Liao Bolin, Najafi Ebrahim, Li Heng, Minnich Austin J, Zewail Ahmed H
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA.
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, USA.
Nat Nanotechnol. 2017 Sep;12(9):871-876. doi: 10.1038/nnano.2017.124. Epub 2017 Jul 3.
Charge carrier dynamics in amorphous semiconductors has been a topic of intense research that has been propelled by modern applications in thin-film solar cells, transistors and optical sensors. Charge transport in these materials differs fundamentally from that in crystalline semiconductors owing to the lack of long-range order and high defect density. Despite the existence of well-established experimental techniques such as photoconductivity time-of-flight and ultrafast optical measurements, many aspects of the dynamics of photo-excited charge carriers in amorphous semiconductors remain poorly understood. Here, we demonstrate direct imaging of carrier dynamics in space and time after photo-excitation in hydrogenated amorphous silicon (a-Si:H) by scanning ultrafast electron microscopy (SUEM). We observe an unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron-hole separation and charge trapping induced by the atomic disorder. Our findings demonstrate the rich dynamics of hot carrier transport in amorphous semiconductors that can be revealed by direct imaging based on SUEM.
非晶半导体中的载流子动力学一直是一个深入研究的课题，受到薄膜太阳能电池、晶体管和光学传感器等现代应用的推动。由于缺乏长程有序性和高缺陷密度，这些材料中的电荷传输与晶体半导体中的电荷传输有根本区别。尽管存在诸如光电导飞行时间和超快光学测量等成熟的实验技术，但非晶半导体中光激发电荷载流子动力学的许多方面仍知之甚少。在这里，我们通过扫描超快电子显微镜（SUEM）展示了氢化非晶硅（a-Si:H）光激发后载流子动力学在空间和时间上的直接成像。我们观察到光激发后立即出现的意外快速扩散状态，以及由原子无序引起的自发电子-空穴分离和电荷俘获。我们的研究结果证明了基于SUEM的直接成像可以揭示非晶半导体中热载流子传输的丰富动力学。
