Free radicals caught in the act with slow spectroscopy

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 these are processes that can take years.  

As such, 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 help us better understand the fundamental 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, two-component systems, with solar cells being a notable example, use a combination of electron donor and acceptor materials to generate free charges even under weak visible light that is too energetically insufficient for direct ionization. When light excites the donor molecule, an electron can jump from the donor to the acceptor, creating free charges via a bound charge transfer state at the interface between the materials.   

Because the free charges quickly disappear through recombination, it has been assumed that they can be observed only at very short timescales up to a few milliseconds. But now, the researchers have found that weak signals originating from accumulated free charges can be detected on much longer timescales.  

These weak and slow signals shed light on minor charge generation processes that have so far received little attention. When a single-component material absorbs weak light that is energetically insufficient for direct ionization, an excited state is formed, but because no charge transfer occurs, no free charges are produced. However, if an excited state absorbs an additional photon within its lifetime, it can reach ionization. Such free charge formation via multiphoton ionization is rare, and in conventional methods that cover only femto- to millisecond timescales, the signals from these events are easily obscured by the much stronger signals from the excited states themselves, making experimental confirmation difficult.  

To investigate this slow, transient decay, the researchers reimagined the conventional spectroscopy setup. Instead of repeatedly irradiating the sample with ultrafast laser pulses and accumulating the signal to observe the conventional fast transient decay, they adopted a simple approach: the sample is excited for an extended period, and the long-timescale response is measured in a single-shot experiment. By expanding both the temporal and intensity dynamic ranges, it became possible to distinguish the signals of excited states from those of free charges for much longer, allowing them to observe, for the first time in single-component organic materials, the charge generation pathways that had previously only been predicted in theory.

“We successfully detected the generation of charge carriers through both donor-acceptor interfaces and single-component multiphoton ionization,” explains Professor Kabe. “Our setup proved effective when an organic material was used as a donor-acceptor interface, producing very clear signals, as well as when the same material stood alone, producing extremely weak signals.”  

Their measurements provide direct evidence for multiphoton pathways, shedding light on the fundamental processes undergirding both theoretical and applied research into organic optics. As Professor Kabe summarizes: “Although their efficiency is far too low for photovoltaics or OLEDs, organic materials universally undergo minor photoionization events, and the charges slowly accumulated through these processes may lead to various forms of photodegradation. With this, we’ve finally got the data to confirm these events, and the tools to further investigate weak charge generation pathways across many different organic materials.”