Key Technologies Of Photogenerated Carrier Dynamics In Perovskite

Organic-inorganic hybrid perovskite cells have made rapid progress in recent years,but the research on its carrier dynamics has been developed less,especially the reliable measurement and analysis of carrier dynamics in the field are still insufficient and in debate.On the one hand,due to the inhomogeneity of the organic-inorganic hybrid perovskite thin film caused by the current preparation method and the intrinsic instability of the perovskite material,it is difficult to obtain accurately carrier dynamics parameters by common time-resolved carrier dynamics research methods.On the other hand,the existing time-resolved carrier dynamics research methods are still lack of the precise description of interface conditions.However,the research on interface carrier behavior is particularly important for the current perovskite thin film device structure system.At present,the research on carrier dynamics in perovskite thin films still needs further development and breakthroughs in measurement methods and theoretical systems.Based on this,this work studied the inherent stability of organic-inorganic hybrid perovskite thin films firstly.Using the strong binding effect of sulfur element in cysteamine hydrochloride and lead element in perovskite,the stability is enhanced by the improved lattice structure.therefore MAPb I₃ perovskite with good photostability and excellent water resistance were further prepared accordlingly.The corresponding solar cell devices can work underwater without any encapsulation and maintain the original efficiency.This work provides a new strategy for the preparation of moisture-resistant and highly light-stable perovskite films and devices,and also provides the basis of stable film preparation for the study of carrier dynamics.Based on the above high-quality thin film systems,this work further investigated the carrier dynamics in perovskite thin films based on time-resolved photoluminescence（TRPL）measurements.At the beginning,we found that the photoluminescence decay curves of regular perovskite films on the front and back sides are quite different,indicating that the front and back sides of the film have different effects on carriers.In the research field,when using TRPL to measure the photoluminescence decay process of perovskite films,the difference between the front and back surfaces of the film is commonly neglected,resulting in inaccuracy in obtained carrier dynamics parameters.This work combined the front（upper surface）and back（perovskite/glass interface）excitation TRPL measurement of perovskite/glass substrate films,and introduced the surface recombination velocity（SRV）to describe the boundary conditions of carrier diffusion-recombination kinetic process,achieving the quantitative description of the nonradiative recombination rate caused by the defects on both the front and back of the film.Compared with conventional methods in the field,the carrier dynamics parameter measurement system based on TRPL realized in this work avoids the harsh assumptions about boundary conditions before and meanwhile decouples the influence of interface defects.The obtained carrier diffusion coefficient D is more intrinsic to the interior of the film.Notably,we observed that the SRV of the perovskite/glass interface is 2-3 orders of magnitude higher than that of the unpassivated free perovskite interface,confirming the non-negligible impact of interfacial differences in thin-film carrier dynamics studies,laying a foundation for further improvement of surface modification strategies for perovskite devices.Meanwhile,this work investigated the influence of low-temperature environmental conditions on the carrier transport process.Although the environmental temperature factor has been widely studied,the effect of temperature-dependent interfacial strain on the carrier dynamics of perovskite films is rarely reported.Perovskite materials are soft,and its thermal expansion coefficients are quite different from the conventional rigid substrates,resulting in a sharp increase in the interfacial strain of perovskite films due to changes in ambient temperature,which is usually ignored by temperature-related studies in the field.We first measured the strain of perovskite thin films at low temperature（160K-room temperature）,and obtained reliable perovskite thin film carrier dynamics parameters varied with temperature by global fitting to in-situ TRPL decays upon front/back-side excitation,including diffusion coefficient,recombination rate,front and back SRV,etc.In a low temperature environment,researchers generally use the intrinsic polaron-phonon scattering law to describe the change of carrier mobility with temperature,that is,the-3/2 power of temperature with mobility.Based on the above-established interfacial carrier dynamics system,the work herein considered the influence of the interfacial strain on the carrier mobility caused by the mismatch of deformation,and a more realistic model of carrier dynamics with temperature is established,clarifying the important effect of interfacial strain on carrier dynamics.Furthermore,based on the in-depth study of the carrier dynamics of perovskite thin films,the blueprint for linking the microscopic carrier hehaviors to the macroscopic performance of devices is also becoming more and more complete.This work proposes to utilize bilayer heterostructures,which refer to bilayer perovskites with different carrier mobilities,to realize high-performance perovskite solar cells.First,we extended and realized the binary carrier diffusion-recombination model to further describe the carrier dynamics of the bilayered perovskite hetero-absorption layer structure.With the help of high-throughput numerical simulation calculations,we predicted the relationship among the interface outflow of photogenerated carriers,different carrier mobilities and layer thicknesses combinations of the bilayered perovskite structure.We revealed the key role of the difference in the diffusion coefficient of the perovskite absorber layers in improvement of the efficiency of the bilayer heterostructure device.We accordingly designed and realized the bilayered device assembled by MAPb I₃/FAPb I₃.Through the optimization of the thickness of each layer,the solar cell device with optimal performance was realized.This work may provide a new strategy for the development of new high-efficiency devices in the future.This work has developed a carrier dynamics description and characterization system based on time-resolved photoluminescence methods,and supplemented the mechanism model of the film interface conditions on its carrier dynamics.This work also provides the foundation of physical mechanism and feasible practical wasy for further improvement of the current device efficiency of perovskite solar cells.