Bold claim: sunlight slowly erodes plastics and paints by driving tiny, almost invisible charges to accumulate over years. And the part most people miss: these faint processes can now be observed directly, thanks to a fresh approach in slow spectroscopy. Researchers at the Organic Optoelectronics Unit of the Okinawa Institute of Science and Technology (OIST) have unlocked a window into how weak charges build up in organic materials long after the initial light exposure. Their results, published in Science Advances, reveal mechanisms previously hidden by the focus on ultrafast timescales.
Traditionally, high-energy ultraviolet light can eject electrons from molecules, a phenomenon exploited in photoelectron spectroscopy. But many real-world devices—like solar cells and OLEDs—operate under gentle, visible light where direct ionization isn’t readily triggered. In tandem systems composed of donor and acceptor materials, charge separation can still occur under these modest energies: an electron moves from the donor to the acceptor, creating free charges at their interface. Yet these charges typically recombine quickly, leading researchers to assume that observable signals must die out within milliseconds.
The new study turns that assumption on its head by tracking signals that persist much longer. The team uncovered weak, slow signals arising from accumulated free charges that would previously have been invisible with conventional techniques. These signals illuminate secondary charge-generation processes that have received little attention until now. In single-component materials, weak light can form excited states without immediate charge transfer. If a subsequent photon arrives within the excited state's lifetime, ionization can occur, producing free charges. Such multiphoton ionization events are rare and easily drowned out by stronger signals from the excited states in typical femtosecond-to-millisecond experiments.
To capture these elusive events, the researchers redesigned the spectroscopy setup. Instead of repeatedly firing ultrafast laser pulses, they exposed the sample for an extended period and then measured the long-timescale response in a single-shot readout. Expanding both the temporal and intensity ranges enabled a clear separation between excited-state signals and genuine free-charge generation, allowing the first direct observation of these pathways in a single-component organic material.
The team mapped how electrons can be excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and beyond toward ionization. They quantified known pathways—photo-induced charge separation at donor–acceptor interfaces, direct photoionization in single-component systems, and non-resonant multiphoton ionization—as well as resonant multiphoton routes that had attracted less attention. In resonant multiphoton excitation, electrons absorb several photons in sequence, climbing through short-lived excited states with each absorption step. In non-resonant multiphoton ionization, multiple photons drive electrons through virtual states without occupying real intermediate states.
“We successfully detected charge-carrier generation via both donor–acceptor interfaces and single-component multiphoton ionization,” says Professor Ryota Kabe. “The approach works particularly well with an organic material acting as a donor or as an interface, yielding clear signals, and even when tested as a standalone material, producing much weaker but detectable signals.”
These measurements offer direct evidence for multiphoton pathways and refine the understanding of the fundamental processes behind organic optics in both theory and application. As Kabe puts it, while the efficiency of these routes is far too low for practical devices like solar cells or OLEDs, every organic material experiences minor photoionization events that can slowly accumulate charges and contribute to photodegradation. The study provides concrete data and a versatile toolkit to probe weak charge-generation pathways across a broad range of organic materials.
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