Nanoscience and Advanced Materials

Fixing Solar’s Weak Spot: Why a tiny defect could be a big problem for perovskite cells

Daniel Morton — Mon, 15 Sep 2025 15:25:36 +0000

Fixing Solar’s Weak Spot: Why a tiny defect could be a big problem for perovskite cells Daniel Morton Mon, 09/15/2025 - 09:25

Daniel Morton

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Solar energy is a crucial part of our clean energy future, but a new, highly efficient solar material has a hurdle that needs to be addressed. A recent study reveals how a microscopic weak spot can lead to total device failure and what we can do about it.

A collaboration between a team led by RASEI Fellow Mike McGehee and scientists at the National Renewable Energy Laboratory (NREL), just published in the scientific journal Joule, provides evidence to help solve one of the key hurdles to large-scale manufacture of next generation perovskite solar cells.

Imagine you have a series of hoses connected end-to-end to water your garden. The water flows from the faucet, through each hose, and out the last nozzle. When every hose is getting enough water, the flow is strong and steady. This is like how a string of solar cells works on a solar panel; the sun’s energy makes electrons (the “water”) that flow through each cell, creating electricity.

But what happens if a single section of the hose gets kinked? The water can’t flow through it anymore, but there is still a lot of pressure coming from the faucet. The pressure will build up and eventually burst the weak spot in the kinked section. This is analogous to what happens when a section of the solar panel is shaded --- the cell becomes ‘kinked’. When just one part of a panel is shaded, the unshaded cells still generate electricity and “force” current backward through the non-producing shaded cell. This is known as reverse bias, and it can cause the shaded cell to permanently degrade and fail.

For conventional silicon-based solar cells, reverse bias is a known problem and engineers have developed a solution: a bypass diode. You can think of this as a small side-channel that allows the water to flow around the kinked hose, keeping the rest of the system running smoothly without building up damaging pressure.

However, the bypass diode solution doesn’t work for perovskite-based solar cells, a leading candidate for the next generation of more efficient and more affordable solar cells, because they are often too “weak”. One of the key pieces in the puzzle to solving this reverse bias problem in perovskite solar cells is understanding how the cell degrades when under reverse bias, and that is the focus of this research collaboration.

The McGehee group has a long history of success in creating and optimizing perovskite solar cells. Beginning in 2018, their focus shifted to a critical challenge: what happens when these cells are in the shade? Many researchers had already observed that even a small amount of reverse bias caused the materials to heat up and "melt," leading to rapid and permanent degradation of the perovskite.

While these observations were widely accepted, the exact reason for the degradation was a mystery and a subject of much debate. "These are complex systems, and it can be very hard to untangle what is going on," explained Ryan DeCrescent, one of the study's lead researchers. This is where the McGehee group's work came in—they set out to find the specific mechanism behind this destructive behavior.

"These are complex systems, and it can be very hard to untangle what is going on," explained Ryan DeCrescent

The perovskite layer is formed through an approach called solution processing. Solution processing is kind of like making a pancake, you make your batter and when you pour it onto a hot griddle several things happen: the water evaporates, the solids set, the thickness is determined by how much you add, and you often get gaps, or holes in your pancake. In these devices, the perovskite ingredients are put into a solvent. The solvent is then dropped onto the earlier layers of the device and warmed up, whereby the solvent evaporates and a film is formed, but often with defects, or gaps. Defects and pinholes are easily formed in such films. This is a particular issue for perovskites, since the precursor solution has low viscosity and during the heating stage defect formation is common.

To better understand the impact of these defects on the performance of the solar cells under reverse bias you need to take a really good look at them. Central to this work is a suite of tools that enabled exceptional examination of the perovskite layer. “A large part of this work was really setting ourselves up to look very carefully at these surfaces” said DeCrescent. Four main techniques were employed to better understand the defects: Electroluminescence (EL) imaging with a high-resolution camera, Scanning Electron Microscopy (SEM), Laser-Scanning Confocal Microscopy (LSCM) and Video Thermography. The strategy was to compare ‘before, during, and after’ pictures of devices that had been exposed to reverse bias. The high-resolution camera showed that “weak spots” in the device were the origin of degradation. To better understand “perfect” device behavior and efficiently scan a large number of samples (~100), the team setup a large number of very small devices, creating thin films with an area of just 0.032 mm^2. To put that in perspective, each device was about the width of two human hairs. The small size of these devices meant that it was possible to create devices that were defect-free, since it is hard to create defect-free films on a larger scale. Through this combination of a large sample size, and advanced imaging, the team was able to rapidly explore many different types of defects.

