By Dave DeFusco

Perovskite solar cells have emerged as a frontrunner in the race for efficient, cost-effective renewable energy technologies. These innovative materials, composed of metal halide perovskites, have captured global attention for their exceptional performance and unique defect tolerance. However, a puzzling phenomenon—the role of shallow traps in perovskite solar cells—has sparked curiosity and debate among scientists.

A recent Department of Applied Physical Sciences study, “Enhancing Charge-emitting Shallow Traps in Metal Halide Perovskites by >100 times by Surface Strain,” published in Joule, has revealed new insights into these traps, reshaping our understanding of their influence on solar cell performance.

In semiconductor materials like perovskites, traps refer to imperfections or defects that capture and hold charge carriers—electrons or holes. Traps can either be deep or shallow, depending on how much energy is required to release the trapped charges. Deep traps are notorious for causing energy loss through non-radiative recombination, effectively sapping efficiency. Shallow traps, on the other hand, are less understood but appear to play a more complex role.

Unlike deep traps, shallow traps can capture charges briefly and then release them back into the conduction or valence band, where they can contribute to the electrical current. This unique behavior has positioned shallow traps as potential allies in enhancing perovskite solar cell efficiency, but their exact properties and impact have remained elusive.

“Shallow traps in some perovskite materials are far more abundant than in traditional semiconductors like silicon or cadmium telluride,” said Jinsong Huang, senior author of the paper and Louis D. Rubin Jr. Distinguished Professor. “These traps are primarily located on the surface of perovskite films and can be amplified significantly—over 100-fold—by introducing surface strain. This strain is induced by attaching specific molecules to the perovskite surface, which shifts the energy levels and creates more shallow traps.”

One of the surprising benefits of these shallow traps is their ability to extend the charge recombination lifetime. By holding onto charges temporarily, they prevent premature recombination where electrons and holes annihilate each other, increasing the overall free charge carrier density. This effect boosts the solar cell’s open-circuit voltage (VOC), a critical parameter for achieving high efficiency. In one experiment, a perovskite solar cell’s VOC loss was reduced to just 317 millivolts, setting a new benchmark for p-i-n structured devices.

Unlike traditional semiconductors, perovskites exhibit an exceptional tolerance to defects. The flexibility of their crystal structure allows these materials to manage and even exploit imperfections to enhance performance. This unique property has made perovskites a preferred material for next-generation solar cells, with efficiencies rivaling those of traditional silicon cells.

Interestingly, tests on other materials, such as silicon and CdTe, showed no signs of a similar level of shallow traps. This suggests that perovskites possess a fundamentally different mechanism for managing charge carriers, giving them a competitive edge.

The study also delved into the direct effects of shallow traps on solar cell operation. By developing advanced measurement techniques, researchers quantified how shallow traps influence charge dynamics. They found that the presence of these traps increases the concentration of free holes—positively charged carriers—while maintaining a stable free electron population. This balance reduces energy losses and improves the device’s overall power output.

Furthermore, shallow traps help mitigate the effects of deep traps by “buffering” charges, preventing them from falling into deeper, non-radiative states. This dual role enhances both the stability and efficiency of perovskite solar cells, making them more attractive for commercial applications.

Understanding shallow traps opens new avenues for optimizing perovskite solar cells. Engineers can now explore ways to intentionally manipulate surface strain or tailor material composition to maximize the beneficial effects of these traps. By fine-tuning trap density and distribution, it may be possible to push the efficiency of perovskite solar cells even closer to their theoretical limits.

As scientists continue to unlock the secrets of shallow traps, the potential for scalable, high-performance solar technology grows ever brighter. With further advancements, perovskite solar cells could revolutionize the energy industry, offering a sustainable solution to meet the world’s growing power demands.

“Shallow traps aren’t just defects—they’re a doorway to innovation in renewable energy,” said Huang.

November 20, 2024