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铅卤钙钛矿的光稳定性研究进展

楼浩然 叶志镇 何海平

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铅卤钙钛矿的光稳定性研究进展

楼浩然, 叶志镇, 何海平

Recent advances in photo-stability of lead halide perovskites

Lou Hao-Ran, Ye Zhi-Zhen, He Hai-Ping
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  • 铅卤钙钛矿可实现溶液法制备且具有诸多优异的光电特性, 在高效太阳能电池、发光二极管、激光、光探测器等光电子领域具有广阔的应用前景. 但是铅卤钙钛矿在光辐照下存在不稳定问题, 严重影响其相关光电器件的寿命和性能稳定性. 因此, 铅卤钙钛矿在持续光照下的不稳定性现象及其机理正受到越来越多的关注. 本文综述了铅卤钙钛矿在持续光照下的四类主要不稳定现象, 即光修复现象、光解现象、光致相分离现象以及光致相变现象, 并介绍目前已提出的相关机理, 分别从缺陷态、离子迁移、热力学原理、化学键等角度来解释其光照不稳定性. 最后, 本文简要讨论了钙钛矿中光稳定性研究的复杂性及未来需要解决的问题.
    Lead halide perovskites, which can be grown by solution process, are very suitable for various optoelectronic applications, such as in solar cells, light-emitting diodes, lasers and photo-detectors, due to their outstanding electrical and optical properties. However, lead halide perovskites are unstable under continuous light illumination, which severely degrades the performance and lifetime of optoelectronic devices based on them. Therefore, the photo-stability of perovskites and the mechanisms have received more and more attention. In this article we review the main phenomena in perovskites, induced by continuous light illumination, namely photo curing, photo dissociation, photo-induced phase separation, and photo-induced phase transition. The proposed mechanisms for the instability from the perspectives of defect states, ion migration, thermodynamics and chemical bonds, respectively, are also introduced. Finally, the complexity presented in the studying of photo-stability, and the issues should be addressed in the future are also briefly discussed.
      通信作者: 何海平, hphe@zju.edu.cn
    • 基金项目: 国家自然科学基金(批准号: 51772271)资助的课题.
      Corresponding author: He Hai-Ping, hphe@zju.edu.cn
    • Funds: Project supported by the National Natural Science Foundation of China (Grant No. 51772271).
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  • 图 1  铅卤钙钛矿ABX3的结构示意图[1]

    Fig. 1.  Structure of lead halide perovskites[1].

    图 2  铅卤钙钛矿及其太阳能电池在持续光照下发生光修复现象时的性能变化 (a) CH3NH3PbI3薄膜在持续光照下的时间分辨PL衰减随光照时间的变化[57]; (b) CH3NH3PbI3薄膜在持续光照下以及不同温度下的PL强度随光照时间的变化[57]; (c) 铅卤钙钛矿太阳能电池ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al在持续光照下的J-V曲线变化[62]; (d) 铅卤钙钛矿太阳能电池ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al在持续光照下的光电转化效率的变化曲线[62]

    Fig. 2.  Performance changes of lead halide perovskite and its solar cells under continuous illunination: (a) A series of time-resolved PL decays from CH3NH3PbI3 films measured over time under continuous illumination[57]; (b) the normalized PL intensity of CH3NH3PbI3 films measured over time under continuous illumination at different temperatures[57]; (c) the J-V characteristics of lead halide perovskite solar cell with the architecture of ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al under continuous illumination[62]; (d) the reversibility of power conversion efficiency of lead halide perovskite solar cell with the architecture of ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al under continuous illumination[62].

    图 3  CH3NH3PbI3薄膜发生光修复现象的机理[57] (a) 在发光较弱的晶粒中分布有高浓度的缺陷态以及过量的碘离子; (b) CH3NH3PbI3薄膜经过光照后, 电子迅速填充缺陷, 产生电场, 造成碘离子从照射区域开始迁移, 填充空位; (c)照射区域中的缺陷态密度和碘离子浓度降低, 薄膜最终达到稳定的状态; (d) 当移除光照后, 碘离子存在浓度梯度, 驱使其扩散回弱发光区域, 直至达到新的碘离子浓度平衡

    Fig. 3.  Mechanism of photo curing in CH3NH3PbI3 film[57]: (a) Iodide in a "dark spot" is initially high with a corresponding excess of trap density; (b) electrons quickly fill traps under illumination, inducing an electric field that results in iodide to migrate away from the illuminated region and fill vacancies; (c) the fill will reaches a stabilized state, with the reduction of the density of the traps and the concentration of the iodide in illuminated region; (d) when the illumination is removed, there may be a concentration gradients driving some iodides in to the dark spot and the film will reaches a new equilibrium with a net redistributed iodide profile.

    图 4  (a) 传统旋涂法制备的CH3NH3PbI3薄膜在激发密度0.2和0.65 W/cm2下的光致发光强度随时间的变化; (b) 气体辅助溶液工程技术制备的CH3NH3PbI3薄膜在激发密度0.90, 1.55和2.00 W/cm2下的光致发光强度随时间的变化; (c) 传统旋涂法以及气体辅助溶液工程技术制备的CH3NH3PbI3薄膜的光致发光谱和吸收光谱[49]

    Fig. 4.  (a) PL intensities over time of CH3NH3PbI3 film fabricated by conventional spin coating under laser excitations at 0.2 and 0.65 W/cm2; (b) PL intensities over time of CH3NH3PbI3 film fabricated by gas-assisted solution process technique under laser excitations at 0.9, 1.55 and 2.00 W/cm2; (c) absorption and PL spectra of samples fabricated conventional spin coating and gas-assisted solution process technique[49].

