Slow spectroscopy sheds light on photodegradation

09 Dec 2025

Slow spectroscopy. The experiment detected extremely faint, long-timescale signals emitted by luminescent materials. This material is illuminated by a laser, and faint signals that persist long after excitation are not visible to the naked eye. (Courtesy: OIST)

Using a novel spectroscopy technique, physicists in Japan have revealed how organic materials accumulate electrical charge under long-term illumination by sunlight, leading to material degradation. Ryota Kabe and colleagues at the Okinawa Institute of Science and Technology have shown how charge separation occurs gradually via a rare multi-photon ionization process, offering new insights into how plastics and organic semiconductors degrade in sunlight.

In a typical organic solar cell, an electron-donating material is interfaced with an electron acceptor. When a donor absorbs a photon, one of its electrons may jump across the interface, creating a bound electron-hole pair that may eventually dissociate, yielding two free charges from which practical electrical work can be extracted.

Although such an interface substantially increases the efficiency of the process, charge separation can occur in the absence of the interface when an electron donor is illuminated. “Even single-component materials can generate tiny amounts of charge via multiphoton ionization,” Kabe explains. “However, experimental evidence has been scarce due to the extremely low probability of this process.”

To trigger charge separation in this way, an electron needs to absorb one or more additional photons while in its excited state. Since the vast majority of electrons fall back into their ground states before this can happen, the spectroscopic signature of this charge separation is very weak. This makes it extremely difficult to detect using conventional spectroscopy techniques, which typically observe on timescales of up to a few milliseconds.

The opposite approach

“While weak multiphoton pathways are easily buried under much stronger excited-state signals, we took the opposite approach in our work,” Kabe describes. “We excited samples for long durations and searched for traces of accumulated charges in the slow emission decay.”

Key to this approach was an electron donor called NPD. This organic material has a relatively long triplet-state lifetime, during which an excited electron is prevented from returning to its ground state. As a result, these molecules emit phosphorescence over relatively long timescales.

In addition, Kabe’s team dispersed their NPD samples into different host materials with carefully selected energy levels. In one medium, the energies of both the highest-occupied and lowest-unoccupied molecular orbitals lie below NPD’s corresponding levels, so that the host material acts as an electron acceptor. As a result, charge transfer occurred in the same way as it would across a typical donor-acceptor interface.

Yet in another medium, the host’s lowest-unoccupied orbital lay above NPD’s – blocking charge transfer, and allowing triplet states to accumulate instead. In this case, the only way for charge separation to occur was through multi-photon ionization.

Slow emission decay analysis

Since NPD’s long triplet lifetime allowed its electrons to be excited gradually over an extended period of illumination, its weak charge accumulation became detectable through slow emission decay analysis. In contrast, more conventional methods employ multiple ultrafast laser pulses, severely restricting the timescale over which measurements can be made. Overall, this approach enabled the team to clearly distinguish between the two charge-generation pathways.

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“Using this method, we confirmed that charge generation occurred via resonance-enhanced multiphoton ionization mediated by long-lived triplet states, even in single-component organic materials,” Kabe describes.

This result provides insight into how plastics and organic semiconductors degrade under sunlight over years to decades. The conventional explanation is that sunlight generates free radicals. These are molecules that lose an electron upon ionization, leaving an unpaired electron that readily reacts with other molecules in the surrounding environment. Since photodegradation unfolds over such a long timescale, researchers could not observe this charge generation in single-component organic materials – until now.

“The method will be useful for analysing charge behaviour in organic semiconductor devices and for understanding long-term processes such as photodegradation that occur gradually under continuous light exposure,” Kabe says.

The research is described in Science Advances.

Sam Jarman is a science writer based in the UK.

FROM PHYSICSWORLD.COM     25/12/2025