聚乙烯吡咯烷酮链段长度依赖的钙钛矿结晶动力学和电致发光研究

Polyvinyl Pyrrolidone Segment Length Dependent Perovskite Crystallization Kinetics and Electroluminescence Performance

基于低温溶液工艺的钙钛矿发光二极管由于低成本,高色纯度,宽色域等优势受到广泛关注。然而,在溶液结晶成膜过程中,钙钛矿晶粒无规形核、快速生长,导致薄膜存在大量的针孔和缺陷,严重影响器件性能。本文通过原位荧光和结构表征技术证实,聚乙烯吡咯烷酮(PVP)添加剂在特定的链段长度下,能优化调控钙钛矿结晶动力学,延缓晶粒生长速度,使得钙钛矿薄膜粗糙度由1.489 nm降低到0.954 nm,缺陷密度从1.55×10^(18)cm^(-3)降低至1.05×10^(18)cm^(-3)。相应地,薄膜荧光量子产率从16.9%提升到58.9%,器件外量子效率从8.55%提高到了18.00%。本文通过原位表征技术,全方位解析了添加剂分子结构对钙钛矿结晶动力学的调控机理,为添加剂分子的优化设计提供了直观依据。

Solution-processed Perovskite Light-Emitting Diodes (PeLEDs) has drawn much attention due to their low cost,narrow emission spectra and wide color gamut.However,undesirable pinholes and defects of perovskite films impair device performance,which is attributed to the atactic nucleation and rapid crystallization during solution processing.In general,the antisolvent is added to the perovskite spin coating process to quickly remove excess solvent and form an intermediate phase.Combining with annealing treatment,mesophase gradually transforms into the perovskite phase and dense perovskite film can be obtained.At the same time,the larger organic cation (PEA+,TEA+,etc.) will cut continuous lead halide octahedron into the periodic quantum well structure to form quasi-two-dimensional perovskite which has a larger exciton binding energy and rapid energy transfer.By adding the large polymers to the precursor,long segments cannot enter the perovskite lattice and act as the“framework”in the perovskite crystallization process,which can restrict nucleation sites,inhibit the rapid growth of grains and further optimize the film quality.However,the size,fluidity and solubility of the polymer change with the increase of the segment length,which significantly affects the growth of perovskite crystals.Therefore,it is crucial to systematically study the regulation law of polymer segment length on the crystallization kinetics of perovskite to improve the quality of perovskite thin films and thus promote the electroluminescence properties.Polyvinyl Pyrrolidone (PVP),a common non-ionic polymer,its structure is relatively simple and carbon segment length can be accurately controlled by molecular weight.Moreover,PVP contains a carbonyl group,which can effectively passivate the defects in perovskite.In this work,perovskite crystallization kinetics has been tuned by the incorporation of Polyvinyl Pyrrolidone (PVP) with different segment length.Based on in-situ photoluminescence and X-Ray Diffraction (XRD) spectra,it showed that perovskite crystallization rate is retarded to inhibit small n phase formation by increasing the PVP segment length,which played an important role in determining perovskite film quality,such as crystallinity,charge carrier recombination and roughness,etc.After different segment PVP was added to the precursors,the absorption peaks at 403 nm(n=1),434 nm(n=2) and 465 nm(n=3) in the Ultraviolet-Visible Spectrum of the quasi-two-dimensional perovskite low-dimensional phase are significantly inhibited,which are considered adverse to Photoluminescence performance.According to space charge limited current tests,compared with films without additives,the defect density of the films decreased significantly after adding different segment PVP,reaching the minimum at PVP-10 kDa.And with the segment length continuing to increase (10 kDa),the defect density will increase slightly.The short PVP segment(≤3 kDa) was not effective on adjusting the crystallization rate,resulting in minor improvement in roughness and photoluminescence quantum yield (PLQY) of the perovskite film.While the too-long PVP segment(≥30 kDa) inhibited crystallization,leading to high roughness,low crystallinity and deteriorated PLQY of the perovskite film.1With the suitable PVP segment length (~10 kDa),the perovskite crystallization kinetics can be optimized,which exhibited decreased roughness from 1.489 nm to 0.954 nm,decreased defect density from 1.55×10^(18) cm^(-3) to 1.05×10^(18) cm^(-3),improved PLQY from 16.9%to 58.9%.Combined with superior morphology and PLQY of the perovskite films with PVP-10 kDa,it is promising to achieve a high electroluminescence performance.The PeLEDs before and after incorporation of PVP-10 kDa were prepared with a stacking structure ITO/PVK/Perovskite/TPBi/LiF/Al.Compared with the control devices,the PVP-10 kDa devices showed little change in luminance but significantly reduced current density,which benefited improved maximum external quantum efficiency (EQE) from 8.55%to 18.00%.This work systematically investigated polymer segment length dependent perovskite crystallization and electroluminescence performance,which provides a guideline to design polymer additives to further promote the development of PeLEDs.