1.Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, Tianjin 300350, China 2.Key Laboratory of Photoelectronic Thin Film Devices and Technology of Tianjin, Tianjin 300350, China 3.Engineering Center of Thin Film Photoelectronic Technology of Ministry of Education, Tianjin 300350, China 4.Sino-Euro Joint Research Center for Photovoltaic Power Generation of Tianjin, Tianjin 300350, China 5.Department of Chemistry, Nankai University, Tianjin 300071, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB1500103), the National Natural Science Foundation of China (Grant No. 61674084), the Overseas Expertise Introduction Project for Discipline Innovation of Higher Education of China (Grant No. B16027), Tianjin Science and Technology Project, China (Grant No. 18ZXJMTG00220), and the Fundamental Research Funds for the Central Universities, Nankai University, China (Grant Nos. 63191736, ZB19500204).
Received Date:26 February 2019
Accepted Date:08 April 2019
Available Online:06 June 2019
Published Online:20 June 2019
Abstract:Organometal halide perovskites featuring solution-processable characteristics, high photoluminescence quantum yield (PLQY), and color purity, are an emerging class of semiconductor with considerable potential applications in optoelectronic devices. Electron injection layer is an important component of perovskite light-emitting device, which determines the growth of perovskite film directly. In this paper, the perovskite light-emitting diodes (PeLEDs) based on n-type nanocrystalline silicon oxide (n-nc-SiOx:H) electron injection layer are designed and realized. This novel electron injecting material is prepared by the plasma enhanced chemical vapor deposition (PECVD), and its smooth surface and matched energy band result in superior perovskite crystallinity and low electron injection barrier from the electron injecting layer to the emissive layer, respectively. However, the external quantum efficiency (EQE) of PeLED is as low as 0.43%, which relates to defects and leakage current due to the incomplete surface coverage of perovskite film. The fast exciton emission decay (< 10 ns) stems from strong non-radiative energy transfer to the trap states, and represents a big challenge in fabricating high-efficiency PeLEDs. In order to obtain desirable perovskite film morphology, an excessive proportion of methylammonium bromide (MABr) is incorporated into the perovskite solution, and a volume of benzylamine (PMA) is added into the chlorobenzene antisolvent. The perovskite films suffer low PLQY and short PL lifetime if only MABr or PMA is introduced. When the molar ratio of MABr is higher than 60%, the luminescence quenching arising from Joule heating is depressed by employing PMA, contributing to a higher PLQY (> 30%) and a longer carrier lifetime. The synergistic effect of MABr and PMA increase the coverage and reduce the trap density of perovskite film, inhibit the luminescence quenching in the annealing process, and thus facilitating the perovskite film with higher quality. Finally, the n-i-p PeLED exhibits green-light emission with a maximum current efficiency of 7.93 cd·A-1 and a maximum EQE up to 2.13% is obtained. These facts provide a novel electron injecting material and a feasible process for implementing the PeLEDs. With further optimizing the perovskite layer and device configuration, the performance of n-i-p type PeLEDs will be improved significantly on the basis of this electron injection material. Keywords:perovskite/ light-emitting diodes/ n-type nanocrystalline silicon oxide/ photoluminescence quantum yields
从图2(a)可以看出, n-nc-Si:H的表面粗糙度(13.2 nm)明显较n-nc-SiOx:H (5.4 nm)更高, 因而基于这两种电子注入层所生长的MAPbBr3钙钛矿薄膜因衬底不同而有所差异. 在衬底上使用一步旋涂溶液法制备钙钛矿薄膜, 并对所制备的薄膜进行X射线衍射分析, 结果如图2(b)所示. 以n-nc-Si:H和n-nc-SiOx:H为电子注入层所生长的钙钛矿薄膜均在14.94°, 30.14°出现了明显的峰值, 分别对应MAPbBr3晶体的(001), (002)衍射峰. 随着电子注入层从n-nc-Si:H到n-nc-SiOx:H的改变, 对应于(001), (002)衍射峰也逐渐提高, 说明晶体的结晶性有所提高. 图2(c)中, 对所制备的薄膜进行PL光谱分析, 可以看到, 基于n-nc-Si:H电子注入层的钙钛矿薄膜的PL峰值较低, 其半高宽为22 nm. 当采用n-nc-SiOx:H作为电子注入层制备钙钛矿薄膜, 其PL峰值增高, 更利于载流子的辐射复合发光, 同时PL峰半高宽略有降低, 亦证明此时钙钛矿薄膜的结晶质量更高. 如表1所列, 以n-nc-SiOx:H为电子注入层所制备的器件, 其性能显著提升, 最大亮度(Lmax)达到2100 cd·m–2, 最大电流效率(CE)为1.37 cd·A–1, 最大EQE为0.43%, 实验上表明了n-nc-SiOx:H可以作为新型电子注入材料应用于PeLEDs中. 图 2 不同衬底对钙钛矿薄膜的影响 (a)不同衬底表面的原子力显微镜图; (b)不同衬底上生长的钙钛矿薄膜X射线衍射图; (c)不同衬底上生长的钙钛矿薄膜PL光谱图 Figure2. Influence of different substrates on perovskite films: (a) Atomic force microscopy images of different substrate surfaces; (b) X-ray diffraction patterns of perovskite films on different substrates; (c) photoluminescence spectra of perovskite films on different substrates.
