In a study published in Nature Materials<\/a>, the Cambridge-led team reports the first observation of \u201cintrinsic intermolecular photoinduced charge separation\u201d in an organic radical semiconductor\u2014a material built from molecules that each contain an unpaired electron.\u00a0<\/p>\n The discovery paves the way for a new generation of lightweight, flexible devices that can both harvest and emit light<\/a>, potentially powering themselves from sunlight without external circuitry.<\/p>\n According to researchers, the key lies in how their material\u2014a triphenyl-substituted version of tris(2,4,6-trichlorophenyl)methyl, or \u201cP3TTM\u201d\u2014behaves when its molecules come into contact. When light strikes the material, electrons naturally hop between neighboring radicals, producing positive and negative charges that can move independently under an electric field.\u00a0<\/p>\n In effect, the researchers have demonstrated a \u201chomojunction\u201d\u2014a charge separation process occurring within a single organic compound.<\/p>\n \u201cThis opens possibilities for light harvesting using single-material molecular semiconductors,\u201d the researchers write, emphasizing that the phenomenon could eliminate the need for traditional multi-layer designs used in solar cells and LEDs.<\/p>\n \u201cThis is the real magic,\u201d co-author and researcher at the Cavendish Laboratory, Biwen Li, said in a press release<\/a>. \u201cIn most organic materials, electrons are paired up and don\u2019t interact with their neighbors. But in our system, when the molecules pack together, the interaction between the unpaired electrons on neighboring sites encourages them to align themselves alternately up and down, a hallmark of Mott-Hubbard behavior. Upon absorbing light, one of these electrons hops onto its nearest neighbor, creating positive and negative charges which can be extracted to give a photocurrent (electricity).\u201d<\/p>\n Most organic semiconductors rely on pairs of molecules, a donor and an acceptor,\u00a0 to create a junction where photoexcited electrons and holes can split apart. In contrast, radical semiconductors contain molecules with one unpaired electron that sits in what\u2019s called a singly occupied molecular orbital (SOMO).\u00a0<\/p>\n Traditionally, these radicals were prized for their bright light emission and stability, making them promising for organic LEDs. But their potential for charge generation remained largely unexplored.<\/p>\n Researchers\u2019 experiments show that when P3TTM molecules are closely packed, they spontaneously form charge pairs after absorbing light. Using time-resolved spectroscopy, the researchers observed that the material emitted two distinct colors: a rapid glow at 645 nanometers, followed by a delayed redshifted emission around 800 nanometers. That faint afterglow turned out to be the smoking gun \u2014 evidence of recombination between P3TTM anions and cations, or the separated charges.<\/p>\n Further tests using diode structures,\u00a0 thin-film devices made solely of P3TTM sandwiched between electrodes,\u00a0 revealed that under reverse bias, the material could achieve nearly 100% charge collection efficiency. In other words, almost every absorbed photon produced a usable charge.<\/p>\n What makes this result extraordinary is that charge separation typically requires a heterojunction, a boundary between two materials with different energy levels. In silicon solar cells, for example, light generates weakly bound electron-hole pairs called excitons, which must be split apart by an electric field or material interface. In P3TTM, however, that separation happens naturally, within the same molecular network.<\/p>\n The mechanism relies on a delicate balance of electronic energy levels. The researchers discovered that the \u201cextra energy\u201d required to add a second electron to the radical\u2019s SOMO orbital is lower than the energy of the initial excited state. This allows the photoexcited electron to jump to a neighboring molecule, leaving behind a positively charged partner,\u00a0 a self-contained anion-cation pair.<\/p>\n In their diode tests, this radical-driven process produced a photocurrent that saturated at 45 milliamps per square centimeter, dwarfing the performance of control devices made with rubrene, a conventional organic semiconductor. The results confirm that the effect is intrinsic to the radical material itself, not dependent on any external interface or dopant.<\/p>\n The discovery could have significant implications<\/a> for renewable energy and next-generation electronics. The ability to generate electricity from a single organic film could enable self-powered sensors, wearable medical devices, or even solar-charging OLED displays<\/a> that require no external wiring.<\/p>\n \t\t
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Because the materials are lightweight, flexible, and potentially low-cost to produce, they could also help bridge the gap between traditional silicon photovoltaics and emerging organic optoelectronic systems.<\/p>\n Additionally, organic radicals are already being explored for quantum information, spintronics, and chiral optoelectronics\u2014fields where control over charge and spin interactions is crucial. The ability to induce charge separation directly within a radical system could make such technologies more efficient and robust.<\/p>\n The study also challenges a long-held assumption in materials science that organic molecules must work in pairs to conduct and separate charge. By showing that a single radical material can perform both roles, the Cambridge researchers open the door to a new class of self-sufficient electronic systems.<\/p>\n Ultimately, the findings suggest that the boundary between materials that emit light and those that harvest it may soon disappear. In principle, it could be possible to create a device that both glows and powers itself\u2014all using the same material.<\/p>\n If that vision proves true, the humble organic radical could become the foundation for a new generation of energy-harvesting technology. These self-powered systems may bridge the gap between the living and the electronic\u2014bringing us closer to devices capable of truly sustaining themselves.<\/p>\n \u201cWe are not just improving old designs,\u201d co-author and professor of functional materials at Cambridge, Dr. Hugo Bronstein, explained. \u201cWe are writing a new chapter in the textbook, showing that organic materials are able to generate charges all by themselves.\u201d<\/p>\n Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter:\u00a0@LtTimMcMillan. \u00a0<\/a>Tim can be reached by email:\u00a0tim@thedebrief.org<\/a>\u00a0or through encrypted email:\u00a0LtTimMcMillan@protonmail.com<\/a>\u00a0<\/p>\n \t\t\t\t\t\t\t\t\t
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