Research On The Relationship Between Microstructures Within Perovskite Thin Films And The Device Performance

Metal halide perovskites with the general formula ABX3 are ideal candidates for a myriad of applications as a result of their unique optoelectronic properties including large absorption coefficient,high mobility,and long diffusion length.The state of art photovoltaic device based on perovskite materials features outstanding open circuit voltage deficit and external quantum efficiency,approaching the commercially available c-Si counterpart.To assure the best attainable power output,research efforts are allocated into two major aspects: 1)to generate sufficient photo carriers by realizing high quality perovskite crystals with prolonged carrier lifetime and/or optical confinement in the device configurations and 2)to improve external extraction efficiency of the photo-carriers mostly at the adjacent contact by interface engineering.These attempts provide significant progresses in perovskite materials/devices in macroscopic scale towards high efficiency,however,it is less exploited for the hybrid perovskites in the micro/mesoscopic scale?e.g.intra-grain scale?so far,which is intuitively responsible for efficient photo-carrier behavior in the materials and devices.A profound understanding and exquisite control of perovskite crystals in the perspective of microstructural arrangement,are recently considered as an intriguing strategy to boost the photovoltaic device performance.For the consideration mentioned above,we carried out our work focused on the relationship between microstructure within perovskite thin films and the device performance.The summary of related works is as following:a)we demonstrate a simple but efficient method to enhance the crystallinity of perovskite films and open-circuit voltage of perovskite solar cells by introduction of an amidine-based additive?Aa Hc?in the perovskite precursor.We investigated samples with and without additive by SEM and XRD measurements,the results indicated an improved the morphology and crystallinity of perovskite films with additive compared to reference samples which is vitally important for carrier dynamics,interface engineering and performance of perovskite solar cells.More interesting,the corresponding perovskite film added with acetamidine-based salt showed extraordinarily long carrier lifetime,or rather,10 times longer than the reference samples.The open-circuit voltage for devices with Aa Hc increased significantly with increased content of additive from 1.022 V to 1.084 V.The amidine based additive would be another interesting and important parameter for the further exploration of interaction between organic and inorganic species in the hybrid materials system.b)we demonstrate a toolbox to effectively manipulate the crystal facet orientation upon crystallographic plane stacking via “cation cascade” doping in the mixed perovskite thin films for the first time.Specifically,a spectrum of Cs+,Rb+ and K+ were accumulatively introduced in sequence during film growth to create A site “cation cascade” in FAMA mixed perovskites.It led to significant crystal facet rotation along both in-plane and out-of-plane directions,as evidenced by twodimensional synchrotron radiation grazing incidence X-ray scattering?GIWAXS?measurement.This observation not only suggests the facet orientation induced by A cation cascade doping as one inferential parameter for the improved perovskite performance,but also provides a feasible approach to probe the relation between crystal stacking orientation of crystalline structure and consequent properties of hybrid perovskite materials in the intra-grain scale.c)we systematically investigated the impact of precursor composition on crystal orientation within Cs5?FAMA?95Pb?I,Br?3 films using two dimensional synchrotron radiation grazing incidence wide-angle X-ray scattering?GIWAXS?technique.Different from previous studies that were mostly concentrated on the precursor chemistry or carrier dynamics,we focused on the microstructure within the perovskite thin films and systematically explored the microstructure variation for samples with different PbX₂/AX ratios using the two-dimensional synchrotron radiation GIWAXS technique and provides a profound understanding about the impact of PbX₂/AX ratio engineering on the perovskite film from the perspective of microstructure,e.g.crystal orientation.We found that the perovskite samplesd)contained excessive AX?FAI and MABr?or PbX₂?Pb I2 and Pb Br2?possessed the same crystallographic orientation but different crystal plane stacking mode.The excessive PbX₂ can strengthen the initial preferred orientation as well as enhance the long-range order,resulted in highly crystalline perovskite thin films.Meanwhile,excessive AX in the film promoted the?001?plane stacking along the out-of-plane direction with enhanced long-range order instead of the relatively disordered stacking in the reference.It is believed that the alteration of stacking ways for perovskite?001?plane partially contributed to the performance variations,which facilitates further exploration of the impact of microstructure on the optoelectronic properties of perovskites.