Photoluminescence and electroluminescence properties of aligned CsPbBr3 nanowire films prepared by off-center spin-coating
Introduction
After decades of development, liquid crystal display (LCD) devices have occupied an important position in human life, such as mobile devices, television equipment, and in-vehicle equipment. Polarized light sources are important to enhance optical efficiency and reduce the dependence on polarizer in LCD backlighting. [1,2] There have been many reports on polarized light-emitting diodes (LEDs) based on III-V group compound (InGaN-GaN) which needs expensive fabricating equipment and complicated process [[3], [4], [5], [6]]. Solution-processing polarized LEDs are therefore meaning to reduce the costs [7,8].
Solution-processing polarized LEDs based on anisotropic one-dimensional nanomaterials have attracted interest recently. [9,10] Hikmet et al. reported polarized LEDs using core-shell CdSe/CdS quantum rod as the emission material and the inorganic film was rubbed to produce luminance polarization [9]. The ratio of the intensities for orthogonal polarization directions for photoluminescence and electroluminescence spectra was 2.2 and 1.6 respectively. Rizzo et al. assembled CdSe/CdS nanorods in smectic and nematic phases on the surface of the water and then transferred the film onto organic layers by microcontact printing [10]. The resulted LED delivered a maximum brightness of 170 cd/m2 at 19 V and a maximum polarization ratio of 0.34. Due to the superior photoluminescence property of lead halide perovskite material, there have been some reports on their polarized photoluminescence. Wang et al. found that inorganic CsPbX3 perovskite quantum dots showed high polarization properties and the polarization ratio reached up to 0.36 in hexane and 0.40 in film. [11] Perovskite/polymer composite film with polarized photoluminescence has also been fabricated by mechanical stretching and electrospinning [12,13]. However, to the best of our knowledge, there have been few reports on polarized electroluminescence of lead halide perovskite up to now.
Various strategies have been developed to align one-dimension nanomaterials into ordered patterns, including template-assisted assembly, [[14], [15], [16], [17]] Langmuir-Blodgett deposition [18], mechanical rubbing [19,20], mechanical stretching [[21], [22], [23], [24]] and off-center spin-coating (OCSC) [[25], [26], [27]]. For the OCSC method developed by Yuan et al., [25] the substrate is placed away from the spin coater's center instead of in the center. When the spin coater starts to spin, the materials in the solvent are ordered by the outward centrifugal force. Due to the easy process which is free of the complicated film transfer process, OCSC method was used here to align CsPbBr3 nanowires into ordered patterns for polarized PL and EL.
Section snippets
Experimental section
Materials. Lead(II) bromide (99.9 %), cesium bromide (99.9 %), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Clevios P AI 4083), poly[bis(4-phenyl)(4-butylphenyl)amine] (Poly-TPD), 2′,2′-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) and LiF were purchased from Xi’an Polymer Light Technology Corp. Hexane and toluene were purchased from Sinopharm Chemical Reagent Co., Ltd. All materials were used as received without further purification.
Synthesis of CsPbBr3
Results and discussion
The CsPbBr3 nanowires were synthesized using the approach developed by Zhang et al. [28] Briefly, as-prepared Cs-oleate precursor solution was injected into a flask to react with PbBr2 to produce the CsPbBr3 nanowires. The CsPbBr3 nanowires show an absorption peak at 509 nm and the PL emission peak is 520 nm with a narrow FWHM of 20 nm (Fig. 1a). The diameter and length of the nanowires are about 15 nm and up to several micrometers respectively (Fig. 1b and c). The nanowires tend to form
Conclusions
In summary, we demonstrated PL polarized emission of CsPbBr3 nanowires aligned by the OCSC method, optimized the condition parameters of the OCSC method and achieved a polarization ratio of 0.46. We fabricated the CsPbBr3 nanowire LED with a turn-on voltage of 10.9 V. The film morphology plays a crucial role in the device performance and needs further investigation.
CRediT authorship contribution statement
Jianyue Wang: Methodology, Investigation, Writing - original draft. Yaping Wei: Investigation, Resources. Yinyan Xu: Visualization. Qian Wang: Visualization. Hongbo Lu: Resources. Longzhen Qiu: Resources, Supervision. Jun Zhu: Conceptualization, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Distinguished Youth Foundation of Anhui Province (1708085J09), the National Key Research and Development Program of China (2017YFE0133800) and the Fundamental Research Funds for the Central Universities (JZ2018HGPB0276). The Key Lab of Photovoltaic and Energy Conservation Materials, Chinese Academy of Sciences is gratefully acknowledged by the authors.
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