Researchers at the University of Cambridge in the United Kingdom claim to have discovered photovoltaic properties in a glowing organic semiconductor molecule known as poly(3‑triphenylmethyl‑thiophene (P3TTM).
Unlike conventional organic semiconductors, which have paired electrons, organic radical semiconductors contain at least one unpaired electron per molecule, giving them an “open-shell” character.
“In most organic materials, electrons are paired up and don’t interact with their neighbors,” said lead author Biwen Li. “But in our system, when the molecules pack together, the interaction between the unpaired electrons on neighboring sites encourages them to align alternately up and down—a hallmark of Mott-Hubbard behavior.”
Mott-Hubbard behavior arises in materials where electron-electron interactions are particularly strong. In P3TTM, each molecule can be thought of as a house containing a single electron. When light excites an electron, it can hop to a neighboring molecule. “Upon absorbing light, one of these electrons moves to its nearest neighbor, creating positive and negative charges that can be harvested as a photocurrent,” the team explained.
To test this, the researchers fabricated an experimental solar cell using a P3TTM film. The device includes a PEDOT:PSS layer on indium tin oxide (ITO), a buckminsterfullerene (C60) layer, a phenyl-C61-butyric acid methyl ester (PCBM) spacer, and an aluminum (Al) contact.
Under standard illumination, the solar cell achieved an almost-unity charge collection efficiency, the team reported.
“This means nearly every photon of light was converted into a usable electrical charge,” the researchers said. “In conventional molecular semiconductor solar cells, photon-to-charge conversion typically occurs only at interfaces between two materials—one acting as an electron donor, the other as an acceptor—limiting overall efficiency.”
“In contrast, in these new materials, after photon absorption, the energy drives an electron from one molecule to an identical neighbor, creating electrical charges,” they added. “The energy required for this—known as the ‘Hubbard U’—represents the electrostatic cost of double electron occupancy on the negatively charged molecule.”
The researchers emphasized that this breakthrough could enable the fabrication of solar cells from a single, low-cost, lightweight material.
Their findings are available in the study “Intrinsic intermolecular photoinduced charge separation in organic radical semiconductors,” published in nature materials.
“This work provides an avenue for the exploration of power generation and solar-driven chemistry in both solution and solid state using only a single component,” they concluded.
Their findings are available in “Intrinsic intermolecular photoinduced charge separation in organic radical semiconductors,” published in nature materials.
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