This approach of using advanced imaging proved to be an incredibly effective way not only to identify the defects but also to understand exactly what happens to them. "Video thermography and electroluminescence imaging are really powerful techniques for such devices; for example, defects that are sometimes difficult to spot really stand out using these approaches," explained Ryan. Using the thermography technique the defects glow brightly, and in the electroluminescence technique the defects show as dark. Using these techniques in combination provided a very reliable and effective way of mapping the defects. The techniques clearly revealed where the degradation was occurring.

The team’s evidence strongly supports the argument that defects, like pinholes and thin spots in the perovskite layer, are the precise locations where reverse-bias breakdown begins. The thermography images showed that these sites are where the material rapidly heats up and melts, essentially shorting between the two contact layers. In contrast, defect-free devices showed remarkable stability, surviving hours of reverse bias without any significant degradation.

This level of detailed understanding is crucial for the future of this technology. The team's research provides a clear path forward for scientists and engineers: to develop more robust and stable perovskite solar cells, they must prioritize making pinhole-free films and using more robust contact layers to prevent this kind of abrupt and permanent thermal damage.

This work represents a critical step in the journey toward commercializing perovskite solar cells. It highlights the fact that detail-driven, rigorous scientific approaches are needed to understand complex problems. With this knowledge in hand, scientists can now engineer devices that are designed for longevity, ensuring these promising materials can fulfill their potential.

September 2025

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Liquid Crystals that Keep Time: Scientists Create Matter that Dances to Its Own Beat

Daniel Morton — Fri, 05 Sep 2025 19:29:45 +0000

Liquid Crystals that Keep Time: Scientists Create Matter that Dances to Its Own Beat Daniel Morton Fri, 09/05/2025 - 13:29

Adapted from an article run in ��ý�Ļ��Ʒ Today by Daniel Strain

A team led by RASEI Fellow Ivan Smalyukh has discovered a new type of liquid crystal that exists in perpetual, rhythmic motion, creating, for the first time, time crystals visible to the naked eye.

Reporting their findings in Nature Materials, the team demonstrates how liquid crystals, the same materials found in your phone display, can form a phase of matter that spontaneously breaks both space and time symmetries. Unlike previous time crystals that existed only in quantum systems invisible to the naked eye, these can be observed directly under a microscope.

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The researchers designed special glass cells filled with liquid crystals and coated with light-sensitive dye molecules. When illuminated with blue light, something remarkable happens: like dancers following a lead, the liquid crystal molecules respond to cues from the dye molecules, creating an elaborate molecular waltz that repeats its steps over and over.

Here's how the molecular choreography works: The azobenzene dye molecules at the surface respond to light by rotating, which then guides neighboring liquid crystal molecules to reorient. This creates a feedback loop where the changing liquid crystal orientation affects how light polarizes as it passes through, which then influences more dye molecules at the bottom surface. The result is a self-sustaining temporal rhythm.

The researchers discovered that these time crystals are built from special molecular arrangements called ‘topological solitons’, think of it like stable whirlpools in a stream that maintain their shape while the water flows around them. These soliton "particles" interact with each other through the liquid crystal's elasticity, forming arrays that oscillate in time with remarkable precision.

What makes these time crystals remarkable is their resilience, similar to a heartbeat that continues despite disturbances, these patterns persist even when perturbed. The team demonstrated that the crystals maintain their rhythm when subjected to random light fluctuations and recover their ordered state after disruptions, meeting stringent criteria that distinguish true time crystals from simple periodic behavior.

The temporal periods can be tuned from milliseconds to tens of seconds by adjusting temperature and light intensity, while the spatial patterns can extend over areas larger than a square millimeter—making them easily visible and potentially practical for applications.

There are many potential applications, particularly in optoelectronics and security. The time crystals could serve as dynamic optical elements that modulate light in both space and time, enable new forms of optical communication, or provide sophisticated anti-counterfeiting features through their unique spatiotemporal "fingerprints." The ability to create 2+1 dimensional barcodes (two spatial dimensions plus time) could revolutionize information storage and encoding.

As is often found with breakthrough discoveries, the most transformative applications are likely yet to be imagined. But for now, scientists have created matter that literally keeps time.

September 2025

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Powering the Future: U.S. Students Gain International Experience Through Photovoltaics Research in Berlin

Daniel Morton — Tue, 26 Aug 2025 19:43:01 +0000

Powering the Future: U.S. Students Gain International Experience Through Photovoltaics Research in Berlin Daniel Morton Tue, 08/26/2025 - 13:43

Lauren Scholz

A summer of international research concludes as U.S. students contribute to solar innovation in Berlin while gaining hands-on training and global scientific perspective through the NSF-IRES Program.