    图 5  (a) CH3NH3PbI3薄膜在黑暗-光照-黑暗循环中的光致发光强度随时间的变化; (b) CH3NH3PbI3薄膜的光致发光谱随黑暗或光照下放置时间的变化; (c) CH3NH3PbI3薄膜的实验中和理论上的拉曼光谱以及CH3NH3PbI3薄膜在黑暗环境和光照20, 40和60 min后的拉曼光谱[58]

    Fig. 5.  (a) PL measurements performed throughout a dark-light-dark cycle, showing the effect of white-light illumination on the PL intensity of the CH3NH3PbI3 film; (b) a graph showing six PL measurements points from the dark-light-dark cycles at different times; (c) a graph showing Experimental (taken in the dark) and theoretical Raman spectra of a CH3NH3PbI3 film as well as the Raman spectra of the same film taken in the dark and following illumination for 20, 40 and 60 min[58]

    图 6  (a) 铅卤钙钛矿太阳能电池ITO/PEDOT:PSS/MAPbI3/PCBM/Al在持续光照下的光电转化效率的变化趋势, 其中工作条件A对应于J = 0, V = Voc; 工作条件B对应于J = Jsc, V = 0; (b) 铅卤钙钛矿材料发生可逆光解现象的机理[76]

    Fig. 6.  (a) Time evolution of power conversion efficiency of lead halide perovskite solar cell with the architecture of ITO/PEDOT:PSS/MAPbI3/PCBM/Al under constant 1-Sun illumination. The device is stressed at point A (red circles, J = 0, V = Voc) or point B (blue squares, J = Jsc, V = 0); (b) schematics of the photocurrent degradation and self-healing mechanism[76].

    图 7  (a) 铅卤钙钛矿太阳能电池FTO/TiO2/mp-TiO2/(FAPbI3)x(MAPbBr3)y/spiro-OMeTAD/Au在滤过紫外光后的LED灯光持续照射下的最大功率输出的变化趋势; (b) 钙钛矿层中的离子分布的变化趋势[59]

    Fig. 7.  (a) Maximum power output tracking for 3 identically prepared perovskite solar cells (device A, B and C), with the architecture of FTO/TiO2/mp-TiO2/(FAPbI3)x(MAPbBr3)y/spiro-OMeTAD/Au, measured under UV-filtered 1-Sun LED illumination. Experimental data were fitted to an exponential decay (single or double). (b) Schematics of the evolution of ion distribution within the perovskite layer[59].

    图 8  (a) CH3NH3Pb(Br0.6I0.4)3在激发密度50 mW/cm2下光照5 min前(黑线)和后(红线)的XRD图像, 其中虚线分别为CH3NH3Pb(Br0.2I0.8)3和CH3NH3Pb(Br0.7I0.3)3的XRD图像[47]; (b) CH3NH3Pb(Br0.4I0.6)3在激发密度为15 mW/cm2的457 nm激光持续照射下的光致发光谱的变化趋势[47]; (c) CH3NH3Pb(Br0.9I0.1)3在激发密度为50 mW/cm2的405 nm激光持续照射下的阴极发光图像随时间的变化[86]; (d) CH3NH3Pb(Br1–xIx)3在光照下发生光致相分解现象的机理[46]

    Fig. 8.  (a) The XRD peak of CH3NH3Pb(Br0.6I0.4)3 film before (black) and after (red) white-light soaking for 5 minutes 50 mW/cm2. XRD patterns of CH3NH3Pb(Br0.2I0.8)3 and CH3NH3Pb(Br0.7I0.3)3 films (dashed brown) are included for comparison[47]; (b) photoluminescence spectra of CH3NH3Pb(Br0.4I0.6)3 film over 45 s in 5 s increments under 457 nm, 15 mW/cm2 at 300 K[47]; (c) a series of cathodo luminescence image after light soaking under a 405 nm LED at 50 mW/cm2[86]; (d) the schematic of two-step mechanism for light-induced phase separation[46].

    图 9  铅卤钙钛矿在持续光照下发生的主要四种不稳定现象 (a) 碘间隙离子/碘空位缺陷对的消除引起光修复现象; (b) 碘空位与阳离子空位的迁移引起光解现象; (c) 亥姆霍兹自由能最小化促使铅卤钙钛矿内部的卤素离子从无序排列向有序排列, 引起光致相分离现象; (d) 光生载流子迁移率差异产生电位差, 造成PBI64–六面体扭力方向的改变, 引起光致相变现象

    Fig. 9.  Main four kinds of instability of lead halide perovskites under continuous illumination: (a) Photo-curing induced by the elimination of iodine interstitial ions/iodine vacancies; (b) photo dissociation induced by the migration of iodine vacancies and cation vacancies; (c) phase separation induced by the minimization of Helmholtz free energy; (d) phase transition induced by the difference in the mobility of electrons and holes.

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出版历程
  • 收稿日期:  2019-03-07
  • 修回日期:  2019-04-12
  • 上网日期:  2019-08-01
  • 刊出日期:  2019-08-05

铅卤钙钛矿的光稳定性研究进展

  • 浙江大学材料科学与工程学院, 硅材料国家重点实验室, 杭州 310027
  • 通信作者: 何海平, hphe@zju.edu.cn
    基金项目: 国家自然科学基金(批准号: 51772271)资助的课题.

摘要: 铅卤钙钛矿可实现溶液法制备且具有诸多优异的光电特性, 在高效太阳能电池、发光二极管、激光、光探测器等光电子领域具有广阔的应用前景. 但是铅卤钙钛矿在光辐照下存在不稳定问题, 严重影响其相关光电器件的寿命和性能稳定性. 因此, 铅卤钙钛矿在持续光照下的不稳定性现象及其机理正受到越来越多的关注. 本文综述了铅卤钙钛矿在持续光照下的四类主要不稳定现象, 即光修复现象、光解现象、光致相分离现象以及光致相变现象, 并介绍目前已提出的相关机理, 分别从缺陷态、离子迁移、热力学原理、化学键等角度来解释其光照不稳定性. 最后, 本文简要讨论了钙钛矿中光稳定性研究的复杂性及未来需要解决的问题.

