Chemistry Student Uses Laser Flashes, a Powerful New Technique, to Track Electricity Inside Solar Cells
Saba Mahmoodpour is the lead author of a recent Journal of Physical Chemistry paper, which focuses on a powerful new technique called nonlinear photocurrent spectroscopy, or NLPC, which uses pairs of short laser pulses to measure how quickly electrical charges move inside thin-film solar cells.
January 9, 2026 I By Dave DeFusco
Understanding how electricity flows inside a solar cell may sound like a job for engineers equipped with wires and meters but at UNC-Chapel Hill, chemistry Ph.D. student Saba Mahmoodpour is showing that sometimes the best way to study a solar cell is with carefully timed flashes of laser light.
Mahmoodpour is the lead author of a recent Journal of Physical Chemistry paper, “Two-Body Origin of Recombination-Induced Photocurrent Signals for Drift Velocity Resolution in Perovskite Solar Cells,” which focuses on a powerful new technique called nonlinear photocurrent spectroscopy, or NLPC, using pairs of short laser pulses to measure how quickly electrical charges move inside thin-film solar cells.
NLPC lets researchers watch electrons and holes—the negative and positive charge carriers created when light hits a solar cell—race toward the device’s electrodes. Being able to measure that motion is crucial for building better solar technologies. Traditional tools, however, can only see this movement on timescales of hundreds of nanoseconds, which is far too slow to capture the dynamics within thin-film photovoltaic devices.
“Our method can see charge motion about a thousand times faster than older techniques,” said Mahmoodpour. “By using two quick laser pulses, we can track how fast electrical charges move inside a solar cell in trillionths of a second. That’s something researchers haven’t been able to do before.”
In NLPC, the first laser pulse acts like a starter pistol by creating a burst of excited charges in the solar cell. Those charges immediately begin drifting toward the electrodes, just as they would when sunlight hits a working solar panel. A short moment later, a second pulse arrives. Mahmoodpour varies the delay between the two pulses, sometimes by billionths of a second, to see how many charges are still left in the device.
“The first pulse turns the solar cell ‘on’ by creating moving electrical charges,” she said. “After a very short and carefully controlled pause, the second pulse takes a snapshot of what’s left. That snapshot shows how the charges move through the material and eventually disappear.”
When the second pulse arrives, the electrical signal stops increasing the way it normally would. This change shows that the charges created by the two pulses are meeting and canceling each other out. By carefully tracking this effect, researchers can determine how fast the charges are moving inside the solar cell.
“Because of this effect, the electrical signal doesn’t increase in a simple, straight-line way as the laser power goes up,” said Mahmoodpour. “Instead, the positive and negative charges meet and cancel each other out. In this study, I show that the signal comes from pairs of charges recombining—one electron and one hole—not from more complicated interactions.”
This was a key contribution of the paper. Until now, scientists often had to run separate control experiments to figure out whether the nonlinear signal predominantly came from two-body recombination. Mahmoodpour’s method shows this directly, while simultaneously measuring charge transport.
While Mahmoodpour’s advisor, Professor Andrew Moran, specializes in the development of laser spectroscopies, the expertise in materials and device fabrication necessary for this interdisciplinary project was provided by Wei You, Cary C. Boshamer Distinguished Professor of Chemistry and Applied Physical Sciences.
Under their guidance, Mahmoodpour works with perovskite solar cells, an emerging class of materials that promise high efficiency at low cost. Unlike traditional silicon panels, perovskites can be cheaply fabricated in a lab.
“Perovskite is really popular because it’s cheap and easy to make,” she said. “My strength is that I make the solar cells myself and I do the spectroscopy myself, so I can change everything to figure out what is happening inside the material.”
The tradeoff is that perovskites are delicate—sensitive to humidity and prone to burning out under strong electrical bias. Mahmoodpour experienced these challenges firsthand.
“When I wanted to add bias to my devices, it burned them,” she said. “We could not measure mobility that way. There are some limitations.”
Alongside the experiments, the team developed a simplified mathematical model to interpret the nonlinear photocurrent signals. The goal was to extract the essential physics without relying on more complicated and less practical approaches.
“Our motivation was to capture the essential physics from the data,” said Mahmoodpour. “While computationally expensive models are often employed to simulate device operations, the strength of our approach is that it is expressed analytically using measurable parameters. Despite its simplicity, this model still captures the most important fundamental information.”
Their analysis shows that recombination-induced NLPC can provide the same time-of-flight information as more advanced “higher-order” femtosecond laser techniques, so long as recombination happens quickly compared to charge transport. This finding helps unify different laser-based approaches that probe material dynamics.
Mahmoodpour hopes that NLPC can be applied across many different solar materials, not just perovskites, to identify patterns that can guide future design.
“If we can apply these techniques to other materials and compare the results, we can get patterns,” she said. “In the future, that can help us increase the efficiency of perovskite solar cells. We need lots of materials and lots of experiments, but we will get there.”