UC Santa Barbara

The utilization of sustainable energy sources, such as solar energy and thermoelectric energy, has gained significant attention in addressing the global energy challenge. Despite their potential, the efficiency of the energy devices still requires improvement. A comprehensive understanding of the underlying mechanisms is essential for rationalizing efficient energy conversion systems. However, the conventional characterization techniques limited by their resolutions are insufficient for the mechanism study. To overcome this limitation, it is imperative to develop new characterization techniques to probe carrier and energy transport in the energy devices. This thesis employs theoretical and experimental approaches to investigate carrier and energy transport using three distinct novel techniques: a theoretical study of photo-carrier dynamics through Monte Carlo simulations in scanning ultrafast electron microscopy (SUEM), transient grating spectroscopy for simultaneous characterization of thermal transport and photocarrier dynamics in organic semicondutors, and customized cryogenic steady-state transport measurements to explore thermoelectric transport in topological materials.

Understanding the transport of photogenerated charge carriers in semiconductors is crucial for improving the performance of photovoltaics. While recent experimental studies using SUEM have demonstrated that the local change in the secondary electron emission induced by photoexcitation enables direct visualization of the photocarrier dynamics in space and time, the origin of the corresponding image contrast still remains unclear. Here, we investigate the impact of photoexcitation on secondary electron emissions from semiconductors using a Monte Carlo simulation aided by time-dependent density functional theory. Particularly, we examine two photoinduced effects: the generation of photocarriers in the sample bulk and the surface photovoltage (SPV) effect. Using doped silicon as a model system and focusing on primary electron energies below 1 keV, we found that both the hot photocarrier effect immediately after photoexcitation and the SPV effect play dominant roles in changing the secondary electron yield (SEY), while the distribution of photocarriers in the bulk leads to a negligible change in SEY. Our work provides insights into electron–matter interaction under photo-illumination and paves the way toward a quantitative interpretation of the SUEM contrasts.

In addition, the thermal management as well as carrier dynamics are both keys to the operational efficiency and stability for energy devices. Experimental techniques that can simultaneously characterize both properties are still lacking. We manage to characterize thin films of the archetypal organic semiconductor regioregular poly(3-hexylthiophene) and its blends with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester on glass substrate. While the thermal responses from the thin film and the substrate cannot be distinguished due to their similar thermal diffusivities, we show that the recombination dynamics of photocarriers in the organic semiconductor thin films occur on a similar timescale and can be separated from the thermal response. Our measurements indicate that the photocarrier dynamics are determined by multiple recombination processes and our extracted recombination rates are in good agreement with previous reports using other techniques. We further apply TG spectroscopy to characterize another conjugated polymer and a molecular fluorescent material to demonstrate its general applicability. Our study indicates the potential of transient grating spectroscopy to simultaneously characterize thermal transport and photocarrier dynamics in organic optoelectronic devices.

In addition to semiconductors, topological semimetals have been shown to possess intriguing thermoelectric properties promising for energy harvesting and cooling applications. However, thermoelectric transport in heterostructures formed by topological and conventional materials remains less explored. Here, we systematically examine thermoelectric transport in a series of topological Dirac semimetal (Cd3As2)/ semiconductor(GaSb) heterostructures by employing a customized cryogenic steady-state transport measurement. Surprisingly, we found a significantly enhanced Seebeck coefficient at cryogenic temperatures when the Cd3As2 layer was thin. Combining angle-dependent quantum oscillation analysis, magneto-thermoelectric measurement, transport modeling and first-principles simulation, we reveal the contribution from multiple conducting channels in the heterostructure, including bulk and topological surface states. Our analysis showcases the rich transport physics in a topological Dirac semimetal/semiconductor heterostructure and suggests new routes to achieving unusual thermoelectric performance at cryogenic temperatures.