稳定高效的全无机钙钛矿发光二极管：抗溶剂处理详解

有机和无机卤化物钙钛矿具有独特的光学和电学特性，如宽带波长吸收、光致发光（PL）效率高、窄半峰全宽（FWHM）、带隙可调性以及较好的光物理稳定性等。作为光电子器件的活性材料，已经受到各国研究人员的广泛关注，并可能在光电子器件，如钙钛矿发光二极管 PeLED，得到突破性的应用。

在钙钛矿发光二极管、激光以及背光源应用中，钙钛矿材料的光电特性对发光器件的性能至关重要。特别是在PeLED中，人们不仅优化发光二极管器件结构提高发光性能，还通过改进钙钛矿薄膜的形貌和结晶性能进一步提高发光二极管的性能。PeLED的激子发射衰减很快，这说明它可以将非辐射能量转移到陷阱态。

与有机钙钛矿相比，全无机钙钛矿因具有较高的耐热性而备受关注。例如，基于铯-溴化铅（CsPbBr3）多晶薄膜的PeLED凭借更强的热稳定性提高了器件性能，成为一种极具潜力的替代技术。然而，对湿度、光和热的敏感性仍然是多晶薄膜PeLED面临的主要挑战。基于多晶CsPbBr₃薄膜的PeLED往往因为薄膜覆盖率不足以及光致发光效率（PLQY）较低，从而导致发光二极管亮度不高以及能量效率较低。

通过采用一些有机化合物，如甲基丙烯酸甲酯（MA）、氯苯（CB）、乙醇（EtOH）和异丙醇（IPA）等对薄膜进行后处理，可以提高成膜均匀性，并抑制漏电流。均匀的CsPbBr3多晶膜可以完全覆盖发光区域，是提高PeLED效率和亮度的重要前提。

东南大学研究小组的Hussain博士提出一种稳定高效的全无机钙钛矿发光二极管，相关论文发表在Chinese Optics Letters第19卷，第3期的金属卤化物钙钛矿及其应用专题中（S. Hussain, et. al., Stable and high performance all-inorganic perovskite light-emitting diodes with anti-solvent treatment）。来自东南大学的Qasim Khan教授和雷威教授对这项工作进行了指导。研究人员对基于CsPbBr3的器件性能进行了研究分析。

从SEM图像可以看出，改进钙钛矿薄膜的成膜特性后，PeLED的漏电流明显减小，最终器件的最低电流密度表示高结晶钙钛矿薄膜会阻挡器件中的短路电流路径。钙钛矿薄膜的表面覆盖率越高，发射层中的电荷注入越强，器件性能越高。采用抗溶剂处理的钙钛矿薄膜使器件的亮度得到了改善，短路电流得以最小化，电流效率得到了惊人的提高。通过抗溶剂处理可改进钙钛矿薄膜的表面形貌，同样也提高了发光二极管的外量子效率EQE。

图（a）钙钛矿发光二极管器件的示意图和（b）亮度-电压特性。(b)

制备工艺如下：

1. 1.首先，将图案化的氧化氟锡(FTO)透明导电玻璃用清洗剂、去离子水、丙酮、乙醇和异丙醇各超声清洗15分钟，然后使用紫外线臭氧处理30分钟。
2. 2.将氧化锌（ZnO）纳米颗粒（在25 mg ml^-1的丁醇中，通过0.22 μm N66滤头过滤）以2000 rpm的速度在基板上旋涂40 s，然后以140 °C退火10分钟。
3. 3.将摩尔比为1.2 : 1（0.3 M）CsBr和PbBr2与无水二甲基亚砜（DMSO）混合，在80^oC下以800 rpm充分搅拌4小时，形成钙钛矿前驱液。
4. 4.将钙钛矿前驱液以2000 rpm的速度在ZnO纳米颗粒上旋转50秒，延迟5秒以后再滴加乙酸甲酯。形成钙钛矿薄膜以后，再用氯苯作为反溶剂处理钙钛矿层。
5. 5.然后，将poly-TPD（10 mg ml^-1 in CF）以2500 rpm转速旋涂到钙钛矿薄膜上40秒，将其作为空穴传输层。
6. 6.最后，采用热蒸镀的方法通过掩模沉积金电极，制备的发光二极管有效面积为0.04 cm^-2。该发光二极管结构简单，且可以在大气环境下制备。

研究人员提出了一种简单易行的方法来制备高质量CsPbBr3钙钛矿薄膜，并由此提高PeLED的效率。 将抗溶剂氯苯滴到经过乙酸甲酯处理的CsPbBr3膜上可以提高钙钛矿薄膜的覆盖率，并且改善结晶质量。实验结果表明，该发光二极管的开启电压仅为2.0V，最大亮度达到17866 cd m^-2，电流效率为45.8 cd A^-1。