English Abstract

    • 铅卤钙钛矿是指一类具有ABX3结构的材料, 如图1[1]所示, 其中A是有机阳离子(如甲胺离子MA+、甲脒离子FA+)或无机Cs+离子, B是Pb2+离子, X是卤素阴离子. 铅卤钙钛矿是双极性材料[2-4], 能同时有效地传输电子和空穴[5,6], 具有较长的载流子扩散距离[7,8]和寿命[9,10], 同时还具备高的光吸收系数[11]和光发射效率[12]、高的光增益系数[13]等特性. 尤为重要的是, 这类材料可以用简便的溶液法制备, 获得具有高质量、低缺陷密度的薄膜. 这些优异的特性使得铅卤钙钛矿在太阳能电池[14-18]、发光二极管(LED)[19-21]、激光[22-25]、光探测器[26-29]等领域具有广阔的应用前景. 从2009年Kojima等[14]首次在染料敏化太阳能电池中使用CH3NH3PbI3和CH3NH3PbBr3作为光吸收材料开始, 短短几年内钙钛矿太阳能电池的光电转化效率迅速提高, 目前已经超过23%[30]. 与此同时, 钙钛矿在发光器件方面也取得快速进展, 最近已有数个研究组报道钙钛矿LED的外量子效率超过20%[31-33].

      图  1  铅卤钙钛矿ABX3的结构示意图[1]

      Figure 1.  Structure of lead halide perovskites[1].

      但是铅卤钙钛矿材料存在比较严重的稳定性问题, 目前报道的钙钛矿太阳能电池的寿命通常都未超过2000 h[34], 钙钛矿LED在最大功率下持续工作300 h后也仅保持最初性能的80%左右[35], 这与应用要求相去甚远. 影响钙钛矿材料及其光电子器件稳定性的因素有很多, 包括湿度[36-40]、氧气[41-43]、光照[44-50]、温度[51-53]等. 通过成熟的器件封装技术[54-56], 可以有效抑制甚至避免空气中的水和氧引起的钙钛矿分解或失效, 但是光照所引发的不稳定现象依然是横亘于钙钛矿器件, 尤其是与光辐照密切相关的各类光电子器件面前的难题. 因此, 研究钙钛矿的光照不稳定性现象和机理, 并期望在此基础上对材料进行改性、对器件进行设计, 对深入理解钙钛矿材料特性以及开发钙钛矿器件应用潜力, 具有重要的科学意义和实用价值.

      相较于钙钛矿太阳能电池和发光器件的快速发展而言, 目前关于铅卤钙钛矿的光稳定性的研究还比较少. 本文将综述铅卤钙钛矿由于光照所引发的各种不稳定现象, 并讨论这些现象背后的可能机理. 需要指出的是, 探究光照在铅卤钙钛矿中引发的变化是比较复杂的, 这主要表现为三方面: 其一是铅卤钙钛矿在持续光照下发生的现象多变, 主要包括光修复现象、光解现象、光致相分离现象、光致相变现象四类; 其二是同种成分的钙钛矿由于制备条件、实验条件等不同会发生相反的现象, 并可能同时诱发多种变化, 例如deQuilettes等[57]报道CH3NH3PbI3钙钛矿薄膜在持续光照下光致发光(PL)强度会持续上升直至稳定, 但是Gottesman等[58]报道CH3NH3PbI3钙钛矿薄膜在持续光照下光致发光强度却持续下降, 1 h后下降至最初强度的60%, 而Domanski等[59]报道混合卤素钙钛(FAPbI3)x(MAPbBr3)y太阳能电池在持续光照下光电转化效率会持续下降, 同时钙钛矿薄膜中发生相分离现象; 其三是影响铅卤钙钛矿在光照下的行为的可能因素很多, 包括缺陷态[50,57]、阴离子或阳离子[49,59]、化学键[58,60]、亥姆霍兹自由能[61]等, 这些因素在上述不同现象中可能单独或联合发挥作用. 因此, 研究铅卤钙钛矿光照不稳定性的机理是相当困难的, 目前文献报道的相关机理尚不能对各种现象进行统一的解释, 还有待研究者们系统深入的研究.

      本文根据钙钛矿的变化特征, 将光辐照引起的效应分为四类: 光修复(photo curing)、光解(photo dissociation)、光致相分离(photo-induced phase separation)、光致相变(photo-induced phase transition). 针对每种效应介绍其主要现象及可能机理. 最后, 对钙钛矿光稳定性研究中尚存在的问题进行了简要讨论.

    • 一些课题组研究CH3NH3PbI3薄膜在氮气或氩气环境下的光辐照效应时, 发现在持续光照下, CH3NH3PbI3薄膜的PL强度[49,57]和寿命[50]都有显著提高, 同时CH3NH3PbI3太阳能电池的开路电压和短路电流也不断增强, 相应地, 光电转化效率也随之不断提高, 直至达到稳定值[62,63], 如图2所示. 这种铅卤钙钛矿薄膜及其相关光电器件在持续光照下发生的性能不断增强的现象被称为光修复现象. Cheng课题组[63]通过将CH3NH3PbI3太阳能电池先后放置于光照和黑暗环境下各12 h, 并不断重复这个过程, 发现当太阳能电池处于黑暗环境时, 它的光电转化效率不断下降, 12 h后会减少至最大值的50%以下; 当太阳能电池重新放置于光照环境下时, 光电转化效率又会重新缓慢提高, 直至达到最大值, 证明光修复现象是可逆的.

      图  2  铅卤钙钛矿及其太阳能电池在持续光照下发生光修复现象时的性能变化 (a) CH3NH3PbI3薄膜在持续光照下的时间分辨PL衰减随光照时间的变化[57]; (b) CH3NH3PbI3薄膜在持续光照下以及不同温度下的PL强度随光照时间的变化[57]; (c) 铅卤钙钛矿太阳能电池ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al在持续光照下的J-V曲线变化[62]; (d) 铅卤钙钛矿太阳能电池ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al在持续光照下的光电转化效率的变化曲线[62]

      Figure 2.  Performance changes of lead halide perovskite and its solar cells under continuous illunination: (a) A series of time-resolved PL decays from CH3NH3PbI3 films measured over time under continuous illumination[57]; (b) the normalized PL intensity of CH3NH3PbI3 films measured over time under continuous illumination at different temperatures[57]; (c) the J-V characteristics of lead halide perovskite solar cell with the architecture of ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al under continuous illumination[62]; (d) the reversibility of power conversion efficiency of lead halide perovskite solar cell with the architecture of ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/Al under continuous illumination[62].

