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Image of molecular structure
Researchers at Queen Mary University of London, working closely with collaborators at Imperial College London and the Spanish National Research Council (CSIC), have uncovered how the latest generation of organic solar cell materials achieve record‑breaking efficiencies of over 20%. Their findings provide long‑sought answers to a major puzzle in the field and lay out new design rules for future molecular photovoltaics.
Organic solar cells - which use carbon-based molecules or polymers to absorb sunlight - offer a lightweight, flexible and potentially more sustainable alternative to traditional silicon photovoltaics. Over the past two decades, their power‑conversion efficiency has climbed from around 2% to over 20%, thanks largely to a new class of molecules known as non‑fullerene acceptors (NFAs), particularly the highly successful "Y‑family" of materials such as Y6. But until now, scientists have not fully understood how these materials reach such high efficiencies.
Rethinking How Charges Are Created
Traditionally, organic solar cells rely on a junction between two molecular materials - an electron donor and an electron acceptor - to split tightly bound excitons into free charges. This process normally requires a large energetic "offset" between the materials, which comes at a cost: the larger the offset, the lower the voltage and overall efficiency of the device.
However, the latest NFAs break this rule, achieving high efficiencies with much smaller energy offsets. Some studies have even suggested that charges could be generated directly within the molecular film, without needing a clear donor-acceptor interface.
A Combined Experimental-Computational Breakthrough
To solve this puzzle, a team from Queen Mary's School of Physical and Chemical Sciences, with researchers at Imperial College London, combined experimental device measurements with a new computational model capable of simulating how excited electronic states spread out, or delocalise, across the molecular network.
By comparing simulated and experimental data, the team found that this delocalisation plays a critical role in enabling efficient charge generation at low energetic cost.
"What our results make clear is that we can no longer look at these molecules in isolation," said Dr Flurin Eisner, Lecturer in Green Energy at Queen Mary University of London and co-author of the study. "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."
Lead author Lucy Hart, Postdoctoral Research Fellow at Imperial College London, added: "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. By combining our experimental measurements with a new computational approach, we were able to pinpoint the molecular features driving this efficient charge generation."
Co-author Daniel Medranda (Imperial College London) highlighted the challenge of studying these ultrafast processes: "These mechanisms occur at incredible speeds and at the molecular scale. Our integrated approach acts like an advanced magnifying glass, allowing us to see how the specific shape and packing of these molecules dictate the performance of the entire solar cell."
New Rules for Molecular Design
The team identified key structural characteristics of the highest‑performing materials - including both their chemical structure and their nanoscale arrangement - that make them exceptionally effective at transferring energy across the film.
The researchers also tested whether the new materials were capable of generating photocurrent without a traditional heterojunction interface. While this is not yet achievable, the results point clearly to how the materials could be improved to move closer to this goal.
Towards Next‑Generation Solar Materials
This work provides practical, evidence‑based design rules for chemists and materials scientists looking to push organic solar cell performance even further. Future efforts, the team suggests, should focus on:
- lowering the energy required for molecular reorganisation
- reducing structural disorder
- increasing intermolecular interactions
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).
