Department of Chemical Engineering, Stanford University , Stanford, California 94305, United States.
Molecular Vista , 6840 Via Del Oro, Suite 110, San Jose, California 95119, United States.
ACS Nano. 2018 Feb 27;12(2):1473-1481. doi: 10.1021/acsnano.7b07865. Epub 2018 Jan 22.
Rapid nanoscale imaging of the bulk heterojunction layer in organic solar cells is essential to the continued development of high-performance devices. Unfortunately, commonly used imaging techniques such as tunneling electron microscopy (TEM) and atomic force microscopy (AFM) suffer from significant drawbacks. For instance, assuming domain identity from phase contrast or topographical features can lead to inaccurate morphological conclusions. Here we demonstrate a technique known as photo-induced force microscopy (PiFM) for imaging organic solar cell bulk heterojunctions with nanoscale chemical specificity. PiFM is a relatively recent scanning probe microscopy technique that combines an AFM tip with a tunable infrared laser to induce a dipole for chemical imaging. Coupling the nanometer resolution of AFM with the chemical specificity of a tuned IR laser, we are able to spatially map the donor and acceptor domains in a model all-polymer bulk heterojunction with resolution approaching 10 nm. Domain size from PiFM images is compared to bulk-averaged results from resonant soft X-ray scattering, indicating excellent quantitative agreement. Further, we demonstrate that in our all-polymer system, the AFM topography, AFM phase, and PiFM show poor correlation, highlighting the need to move beyond standard AFM for morphology characterization of bulk heterojunctions.
快速纳米尺度成像技术是研究有机太阳能电池中本体异质结的关键，对于高性能器件的持续发展至关重要。不幸的是，常用的成像技术，如隧道电子显微镜（TEM）和原子力显微镜（AFM），都存在明显的缺陷。例如，从相位对比度或形貌特征来推断畴结构可能会导致不准确的形态学结论。在这里，我们展示了一种称为光致力显微镜（PiFM）的技术，用于对具有纳米级化学特异性的有机太阳能电池本体异质结进行成像。PiFM 是一种相对较新的扫描探针显微镜技术，它将 AFM 探针与可调谐红外激光相结合，以诱导用于化学成像的偶极子。通过将 AFM 的纳米分辨率与调谐 IR 激光的化学特异性相结合，我们能够以接近 10nm 的分辨率对模型全聚合物本体异质结中的给体和受体畴进行空间映射。从 PiFM 图像得出的畴大小与共振软 X 射线散射的体平均结果进行了比较，表明存在极好的定量一致性。此外，我们还证明了在我们的全聚合物体系中，AFM 形貌、AFM 相位和 PiFM 之间的相关性很差，这突出了需要超越标准 AFM 来对本体异质结的形貌进行表征。