      目前关于导致CH3NH3PbI3薄膜发生光修复现象的机理并没有得到统一的解释, 但是缺陷态在其中发挥重要作用已基本达成共识. Mosconi等[50]在室温下使用脉冲激光照射CH3NH3PbI3薄膜一段时间后, CH3NH3PbI3薄膜的寿命和光致发光强度不断提高直至达到稳定的状态. 根据Stranks等[10]提出的缺陷态模型, Mosconi等[50]推导出CH3NH3PbI3薄膜中的缺陷态密度从最开始的1017 cm–3下降至1016 cm–3后达到稳定状态. 由于缺陷态在CH3NH3PbI3薄膜中起非辐射复合中心的作用[10], 光致发光量子效率(photoluminescence quantum efficiency)可以很好地反映薄膜内部光生载流子的复合过程[64], 量子效率高意味着非辐射复合中心的密度减少. 迄今已有多个课题组报道了CH3NH3PbI3薄膜在持续光照下发生发光量子效率不断提高的现象, 并将这种现象归因于光照会消除薄膜内部的缺陷态[65,66]. Brenes等[67]结合光照和热处理消除CH3NH3PbI3薄膜中的浅能级表面态, 将发光量子效率提高至单晶的水平.

      关于这些缺陷态的具体指认, 也已开展了研究. 在CH3NH3PbI3薄膜内部存在多种本征点缺陷, 包括碘空位VI、碘间隙离子Ii、铅空位VPb等, 它们都具有较低的形成能[68-70]. 但是, 与铅离子相关的缺陷的迁移能较高, 约为0.8—1.4 eV[71,72], 而碘空位和碘间隙离子则具有更低的迁移能[72,73], 约为0.1 eV[71], 因此碘空位/碘间隙离子弗兰克尔缺陷对(Frenkel defect pairs)更容易在CH3NH3PbI3薄膜中发生迁移. Mosconi等[50]从点缺陷的角度解释光修复现象发生的原因, 认为光照可以加速碘空位/碘间隙离子组成的弗兰克尔缺陷对的湮灭. 这一方面可以降低CH3NH3PbI3薄膜中与碘离子相关的非辐射复合中心的密度, 另一方面使得薄膜内部局部的结晶达到更好的状态.

      deQuilettes等[57]定量分析了CH3NH3PbI3薄膜在持续光照下光致发光强度增强与缺陷态密度降低之间的联系. CH3NH3PbI3薄膜在最开始光照时的缺陷态密度为1.7 × 1017 cm–3, 在持续光照约15 min后缺陷态密度减小至2.5 × 1016 cm–3并达到稳定. 缺陷态密度的数量级下降直接导致CH3NH3PbI3薄膜的发光强度和寿命的提高[50,57]. 他们将离子迁移模型引入解释光修复现象的机理中, 如图3所示. 当CH3NH3PbI3薄膜刚被制备出来时, 在表面和晶界处分布大量的碘空位和碘间隙离子等缺陷[9]. CH3NH3PbI3薄膜接受光照后, 会产生光生载流子, 它们在薄膜表面处的浓度最高[57]. 因此, 大量的电子会被薄膜表面和晶界处的空位所捕获, 从而在薄膜内部形成电场, 促进碘离子的迁移, 使得大量的碘间隙离子填充原已存在的碘空位. 碘间隙离子和碘空位的密度下降, 意味着CH3NH3PbI3薄膜界面处的非辐射复合中心的密度下降, 更多的光生载流子能够通过辐射复合发光. 当CH3NH3PbI3薄膜重新放置于黑暗环境下时, 残留的的缺陷态会吸引碘离子缓慢回迁, 最终导致CH3NH3PbI3薄膜的光修复现象部分可逆.

      图  3  CH3NH3PbI3薄膜发生光修复现象的机理[57] (a) 在发光较弱的晶粒中分布有高浓度的缺陷态以及过量的碘离子; (b) CH3NH3PbI3薄膜经过光照后, 电子迅速填充缺陷, 产生电场, 造成碘离子从照射区域开始迁移, 填充空位; (c)照射区域中的缺陷态密度和碘离子浓度降低, 薄膜最终达到稳定的状态; (d) 当移除光照后, 碘离子存在浓度梯度, 驱使其扩散回弱发光区域, 直至达到新的碘离子浓度平衡

      Figure 3.  Mechanism of photo curing in CH3NH3PbI3 film[57]: (a) Iodide in a "dark spot" is initially high with a corresponding excess of trap density; (b) electrons quickly fill traps under illumination, inducing an electric field that results in iodide to migrate away from the illuminated region and fill vacancies; (c) the fill will reaches a stabilized state, with the reduction of the density of the traps and the concentration of the iodide in illuminated region; (d) when the illumination is removed, there may be a concentration gradients driving some iodides in to the dark spot and the film will reaches a new equilibrium with a net redistributed iodide profile.

      之后, Chen等[49]进一步确认CH3NH3PbI3薄膜的光修复现象与薄膜的缺陷态浓度以及光的激发密度密切相关. 他们分别使用传统的旋涂技术和气体辅助溶液工程技术[74]制备了两组CH3NH3PbI3薄膜样品, 后者的缺陷态密度要远大于前者[75]. 在低的激发密度下, 传统的旋涂技术制备的CH3NH3PbI3薄膜发生光修复现象, 而气体辅助溶液工程技术制备的CH3NH3PbI3薄膜的光致发光强度基本保持不变, 且后者的光致发光强度远弱于前者, 如图4所示. Chen等[49]提出, 在低的激发密度下, 由于CH3NH3PbI3薄膜的光致发光强度取决于光生载流子的辐射复合与缺陷态非辐射复合之间的竞争, 当薄膜内部存在很高浓度的缺陷态时, 光照并不能有效地降低缺陷态的密度, 因此表现出稳定的弱发光. 有趣的是, 当激发密度提高至一定程度后, 两种制备方法获得的CH3NH3PbI3薄膜都发生光解现象. 他们的研究表明, 不同的制备工艺技术以及不同的激发密度对于CH3NH3PbI3薄膜在持续光照下的光效应具有显著影响.