电子注入层
Lmax/cd·m–2
CE/cd·A–1
EQE/%
n-nc-Si:H
650
0.4
0.1
n-nc-SiOx:H
2100
1.37
0.43
表1基于两种不同电子注入层的PeLEDs器件性能的比较 Table1.Performance of PeLEDs based on different electron injection layers.
23.3.基于n-nc-SiOx:H的PeLEDs器件性能提高的研究 -->
3.3.基于n-nc-SiOx:H的PeLEDs器件性能提高的研究
为了进一步降低电子注入势垒, 采用无机CsPbBr3钙钛矿材料, 其导带约为3.6 eV, 相较于MAPbBr3材料, 电子注入势垒降低了一半. 而且, 无机CsPbBr3钙钛矿具有更好的热稳定性, 更平衡的电子空穴迁移率-寿命乘积(μτ), 更长的载流子寿命(2.5 μs)等优点[32]. 但是无机CsPbBr3钙钛矿薄膜覆盖率较低, 薄膜孔洞较多导致缺陷复合严重. 图3(a)所示为采用三种钙钛矿成膜工艺来试图改进钙钛矿薄膜质量. 方法一是在采用一步法旋涂工艺, 采用氯苯反溶剂, 得到的CsPbBr3钙钛矿薄膜, 其表面粗糙度为28.7 nm, 从图3(b)可看出, 钙钛矿薄膜的孔洞很多, 制备的钙钛矿发光器件无法启亮, 无器件效率. 图 3 钙钛矿成膜工艺 (a)三种钙钛矿薄膜制备工艺及对应的原子力显微镜图和实物图; (b)三种工艺下钙钛矿薄膜表面的扫描电子显微镜图 Figure3. Synthesis of perovskite film: (a) Different fabrication processes of perovskite films and the corresponding atomic force microscopy images and photographs; (b) planar scanning electron microscopy images of the perovskite films based on different fabrication processes.
其中, A(t)为t时刻的标准化PL强度; t0为起始时间点; τ1表示快衰退过程的载流子寿命, 与界面非辐射复合相关, τ2表示慢衰退过程的载流子寿命, 与晶粒内部辐射复合相关; A1和A2为两个衰退过程所占比例. 从图5(a)可进一步看出在反溶剂中不加入PMA的情况下, 不同浓度MABr的钙钛矿薄膜经退火处理后, 其载流子寿命小于10 ns, 这表明钙钛矿薄膜发光猝灭现象严重. 从图5(b)可以发现, 在反溶剂中加入1.0 vol.%的PMA的情况下, 当MABr比例为80% (0.8-PMA)时, 钙钛矿薄膜的载流子寿命达到最高, τ1提高到5.09 ns, τ2提高到28.6 ns, 此时钙钛矿薄膜内部缺陷最少, 更利于薄膜的辐射复合发光, 进一步表明了和PMA的协同作用对钙钛矿薄膜质量的显著改善. 图 5 钙钛矿薄膜在n-nc-SiOx:H基底下的TRPL图 (a)不加PMA时, 不同MABr浓度下钙钛矿TRPL图; (b)加入PMA时, 不同MABr浓度下钙钛矿TRPL图 Figure5. TRPL spectra of perovskite films on n-nc-SiOx:H: (a) TRPL spectra of perovskite films at different MABr concentrations without PMA additive; (b) TRPL spectra of perovskite films at different MABr concentrations with PMA additive.
将优化后的钙钛矿成膜工艺应用到以n-nc-SiOx:H为电子注入层的钙钛矿发光器件中, 得到的PeLEDs电致发光表现如图6(a)—(c)所示. 随着外置电压的升高, 注入电子空穴能力更强, 器件的电流密度随之提高. 不断注入的电子空穴使得钙钛矿发光层内辐射复合概率增加, 器件的光强也随之不断变高. 可看出, 器件的开启电压为3.4 V, 在偏置电压为5 V, 电流密度为4.7 mA·cm–2时, 器件的CE及EQE达到了最高值, 之后器件效率开始滚降, 随着电压的升高, 器件性能不断衰退, 这可能是源于俄歇复合所导致的发光猝灭, 或是由电流热效应导致钙钛矿内部不断积聚的热量所引发的[8,37]. 图6(d)所示为PeLEDs器件的CIE 1931色度图, 根据发光的CIE坐标可知该PeLEDs为绿光器件. 最终基于n-nc-SiOx:H电子注入材料, 通过优化钙钛矿成膜工艺, 获得了最大CE为7.93 cd·A–1, EQE为2.13%的n-i-p型PeLEDs, 器件性能较钙钛矿层优化前的 0.43%有了明显提升, 后续可通过继续改进器件结构, 提高钙钛矿薄膜质量来进一步提升PeLEDs的发光性能. 图 6 PeLEDs的电致发光表现 (a)器件的电流密度、光强随电压的变化; (b)器件的EQE随电流密度的变化; (c)器件的EQE随电压的变化; (d)器件发光对应的CIE坐标 Figure6. Electroluminescence of PeLEDs: (a) Current density and luminance of the device as a function of voltage; (b) EQE of the device as a function of current density; (c) EQE of the device as a function of voltage; (d) the corresponding CIE coordinate.