钙钛矿光伏器件中异常迟滞现象的扫描探针研究

Hysteresis is an perennial problem in photovoltaic devices, which manifests as different shapes of current-voltage curves when voltage sweeping parameters, e.g. scanning direction, scan rate et al, has been changed, thus hinders the measurement of true power conversion efficiency (PCE). Hysteresis is originated from the change of elementary photovoltaic processes, such as the generation, separation, transport, recombination and collection of charge carriers, when different scan parameters are used. However, it is naive to explain hysteresis of the state-of-the-art organometallic perovskite photovoltaic devices based on mechanism of that in traditional photovoltaic devices. To explore the reasons of anomalous hysteresis in perovskite devices, it is necessary to investigate the transport properties of photogenerated carriers in situ, and understand the behaviour of carriers with different scan parameters. Thus, based on our previous work, we proposed synergetic application of scanning Kelvin probe microscopy (SKPM) and dielectric force microscopy (DFM) in cross-sectional organometallic perovskite devices, to investigate the relationship between behavior of carriers and hysteresis. Furthermore, understanding this relationship is important to explore the mechanism of hysteresis and its connection with perovskite polarization and surface/interface defect states, which is helpful to resolve hysteresis and further improve device performance.

光伏器件在性能测试过程中常会表现出电流-电压曲线随电压扫描方向、扫描速度等参数的选择而变化的迟滞现象，影响测得的器件光电转换效率的准确性。这种迟滞现象源于不同扫描参数下，载流子产生、分离、复合、输运和收集等微观过程的变化。但是对于目前光伏器件中的明星材料有机金属钙钛矿而言，其表现出的迟滞现象难以用解释传统光伏器件的机理来理解。为了解析钙钛矿器件异常迟滞现象的成因，我们需要原位研究钙钛矿中光生载流子的输运性质，获得不同扫描参数下载流子微观运动过程的变化规律。为此，我们提出利用已建立的器件截面制备技术，将扫描开尔文探针显微镜和自主开发的介电力显微镜协同应用于载流子微观运动过程与迟滞现象关系的研究中，理解钙钛矿极化和表界面缺陷态等关键科学问题对迟滞现象的影响，从而有效抑制迟滞现象并为器件性能优化提供依据。

有机-无机钙钛矿化学通式为ABX3，其中A代表正一价有机阳离子（如MA+、FA+等），B代表正二价金属阳离子（如Pb2+等），X代表负一价卤素阴离子（如Cl-、Br-、I-），目前其太阳能电池认证效率已经超过23%，极具应用前景。理论计算表明Pb2+参与成键，显著影响钙钛矿结晶动力学、薄膜形貌、能级结构和光电特性等。因此，通过注入金属离子替换Pb2+调控钙钛矿结晶和成膜过程，是提高其太阳能电池性能的有效手段之一。但是，金属离子替换Pb2+对器件异常迟滞现象的影响却缺乏深入的研究。本项目分别以非等价Ag+和等价Cd2+替换MAPbI3中的Pb2+，研究不同金属离子替换浓度下钙钛矿材料特性及其平面异质结光伏电池性能和迟滞效应的变化趋势。研究结果表明，优化的金属离子替换比例能有效增强钙钛矿结晶性、改善薄膜形貌和光生载流子动力学，使得器件性能大幅提高且未见明显迟滞现象。而金属离子替换比例过高时，钙钛矿结晶性下降，薄膜形貌变差以及非辐射复合加剧，不仅器件性能大幅下降而且迟滞效应加剧。我们利用扫描探针显微镜证实，注入过量的金属离子引起的器件异常迟滞现象，与钙钛矿卤素离子空位缺陷增加使得离子迁移加剧有关。