1.Mechanical Engineering Institute, Nanjing Institute of Technology, Nanjing 211167, China 2.Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0404501), the National Natural Science Foundation of China (Grant No. 61274065), the Science Fund for Distinguished Young Scholars of Jiangsu Province, China (Grant No. BK20160039), and the Major Projects of Innovation Fund of Nanjing Institute of Technology (Grant No. CKJA201602)
Received Date:23 August 2020
Accepted Date:21 September 2020
Available Online:04 February 2021
Published Online:20 February 2021
Abstract:Appreciable progress of organometal halide perovskite materials has been achieved in recent years due to their controllable synthesis and excellent optoelectronic properties. And the potential uses of these perovskites in photovoltaics, light-emitting diodes (LEDs), photodetectors and lasers have been successfully demonstrated. Although organometal halide perovskites appear as emitters with extremely high color purity and low cost, the device performance is significantly limited by poor morphology of the perovskite layer. The addition of the polymer into the perovskite layer is a convenient and effective method to improve the homogeneity of the spin-coated perovskite film. In this work, we fabricate green perovskite light emitting diodes (PeLEDs) with poly(styrenesulfonate) (PSS)-modified poly(3,4-ethylenedioxythiophene):PSS (PEDOT:PSS) as the hole injection layer (HIL) and a single spin coating composite film consisting of methylammonium lead tribromide (MAPbBr3) and poly(ethylene oxide) (PEO) as the emissive layer. The PSS doping increases the work function of PEDOT:PSS and reduces the injection barrier between PEDOT:PSS HIL and MAPbBr3 perovskite, thus balancing the carriers within the PeLEDs. The PEO doping enables the MAPbBr3 to become a dense and uniform perovskite film with a ~100% coverage. With the above approaches, highly efficient PeLEDs with maximum luminance and current efficiency of 2476 cd·m–2 and 7.6 cd·A–1 are eventually acquired. This work provides a method of fabricating the high-coverage and high-efficiency PeLEDs. Keywords:perovskite/ light emitting diode/ composite film/ charge injection
图 2 不同PSS与PEDOT:PSS体积比的PeLEDs的特性 (a)亮度-电压特性; (b) 电流密度-电压特性; (c) 电流效率-电压特性 Figure2. The characteristics of PeLEDs with varying volume ratio of PSS and PEDOT:PSS: (a) Luminance-voltage; (b) current density-voltage; (c) current efficiency-voltage.
图 4 本征MAPbBr3钙钛矿和MAPbBr3:PEO(1∶1)钙钛矿薄膜的XRD. 图中Pe为MAPbBr3的简写 Figure4. The XRD of the perovskite films of pristine MAPbBr3 and MAPbBr3:PEO (1∶1). Here, Pe is the abbreviation of MAPbBr3.
图5给出了纯MAPbBr3和MAPbBr3:PEO(1:1)复合膜的光致发光(PL)光谱. 从图中可以看出, 两者的PL谱形状相似, 峰值位置均为539 nm, 可见PEO掺杂并不影响钙钛矿膜层本身的发光. 此外, 可以看出MAPbBr3:PEO复合膜的PL强度略低于标准钙钛矿样品, 这与PEO的加入对钙钛矿产生稀释致使PL强度降低有关. 以上分析结果表明, MAPbBr3:PEO复合膜不仅改善了膜层质量, 提高了覆盖率, 还保持了MAPbBr3的光学特性, 因此有望在器件应用中获得良好的效果. 图 5 MAPbBr3和MAPbBr3:PEO (1∶1)复合薄膜的PL光谱 Figure5. Photoluminescence spectra of the MAPbBr3 and MAPbBr3:PEO (1∶1) composite thin films.
我们在上述膜层分析的基础上, 制备了不同PEO掺杂比的PeLEDs, 器件的亮度、电流密度、电流效率和电致发光光谱曲线如图6所示. 图中的MAPbBr3:PEO的掺杂比分别为1∶0, 1∶0.75, 1∶1, 1∶1.25. 基于上述掺杂比的PeLEDs的启亮电压(1 cd·m–2)分别为3.5, 3.5, 3.2, 3.3 V. 1:1器件在5 V条件下, 亮度大于1000 cd·m–2, 在6.5 V偏压下达到最大, 为6762 cd·m–2. 1∶0.75器件的发光亮度最大为2794 cd·m–2, 1∶1.25器件最大为2476 cd·m–2, 后两者最大亮度均明显低于1:1器件. 在1∶0.75, 1∶1, 1∶1.25器件中, 最高的电流效率分别达到2.0, 6.2, 7.6 cd·A–1, 均高于未掺杂PEO标准器件的0.39 cd·A–1. 其中, 最佳掺杂比器件(1∶1)效率比标准未掺杂器件高出18.5倍, 可见均匀性和高覆盖率的膜层对器件发光性能的影响非常大. 从图6(e)电致发光光谱可以看出, PEO掺杂未对MAPbBr3发光光谱产生影响, 光谱的峰值(524 nm)保持不变. 此外, 我们测试了MAPbBr3:PEO掺杂比分别为1∶0, 1∶0.75, 1∶1, 1∶1.25时器件的半衰期寿命(即亮度降至初始值(100 cd·m–2)一半时的寿命), 发现掺杂适量PEO有助于改善器件的稳定性, 半衰期寿命值从标准器件的15 s分别提升至125 s (1∶0.75)、275 s (1∶1)和203 s (1∶1.25). 器件寿命的提升一方面得益于致密均匀的钙钛矿膜层质量, 另一方面与PEO包覆于钙钛矿晶体表面, 对其表面缺陷存在一定的钝化效果有关[27]. 图 6 (a) 使用了MAPbBr3:PEO的器件结构示意图; 不同MAPbBr3:PEO体积比钙钛矿所制备出发光器件的(b)亮度-电压, (c)电流密度-电压, (d)电流效率-电压和(e)电致发光光谱曲线 Figure6. (a) PeLED structure using MAPbBr3:PEO, and the (b) luminance, (c) current density-voltage, (d) current efficiency-voltage and (e) electroluminant curves of PeLEDs with varying volume ratio of MAPbBr3:PEO.