      图  4  (a) 传统旋涂法制备的CH3NH3PbI3薄膜在激发密度0.2和0.65 W/cm2下的光致发光强度随时间的变化; (b) 气体辅助溶液工程技术制备的CH3NH3PbI3薄膜在激发密度0.90, 1.55和2.00 W/cm2下的光致发光强度随时间的变化; (c) 传统旋涂法以及气体辅助溶液工程技术制备的CH3NH3PbI3薄膜的光致发光谱和吸收光谱[49]

      Figure 4.  (a) PL intensities over time of CH3NH3PbI3 film fabricated by conventional spin coating under laser excitations at 0.2 and 0.65 W/cm2; (b) PL intensities over time of CH3NH3PbI3 film fabricated by gas-assisted solution process technique under laser excitations at 0.9, 1.55 and 2.00 W/cm2; (c) absorption and PL spectra of samples fabricated conventional spin coating and gas-assisted solution process technique[49].

    • 近些年来, 不少研究组报道了铅卤钙钛矿在经过持续光照后发生性能衰减的现象[58,76], 例如光致发光强度下降, 太阳能电池器件的光电转化效率下降等. Gottesman等[58]在手套箱中使用顺序制备法在玻璃衬底上制备CH3NH3PbI3薄膜后, 使用树脂和0.5 mm厚的盖玻片进行封装, 隔绝水蒸气、氧气对CH3NH3PbI3薄膜性能的影响. 他们使用白光LED (< 0.1 sun)作为光源, 持续照射封装后的CH3NH3PbI3薄膜. 在1 h的持续光照过程中, CH3NH3PbI3薄膜的光致发光强度逐渐下降, 最终减少至初始强度的60%. 在关闭白光LED后, CH3NH3PbI3薄膜一直处于黑暗环境下, 此时光致发光强度开始缓慢恢复, 10 h后恢复至初始强度的85%左右, 12—15 h后可以完全恢复. 在光照-黑暗放置过程中, CH3NH3PbI3薄膜的光致发光谱只有峰强发生变化, 峰形和峰位都保持不变, 如图5(a)图5(b)所示. Gottesman等[58]随后记录了CH3NH3PbI3薄膜在黑暗环境下及不同光照时间长度下的拉曼光谱变化, 如图5(c)所示. 在持续光照下发现CH3NH3PbI3薄膜的拉曼光谱在逐渐发生变化, 意味着此时钙钛矿结构正在发生转变; 但是当关闭白光LED后, CH3NH3PbI3薄膜的拉曼光谱又逐渐恢复至最开始的形状, 意味着由于光照所引发的结构转变是可逆的. Gottesman等随后提出[58], 光照会削弱CH3NH3+阳离子与无机阴离子之间的氢键强度, 从而提高CH3NH3+阳离子的旋转自由度[77], 为适应CH3NH3+阳离子的分布以及增强的旋转, 无机Pb—I键会重新排列, 驱动CH3NH3PbI3薄膜内部的结构转变.

      图  5  (a) CH3NH3PbI3薄膜在黑暗-光照-黑暗循环中的光致发光强度随时间的变化; (b) CH3NH3PbI3薄膜的光致发光谱随黑暗或光照下放置时间的变化; (c) CH3NH3PbI3薄膜的实验中和理论上的拉曼光谱以及CH3NH3PbI3薄膜在黑暗环境和光照20, 40和60 min后的拉曼光谱[58]

      Figure 5.  (a) PL measurements performed throughout a dark-light-dark cycle, showing the effect of white-light illumination on the PL intensity of the CH3NH3PbI3 film; (b) a graph showing six PL measurements points from the dark-light-dark cycles at different times; (c) a graph showing Experimental (taken in the dark) and theoretical Raman spectra of a CH3NH3PbI3 film as well as the Raman spectra of the same film taken in the dark and following illumination for 20, 40 and 60 min[58]

      随后Nie等[76]和Domanski等[59]也报道了相似的实验现象. 如图6(a)所示, Nie等[76]观察到在持续光照下, CH3NH3PbI3钙钛矿太阳能电池的光电转化效率逐渐提升, 这与Mosconi等[50]报道的光修复现象一致. 但是经过几分钟的持续光照后, 太阳能电池的性能衰减现象发生. 经过2 h的持续光照后, 处于V = Voc工作条件下的, CH3NH3PbI3钙钛矿太阳能电池的光电转化效率下降了30%. 在此过程中, 研究者使用红外吸收光谱和阻抗谱观测到光照诱发CH3NH3PbI3薄膜中产生亚稳的缺陷态. 当撤除光照后, 大部分的亚稳态缺陷会在黑暗下逐渐湮灭, CH3NH3PbI3太阳能电池的光电转化效率又会重新缓慢恢复, 恢复速度与温度密切相关. 图6(b)描述了CH3NH3PbI3钙钛矿太阳能电池在光照下发生的性能衰减现象以及在黑暗环境下性能逐渐恢复的微观机理. 当CH3NH3PbI3薄膜处于持续光照时, 会在薄膜内部持续诱发形成亚稳的缺陷态, 这些缺陷态经过长时间的积累后会在薄膜中形成大量的带电区域, 最终导致CH3NH3PbI3钙钛矿太阳能电池的光电流和光电转化效率下降, 即发生光解现象. 然而, 当CH3NH3PbI3薄膜重新放置于黑暗环境后, 这些光诱发的缺陷态又会逐渐湮灭, 使得CH3NH3PbI3钙钛矿太阳能电池的光电流和光电转化效率又重新恢复至最开始的状态. Nie等[76]认为光照会在薄膜中诱发局域极化子产生, 这些极化子处于带隙深处, 发挥着缺陷态的作用.

