Polymer/Perovskite Composites Materials Luminescent Devices

In recent years,organic-inorganic halide perovskites have become a new generation of semiconducting materials possessing the enticing advantages such as solution processability,low carrier trap density,high absorption coefficients and long-range charge carrier diffusion lengths and therefore showing great prospects in solar cells,lasers,high-gain photodetectors and light-emitting diodes?LEDs?.In this thesis,organic-inorganic halide perovskite light emitting diodes are mainly studied.Due to the rapid crystallization speed of the perovskite materials,the formation of smooth,pin-hole free and flat perovskite films is the prerequisite to achieve high performance light emitting devices.To date,several methods have been reported to improve the perovskite film morphology,including anti-solvent aided fast crystallization,the manipulation of the ratio of the organic to inorganic compounds in the perovskite precursor solutions and the formation of polymer:perovskite composites.Among the above methods,the formation of polymer:perovskite composite is a simple,universal and reliable method to effectively control the morphology of the perovskite films.Besides,polymer additives enable to improve the environmental stability and mechanical properties of the perovskites.In this article,the morphology of polyethylene oxide?PEO?:CH₃NH₃PbBr₃ composite films and in turn the luminance efficiency of the related light emitting devices are improved by adjusting the molar ratio between CH₃NH₃Br and PbBr₂.We further study the influence of PEO contents on the properties of the PEO:CH₃NH₃PbBr₃ composite materials.On this basis,we investigate the properties of the composite materials using a polyacrylonitrile?PAN?additive and analysize the influences of the polymer additive on the morphological,structural,photo-physical and electroluminescent properties of the resultant materials.The paper is divided into the following three parts:?1?We have prepared the samples from the CH₃NH₃PbBr₃ precursor solutions with various CH₃NH₃Br:PbBr₂ ratios.Scanning electron microscope?SEM?measurements indicate that CH₃NH₃PbBr₃ grain size decreases and the homogeneity of the samples is improved with increasing CH₃NH₃Br:PbBr₂ ratio.PbBr₂ diffraction peak at 18.7^o appears in the diffractogram of the sample prepared from the precursor solution with CH₃NH₃Br:PbBr₂ ratio of 1.05:1,indicating that PEO disturbs the formation of of CH₃NH₃PbBr₃.When CH₃NH₃Br:PbBr₂ ratio reaches 2:1,PbBr₂ impurity peak disappears.As CH₃NH₃Br:PbBr₂ ratio is further increased to 3:1,the impurity peak at12.6^o originating from CH₃NH₃⁺point defects emerges,suggesting excess CH₃NH₃PbBr₃ may disturb the formation of CH₃NH₃PbBr₃ in the PEO:CH₃NH₃PbBr₃composite films.We have prepared the PEO:CH₃NH₃PbBr₃ composite light emitting devices with the sturcuture of ITO/PEDOT:PSS?70 nm?/PEO:CH₃NH₃PbBr₃?50 nm?/TmPyPB?60 nm?/CsF?1 nm?/Al?100 nm?from the solutions with various CH₃NH₃Br:PbBr₂ ratios.When CH₃NH₃Br:PbBr₂=2:1,the luminance efficiency of the devices reaches 6.1 cd/A,which is correlated with relatively uniform morphology and high CH₃NH₃PbBr₃ phase purity of the samples.?2?We proceed to study the influences of PEO content on the properties of PEO:CH₃NH₃PbBr₃ composites.SEM measurements indicate that CH₃NH₃PbBr₃ grain size decreases and the morphology of the samples is improved simultaneously.The diffractograms of the samples with PEO:CH₃NH₃PbBr₃ ratios of 0.75:1 and 1:1 show PbBr₂ diffraction peak,reiterating the disturbing effect of PEO on the CH₃NH₃PbBr₃formation.Photoluminescence intensity of the samples decreases with increasing PEO content.We have prepared the PEO:CH₃NH₃PbBr₃ composite light emitting devices with the sturcuture of ITO/PEDOT:PSS?70 nm?/PEO:CH₃NH₃PbBr₃?50 nm?/TmPyPB?60 nm?/CsF?1 nm?/Al?100 nm?with various PEO contents.When PEO:CH₃NH₃PbBr₃=0.5:1,the optimal luminance efficiency of the devices is 6.1 cd/A.?3?On the basis of the above results,we further investigate the properties of the composite materials using a PAN additive and make a direct comparison between the properties of the composites using a PEO or PAN additive to understand the role of polymer additive.The results indicate that the addition of PAN significantly improves the morphology of the derived films and decreases CH₃NH₃PbBr₃ grain size.The selecton of polymer additive significantly affects the photoluminance and electroluminescence characteristics of the composite films:while photoluminescence intensity of PAN:CH₃NH₃PbBr₃ sample is ca.two times stronger than that of the analogousPEO:CH₃NH₃PbBr₃composite,theluminanceefficiencyof PAN:CH₃NH₃PbBr₃ devices is only one third of that of the PEO:CH₃NH₃PbBr₃counterpart.The current density—voltage characteristics of hole-dominated and electron-dominated devices indicates that hole current density is similar to electron current density for PEO-based devices,whereas hole current density is ten times smaller compared with electron current density for PAN-based devices,revealing that difference in the luminance effiency of the devices using various polymer addtives results from varied charge-balance factors.