UC Riverside researchers, led by Ming Liu and Ruoxue Yan, have developed a groundbreaking 3D imaging technique that provides unprecedented insight into electron movement within ultrathin solar chips. This innovation allows scientists to precisely observe how both light and heat contribute to electricity generation at the nanoscale, challenging long-held assumptions in photovoltaic design and opening new avenues for device optimization.

Traditional solar power relies on the photovoltaic effect, where photons directly dislodge electrons to create current. However, in ultrathin semiconductor materials like single-layer molybdenum disulfide, the photothermoelectric effect—where light heats charge carriers, driving them towards cooler regions—plays an equally, if not more, significant role. Previously, engineers understood both effects were present but lacked the tools to spatially differentiate their contributions or determine which dominated in specific regions of a device.

The UC Riverside team utilized a specialized scanning microscope, focusing a laser beam to a few billionths of a meter. By scanning this tip across ultrathin chips composed of metal and a light-sensitive material, and analyzing signal changes based on tip proximity, they successfully isolated the two current types. Sharp signal changes indicated the photovoltaic effect, while slower, more stable variations revealed heat-driven movement. Their findings revealed that heat-driven current spread much farther into the chip than anticipated, contradicting conventional wisdom and explaining why earlier devices often underperformed simulated predictions.

To demonstrate control over this phenomenon, the researchers introduced an additional layer of hexagonal boron nitride, just a few atoms thick. This layer facilitated sideways heat dissipation, preventing localized heat buildup. This thermal routing significantly enhanced the photothermoelectric current, tripling it in the covered region despite a lower absolute temperature rise. As Liu stated, "The idea that we can fine-tune a photodetector’s performance using heat flow is really exciting." This 'design knob' offers a novel approach to device engineering, as the boron nitride sheet minimally impacts the device's electric field.

This breakthrough provides engineers with the ability to rethink optoelectronic circuit design. By visualizing the interplay of heat-driven and photon-driven currents, designers can now match material properties to specific effects, boosting sensitivity in detectors for applications like infrared cameras or medical sensors, or minimizing waste in energy-harvesting systems. The technique also enables early detection of manufacturing flaws, such as cracks or rough edges, by revealing their impact on electricity generation. Published in Science Advances, this study marks a pivotal step towards more efficient, flexible, and high-performance nanoscale optoelectronic devices.