The core of the problem lies at the boundary between the two light-absorbing materials inside an organic photovoltaic cell. When a photon is absorbed, it creates a tightly bound electron-hole pair, an exciton, which must travel to the interface between the electron-donating polymer and the electron-accepting molecule to be split into free charges. In the process, an interfacial charge-transfer state forms, in which the electron and hole sit on adjacent molecules. These charge-transfer excitons are the workhorses of charge generation, but they are also a liability. Because they are spatially separated, their electron and hole wavefunctions overlap only weakly, which means they couple poorly to light. They cannot easily emit a photon, and instead they tend to decay non-radiatively, releasing their energy as molecular vibrations and heat. Every charge-transfer exciton lost this way erodes the open-circuit voltage, the maximum voltage the cell can produce, and therefore the overall efficiency.

The research team, led by Yingze Zhang and Sai Ho Pun of the Hong Kong University of Science and Technology together with Gang Li of the Hong Kong Polytechnic University and colleagues, approached the problem not by redesigning the morphology of the blend or tweaking the energy levels of the frontier orbitals, the two conventional levers of organic photovoltaic optimization, but by adding an entirely new photophysical channel. Their idea was to introduce a small fraction of a solid-state emissive guest acceptor, a molecule based on a tetraphenylethylene, or TPE, core, into a state-of-the-art host system built around Y-series non-fullerene acceptors. The guest is chosen so that its localized excited state, the excited state confined to a single molecule, sits energetically close to the charge-transfer manifold at the interface.

This energetic resonance is the key to the whole scheme. When an interfacial charge-transfer exciton forms, it normally faces a choice between radiative decay, which is slow and improbable, and non-radiative decay, which is fast and dominant. But if a localized excited state of the host acceptor lies just slightly above the charge-transfer state, thermal energy at room temperature is enough to push the exciton back up onto that localized state. From there, the excitation can hop onto the emissive guest molecule through resonance energy transfer, the same near-field mechanism that transfers energy between closely spaced fluorescent dyes. The guest, engineered to luminesce strongly even in the solid state, then has a real chance of emitting a photon. The result is an indirect but effective radiative recombination pathway: charge-transfer excitons that would otherwise be lost as heat are thermally recycled into bright, emissive states.

Establishing that this counterintuitive route actually operates required an extensive experimental and computational campaign. The team grew single crystals of the TPE-based acceptors to resolve their molecular packing, confirming that the twisted tetraphenylethylene core suppresses the tight pi-pi stacking that normally quenches emission in organic solids. Photoluminescence quantum yield measurements showed that the guest molecules retain strong solid-state emission when blended into the host. Transient spectroscopy, including time-resolved photoluminescence and electroluminescence measurements performed with collaborators at Zhejiang University, traced the flow of excitation from the charge-transfer states to the localized states and finally to the emissive guest. Molecular dynamics simulations, run with the GROMACS package, mapped the distribution of intermolecular distances and demonstrated that the energetic resonance between the charge-transfer and localized manifolds is thermally accessible across realistic molecular configurations.

The device-level consequences are striking. Cells containing the emissive guest acceptor showed markedly enhanced electroluminescence quantum efficiency, the fraction of injected charges that emerge as photons when the device is driven like a light-emitting diode. Because electroluminescence and the reverse process of non-radiative recombination are two sides of the same coin, this enhancement translates directly into suppressed voltage loss. The team measured a non-radiative voltage loss as low as 0.142 electronvolts, an ultralow figure by the standards of organic photovoltaics, where losses of 0.2 to 0.3 electronvolts are common. Crucially, the guest did not compromise the other pillars of device performance: charge generation remained efficient, charge transport was preserved, and devices still delivered power conversion efficiencies of around 17 percent in the binary host system.

The full payoff came when the strategy was combined into a ternary blend, in which the donor polymer, the host acceptor and the emissive guest acceptor coexist in a single active layer. The resulting single-junction devices achieved a power conversion efficiency of 21.2 percent, independently certified at 20.84 percent, with an open-circuit voltage of 0.93 volts. For a single-junction organic solar cell, these numbers place the work at the very front of the field, and the certified figure matches or exceeds the best values reported for any organic photovoltaic architecture to date. The fact that the gain comes from reducing a fundamental loss channel, rather than from squeezing more current out of the absorber, suggests the approach is complementary to, rather than competing with, existing optimization strategies.

What makes the result conceptually important is the reframing of how energy loss should be fought. For years, the community has pursued two main routes: controlling the nanoscale morphology of the donor-acceptor blend so that charges separate cleanly, and engineering the frontier orbital energies of the molecules to minimize the energetic offset at the interface. The new work adds a third, distinctly photophysical lever. By deliberately engineering a thermally accessible bridge between dark charge-transfer states and bright localized states, and by supplying a luminescent sink that gives those excitons somewhere to go, the researchers effectively convert a loss channel into a radiative one. The same principle of charge-transfer to localized-state coupling has been discussed theoretically, notably in work on hybridized exciton states, but demonstrating it with a practical guest-acceptor additive in a record-efficiency device is a significant step from theory to technology.

There are, of course, questions that will shape the next phase of this research. The guest acceptor strategy was demonstrated in one particular host system, and extending it to other donor-acceptor pairs will require careful matching of the localized excited state energy to each interface. Long-term operational stability, always a central concern for organic photovoltaics, will need to be verified for the ternary blends. And the mechanism itself, involving thermal repopulation of higher-energy states, invites scrutiny of whether the small energetic penalty involved costs anything in photocurrent at different operating temperatures. Still, the combination of certified efficiency, spectroscopic evidence and molecular simulation presented in the study makes a compelling case that solid-state luminescence can be harnessed as a design tool. If the approach generalizes, the path from twenty-one percent toward the Shockley-Queisser-inspired limits of organic photovoltaics may run not through new absorbers alone, but through molecules engineered to glow.

Subject of Research: Suppression of non-radiative recombination in organic solar cells using solid-state emissive acceptors

Article Title: Solid-state emissive acceptors enable thermally accessible radiative pathways for efficient organic solar cells

Article References: Solid-state emissive acceptors enable thermally accessible radiative pathways for efficient organic solar cells. (n.d.). https://doi.org/10.1038/s41566-026-02009-8

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02009-8

Keywords: organic solar cells, non-fullerene acceptors, non-radiative recombination, charge-transfer states, electroluminescence, open-circuit voltage, ternary blend, photophysics, energy loss, power conversion efficiency, resonance energy transfer, Nature Photonics