Study On Lead-Free Double Perovskites And Their Optoelectronic Devices Via Vapor Deposition Processing

Organic inorganic lead halide perovskites have been widely studied because of their superior optoelectronic properties such as high defect tolerance,high absorption coefficient,long carrier diffusion length and large carrier mobility.Optoelectronic devices based on perovskite functional layers have achieved leading performance.For example,perovskite solar cells have gained a certificated power conversion efficiency（PCE）of25.5%and the detectivity of perovskite photodetectors is above 10¹³Jones.However,the toxicity and instability hamper further commercialization of the perovskite-based optoelectronic devices.By substituition of the lead site with isovalent and non-toxic metal elements,novel all-inorganic double perovskites obtain the lead-free merit and stability because of the absence of volatile organic conponent.Besides,double perovskites possess long carrier recombination lifetime,dimension and band tunability,and become the promising substitution of lead halide perovskites for optoelectronic devices.However,the fabrication of high-quality double perovskite films still remains challenging in common solution processing methods,because of the different solubility of the components.This work innovatively adopts vapor deposition method to prepare Cs₂AgBiX₆（X=Br,Cl）double perovskite films and demonstrates their applications in solar cells and photodetectors.The main research contents of this thesis are as below:（1）Sequential vapor deposition was used to prepare high-quality Cs₂AgBiBr₆ films.A systematic study of the relationship between fabrication and properties of the films was carried out.By optimizing the ratio of deposition amounts of different sources and adopting a two-step annealing process,Cs₂AgBiBr₆ films of high-crystallinity and smooth and uniform morphology were obtained.The defect density of this film was2.13’10¹⁶ cm^-3,characterized by the space-charge-limit-current（SCLC）method.Besides,the carrier diffusion length of the vapor-deposited Cs₂AgBiBr₆ films was obtained by time-resolved photoluminescence（PL）measurements fitted by a one-dimension carrier diffusion-recombination model.The diffusion lengths were 147 nm and 232 nm for electron and hole,respectively.The resulted double perovskite solar cells achieved a PCE of 1.37%with a high V_oc of 1.12 V.Further stability research revealed great thermal stability of the Cs₂AgBiBr₆ films and good storage stability of their devices.（2）For the double perovskite-based photodiodes,the selective detection function was realized by utilizing the sharp absorption characteristic of Cs₂AgBiCl₆ films in the UV region.The sequential vapor deposition was also used to prepare Cs₂AgBiCl₆ films,and the tunability of composition and thickness as well as the influence of annealing temperature on cystal phase,crystallinity and morphology of the films were studied.The stoichiometry-balanced film was obtained by adjusting the deposition ratio;and the thickness can be tuned by increasing or decreasing the deposition cycle number.The deposited double perovskite films show compact morphology and high crystallinity.The films exhibit an indirect bandgap of 2.41 eV determined by measurements of absorption spectra.The ultraviolet detector was fabricated based on a photodiode structure and exhibited a responsivity over 10 mA×W^-1,a detectivity over 10¹² Jones at 370 nm.Especially,a selective narrow detection band centered at 370 nm with a band width of 67nm was realized.