该方法可以便捷地控制钙钛矿薄膜光电子器件中载流子的注入以及降低表面缺陷，可以低成本地提高发光和显示器件性能。此外，经过溶剂有效处理后，发光二极管抗水和氧性能提高，器件的寿命和稳定性也随之改善。因此经过抗溶剂处理的钙钛矿薄膜可望成为高效率CsPbBr₃发光二极管的优秀活性材料。

Stable and high performance all-inorganic perovskite light-emitting diodes with anti-solvent treatment

Organic and inorganic halide perovskites have attracted a lot of attention due to their extensive use in various optoelectronic applications given their unique properties like broad band wavelength absorption, high photoluminance (PL), narrow full width at half-maximum (FWHM), band gap tunability, and intrinsic photophysical stability. Perovskite material is widely applied in light- emitting diodes (LEDs), lasers, and photodetectors. Particularly, perovskite LED (Pe-LED) has been studied, and its efficiency has been significantly improved by enhancing the crystallinity and morphology of perovskite film. Despite the advantages of the fast free exciton emission attenuation described above, it may help transfer non-radiant energy to trapped states.

All-inorganic perovskites have attracted extraordinary attention due to their high heat resistance compared to organic perovskites. For example, cesium lead bromide (CsPbBr₃) polycrystalline film-based Pe-LEDs offer an auspicious substitute technique increasing the device performance taking account of their enhanced thermal stability. However, sensitivity to humidity, light, and heat are still the main challenges Pe-LEDs facing. Nonetheless, various polycrystalline CsPbBr₃-based Pe-LEDs suffer from low brightness (L) and inefficiency owing to their low film surface coverage and PL quantum yield (PLQY). Through post-treatment of organic compounds such as methyl acetate (MA), chlorobenzene (CB), ethanol (EtOH), and isopropanol (IPA) plays an important role for achieving uniform thin films and prevent leakage current, due to completely covered CsPbBr₃ polycrystalline film with high PLQYs have been verified as a significant prerequisite to improve both efficiency and luminance of Pe-LEDs.

The device introduced by Dr. Hussain from a research group in Southeast University China published in Chinese Optics Letter (COL), Volume 3, Issue 01, 2021 （S. Hussain, et. al., Stable and high performance all-inorganic perovskite light-emitting diodes with anti-solvent treatment）. This work was supervised by Prof. Qasim Khan and Prof. Lei Wei from Southeast University China who believe that these characteristics are of great importance for optoelectronic devices. The performance of CsPbBr₃-based device is studied. The lowest current density (J) of final device is an indication of the high crystalline perovskite film blocking the short-circuit current paths in the device, as revealed by the SEM images. The better surface coverage of the perovskite film in turn results in higher device performance due to better charge injection in the emissive layer (EML). The improvement in luminance of device with anti-solvent-treated perovskite film, indicate that the minimized short circuit as well as an incredible enhancement in current efficiency (CE) is found, and higher external quantum efficiency (EQE) is dedicated to the improved surface morphology and affects the performance of the device.

(a) Schematic diagram of Perovskite Light-emitting diode device and (b) Luminance-Voltage characteristics.

The patterned fluorine-doped tin oxide (FTO) substrates were cleaned with detergent, deionized (DI) water, acetone, EtOH, and IPA for 15 min each using the ultrasonication cleaner and treated with UV ozone for 30 min before use. The ZnO nanoparticles (NPs) (25 mg ml ⁻¹ in butanol, filtered through a 0.22 μm N66 filter) was spin-coated on the substrate at 2000 rpm for 40 s and annealed at 140°C for 10 min. The active layer perovskite precursor solution CsBr and PbBr₂ with a molar ratio of 1.2:1 (0.3 M) was mixed in anhydrous dimethyl sulfoxide (DMSO, 80°C, 4 h, 800 rpm), spun onto the ZnO NPs at 2000 rpm for 50 s along the dropped MA delay of 5 s after starting the spin coater, and finally treated with an anti-solvent CB perovskite layer. Then, poly-TPD (10 mg ml ⁻¹ in CF) as an HTL was spin-coated onto the perovskite film at 2500 rpm for 40 s. Finally, Au was deposited by thermal deposition technique through a shadow mask. The active area of our fabricated devices was 0.04 cm⁻². This device presents a simple structure and the fabrication was carried out in ambient conditions.

We have presented a simple and facile method for obtaining high quality CsPbBr₃ perovskite film for efficient Pe-LEDs. Dropping anti-solvent CB onto the treated MA CsPbBr₃ film can obtain high coverage of the active layer with better crystallinity. As a result, the devices showed a low turn on voltage (V_T) of 2.0 V and maximum luminance of 17,866 cd m⁻² with current efficiency of 45.8 cd A⁻¹. This initiative suggests a simple way of controlling the carriers' injection along with reducing surface defects in optoelectronic devices and lighting technologies based on perovskites for their utilization in high resolution displays and cost-effective technologies. Most significantly, the process treated with solvent enabled the fabrication of all-inorganic CsPbBr3-based LEDs with high efficiency, taking advantages of the longevity of the device and stability against environmental moisture and oxygen.