Reducing voltage losses while maintaining efficient charge generation is a key challenge in developing efficient organic solar cells. Researchers from Chiba University, Japan, investigated how the energy levels of different donor–acceptor combinations affect solar cell performance. They identified a thermodynamic boundary for photocurrent generation and found that more rigid donor molecules were associated with lower radiative voltage losses. These findings could guide the design of more efficient organic solar cells.
A key goal in solar cell research is to improve efficiency toward the Shockley–Queisser limit, the theoretical maximum efficiency for converting sunlight into electricity under specified conditions. Organic solar cells use carbon-based semiconducting materials and offer advantages such as being lightweight, potentially low-cost, and suitable for flexible applications. Their power conversion efficiencies have approached 20%, but voltage losses remain a major obstacle to further improvements.
In organic solar cells, the donor material absorbs light and creates excited states. An electron can then transfer 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.
One challenge is to identify the energy-level relationships that allow efficient charge generation while minimizing voltage losses. In particular, researchers need to understand 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 investigate this, researchers led by Professor Hirohiko Fukagawa from the Center for Frontier Science, Chiba University , Japan, measured the energy levels of different donor–acceptor combinations to develop a framework for understanding how they influence charge generation and voltage losses.
The study was published online in the journal Advanced Materials on October 7, 2026. The research team included Dr. Takahisa Shimizu and Dr. Taku Oono from NHK Science & Technology Research Laboratories, Japan, and Professor Takuji Hatakeyama from Kyoto University, Japan.
"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," says Prof. Fukagawa.
To investigate how the energy levels of the two materials relate to charge generation and voltage losses, the researchers built simplified organic solar cells with separate layers of donor and acceptor materials. They designed the cells so that the donor absorbed visible light, while the acceptor remained transparent to sunlight. They tested five donor materials with different molecular structures and degrees of rigidity, pairing them with 15 acceptor materials with varying electron affinities between 2.0 and 3.1 eV.
They measured the materials' ionization energies and electron affinities in the solid state using ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy, capturing energy levels under conditions more representative of those in an actual solar cell.
The researchers found that the difference between the donor's optical energy gap and the energy of separated charges was important for determining whether photocurrent could be maintained. When these energy values approached one another, the researchers identified a practical thermodynamic boundary for photocurrent generation. Power conversion efficiency also tended to increase near this boundary. The findings suggest that this energy relationship could help researchers maintain photocurrent while minimizing voltage losses.
The researchers also 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 eV or less, indicating that the CT-state energy was close to that of the separated charges. These results suggest that controlling the CT state's energy through molecular design could help reduce voltage losses. However, 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.
These findings could provide design guidelines for developing more efficient organic solar cells and help researchers explore the limits of their achievable efficiency.
"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. In the longer term, such devices could make solar energy harvesting more practical in lightweight, flexible, and space-limited applications that are difficult for conventional silicon modules," says Prof. Fukagawa.
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About Professor Hirohiko Fukagawa from Chiba University, Japan
Dr. Hirohiko Fukagawa obtained a Ph.D. in Engineering from Chiba University in 2007. After over a decade of research at the Broadcasting Technology Research Institute, Japan Broadcasting Corporation, he joined the Center for Frontier Science at Chiba University in 2024, where he currently serves as a Full-Time Professor. He specializes in organic electronics, surface and interface engineering, and advanced optoelectronics. He has authored over 100 publications and has received numerous awards, including the Achievement Award at the OLED Symposium in 2019.
Funding:
This work was supported by the Japan Science and Technology Agency (JST) CREST (Grant No. JPMJCR22B3). This work was also supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant-in-Aid for Scientific Research (Grant No. 24K23071) and JKA through its promotion funds from the KEIRIN (Grant No. 2025M-340).
Reference:
Authors: Hirohiko Fukagawa1,2,3, Taku Oono4, Yusuke Aoki5, Tsubasa Sasaki4, Haruto Shoji5, Manato Tateno3, Seita Kyogoku3, Shintaro Mitsui3, Hisao Ishii1,2,3, Feiran Liu6, Takuji Hatakeyama6, and Takahisa Shimizu4
Affiliations:
1Center for Frontier Science, Chiba University, Japan
2Molecular Chirality Research Center, Chiba University, Japan
3Graduate School of Science and Engineering, Chiba University, Japan
4Science & Technology Research Laboratories, Japan Broadcasting Corporation (NHK), Japan
5Department of Applied Physics, Tokyo University of Science, Japan
6Department of Chemistry, School of Science, Kyoto University, Japan
DOI: 10.1002/adma.75304
