Visualization of carrier dynamics on photoactive material surfaces

Despite extensive research efforts over the past few decades, detailed information about surface-dynamical processes (e.g., charge-carrier trapping and recombination) is still extremely limited. A better understanding of surface-dynamical processes is therefore urgently needed to address various fundamental problems related to the performance of solar cells (and many other optoelectronic devices). In particular, one of the largest challenges in this field is the need to selectively map the ultrafast dynamics of charge carriers on material surfaces and interfaces. In the past, such selective mapping has been impractical with conventional time-resolved laser techniques (e.g., time-resolved laser spectroscopy and static-electron imaging). This is because these methods are limited by the laser's relatively large penetration depth. As a consequence, these techniques can only be used to record bulk information. Although many advances have been made in both the electron microscopy and laser spectroscopy communities to achieve very high spatial and temporal resolutions, it is still extremely difficult to obtain both simultaneously (i.e., in one experiment).