McGehee

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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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.

Find out more about the 2025 IRES Cohort

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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2025 BEaRS Workshop

Daniel Morton — Mon, 21 Jul 2025 22:38:29 +0000

2025 BEaRS Workshop Daniel Morton Mon, 07/21/2025 - 16:38

Daniel Morton

The 2025 BEaRS Workshop brought together researchers from across the front range (and beyond, with our guests from Japan) to participate in cross-cutting interdisciplinary discussion focused on energy and resilience.

This inaugural Boulder Energy and Resilient Sustainability (BEaRS) workshop brought together scientists from across a range of disciplines, from the social sciences to fundamental physics, from buildings and control to electronics and computing, all discussing how we can better use energy to develop a more resilient future.

The program for the three-day workshop was made up from seven session, with topics including sustainability and the built environment, active matter and chiral materials, building energy technologies, energy systems and the economics of renewable integration, energy policy, grid optimization and energy efficient computing. We welcomed guests from across the Front Range, from Golden to Fort Collins, and further afield, with collaborators from the WPI-SKCM2, based at Hiroshima University.

The program was designed to include a range of speakers, from invited plenary speakers to student researchers, the aim was to provide opportunities for connection and discussion and to seed cross-disciplinary ideas. The plenary speakers really set the tone for each of the days, striking notes of optimism and encouragement, while also instilling urgency and impatience for change.

Amory Lovins, a co-founder of the Rocky Mountain Institute, joined us from Stanford to present on how integrative design can be used to enhance efficiencies and drive down costs in buildings, everything from the house Amory lives in, to factories for Intel. Using this design principles for today’s technologies can have significant lasting impact in reducing energy use. We have the technology, and we know how to design for it.

Chuck Kutscher, who has spent decades at the cutting-edge of renewable energy research at NREL and now as a RASEI Fellow, gave an overview talk on the clean energy transition with a focus on air pollution and human health. Chuck highlighted all the ways in which we can drive down air pollution as we move to cleaner energy and the impacts, both on human health and societal economics that these changes would make. Central to this change is the drive toward electrification, it will provide cheaper electricity that will significantly reduce the impact on human health.

Ian Lange, an economist from the Colorado School of Mines, an expert in global mineral policy, gave an insightful overview of the status of the global mineral market and provided perspectives on how to think about and understand this industry. The knowledge imparted, both on how to think about the minerals in the soil, and some of the factors that are real drivers in this discussion provided a great way to better understand this important market for technology.

Thank you to everyone who was able to attend and participate in the lively questions, panels, and discussion. The opportunity to spend time with experts from a wide range of fields and approach current ideas from new perspectives is extremely valuable and a great opportunity to explore some of the intersectional ways in which disparate fields can work together.

The workshop was capped off with some of the workshop participants, after a busy three days, finding the motivation to hike up Bear Peak on Saturday morning,

July 2025

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Dynamic windows based on reversible silver electrodeposition with improved resting stability and cyclability

Daniel Morton — Wed, 21 May 2025 22:12:53 +0000

Dynamic windows based on reversible silver electrodeposition with improved resting stability and cyclability Daniel Morton Wed, 05/21/2025 - 16:12

CELL REPORTS PHYSICAL SCIENCE, 2025, 6, 5, 102559

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RASEI Entrepreneurship Highlighted at Startup Showcase

Daniel Morton — Wed, 26 Mar 2025 20:46:46 +0000

RASEI Entrepreneurship Highlighted at Startup Showcase Daniel Morton Wed, 03/26/2025 - 14:46

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Four RASEI-connected startups were highlighted in a showcase event for Embark Deep Tech Startup Creator and Destination Startup.

This showcase was a platform for cutting-edge startups, breakthrough research and high-growth potential ventures to connect university innovation with industry leaders and investors.

Ecovaleric (Adam Holewinski), WindSenseAI (Lucy Pao), Otoro Energy Inc. (Mike Marshak), and Tynt Technologies Inc. (Mike McGehee), were all featured in this event. With a range of ��ý�Ļ��Ʒ technologies ready to make a real-world impact, connecting these motivated entrepreneurs to investors and industry leaders provides support in growing these companies.

March 2025

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2024 Clarivate Highly Cited Researcher Awards

Daniel Morton — Tue, 19 Nov 2024 20:12:12 +0000

2024 Clarivate Highly Cited Researcher Awards Daniel Morton Tue, 11/19/2024 - 13:12

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2024 Clarivate Highly Cited Researchers

Five RASEI Fellows awarded the Highly Cited Researcher designation by Clarivate.

Each year Clarivate analyzes the citations of articles from researchers from across the sciences. From this analysis Clarivate highlights the researchers who have authored multiple highly cited papers which rank in the top 1% by citations for their field(s) and publication year in the Web of Science over the past decade.

This year five RASEI Fellows featured in the 2024 Highly Cited Researcher list, which includes 6,886 researchers. This year Matt Beard, Joe Berry, Joey Luther, Mike McGehee and Mike Toney were selected. For Matt, Mike and Joey, this is the fifth year in the row, the fourth year in a row for Joe Berry and the third year in a row for Mike Toney.

Congratulation to everyone for the recognition of the impact of their research.

November 2024

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2024 TEAM PV Meeting

Daniel Morton — Tue, 12 Nov 2024 00:02:15 +0000

2024 TEAM PV Meeting Daniel Morton Mon, 11/11/2024 - 17:02

In November of 2024 RASEI hosted the 2024 TEAM PV Consortium meeting. This event brought together researchers from ��ý�Ļ��Ʒ, NREL, Humboldt University and Hemholtz-Zentrum Berlin.

The goals of this workshop was to develop nascent collaborative connections, discuss the best practices for collecting, sharing, and analyzing data around the parametrization of perovskites.

RASEI Fellows Steve Barlow, Joe Berry, Seth Marder, Mike McGehee, Laura Schelhas, and Mike Toney participated in the event that included many students and researchers both from Colorado and Germany.

11/11/2024

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Improved reverse bias stability in p–i–n perovskite solar cells with optimized hole transport materials and less reactive electrodes

Anonymous — Wed, 07 Aug 2024 06:00:00 +0000

Improved reverse bias stability in p–i–n perovskite solar cells with optimized hole transport materials and less reactive electrodes Anonymous (not verified) Wed, 08/07/2024 - 00:00

NATURE ENERGY, 2024

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Moisture Uptake Relaxes Stress in Metal Halide Perovskites at the Expense of Stability

Anonymous — Tue, 30 Jul 2024 06:00:00 +0000

Moisture Uptake Relaxes Stress in Metal Halide Perovskites at the Expense of Stability Anonymous (not verified) Tue, 07/30/2024 - 00:00

ACS ENERGY LETTERS, 2024, 9, 4153-4161

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Why Perovskite Thermal Stress is Unaffected by Thin Contact Layers

Anonymous — Wed, 05 Jun 2024 06:00:00 +0000

Why Perovskite Thermal Stress is Unaffected by Thin Contact Layers Anonymous (not verified) Wed, 06/05/2024 - 00:00

ADVANCED ENERGY MATERIALS, 2024, 2400764

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