Photo-excited hot carrier dynamics in hydrogenated amorphous silicon imaged by 4D electron microscopy

Charge carrier dynamics in amorphous semiconductors has been a topic of intense research that has been propelled by modern applications in thin-film solar cells¹, transistors and optical sensors². Charge transport in these materials differs fundamentally from that in crystalline semiconductors^{3, 4} 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^{5, 6, 7, 8} and ultrafast optical measurements^{9, 10, 11, 12}, 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)^{13, 14}. We observe an unexpected regime of fast diffusion immediately after photoexcitation, together with spontaneous electron–hole separation¹⁵ and charge trapping³ 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.