      图  6  (a) 铅卤钙钛矿太阳能电池ITO/PEDOT:PSS/MAPbI3/PCBM/Al在持续光照下的光电转化效率的变化趋势, 其中工作条件A对应于J = 0, V = Voc; 工作条件B对应于J = Jsc, V = 0; (b) 铅卤钙钛矿材料发生可逆光解现象的机理[76]

      Figure 6.  (a) Time evolution of power conversion efficiency of lead halide perovskite solar cell with the architecture of ITO/PEDOT:PSS/MAPbI3/PCBM/Al under constant 1-Sun illumination. The device is stressed at point A (red circles, J = 0, V = Voc) or point B (blue squares, J = Jsc, V = 0); (b) schematics of the photocurrent degradation and self-healing mechanism[76].

      Domanski等[59]观察到铅卤钙钛矿太阳能电池在持续光照下发生光电转化效率衰减的现象, 如图7(a)所示. 但是不同的是, Domanski等[59]将光电转化效率下降过程划分为两个不同的过程: 第一个过程中太阳能电池的光电转化效率下降迅速, 但是可逆; 第二个过程中太阳能电池的光电转化效率下降缓慢, 但是不可逆. 他们将处于效率快速下降过程中的太阳能电池重新放置于黑暗环境后, 太阳能电池的效率又重新恢复, 这与之前Gottesman等[58]以及Nie等[76]报道的现象一致. Domanski等[59]将铅卤钙钛矿太阳能电池在光照下发生可逆的性能衰减现象归因于离子空位的迁移. 在铅卤钙钛矿结构ABX3中, X空位(包括X间隙离子)被认为是最容易发生迁移的空位, 其次才是A阳离子空位与B阳离子空位[73,78,79]. Domanski等[59]观察到在(FAPbI3)x(MAPbBr3)y钙钛矿薄膜中, 卤素离子空位和阳离子空位都是会发生迁移的(后者的迁移率远小于前者), 它们在钙钛矿层中的分布将会显著影响到太阳能电池中的电荷提取, 进而对光电转化效率产生影响. 图7(b)描述了钙钛矿太阳能电池在持续光照下发生性能衰减的机理. 当钙钛矿没有受到光照时, 薄膜中的卤素离子空位和阳离子空位会在钙钛矿的晶格中随机分布[80]. 经过几分钟的光照后, 卤素离子空位发生迁移, 在界面处形成具有空位选择性的徳拜层, 而迁移率较小的阳离子依然还停留在原来的位置上. 在经过几小时的持续光照后, 阳离子也发生迁移, 在界面处形成另一个具有电子选择性的徳拜层. 界面电荷的产生引起能带弯曲, 阻碍太阳能电池中的电荷抽取过程[81], 最终导致效率不断下降. 当撤离光源后, 处于界面两端的阴离子和阳离子又会由于内建电场的作用, 重新缓慢移动, 恢复至原来的随机分布状态. 值得指出的是, Domanski等[59]使用的铅卤钙钛矿是混合两种卤素元素的(FAPbI3)x(MAPbBr3)y钙钛矿, 这类钙钛矿在持续光照下还会发生相分离现象, 或许会对报道的结果有一定的影响. 最近, Zu等[82]报道了基于CH3NH3PbI3钙钛矿的太阳能电池中也观察到类似的现象, 即迅速但可逆的性能衰减和缓慢但不可逆的性能衰减两个过程. 但是目前, 关于后一种不可逆的性能衰减过程的研究尚少, 其机理还在探究中.

      图  7  (a) 铅卤钙钛矿太阳能电池FTO/TiO2/mp-TiO2/(FAPbI3)x(MAPbBr3)y/spiro-OMeTAD/Au在滤过紫外光后的LED灯光持续照射下的最大功率输出的变化趋势; (b) 钙钛矿层中的离子分布的变化趋势[59]

      Figure 7.  (a) Maximum power output tracking for 3 identically prepared perovskite solar cells (device A, B and C), with the architecture of FTO/TiO2/mp-TiO2/(FAPbI3)x(MAPbBr3)y/spiro-OMeTAD/Au, measured under UV-filtered 1-Sun LED illumination. Experimental data were fitted to an exponential decay (single or double). (b) Schematics of the evolution of ion distribution within the perovskite layer[59].

    • CH3NH3PbBrxI3–x是一种重要的铅卤钙钛矿材料, 可以通过改变卤素元素的成分及计量比, 即x从0变化为3, 实现带隙从1.6 eV到2.3 eV的连续变化[83], 因此十分适合用于作为串联太阳能电池中的大带隙光吸收材料[84]. 然而当CH3NH3PbBrxI3–x作为太阳能电池的光吸收材料被使用时, 并没有观察到太阳能电池的最大开路电压随带隙的增大呈现线性变化的现象[83,85]. 同时Hoke等[47]观察到, 在457 nm的激光持续照射下, CH3NH3PbBrxI3–x薄膜中发生可逆的相分离现象, 最终形成富碘相和富溴相. 如图8(a)所示, CH3NH3PbBrxI3–x薄膜经过光照后, XRD衍射峰发生分裂, 说明此时薄膜内两种晶相共存; 如图8(b)所示, 在15 mW/cm2的457 nm激光持续光照下, CH3NH3PbBrxI3–x薄膜的光致发光光谱中低能量的发光峰强度快速增长, 而高能量的发光峰强度随之减弱. 这表明CH3NH3PbBrxI3–x薄膜经过光照发生相分离后, 小带隙的富碘相成为新的载流子辐射复合中心, 因此大部分光生载流子都被低能量态的富碘相所捕获. 但是, 这种相分离现象是可逆的. Hoke等[47]将光照后的CH3NH3PbBrxI3–x薄膜重新放置于黑暗环境下几分钟后, CH3NH3PbBrxI3–x薄膜的光致发光谱、XRD图谱、吸收光谱又恢复至初始的状态. CH3NH3PbBrxI3–x薄膜在光照下发生的可逆的相分离现象又被称为霍克效应(Hoke effect). 之后, Bischak等[86]使用阴极发光观察了CH3NH3Pb(Br0.1I0.9)3薄膜在微观尺度上的相分离的演变过程, 结果如图8(c)所示. 他们确定了CH3NH3Pb (Br0.1I0.9)3薄膜发生霍克效应, 并指出富碘相最开始是在晶界处开始生长, 在尺寸上增长很快, 但是数量上增长相对缓慢. 值得注意的是, 在CH3NH3PbBrxI3–x材料中, 只有x > 0.6时, CH3NH3 PbBrxI3–x薄膜才会发生霍克效应.

