1.Institute of Photoelectronic Thin Film Devices and Technology, Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Tianjin 300350, China 2.Third central hospital of Tianjin, Tianjin 300170, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grants Nos. 61076006, 61377031), National High Technology Research and Development Program of China (863 Plan) (Grant No. 2002AA303260), and Basic Scientific Research of Nankai University in the 2019 Project
Received Date:12 May 2020
Accepted Date:06 June 2020
Available Online:15 June 2020
Published Online:05 October 2020
Abstract:Despite the fact that three-dimensional organic-inorganic hybrid perovskite is regarded as a promising material in the field of optoelectronics and microelectronics due to its excellent photoelectric properties, however, the instability under the moisture environment and the gate-voltage screening effect associated with ionic transport are still serious, which restricts the development of perovskite devices. Here in this work, the lead iodide perovskite (PEA)2(MA)n–1PbnI3n+1 series are successfully prepared by one-step solution method, including pure-two-dimensional (pure-2D), quasi-two-dimensional (quasi-2D) and traditional three-dimensional (3D) perovskite materials. The dimension and microstructure of the perovskites are regulated, and the effects of dimensions on the performance of organic-inorganic hybrid perovskite materials are investigated firstly. The crystallization of the 2D perovskites and 3D perovskite films are observed obviously. Moreover, the surface of pure-2D perovskite film with discoid, regular and micron-sized grains is smoother than that of 3D perovskite film. And also, the unapparent grain boundary is exhibited in the quasi-2D perovskites. A uniform perovskite film with full coverage and inconspicuous grain boundaries facilitates the transmission capacity of the charge carriers in the channel layer due to the reduction of defects caused by the grain boundaries. And benefited from the high-quality films with inconspicuous grain boundary as demonstrated, the quasi-2D hybrid perovskite film exhibits a longer carrier lifetime (τns) than traditional 3D MAPbI3 perovskite film, revealing that the layered 2D structure is more favorable for carrier transport due to the fewer defects in it. In addition, under the condition of the same environment humidity, the 2D perovskite materials show better moisture stability. Then, to investigate the influences of dimensional structure on the perovskite field-effect devices, we fabricate the bottom-gate and top-contact thin film transistors (TFTs) based on the perovskite materials with different dimensions. As a result, the instability and ion migration effect for each of the devices are suppressed effectively due to the distinct 2D layer-structure and quantum confinement effect, which leads the device performance to be further improved. The device based on quasi-2D (n = 6) channel TFT achieves a hole mobility (μhole) of 3.9 cm2/(V·s), an on-off current ratio of 104 and more, and a 1.85V turn-on voltage of 1.85 V. The first application of quasi-2D organic and inorganic hybrid perovskite materials to thin film transistors provides a new idea for preparing the high-performance and stable thin film transistor devices. Keywords:two-dimensional perovskite/ environmental stability/ thin film transistors/ hole mobility
首先, 测试分析了维度对有机无机杂化钙钛矿微观结构的影响. 不同维度有机无机杂化钙钛矿材料的SEM如图3所示, 纯二维钙钛矿薄膜具有规则的圆盘状、微米量级晶粒以及相对光滑的形貌和模糊的晶界, 这有利于减少薄膜中的缺陷态, 提高场效应器件沟道层中载流子输运能力. 传统的3D钙钛矿薄膜晶粒尺寸虽然也能达到微米量级, 但从图4所示AFM图谱可以看出, 相对于2D钙钛矿薄膜, 其不规则的晶粒形状及明显的晶界使得钙钛矿薄膜表面更加粗糙 (RMS = 49.9 nm). 图 3 不同维度钙钛矿薄膜扫描电子显微镜图 (a)纯二维; (b) n = 3的准二维; (c) n = 6的准二维; (d) 三维钙钛矿. 扫描电子显微镜图比例尺为1 μm Figure3. SEM images of perovskite films with different dimensions (a) Pure-2D; (b) Quasi-2D (n = 3); (c) Quasi-2D (n = 6); (d) 3D. The scale bar is 1 μm for the SEM images.
