Organic solar cells are made from carbon-based semiconductor materials. They are lighter than silicon analogues, can be flexible and are potentially cheaper to produce. The efficiency of converting light into electricity in such devices is already approaching 20%, but further growth is hindered by voltage losses.
In an organic cell, the donor material absorbs light and creates excited states. An electron then transfers to the acceptor material, forming a charge-transfer state (CT state) - a bound pair of an electron and a hole. For the cell to produce current, this pair must separate into free charges. The energy of the CT state affects the voltage the cell can deliver, and radiative recombination involving such states contributes to voltage losses.
A group led by Professor Hirohiko Fukagawa of the Center for Frontier Science at Chiba University (Japan) measured the energy levels of various donor-acceptor pairs to understand how they relate to charge generation and voltage losses. The work was published online in the journal Advanced Materials on October 7, 2026. The team included Dr. Takahisa Shimizu and Dr. Taku Ono of NHK Science & Technology Research Laboratories, as well as Professor Takuji Hatakeyama of Kyoto University.
The scientists created simplified organic solar cells with separate donor and acceptor layers. The donor absorbed visible light, while the acceptor remained transparent to solar radiation. Five donor materials with different molecular structures and degrees of rigidity were tested in pairs with 15 acceptor materials whose electron affinity (a measure of a molecule's ability to accept an electron) ranged from 2.0 to 3.1 eV (electronvolt - a unit of energy used in microcosm physics).
Ionization and electron affinity were measured in the solid state using ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy. This made it possible to record energy levels under conditions closer to those of a real solar cell.
It turned out that, to preserve photogeneration, the relationship between the donor's optical band gap (the energy difference light must overcome to excite an electron) and the energy of separated charges is important. When these values approached each other, the researchers found a practical thermodynamic boundary of photocurrent generation. Near it, the energy conversion efficiency also tended to increase. This points to the possibility of preserving photocurrent with lower voltage losses.
In addition, more rigid donor molecules were associated with reduced relaxation and spread of CT state energy. In a number of systems with rigid donors, the effective binding energy of the CT state was 0.1 eV or less - that is, the CT state energy was close to the energy of separated charges.
The authors conclude that controlling CT state energy through molecular design may help reduce voltage losses. However, the effective binding energy of the CT state also depends on other factors: charge delocalization, molecular packing and the electrostatic environment at the donor-acceptor interface. These are correlational observations that by themselves do not prove causation.
According to Fukagawa, if these principles can be introduced into practical materials with a bulk heterojunction (a structure with mixed donor and acceptor), they may help create cells that efficiently generate current with lower energy losses in the form of voltage. In the long term, such devices could make solar energy harvesting more practical in lightweight, flexible and area-constrained applications where conventional silicon modules are difficult to use.