Chemistry
Designing Non-Fullerene Acceptors for High-Efficiency Organic Photovoltaics
Quick fact
Replacing buckyball-like fullerene acceptors with flat, fused-ring molecules called non-fullerene acceptors (NFAs) has boosted organic solar cell efficiency from around 11% to over 18% in just a few years, by enabling better light absorption and higher open-circuit voltages.
Why this is interesting
You've probably heard that organic solar cells are cheap and flexible, but for years they lagged behind silicon. Now, a simple molecular change has pushed their efficiency past 18%—what changed?
Read the full explanation
Understanding Designing Non-Fullerene Acceptors for High-Efficiency Organic Photovoltaics
Imagine a solar cell as a sandwich: one layer (the donor) absorbs light and hands off negative charges to the other layer (the acceptor). For decades, the acceptor was a fullerene—a spherical carbon molecule. But fullerenes absorb little visible light and are hard to tune. Non-fullerene acceptors are designed from scratch: they typically have a central electron-rich core, fused rings that keep the molecule flat, and electron-poor end groups that pull electrons. This structure allows chemists to adjust the energy levels (the 'incentive' for electrons to jump) and the band gap (the light wavelengths it absorbs). By tweaking these, NFAs can complement the donor's absorption, capture more sunlight, and reduce energy waste, leading to higher voltages and efficiencies.
A deeper explanation
The mechanism behind NFA success lies in their molecular design. In a donor-acceptor pair, light creates an exciton—a bound electron-hole pair—in the donor. For it to split, the acceptor must have a lower LUMO (lowest unoccupied molecular orbital) than the donor. Fullerenes do this, but their high reorganization energy and poor absorption limit efficiency. NFAs, built with a fused-ring core (e.g., ITIC or Y6), have rigid, planar structures that promote intimate packing with the donor, enhancing charge separation. More importantly, their energy levels can be independently tuned by modifying electron-donating or withdrawing groups. Raising the acceptor's LUMO relative to fullerene reduces the energy loss in charge transfer, boosting the open-circuit voltage. Simultaneously, their strong absorption in the visible-to-near-infrared region allows the active layer to capture more photons, increasing short-circuit current. This dual control—voltage and current—is why NFAs have achieved record power conversion efficiencies, and why they now dominate the field.