图 4 不同维度钙钛矿薄膜表面原子力显微镜图 (a) 纯二维; (b) n = 3的准二维; (c) n = 6的准二维; (d) 三维钙钛矿. 原子力显微镜图比例尺为5μm Figure4. Top-surface AFM images of perovskite films with different dimensions: (a) Pure-2D; (b) quasi-2D (n = 3); (c) quasi-2D (n = 6); (d) 3D. The scale bar is 5 μm for the AFM images.
准二维 (n = 3, 6) 有机无机杂化钙钛矿薄膜形貌较为光滑, 没有出现明显的晶界, 难以确定其晶化情况, 但XRD图谱(图5) 表明准二维 钙钛矿具有良好的结晶性. 纯二维 钙钛矿薄膜在5.57°、10.83°、16.28°和21.79°出现了明显的衍射峰, 分别对应PEA2PbI4 的(002)、(004)、(006)和(008)晶体取向, 表明纯二维钙钛矿材料强烈倾向完全平行于衬底的晶体取向, 理论上这有利于载流子的横向输运. 一旦无机层层数增加(n > 1), 试图将钙钛矿的生长限制在平面内的PEA离子与尝试扩展层外生长的MA离子之间产生竞争[15], 因此制备的准二维 钙钛矿虽然在14.08°、28.41°和31.85°与3D钙钛矿有相似的衍射峰, 但其晶体取向不同. 图 5 不同维度钙钛矿薄膜的XRD图谱 Figure5. XRD patterns of perovskite films with different dimensions.
表2不同维度钙钛矿薄膜的水接触角 Table2.Summary of water contact angle of perovskite films with different dimensions
图 7 不同维度钙钛矿薄膜的水接触角 (a) 纯二维; (b) n = 3的准二维; (c) n = 6的准二维; (d) 三维钙钛矿; (e) 不同维度钙钛矿在30%湿度的空气环境下暴露前后对比图 Figure7. Water contact angle of perovskite films with different dimensions: (a) Pure-2D; (b) Quasi-2D (n = 3); (c) Quasi-2D (n = 6); (d) 3D; (e) images of perovskite films with different dimensions before and after exposed to 30% humidity of the ambient environment.
23.2.维度对有机无机杂化钙钛矿薄膜晶体管(TFTs)性能的影响 -->
3.2.维度对有机无机杂化钙钛矿薄膜晶体管(TFTs)性能的影响
为进一步探究材料维度对实际场效应器件的影响, 制备了以不同维度有机无机杂化钙钛矿材料为半导体沟道层的底栅顶接触TFTs器件, 同时蒸发8 nm 厚的MoO3层作为金属电极与钙钛矿有源层之间的缓冲层, 在改善电极与有源层界面质量的同时, 对钙钛矿材料进行p掺杂[29]. 图8为不同器件的转移特性曲线, 不同维度钙钛矿对应的TFTs器件均表现出明显的场效应特性并呈现出双极性偏P型特点, 且器件电流开关比均高于104. 根据器件转移特性和(2)式提取了TFTs器件在线性区的相关性能参数. 图 8 基于不同维度钙钛矿的薄膜晶体管器件转移特性曲线 (a)Ids-Vg; (b) $ I_{\rm ds}^{1/2} $-Vg 对比图 Figure8. A comparison of typical transfer characteristic curve of the TFT devices based on the perovskite with different dimensions: (a) Ids-Vg; (b) $ I_{\rm ds}^{1/2} $-Vg, respectively.
表3基于不同维度钙钛矿的薄膜晶体管器件的相关性能参数 Table3.Summary of related performance parameters of TFT devices based on the perovskite with different dimensions.
图 9 (a) n = 20的准二维; (b) n = 30的准二维钙钛矿扫描电子显微镜图; (c) n = 20的准二维; (d) n = 30 的准二维钙钛矿原子力显微镜图. 扫描电子显微镜比例尺为500 nm; 原子力显微镜比例尺为5 μm Figure9. SEM images of perovskite films (a) Quasi-2D (n=20); b) Quasi-2D (n=30). The scale bar is 500 nm for the SEM images and 5 μm for the AFM images.