Aubriet Valentin, Courouble Kristell, Bardagot Olivier, Demadrille Renaud, Borowik Łukasz, Grévin Benjamin
Université Grenoble Alpes, CEA, LETI, F-38000 Grenoble, France.
STMicroelectronics, 850 Rue Jean Monnet, F-38926 Crolles Cedex, France.
Nanotechnology. 2022 Mar 8;33(22). doi: 10.1088/1361-6528/ac5542.
In this work, we use pump-probe Kelvin probe force microscopy (pp-KPFM) in combination with non-contact atomic force microscopy (nc-AFM) under ultrahigh vacuum, to investigate the nature of the light-induced surface potential dynamics in alumina-passivated crystalline silicon, and in an organic bulk heterojunction thin film based on the PTB7-PCBM tandem. In both cases, we demonstrate that it is possible to identify and separate the contributions of two different kinds of photo-induced charge distributions that give rise to potential shifts with opposite polarities, each characterized by different dynamics. The data acquired on the passivated crystalline silicon are shown to be fully consistent with the band-bending at the silicon-oxide interface, and with electron trapping processes in acceptors states and in the passivation layer. The full sequence of events that follow the electron-hole generation can be observed on the pp-KPFM curves, i.e. the carriers spatial separation and hole accumulation in the space charge area, the electron trapping, the electron-hole recombination, and finally the electron trap-release. Two dimensional dynamical maps of the organic blend photo-response are obtained by recording the pump-probe KPFM curves in data cube mode, and by implementing a specific batch processing protocol. Sample areas displaying an extra positive SPV component characterized by decay time-constants of a few tens of microseconds are thus revealed, and are tentatively attributed to specific interfaces formed between a polymer-enriched skin layer and recessed acceptor aggregates. Decay time constant images of the negative SPV component confirm that the acceptor clusters act as electron-trapping centres. Whatever the photovoltaic technology, our results exemplify how some of the SPV components may remain completely hidden to conventional SPV imaging by KPFM, with possible consequences in terms of photo-response misinterpretation. This work furthermore highlights the need of implementing time-resolved techniques that can provide a quantitative measurement of the time-resolved potential.
在这项工作中，我们在超高真空条件下将泵浦-探测开尔文探针力显微镜（pp-KPFM）与非接触原子力显微镜（nc-AFM）结合使用，以研究氧化铝钝化的晶体硅以及基于PTB7-PCBM串联的有机本体异质结薄膜中光诱导表面电势动力学的本质。在这两种情况下，我们都证明可以识别并区分两种不同的光诱导电荷分布的贡献，这两种电荷分布会导致具有相反极性的电势偏移，每种电荷分布都具有不同的动力学特征。在钝化晶体硅上获得的数据被证明与硅-氧化物界面处的能带弯曲以及受主态和钝化层中的电子俘获过程完全一致。在pp-KPFM曲线上可以观察到电子-空穴产生之后的完整事件序列，即载流子在空间电荷区的空间分离和空穴积累、电子俘获、电子-空穴复合，以及最终的电子陷阱释放。通过以数据立方体模式记录泵浦-探测KPFM曲线并实施特定的批处理协议，获得了有机共混物光响应的二维动态图。因此揭示了显示出以几十微秒的衰减时间常数为特征的额外正表面光伏（SPV）分量的样品区域，并初步将其归因于富含聚合物的表层与凹陷的受主聚集体之间形成的特定界面。负SPV分量的衰减时间常数图像证实受主簇充当电子俘获中心。无论采用何种光伏技术，我们的结果都例证了某些SPV分量如何可能对传统的KPFM SPV成像完全隐藏，这可能会导致光响应误解。这项工作还强调了实施能够提供时间分辨电势定量测量的时间分辨技术的必要性。