We are proud to celebrate the successful completion of our first cohort of students bound for Berlin as part of the National Science Foundation International Research Experience for Students (NSF-IRES) Program in metal-halide perovskite photovoltaics. Over the course of ten intensive weeks, nine students from universities across the United States immersed themselves in collaborative research at Humboldt-Universität zu Berlin and Helmholtz-Zentrum Berlin. Their work focused on advancing next-generation solar technologies—specifically, the development and optimization of metal-halide perovskite solar cells.

This timely exchange supported critical progress in the field of photovoltaics, where metal-halide perovskites offer promising pathways to higher efficiency and more versatile solar solutions beyond the limits of conventional silicon-based technologies. By engaging directly with leading German research teams, students not only deepened their technical knowledge and experimental skills but also gained valuable cross-cultural experience and a global perspective on scientific collaboration.

Selected for their academic excellence and commitment to renewable energy innovation, the participants—ranging from undergraduate to graduate level—contributed to a variety of interdisciplinary projects in chemistry, physics, materials science, and engineering. Their contributions helped strengthen the scientific partnerships between U.S. and German institutions and demonstrated the impact of international collaboration in addressing global climate and energy challenges.

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2025 NSF IRES-Perovskites participants. Pictured (left to right): Megan Davis, Keya Amundsen, Jiselle Ye, Jack Schall, Keenan Wyatt, Kell Fremouw, Leo Beck, Gabriel Graf. Not pictured: Arial Brookhart.

August 2025

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Autonomous Research for Real-World Science Workshop: AI Could Help Bridge Valley of Death for New Materials

Daniel Morton — Tue, 19 Aug 2025 16:05:51 +0000

Autonomous Research for Real-World Science Workshop: AI Could Help Bridge Valley of Death for New Materials Daniel Morton Tue, 08/19/2025 - 10:05

August 2025

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Point Defect Induced Potential Wells across the m-Plane of Core/Shell GaN Nanowires

Daniel Morton — Tue, 05 Aug 2025 19:52:19 +0000

Point Defect Induced Potential Wells across the m-Plane of Core/Shell GaN Nanowires Daniel Morton Tue, 08/05/2025 - 13:52

PHYSICA STATUS SOLIDI RAPID RESEARCH LETTERS, 2025, 2500145

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Tuning transition metal nanoparticles on a non-traditional support via experimental design

Daniel Morton — Fri, 01 Aug 2025 20:11:16 +0000

Tuning transition metal nanoparticles on a non-traditional support via experimental design Daniel Morton Fri, 08/01/2025 - 14:11

APPLIED CATALYSIS A: GENERAL, 2025, 706, 120464

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Pre-steady-state kinetics of nanocrystal:molybdenum nitrogenase biohybrids reveals hole-scavenging efficiency is critical to N2 reduction

Daniel Morton — Wed, 30 Jul 2025 19:59:40 +0000

Pre-steady-state kinetics of nanocrystal:molybdenum nitrogenase biohybrids reveals hole-scavenging efficiency is critical to N2 reduction Daniel Morton Wed, 07/30/2025 - 13:59

CELL REPORTS PHYSICAL SCIENCE, 2025, 102732

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Narrow-Band Electrochromism of Ferrocene-Substituted Rhodamine B

Daniel Morton — Tue, 29 Jul 2025 19:54:28 +0000

Narrow-Band Electrochromism of Ferrocene-Substituted Rhodamine B Daniel Morton Tue, 07/29/2025 - 13:54

CHEMISTRY OF MATERIALS, 2025, 37, 15, 5558-5568

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Surface Acidity of Oxygen Evolution Intermediates by Excited State Optical Spectroscopy

Daniel Morton — Mon, 28 Jul 2025 19:37:57 +0000

Surface Acidity of Oxygen Evolution Intermediates by Excited State Optical Spectroscopy Daniel Morton Mon, 07/28/2025 - 13:37

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 2025, 147, 31, 28474-28483

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Formation trajectories of solution-processed perovskite thin films from mixed solvents

Daniel Morton — Wed, 16 Jul 2025 20:06:17 +0000

Formation trajectories of solution-processed perovskite thin films from mixed solvents Daniel Morton Wed, 07/16/2025 - 14:06

CELL REPORTS PHYSICAL SCIENCE, 2025, 6, 7, 102655

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