Ultrafast Dynamics Of Photogenerated Carriers In Organolead Halide Perovskites

Energy crisis and environmental pollution are two major problems that threaten the sustainable development of human society.Looking for a renewable clean energy has become an important issue to be solved urgently in human society.Solar energy as a clean energy,with inexhaustible and widely distributed,and other advantages,by the people attention.In recent years,metal organic halide perovskite(CH3NH3Pb X3,X=Cl,Br,I)solar cells because of its simple preparation process and has good photovoltaic properties,causing the strong concern of researchers.Compared with the progress of the device,the perovskite semiconductor material photoelectric physical mechanism of understanding is still not deep.In this paper,the ultrafast spectroscopy method is used to study the ultrafast kinetic process of the photogenerated carriers in the perovskite solar cell from the film to the interface and then transferred to the electrode.The main contents are as follows:1.In perovskite solar cell,the overall photoinduced charge transfer(CT)process comprises both charge diffusion through the bulk to perovskite/electrode interfaces and interfacial electron/hole transfer to electrodes.In this chapter,we decoupled these two entangled processes by investigating the film thickness-dependent CT dynamics from CH3NH3Pb I3 perovskites to PCBM(electron acceptor)and Spiro-OMe TAD(hole acceptor).By fitting ultrafast transient absorption kinetics to an explicit“diffusion-coupled charge transfer”model,we found that the charge diffusion from the film interior to perovskite/electrode interfaces took?200 ps to a few nanoseconds,depending on the thickness of perovskite film;while the subsequent interfacial charge transfer was ultrafast,?6 ps for electron transfer to PCBM and?8 ps for hole transfer to Spiro-OMe TAD,and led to efficient charge extraction(>90%)to electrodes in a 400-nm-thickness film.Our results indicate that the picosecond interfacial charge transfer is a key to high-performance perovskite solar cells.2.From the previous research results,it can be seen that the intra-membrane diffusion of photogenerated carriers is a key factor to control the efficiency of solar cells.In order to improve the diffusion rate of photo-generated charge to the interface,we have prepared a concentration gradient perovskite films.And the ultrafast kinetics of photogenerated charge diffusion and transfer were studied.The CH3NH3Pb I3 perovskite thin film was prepared by one-step spin coating method,then Br-doping was carried out to form CH3NH3Pb I3-x Brx on the surface.The steady-state absorption spectra,fluorescence spectra and transient absorption spectra were used to characterize the diffusion rate of the photogenerated carriers in the films.The migration time of the films was?20 ps.In addition,the doped perovskite film has a better match with the receptor,resulting in a faster charge transfer of the perovskite/charge acceptor interface,and thus has a more gravitational perovskite film with more high charge extraction efficiency.3.Two-dimensional(2D)organolead halide perovskites are promising for various optoelectronic applications.In this chapter,we study a unique spontaneous charge(electron/hole)separation property in multi-layered(BA)2(MA)n-1Pbn I3n+1(BA=CH3(CH2)3NH3~+,MA=CH3NH3~+)2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy.Surprisingly,the 2D perovskite films,although nominally prepared as“n=4”,are found to be mixture of multiple perovskite phases,with n=2,3,4 and??,that naturally align in the order of n along the direction perpendicular to the substrate.Driven by the band alignment between 2D perovskites phases,we observe consecutive photoinduced electron transfer from small-n to large-n phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of?358 nm thickness.This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films,which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices.4.We demonstrated that the TA spectra measured in the MAPb Br3 perovskite films with heterogeneous microstructures do comprise non-negligible TR signal originating from the photoinduced reflection variation at film surfaces or the interface of microstructures.The weight of this TR signal was found to increase as the microstructure size in the film increases,and distort the shape of the TA spectra.Because the TR probes the photoinduced reflection variation at film surface,the presence of TR signal can lead to faster observed carrier kinetics than in the bulk(probed by the intrinsic TA signal)owing to the presence of more surface defects.In principle,the artifacts resulting from TR signal in TA measurements should not only occur in perovskite films,but also may occur in other film materials and suspensions,the similar TR signal was also observed in their TA measurements in Cd S suspensions.Since the transient absorption is one important technique to probe the photoinduced dynamics in semiconductors and many other materials,we pointed out in this work that special attention must be paid in analyzing TA results from samples with heterogeneous microstructures to avoid the possible misinterpretation of carrier dynamics.