Perovskite solar cells have been hailed as the next big thing in renewable energy.
They are cheaper and easier to produce than traditional silicon solar cells and have the potential to be even more efficient.
But despite their promise, these cells still face a critical hurdle on the path to large-scale use: they are vulnerable to a damaging effect called reverse bias, which can destroy them if even a tiny defect is present.
A new study led by Mike McGehee, a Fellow at the Renewable and Sustainable Energy Institute (RASEI) at the University of Colorado Boulder, working with scientists at the National Renewable Energy Laboratory (NREL), sheds light on this problem.
Their research, published in Joule, helps explain why perovskite cells degrade under reverse bias and what can be done to prevent it.
To understand reverse bias, imagine a garden hose system. Water flows smoothly through connected hoses unless one hose is kinked.
The water builds up pressure at the kink until it bursts. Solar panels work in a similar way.
Sunlight creates electrons that flow through each cell like water through a hose.
If one cell becomes shaded, it stops producing electricity but is still forced to carry current from the unshaded cells. This backward flow creates reverse bias, which can permanently damage the shaded cell.
Silicon solar panels already face this issue, but engineers solved it by adding bypass diodes—small “side channels” that reroute the current and protect the shaded cell. Unfortunately, perovskite cells are too delicate for this fix.
Researchers needed to understand why these cells fail so quickly when under reverse bias.
The key lies in how perovskite layers are made. They are created through a process called solution processing, which is a bit like making pancakes. When you pour batter onto a griddle, bubbles and gaps can form.
Similarly, when perovskite materials are deposited from a solution, tiny pinholes and thin spots often appear. These defects turn out to be the weak points where reverse-bias damage begins.
To uncover this, the team used a suite of advanced imaging tools, including electroluminescence cameras, scanning electron microscopes, laser-scanning confocal microscopes, and video thermography.
These techniques let them watch in detail how defects behaved before, during, and after reverse bias stress. The results were striking. Defective spots glowed brightly under thermal imaging, showing intense heat buildup, while defect-free films remained stable for hours under the same conditions.
By creating very small test devices, each about the width of two human hairs, the researchers could make nearly perfect perovskite films without defects. These tiny devices did not degrade under reverse bias, confirming that the problem lies with imperfections in larger-scale films.
The conclusion is clear: defects such as pinholes and thin spots are the exact sites where perovskite cells begin to fail.
Preventing these flaws and reinforcing the contact layers that hold the films together are essential steps to building durable perovskite solar panels.
This research marks an important step toward making perovskite technology commercially viable.
By identifying the weak spots and showing how they trigger breakdown, the study gives engineers a roadmap for designing longer-lasting, more reliable solar cells.
If scientists can produce defect-free films on a larger scale, perovskite solar panels could soon live up to their promise of delivering cheaper, cleaner energy for the world.
