Analytical measurements can predict organic solar cell stability

January 11, 2021

North Carolina State University-led researchers have developed an analytical measurement "framework" which could allow organic solar cell researchers and manufacturers to determine which materials will produce the most stable solar cells prior to manufacture.

Organic solar cells have increased in efficiency over the past decades, but researchers and manufacturers still struggle with determining which material combinations work best and why, as well as with achieving stable morphology and operation.

"There is still a lot of 'trial and error' guesswork involved in identifying promising materials for these solar cells," says Harald Ade, Goodnight Innovation Distinguished Professor of Physics at NC State and co-corresponding author of the research. "However, we found that if you understand two important parameters for the materials being used, you can predict how stable the active layer morphology will be, which in turn affects efficiency over time."

The parameters in question are the elastic modulus and glass transition - essentially how stiff the material is and at what temperature the material transitions from a rigid state to a rubbery or viscous fluid state.

"The most efficient solar cells are composed of a blend of materials that typically have poor miscibility," says Brendan O'Connor, associate professor of mechanical and aerospace engineering at NC State and co-corresponding author of the research. "Ideally, these blends need to be mixed during fabrication to an optimized composition, but over time they can separate or diffuse into domains that are too pure, which leads to device degradation.

"We wanted to understand what drives this instability in composition. We found that the molecular interactions that fundamentally drive diffusion behavior could be captured with the 'proxy-parameters' of elastic modulus and glass transition temperature."

The team, led by NC State postdoctoral researcher Masoud Ghasemi, used secondary ion mass spectrometry (SIMS), to measure the diffusion behavior of small molecules into a pure polymer layer. They also used differential scanning calorimetry (DSC), and a wrinkling metrology approach to measure the glass transition and elastic modulus of a number of materials that are commonly used in organic solar cells.

Overall, the team found that the most stable organic solar cells contained a small molecule with a high glass transition temperature and a polymer with a large elastic modulus; in other words, a highly rigid material.

"The more rigid materials also have the lowest inherent miscibility," Ghasemi says. "Interestingly, this means that the materials that do not like to mix have the lowest diffusion when forced to do so, resulting in the most stable solar cells."

"Our findings are fairly intuitive," Ade says, "but finding that there is a quantitative relationship between elastic modulus, glass transition and the molecular interactions inside these materials allows us to capture interaction forces at a local level, predicting stability in these systems without requiring trial and error."
-end-
The research appears in Nature Materials and was supported in part by the Office of Naval Research and the National Science Foundation. Researchers from the University of North Carolina at Chapel Hill, the University of Kentucky, Imperial College London and the University of Oxford, U.K., also contributed to the work.

Note to editors: An abstract follows.

"A molecular interaction-diffusion framework for predicting organic solar cell stability"

DOI: 10.1038/s41563-020-00872-6

Authors: Masoud Ghasemi, Nrup Balar, Zhengxing Peng, Huawei Hu, Yunpeng Qin, Taesoo Kim, Aram Amassian, Brendan T. O'Connor and Harald Ade, North Carolina State University; Jeromy James Rech, Wei You, University of North Carolina at Chapel Hill; Matthew Bidwell, Imperial College, London; Walter Mask, Chad Risko, University of Kentucky; Iain McCulloch, University of Oxford, U.K.
Published: Online in Nature Materials

Abstract:
Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene-small molecule acceptors (NF-SMAs). Although the morphological stability of these NF-SMA devices critically affects their intrinsic lifetime, their fundamental intermolecular interactions and how they govern property-function relations and morphological stability of OSCs remain elusive. Here, we discover that the diffusion of an NF-SMA into the donor polymer exhibits Arrhenius behaviour and that the activation energy Ea scales linearly with the enthalpic interaction parameters χH between the polymer and the NF-SMA. Consequently, the thermodynamically most unstable, hypo-miscible systems (high χ) are the most kinetically stabilized. We relate the differences in Ea to measured and selectively simulated molecular self-interaction properties of the constituent materials and develop quantitative property-function relations that link thermal and mechanical characteristics of the NF-SMA and polymer to predict relative diffusion properties and thus morphological stability.

North Carolina State University

Related Solar Cells Articles from Brightsurf:

Solar cells of the future
Organic solar cells are cheaper to produce and more flexible than their counterparts made of crystalline silicon, but do not offer the same level of efficiency or stability.

A blast of gas for better solar cells
Treating silicon with carbon dioxide gas in plasma processing brings simplicity and control to a key step for making solar cells.

Record efficiency for printed solar cells
A new study reports the highest efficiency ever recorded for full roll-to-roll printed perovskite solar cells.

Next gen solar cells perform better when there's a camera around
A literal ''trick of the light'' can detect imperfections in next-gen solar cells, boosting their efficiency to match that of existing silicon-based versions, researchers have found.

On the trail of organic solar cells' efficiency
Scientists at TU Dresden and Hasselt University in Belgium investigated the physical causes that limit the efficiency of novel solar cells based on organic molecular materials.

Exciting tweaks for organic solar cells
A molecular tweak has improved organic solar cell performance, bringing us closer to cheaper, efficient, and more easily manufactured photovoltaics.

For cheaper solar cells, thinner really is better
Researchers at MIT and at the National Renewable Energy Laboratory (NREL) have outlined a pathway to slashing costs further, this time by slimming down the silicon cells themselves.

Flexible thinking on silicon solar cells
Combining silicon with a highly elastic polymer backing produces solar cells that have record-breaking stretchability and high efficiency.

Perovskite solar cells get an upgrade
Rice University materials scientists find inorganic compounds quench defects in perovskite-based solar cells and expand their tolerance of light, humidity and heat.

Can solar technology kill cancer cells?
Michigan State University scientists have revealed a new way to detect and attack cancer cells using technology traditionally reserved for solar power.

Read More: Solar Cells News and Solar Cells Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.