightly red-shift with the absorbance at low energy slightly increased.The intensity of peak in XRD increased a lot,and the result of SEM show the formation of larger size of grains than that of without THA.These result demonstrated that THA greatly improves the crystallinity of the films and slightly promotes the formation of large n phase perovskite.These results show that THA can improve the crystallinity of FA-based layered erovdkite,which also lays a foundation for our sequent research.（2）Emission and extraction of hot carriers in layered perovskite.Under low excitation intensity（0.6μJ/cm²）,there is only a single PL peak at low energy（～800 nm）.With the excitation intensity increase,some new features located at higher energy appeared apart PL peak located at about 800 nm,which could be assigned to the perovskite with different n phases.The PL of films with a polarization memory of 12%,which decreased with the wavelength increase.When excited by the laser with shorter wavelength,the polarization memory also decreased.More importantly,a significant increase of open circuit voltage was observed when layered perovskite solar cells excited by a continuous wave laser with the power exceeded 300 m W/cm².These results show that not only the emission of hot photoexcitation states but also the extraction of hot carriers are directly observed in FA-based layered perovskite.（3）Effect of morphology on the perovskite films and devices.The films fabricated with low concentration（0.25 mol/L）solution with the grains size of about 40 nm;the films fabricated with high concentration（1.0 mol/L）solution with the grains size of about 300 nm.The PCE of the devices fabricated with low concentration solution is significantly higher than that of the devices fabricated with high concentration solution.The excitons in the devices from low concentration solution are effectively converted into photocurrent but the devices fabricated with high concentration did not.Our results show that the absorption of an exciton plays the dominant contribution to the photocurrent in the devices fabricated with low concentration solution,which have better photovoltaic performance,and the effective exciton dissociation will be crusial for FA-based layered perovskite solar cells.（4）Stability of layered perovskite films and devices.The films and devices stored in the glove box have almost no attenuation whether there are additives or not.Films and devices without THA degrade obviously after being stored in air for 3 days.The films with THA began to decompose after 90 days in air,while the PCE of devices with THA decreased about20%after 7 days.We found that the samples deposited Ag electrode showed a faster degradation process in the air than the sample without electrode.Our results show that the encapsulated devices are expected to achieve long-term stability,the additives can effectively improve the stability of perovskite films and devices,and Ag electrode is one of the main factors of the degradation of devices in this work.（5）Optimization of FA-based perovskite solar cells.The（BA）₂(FA_1-xMA_x)₃Pb₄I₁₃（x=0,0.2,0.5,0.8,1）films and devices were fabricated by hot-cast technology.When x=0.2,the films have the best optical properties and show best stability,and the corresponding chaimpion device with the PCE of 14.2%,which is the highest value reported PCE of the FA-based layered perovskite solar cell at present.Our results show that the performance of FA-based layered perovskite solar cells could be improved by effectively tunning the composition and proportion of A-site cations.