Researchers at City University of Hong Kong (CityUHK) have reported a breakthrough in organic PV by achieving a power conversion efficiency of 20.5% in an organic solar cell. This milestone was enabled by a novel strategy that re-associates normally non-emissive triplet excitons into extractable free charge carriers, significantly reducing energy losses that have traditionally limited the performance of organic PV cells.

In these cells, triplet excitons are often regarded as loss channels because their long lifetimes and spin-forbidden transitions hinder efficient charge generation. The CityUHK team developed a mechanism that converts these otherwise trapped excitations into free electrons and holes that can be collected at the electrodes, thereby improving photocurrent generation without compromising the device voltage.

Organic solar cells have so far achieved certified efficiencies above 20%, with laboratory-scale devices exceeding 21% through advances in non-fullerene acceptors, morphology control, and reduced energy losses.

The organic solar cell developed by the researchers integrates a small-molecule non-fullerene acceptor (NFA) known as FTh-4F. It is commonly used as the electron-accepting material in organic solar cells and belongs to the family of fused-ring electron acceptors. It is designed to provide strong near-infrared absorption, efficient electron transport, and low energy loss.

“By introducing this acceptor as a ternary component into other host OPV systems, we manage to recover the triplet-mediated losses and improve OPV efficiencies by maximizing the number of extractable photocarriers,” they explained.

They also found that free charge carriers persist much longer than spin-triplet excitons, indicating that triplet excitons can be converted back into free charge carriers rather than being lost as heat. By increasing the triplet exciton population through sensitization, they experimentally confirmed this recycling pathway via interfacial triplet charge-transfer states. Additionally, optimizing the acceptor’s side-chain structure and exciton delocalization reduced the singlet–triplet energy gap (ΔEST), making triplet exciton dissociation more efficient.

The research team said subsequent laboratory experiments have further advanced cell performance, pushing power conversion efficiency beyond 21%, without providing further details.

The new cell technology was described in “Recycling of spin-triplet excitons in organic photovoltaics,” published in nature.

“This study refines the scientific framework regarding the evolution of excitons/charge carriers in organic optoelectronic devices and opens up broad application prospects for systems involving charge separation and charge recombination processes,” the academics stated. “This breakthrough is expected to significantly enhance energy utilisation efficiency and advance the transition towards a cleaner, more efficient and sustainable future.”