      图  8  (a) CH3NH3Pb(Br0.6I0.4)3在激发密度50 mW/cm2下光照5 min前(黑线)和后(红线)的XRD图像, 其中虚线分别为CH3NH3Pb(Br0.2I0.8)3和CH3NH3Pb(Br0.7I0.3)3的XRD图像[47]; (b) CH3NH3Pb(Br0.4I0.6)3在激发密度为15 mW/cm2的457 nm激光持续照射下的光致发光谱的变化趋势[47]; (c) CH3NH3Pb(Br0.9I0.1)3在激发密度为50 mW/cm2的405 nm激光持续照射下的阴极发光图像随时间的变化[86]; (d) CH3NH3Pb(Br1–xIx)3在光照下发生光致相分解现象的机理[46]

      Figure 8.  (a) The XRD peak of CH3NH3Pb(Br0.6I0.4)3 film before (black) and after (red) white-light soaking for 5 minutes 50 mW/cm2. XRD patterns of CH3NH3Pb(Br0.2I0.8)3 and CH3NH3Pb(Br0.7I0.3)3 films (dashed brown) are included for comparison[47]; (b) photoluminescence spectra of CH3NH3Pb(Br0.4I0.6)3 film over 45 s in 5 s increments under 457 nm, 15 mW/cm2 at 300 K[47]; (c) a series of cathodo luminescence image after light soaking under a 405 nm LED at 50 mW/cm2[86]; (d) the schematic of two-step mechanism for light-induced phase separation[46].

      近些年, 关于解释霍克效应的机理不断被提出, 但是并没有得到一个统一的解释. Brivio等[61]认为CH3NH3PbBrxI3–x薄膜在光照后富碘相和富溴相能够共存与局部的亥姆霍兹自由能最小化有关. 其利用密度泛函理论计算得出CH3NH3PbBrxI3–x薄膜的亥姆霍兹自由能与卤素元素的计量比以及温度之间的函数关系, 并指出如果CH3NH3PbBrxI3–x薄膜吸收足够的能量来克服亚稳的混合卤化物状态, 将会发生相分离, 从而形成富碘相和富溴相. 而光照可以提供必要的能量, 满足Br离子和I离子在CH3NH3PbBrxI3–x薄膜内的迁移能, 从而引发相分离. 但是Brivio等[61]的模型并不能解释CH3NH3PbBrxI3–x薄膜的相分离在黑暗环境下是可逆的. 有意思的是, Yang等[46]使用低于500 Hz的脉冲激光照射CH3NH3PbBr1.7I1.3薄膜时, 并没有观察到霍克效应的发生; 但是使用高于1000 Hz的相同强度的脉冲激光作为辐照光时, 发现其光致发光谱的峰位从640 nm逐渐移至750 nm, 因此CH3NH3PbBrxI3–x薄膜是否会发生霍克效应很可能与光子能量没有关系. 同时, Yang等[46]通过使用光调制光谱发现CH3NH3PbBr1.7I1.3薄膜发生霍克效应时, 在带隙间产生缺陷态. 他们提出CH3NH3PbBrxI3–x薄膜发生霍克效应时要经过两步(如图8(d)所示): 首先, CH3NH3PbBrxI3–x薄膜未接受光照时保持刚制备时的相结构, 当接受光照后, CH3NH3PbBrxI3–x薄膜会在带隙中生成缺陷态; 如果其他的光子被缺陷态捕获, 将会诱发亚稳态转换. 这些亚稳态类似于新的能带, 并且在吸收光子后会被填充. 这种模型假设在CH3NH3PbBrxI3–x薄膜中长期存在极化态, 为卤素离子的局部迁移提供驱动力[46,86]. 除此之外, 还有其他关于CH3NH3 Pb(BrxI1–x)3中发生相分离的机理与驱动力的解释. 例如, Barker等[87]提出, 光生载流子在CH3NH3 PbBrxI3–x薄膜内不同深度的产生速率是不一样的, 这种产生速率的梯度为相分离提供驱动力; Draguta等[88]则提出, CH3NH3PbBrxI3–x薄膜中发生相分离的驱动力源于富碘相的带隙减小, 并指出通过合理设计载流子的扩散距离以及注入载流子的密度, 能够有效地降低CH3NH3PbBrxI3–x薄膜发生霍克效应的可能性. 目前, 调整成分组成[89,90]、提高外部压力[91]、优化薄膜形态[92]都已被证实是减弱钙钛矿中霍克效应的有效手段.

