Spatiotemporal Exciton Tracking with a SPAD Camera.

Spatiotemporal microscopy plays an important role in the quest for highly efficient light harvesting materials as it allows direct tracking of the nanoscale transport of excitons, the carriers of the photon energy. Unfortunately, achieving high resolution in both space and time often requires scanning beam spots or delay lines, limiting these techniques to specialized research groups. To overcome this problem, we introduce a novel implementation of photoluminescence-detected exciton tracking using a camera composed of an array of single-photon avalanche diodes (SPADs), gated with ∼150 ps temporal accuracy. The use of such a SPAD camera drastically simplifies the experiment, is free of moving parts, and provides at least 1 order of magnitude increase in photon collection efficiency due to the parallel multipixel acquisition. Moreover, the camera allows one to implement different super-resolution excitation strategies. Here we show both point and structured excitation in the same device by simply changing the optical element. The structured illumination allows direct retrieval of the diffusion from a single time-resolved imaging without fitting, even at fluences far below exciton-exciton annihilation conditions. We tested the SPAD camera effectiveness by studying the exciton diffusion properties of the organic photovoltaic material PM6, where we measured an exciton diffusion length of 45 nm. Certainly our new implementation, boosted by rapid advances in SPAD technology, will extend the range of both users and applications of spatiotemporal microscopy.