Heat meets light: A new way to supercharge solar power

The first thing every textbook says about solar power is that light knocks electrons loose. But inside ultrathin chips just a few atoms thick, those electrons face a complex landscape of heat, boundaries, and materials that defy the rules of ordinary semiconductors.

Now, thanks to Ming Liu and Ruoxue Yan at UC Riverside’s Bourns College of Engineering, scientists can finally watch that chaotic electron dance play out in three dimensions.

The team’s new imaging technique reveals exactly where those electrons go – and how much their journey is driven by heat, not just light.

Competing currents in solar chips

The photovoltaic effect converts photons into current by freeing electrons inside a semiconductor. Those electrons drift toward metal contacts, creating the power that runs a rooftop array.

An equally important process – the photothermoelectric effect – occurs when light heats the charge carriers themselves, causing them to rush toward cooler regions instead of the nearest wire. In single-layer molybdenum disulfide, that heat-driven push can dominate.

“Before now, we knew both effects were happening, but we couldn’t see how much each one contributed or how they were spatially distributed,” said Liu.

Because the two effects pull charges in opposite directions, device engineers struggle to know which one dominates in a given pixel or junction.

Where electricity really forms

Liu’s team used a special microscope that focuses laser light through a tiny tip, narrowing the beam to just a few billionths of a meter.

The researchers scanned this tip across the surface of an ultrathin chip made of metal and a light-sensitive material to see where and how electricity was being generated.

By moving the tip closer and farther from the surface and analyzing how the signal changed, they could separate two different types of current. The sharp changes came from the photovoltaic effect, while the slower, more stable ones came from heat-driven movement.

The final map showed a narrow line of photovoltaic current near the edge where the metal and semiconductor met. But the heat-driven current reached much farther into the chip, spreading over an area much wider than scientists expected.

This goes against the conventional wisdom. The observation explains why earlier devices sometimes delivered less power than their simulated band diagrams predicted.

Thin layer redirects solar heat

To see if they could control how heat moves through the chip, the team added an extra layer just a few atoms thick. This hexagonal boron nitride layer spread heat sideways, preventing it from building up in one spot.

That smoother heat flow lined up better with parts of the chip that respond strongly to temperature changes. As a result, the chip produced even more electric current from heat than before.

In the covered region, the photothermoelectric current more than tripled, even though the absolute temperature rise fell. The uncovered half, by contrast, showed a skinny hot spot and a much weaker signal.

Because the boron nitride sheet is only a few atoms thick, it barely alters the device’s electric field. The boost therefore comes almost entirely from thermal routing, a design knob that traditional photodiodes have ignored.

“The idea that we can fine-tune a photodetector’s performance using heat flow is really exciting,” Liu said. With the microscope acting as both thermometer and ammeter, designers can now visualize the effect of every added layer.

Heat joins the solar toolbox

Being able to untangle heat-driven and photon-driven currents at the nanoscale gives researchers a rare chance to rethink how they build optoelectronic circuits.

Engineers can now match material properties with specific effects, boosting sensitivity in detectors or minimizing waste in energy-harvesting systems.

For example, engineers building infrared cameras or medical sensors could boost heat-based current by adding materials that spread heat more evenly.

But in other cases – like in tiny quantum devices – designers might want to keep the heat more focused to get sharper results in specific areas.

Better solar from heat maps

Smartphones rely on nanoscale photodetectors to translate laser pulses in fiber-optic cables into data. As channels shrink, the heat generated by absorbed light becomes harder to dump, so balancing photovoltaic and photothermoelectric forces grows critical.

Solar researchers, meanwhile, are hunting hybrid panels that harvest both light and heat. Mapping where the two currents help or hinder each other could guide better device layouts. That, in turn, might squeeze a few extra percentage points of efficiency from ultrathin cells.

Ultrathin materials could be used to make flexible, see-through sensors for things like health trackers or self-driving cars.

The new microscope technique reveals how tiny flaws – like cracks or rough edges – impact a material’s ability to generate electricity, helping engineers catch issues early in the manufacturing process.

Finally, the study hints at basic science questions: Why do depleted regions in 2D materials stretch so far, and how does that length scale with carrier density?

The answers may improve models of quantum-scale thermoelectric devices or lead to smarter algorithms for managing hot carriers in materials like perovskites.

The study is published in the journal Science Advances.

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