July 5,2026 -- In organic photovoltaics (OPV), low-energy spin-triplet excitons (T₁) have long been regarded as “energy traps” that often dissipate energy as heat rather than contributing to light-to-electricity conversion. A research team from City University of Hong Kong (CityUHK) has recently overcome a technological bottleneck that has persisted for over a decade. They successfully “turned waste into treasure” by recycling triplet excitons and converting them into effective charge carriers for electricity generation. Using this new approach, they achieved an efficiency of 20.5% in OPV cells.
 
Led by Professor Alex Jen Kwan-yue, Lee Shau Kee Chair Professor of Materials Science at CityUHK, the team studied an organic photovoltaic system incorporating a new acceptor material, FTh-4F. They observed that the lifetime of free charge carriers in this system is significantly longer than that of spin-triplet excitons. This finding challenges the conventional understanding and suggests that triplet excitons can dissociate back into free charge carriers instead of simply decay into heat.
 
In follow-up experiments by increasing the concentration of triplet excitons through sensitization, the team successfully demonstrated that triplet excitons can be re-dissociated into extractable free charge carriers via interfacial triplet charge-transfer states. By fine-tuning the side-chain structure and exciton delocalisation in aggregates, the singlet-triplet band gap (ΔEST) of the acceptor can be effectively reduced to facilitate the dissociation of triplet excitons. FTh-4F was used as a ternary component in organic photovoltaic devices to recycle triplet excitons to successfully increase the power conversion efficiency (PCE) to 20.5%. The findings were published in Nature under the title “Recycling of Spin-Triplet Excitons in Organic Photovoltaics”.
 
Professor Jen noted that subsequent experiments in the lab have pushed the efficiency of organic solar cells beyond 21%, establishing a new scientific foundation and engineering pathway for the development of next-generation, high-performance organic photovoltaic devices.
 
This research builds on years of dedicated efforts by the CityUHK team. In 2022, they developed a novel layer-by-layer engineering strategy to suppress the formation of triplet excitons in active layer, thereby minimising the associated recombination loss. This earlier work set an OPV efficiency record beyond 19% and was published in Nature Energy.
 
Subsequently, while continuing to investigate the excited-state dynamics of different material systems, the team observed the unique phenomenon in the D18:FTh-4F system. Moving beyond the mainstream understanding on triplet excitons to pioneer a new pathway to recycle and utilise triplet excitons. After two years of data collection and validation, they achieved this breakthrough.
 
Professor Jen stated, “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. This breakthrough is expected to significantly enhance energy utilisation efficiency and advance the transition towards a cleaner, more efficient and sustainable future.”