Glowing guest molecules boost organic solar cells past 20% efficiency

Organic solar cells, composed of carbon-based semiconductors, have achieved a power conversion efficiency of 20.84 percent, according to a study published in Nature Photonics. This advancement addresses the issue of non-radiative recombination, a key energy loss mechanism where excited charges dissipate as heat rather than contributing to the cell's voltage. Researchers from Hong Kong, Shanghai, Beijing, Shenzhen, and Hangzhou have introduced a novel strategy involving solid-state emissive guest acceptors to mitigate this loss.

The challenge arises at the interface between the light-absorbing materials in organic photovoltaic cells. When a photon is absorbed, it creates an exciton that must travel to the interface to be split into free charges. Charge-transfer excitons, formed at this interface, are crucial for charge generation but are prone to non-radiative decay due to weak coupling with light. This decay reduces the open-circuit voltage and overall efficiency.

The research team, led by Yingze Zhang and Sai Ho Pun from the Hong Kong University of Science and Technology, along with Gang Li from the Hong Kong Polytechnic University, introduced a new photophysical channel. They incorporated a small amount of a solid-state emissive guest acceptor, based on a tetraphenylethylene (TPE) core, into a host system of Y-series non-fullerene acceptors. This guest acceptor's localized excited state is energetically close to the charge-transfer state, allowing thermal energy to push excitons onto the emissive guest molecule, enabling photon emission.

Extensive experimental and computational efforts confirmed this mechanism. The team resolved the molecular packing of TPE-based acceptors and measured photoluminescence quantum yields, demonstrating strong solid-state emission. Transient spectroscopy and molecular dynamics simulations further validated the energy transfer process and the accessibility of the energetic resonance.

Devices incorporating the emissive guest acceptor showed enhanced electroluminescence quantum efficiency, indicating reduced voltage loss. The non-radiative voltage loss was measured at 0.142 electronvolts, significantly lower than typical losses in organic photovoltaics. The approach maintained efficient charge generation and transport, achieving power conversion efficiencies of around 17 percent in the binary host system.

When integrated into a ternary blend, the strategy resulted in single-junction devices with a power conversion efficiency of 21.2 percent, independently certified at 20.84 percent. This efficiency places the work at the forefront of organic photovoltaic technology, complementing existing optimization strategies by reducing fundamental loss channels.

The study reframes energy loss management by introducing a photophysical lever that converts loss channels into radiative pathways. While the guest acceptor strategy was demonstrated in a specific host system, extending it to other systems will require careful energy matching. Long-term stability and the impact of thermal repopulation on photocurrent also warrant further investigation. Nonetheless, the study presents a compelling case for using solid-state luminescence as a design tool in organic photovoltaics.