    • 近年来, 研究人员发现铅卤钙钛矿在持续光照下会发生相变现象, 同时伴随着尺寸的变化. 最开始, Zhou等[60]观察到CH3NH3PbI3单晶和薄膜在持续光照下都会发生结构的变化, 认为光生载流子会形成电场, 削弱CH3NH3+阳离子与PbI64–阴离子之间的氢键强度, 从而加速CH3NH3+阳离子的旋转, 为适应CH3NH3+阳离子的分布以及增强的旋转, 无机Pb—I键会重新排列, 驱动CH3NH3PbI3内部的结构转变[58]. 随后, He课题组[93]报道CH3NH3PbBr3单晶在持续光照下发生尺寸变化, 并指出这种晶格的显著变化是由光伏效应和分子构型的平移对称性损失共同造成的. 最近, Kirschner等[94]使用时间分辨X射线衍射(TR-XRD)记录了无机钙钛矿在不同激发密度下的晶格响应过程. 当激发密度大于0.34 mJ/cm2时, CsPbBr3纳米晶发生可逆的光致相变现象, CsPbBr3的晶相由正交相转变为立方相. 当激发密度大于2.5 mJ/cm2时, CsPbBr3纳米晶会由晶体转变为非晶相, 这个相变过程也是可逆的. 在高激发密度下, CsPbBr3纳米晶经历显著的脉冲加热, 能量聚集于晶格中, 促进非平衡的相变过程. 但是Kirschner等[94]指出这种光致相变现象与热效应引发的相变过程不同, 相变过程中并没有出现四方中间相[95]. 几乎同时, Xue等[96]报道了CsPbBr3单晶在大于带隙的单色光照射下会发生结构的重组, 晶相由最开始的正交相转变为四方相, 且在相变过程中单晶的温度没有显著提高, 这表明无机钙钛矿仅在光照作用下也会发生相变. CsPbBr3单晶发生光致相变现象的同时, 尺寸也发生改变, 且整个过程是可逆的. 由于在CsPbBr3中, 电子的迁移率约为1000 cm2·V–1·s–1[97], 而空穴的迁移率约为50 cm2·V–1·s–1[98], 因此由于二者不同的迁移能力, CsPbBr3单晶中的光生电子和光生空穴最终会积累在不同的位置, 而这种扩散电位差最终会在CsPbBr3单晶中形成局域电场. Xue等[96]指出, CsPbBr3单晶中发生的光致相变现象是由内部PbBr64–八面体的运动模式改变导致的, 而光生载流子形成的局域电场为PbBr64–八面体的扭力方向的改变提供足够的能量[96]. 不过, 从目前的文献报道来看, 对光致相变现象可逆性背后的原因还鲜有涉及, 同时不同种类铅卤钙钛矿在光照下的相变过程不一致的原理也尚未清晰, 还需要进一步探索.

      结构的动态重组对于钙钛矿材料的能带结构、介电常数、电荷的分离/输运具有重要影响[60]. 实现铅卤钙钛矿可控且可逆的结构重组, 可以为设计具有新功能的器件(如连续激光器)或通过Rashba效应提升光电器件性能提供新颖的思路. 因此, 研究铅卤钙钛矿的光致相变现象的机理, 对于提高铅卤钙钛矿的稳定性以及设计钙钛矿光电器件的新功能具有重要意义.

    • 本文综述了铅卤钙钛矿在持续光照下出现的不稳定现象, 包括单一卤素钙钛矿在持续光照下产生的光修复和光解现象, 混合卤素钙钛矿在持续光照下出现的光致相分离现象, 以及全无机钙钛矿在持续光照下出现的光致相变现象, 如图9所归纳. 同时回顾了近年来研究人员从不同角度解释铅卤钙钛矿在光照下的不稳定性机理: 研究者肯定了缺陷态在光修复现象中的重要作用, 认为碘间隙离子、碘空位的迁移有利于缺陷态的消除[50,57], 但是碘离子迁移的驱动力还有待进一步研究; 解释光解现象的机理有很多, 包括化学键的重新排列[58]、光诱发产生新的缺陷态[76]、离子空位的迁移[59]等, 目前尚未达成共识, 同时光解现象可分为快速但可逆的性能衰减以及缓慢但不可逆的性能衰减[59,82], 其中关于后一种不可逆的光解现象的研究尚少; 混合卤素的铅卤钙钛矿发生光致相分离现象的驱动力仍然处于研究中, 目前提出的可能的驱动力包括亥姆霍兹自由能的最小化[61]、极化态[46]、光生载流子的产生速率梯度[87]等; 光致相变现象由于最近刚被报道, 目前对此研究较少, 研究人员试图从化学键的角度解释该现象[96], 但是关于可逆性的原因鲜有涉及, 同时不同种类的钙钛矿在光照下的相变过程不一致的原理尚未清晰[94,96]. 目前, 人们尚未完全理解铅卤钙钛矿的光不稳定性背后的机理, 仍需进行系统和深入的研究.

      图  9  铅卤钙钛矿在持续光照下发生的主要四种不稳定现象 (a) 碘间隙离子/碘空位缺陷对的消除引起光修复现象; (b) 碘空位与阳离子空位的迁移引起光解现象; (c) 亥姆霍兹自由能最小化促使铅卤钙钛矿内部的卤素离子从无序排列向有序排列, 引起光致相分离现象; (d) 光生载流子迁移率差异产生电位差, 造成PBI64–六面体扭力方向的改变, 引起光致相变现象

      Figure 9.  Main four kinds of instability of lead halide perovskites under continuous illumination: (a) Photo-curing induced by the elimination of iodine interstitial ions/iodine vacancies; (b) photo dissociation induced by the migration of iodine vacancies and cation vacancies; (c) phase separation induced by the minimization of Helmholtz free energy; (d) phase transition induced by the difference in the mobility of electrons and holes.

      从目前的文献报道看, 铅卤钙钛矿光辐照下不稳定行为从实验现象到机理解释都存在相当的复杂性. 这一复杂性至少部分与所研究样品的多样性(如组分、形态等)密切相关. 因此, 在后续的深入研究中, 一方面需要对不同组分、形态的样品进行综合对比, 以提取共同或相似的特性; 另一方面, 也需要在样品制备和选取上进行合理的设计, 例如采用高质量的单晶作为研究对象, 尽可能简化体系的复杂性, 以利于获取可靠的实验数据和清晰的物理图像.

      相比有机-无机杂化钙钛矿而言, 全无机钙钛矿和二维层状钙钛矿表现出更好的稳定性, 有望在钙钛矿光电子器件中获得越来越多的应用. 例如, 近年来在钙钛矿发光器件中, 全无机钙钛矿[99-101]和二维层状钙钛矿[102,103]已逐渐占据主导地位, 并获得了很好的性能(包括发光效率和亮度等). 尽管它们比有机无机杂化钙钛矿更稳定, 但对于长时间的实际应用而言, 我们仍需要对其光稳定性进行研究和评估. 不过, 这方面的研究相对更少. 除了前述一些关于光致相变的工作以外, 目前对全无机以及二维层状钙钛矿的光稳定性还没有形成比较系统的研究.

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