The efficiency of converting photovoltaic energy into electricity in these molecular materials has skyrocketed from just 2% to over 20% in two decades. The latest leap is thanks to new molecules known as non-fullerene acceptors, particularly the highly successful "Y-family" of materials like Y6. Until now, researchers haven’t understood how efficiencies over 20% are possible.
The Low-Offset Mystery
When light hits a traditional organic solar cell, the cell relies on a junction between two different materials (an electron donor and an electron acceptor) to split the tightly bound light-generated excitation (called an exciton) into free electrical charges. Historically, this process required a large energetic "step" or offset between the materials (called a heterojunction) to force the charges apart, and this step reduced the extracted power.
However, the new materials bypass this rule, achieving high power conversion efficiencies with small energy offsets. Some recent reports even suggested that the charges were being generated directly within the molecular film and that the heterojunction was no longer needed.
A Combined Experimental and Computational Approach
To solve this mystery, a team of researchers based at Imperial’s physics department, led by Prof Jenny Nelson, combined experimental measurements of real devices with a new computational model. The model simulates how these excited states spread out (or "delocalize") across the material and how this delocalisation impacts the thermodynamics of energy conversion. Comparing the model to experimental results they found that including this state spreading in their models was critical to understanding the observed behaviour.
“There has been a lot of debate about exactly how these exciting new materials generate electricity so efficiently when the traditional driving forces are so small," says Lucy Hart, lead author of the study and Post-Doctoral Fellow in the Department of Physics at Imperial college. " By combining our experimental measurements with a new computational model, we were able to pinpoint which molecular features drive this efficient charge generation."
Daniel Medranda, co-author and researcher at Imperial College London, highlights the difficulty of observing these hidden mechanics: "Because these processes happen incredibly fast and at a molecular level in the materials, they are notoriously difficult to uncover. Our combined approach essentially acts as a highly advanced magnifying glass, allowing us to see exactly how the specific shape and packing of these molecules dictate the performance of the entire solar cell."
Visualisation of a photogenerated excited state (grey) evolving into separated positively (green) and negatively (red) charged regions of a molecular film. | Co-author Dr Daniel Medranda.
New Rules for Molecular Design
The researchers went on to identify specific properties of the best performing molecules that help to generate photocurrent at low cost in these high-performance materials. Both chemical structure and the arrangement of the molecules matters. When both conditions are met, these molecules are exceptionally good at communicating and transferring energy.
"What our results make clear is that we can no longer look at these molecules in isolation," adds co-author Dr. Flurin Eisner, Lecturer in Green Energy at Queen Mary University of London. "The secret to their high efficiency lies in how the energy is shared and spread out across an entire molecular network. It's this teamwork at the nanoscale that allows the charges to separate so effectively without needing a massive energetic push."
The team also investigated whether the new materials could produce photocurrent on their own, without a heterojunction. The answer was ‘not yet’ but, although they could show how the materials could be improved to make solar energy conversion even more efficient.
This research provides clear design rules for chemists and engineers to push organic solar cells to even higher efficiencies. Future materials development should focus on reducing the energy required for molecules to reorganize, minimizing structural disorder, and increasing how strongly the molecules interact with one another.
“Our findings highlight the value of combining theoretical modelling with experimental physics," says Mariano Campoy-Quiles, co-author and Research Professor at ICMAB-CSIC. "By addressing this ongoing debate in the field, we are able to provide clearer, more concrete guidelines for designing the next generation of efficient organic solar cells."
“It is very exciting to see the first results of the collaboration project DOMMINO, between Imperial College London and CSIC, funded by CSIC through the program JAE-Chair. Prof. Nelson is ICMAB´s JAE-Chair!”, Campoy-Quiles added.
The research was supported by UKRI (ATIP programme grant) the UKRI ERC underwrite scheme (POTENtIAl) and the Spanish CSIC, via collaboration with Prof Campoy-Quiles at ICMAB, Barcelona (project DOMMINO).
Reference article
Unlocking the Material Secrets Behind 20% Efficient Organic Solar Cells
Lucy J. F. Hart, Daniel G. Medranda, Shi Wei Yuan, Linnea Lindh, Jolanda S. Müller, Hanbo Yang, Hugo Gerard, Tianyu Zhao, Arianna Quesada-Ramirez, Mariano Campoy-Quiles, Mohammed Azzouzi, Flurin D. Eisner & Jenny Nelson
Nat. Mater. (2026).
DOI: 10.1038/s41563-026-02509-6
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Unlocking the Material Secrets Behind 20% Efficient Organic Solar Cells | Imperial News | Imperial College London
