Unlocking the Secrets of Photodegradation: How Sunlight Damages Materials

Why does plastic turn brittle and paint fade when exposed to the sun for long periods? Scientists have long known that such organic photodegradation occurs due to the sun’s energy generating free radicals: molecules that have lost an electron to sunlight-induced ionization and have been left with an unpaired one, making them very eager to react with other molecules in the environment. However, the exact mechanisms for how and why the energy from the sun’s photons get stored and released in the materials over very long periods have eluded empirical evidence.

The problem lies in the timeframe.While scientists have access to extremely sophisticated spectroscopy equipment capable of measuring the energy levels of individual electrons at femtosecond to millisecond scales in organic materials, they have paid little attention to time scales beyond seconds – and thes are processes that can take years.

In this very way, slow, transient charge accumulation has presented a disappointing data gap in both applied and theoretical optics.But now, researchers from the Organic Optoelectronics Unit at the Okinawa Institute of Science and Technology (OIST) have addressed this challenge with a new methodology that detects these faint signals. Their findings are published in Science Advances. “We can now capture the exact mechanisms of weak charge accumulation,” explains Professor Ryota Kabe. “This can definitely help us better understand the basic characteristics of excitation in organic materials, and it allows for much more accurate measurements of weak charge accumulation – like in photovoltaics, OLED, and photodegradation.”

The flight of photoexcited electrons

The process by which a material absorbs light and generates free charges is important to many fields. When a material is subjected to strong ultraviolet light with enough energy to directly ionize the molecules, electrons can be ejected from their orbit. This process is central to photoelectron spectroscopy, which is widely used to study material properties across scientific fields.

In contrast to these high-energy ionization events in single-component materials, organic materials often exhibit a more subtle process. The OIST team discovered that when organic materials absorb light, electrons don’t immediately fly off. Instead, they become temporarily trapped in defects within the material. These defects act like tiny holding cells, accumulating charge over time. This slow accumulation of charge is what ultimately leads to the degradation of the material.

“Imagine a bucket slowly filling with water,” explains dr. Manoj Kumar, the first author of the study. “Each drop of water represents a tiny amount of charge accumulating in the material. Eventually, the bucket overflows, and that’s when the material starts to degrade.”

A new methodology for detecting faint signals

To detect this slow charge accumulation, the researchers developed a new technique called Transient Photoconductivity with Charge Trapping (TPCT). This method involves shining a pulse of light on the material and then measuring the resulting electrical current over a long period. By carefully analyzing the shape of the current signal, the researchers can determine how quickly charge is accumulating in the material and where it is being trapped.

“The key to our success was developing a way to filter out the noise and amplify the faint signals,” says Professor Kabe. “This allowed us to observe the charge accumulation process in real-time.”

Implications for future technologies

This new understanding of photodegradation has critically important implications for a wide range of technologies. For example, it could led to the advancement of more durable plastics and paints that are less susceptible to fading and cracking. It could also help improve the efficiency of solar cells and OLEDs by reducing the amount of charge that is lost due to degradation.

“By understanding the fundamental mechanisms of photodegradation, we can design materials that are more resistant to the damaging effects of sunlight,” says Dr. Kumar. “This will not only save consumers money but also reduce the environmental impact of these materials.”

Key Takeaways

• Photodegradation occurs due to the sun’s energy creating free radicals in materials.
• Slow charge accumulation within material defects is a key, previously unobserved, mechanism in this process.
• Researchers at OIST developed a new technique (TPCT) to detect this slow charge accumulation.
• This research has implications for improving the durability of plastics, paints, solar cells, and OLEDs.