Extraordinary Carrier Diffusion on CdTe Surfaces Uncovered by 4D Electron Microscopy

The lack of understanding and control over losses of charge carriers at the surfaces/interfaces of solar cell materials is the major factor limiting overall device conversion efficiency. This work describes a breakthrough in real-space visualization of charge-carrier dynamics at the atomic surface level of CdTe, a leading direct bandgap semiconductor in commercial thin-film solar cells. We present a fundamentally new understanding of charge-carrier diffusion and carrier trapping of CdTe single crystals using a four-dimensional scanning ultrafast electron microscope (4D-SUEM)—the only instrument of its kind currently in operation. We found that the diffusion of charge carriers at surfaces vary within extreme ranges, from extraordinary to virtually trapped when surface orientation was changed from (110) to (211). The work presented here is a milestone in addressing the device performance bottlenecks stemming from surfaces and a new avenue to create CdTe-based optoelectronic devices.