Rigid Molecules and a Sharp Energy Boundary Set the Path to Better Organic Solar Cells

Organic solar cells have long promised lightweight, flexible and potentially low-cost alternatives to silicon, yet their efficiency gains have been throttled by a stubborn problem: voltage losses. Now researchers at Chiba University in Japan, working with colleagues at NHK Science & Technology Research Laboratories and Kyoto University, have mapped the thermodynamic conditions under which an organic photovoltaic device can keep generating current while shedding less energy as voltage. Their study, published online in the journal Advanced Materials on October 7, 2026, identifies a practical energy boundary for photocurrent generation and links stiffer donor molecules to reduced radiative voltage losses, offering a design rule that could push the technology closer to its theoretical ceiling.

Power conversion efficiencies of organic solar cells have approached 20 percent, a remarkable figure for carbon-based semiconductors that can be processed into thin, bendable films. But the Shockley–Queisser limit, the theoretical maximum efficiency for converting sunlight into electricity under specified conditions, remains far above what any laboratory device achieves. Voltage losses remain a major obstacle to further improvements. In an organic solar cell, the donor material absorbs light and creates excited states; an electron then transfers to the acceptor material, forming a charge-transfer (CT) state in which the electron and hole remain bound to each other. For the cell to generate electricity, these charges must separate into free charges. The energy of the CT state also influences the voltage the cell can deliver, and radiative recombination involving these states contributes to voltage losses.

Professor Hirohiko Fukagawa of the Center for Frontier Science at Chiba University led the effort, and he framed the problem in terms of what happens after the initial photoexcitation. “In this study, we focused not only on whether a CT state can be formed, but also on whether electrons and holes can then reach a free charge state in which they are sufficiently separated,” he says. The distinction matters because a bound electron–hole pair is not yet usable electricity. The team wanted to know the energy-level relationships that allow efficient charge generation while minimizing voltage losses, and in particular the conditions under which a solar cell can continue generating photocurrent as the donor’s optical energy gap approaches the energy of separated charges.

To build that picture, the researchers constructed simplified organic solar cells with separate layers of donor and acceptor materials rather than the blended architectures used in commercial devices. Five donor materials with different molecular structures and degrees of rigidity were tested, each paired with 15 acceptor materials whose electron affinities ranged from 2.0 to 3.1 electronvolts.

Measuring the relevant energies accurately was central to the approach. The team determined the materials’ ionization energies and electron affinities in the solid state using ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy.

The central result concerns the difference between the donor’s optical energy gap and the energy of separated charges. The researchers found that this difference was the key quantity determining whether photocurrent could be maintained. When the two energy values approached one another, the team identified a practical thermodynamic boundary for photocurrent generation. Photocurrent can be sustained when the donor optical gap approaches the free-carrier energy, a condition the researchers describe as Eg, Dopt ≈ ECS. Strikingly, power conversion efficiency also tended to increase near this boundary.

The second strand of the work addresses molecular rigidity. The researchers found that more rigid donor molecules were associated with reduced relaxation and energy spreading of the CT state. In several systems using rigid donors, the effective CT binding energy was 0.1 electronvolt or less, indicating that the CT-state energy was close to that of the separated charges. Rigid donor molecules can further reduce CT-state relaxation and effective CT binding, helping to suppress voltage losses near the thermodynamic boundary.

The researchers are careful to note that rigidity is not the whole story. The effective CT binding energy is also influenced by other factors, including charge delocalization, molecular packing, and the electrostatic environment at the donor–acceptor interface. A rigid backbone helps, but how the molecules stack, how far the charges spread across the interface, and how the surrounding dielectric screens the Coulomb attraction all shape the final binding energy. This nuance is important for the field, because it means molecular design is one lever among several rather than a universal fix. Still, the results suggest that controlling the CT state’s energy through molecular design could help reduce voltage losses, and that synthetic chemists now have a measurable target to aim at.

The practical implications extend beyond the laboratory. “If these principles can be incorporated into practical bulk heterojunction materials, they may help researchers develop solar cells that generate current efficiently while losing less energy as voltage,” says Prof. Fukagawa. The team’s framework provides a thermodynamic map for that translation, indicating where in energy-level space a given donor–acceptor pairing should sit.

By measuring solid-state energy levels across a broad matrix of material combinations, the researchers showed that a single energy relationship governs both. Near the boundary where the donor optical gap matches the free-carrier energy, devices generate current efficiently and lose less voltage; rigid donors help devices operate close to that boundary with minimal CT-state relaxation. These findings could provide design guidelines for developing more efficient organic solar cells and help researchers explore the limits of their achievable efficiency.

Subject of Research: Thermodynamic conditions for photocurrent generation and voltage losses in organic solar cells

Article Title: Researchers identify an energy boundary that could guide the design of more efficient organic solar cells

Article References: Researchers identify an energy boundary that could guide the design of more efficient organic solar cells. Original publication (Bibliographic details unavailable.)

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Keywords: organic solar cells, voltage losses, charge-transfer state, photocurrent, donor-acceptor interface, molecular rigidity, thermodynamic boundary, power conversion efficiency, Shockley-Queisser limit, spectroscopy, Researchers, identify