Researchers across Hong Kong have developed a strategy to target non-radiative energy loss in organic photovoltaic cells. Teams in Shanghai, Beijing, Shenzhen, and Hangzhou joined this work, achieving a certified power conversion efficiency of 20.84 percent in a single-junction device.
Recycling Excitons Through Energy Transfer
Organic solar cells use carbon-based semiconductors that function much like printable ink. Non-radiative recombination is a process where excited charges dissipate their energy as heat instead of contributing to usable voltage.
When a photon strikes an organic photovoltaic cell, it generates a bound electron-hole pair known as an exciton. This exciton migrates to the interface between the electron-donating polymer and the electron-accepting molecule.
It splits into free charges via an interfacial charge-transfer state. Non-radiative decay normally outpaces slow, improbable radiative decay.
Researchers engineered a system where a localized excited state sits energetically close to the interfacial charge-transfer manifold. Thermal energy pushes the exciton from the charge-transfer state back up onto the localized state. Room-temperature conditions make this possible, allowing the excitation to hop to an emissive guest molecule through resonance energy transfer.
The guest, engineered to luminesce strongly even in the solid state, then has a real chance of emitting a photon.
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Luminescent Guest Reduces Non-Radiative Voltage Loss
This indirect radiative pathway recycles excitons into bright, emissive states. Electroluminescence and non-radiative recombination operate inversely.
The investigative team documented a non-radiative voltage loss of just 0.142 electronvolts. Typical organic photovoltaic losses range from 0.2 to 0.3 electronvolts.
Introducing the luminescent guest preserved efficient charge generation and transport. It maintained power conversion efficiencies of around 17 percent within the binary host system.
The breakthrough expanded further when integrated into a ternary blend configuration combining the donor polymer, the host acceptor, and the emissive guest acceptor within a single active layer. The resulting devices achieved a power conversion efficiency of 21.2 percent, independently certified at 20.84 percent, alongside an open-circuit voltage of 0.93 volts.
Crystal Analysis Confirms Twisted Core Suppresses Emission Quenching
The team grew single crystals of tetraphenylethylene-based acceptors to examine their molecular packing. They grew these crystals to confirm that the twisted core structure suppresses tight pi-pi stacking that normally quenches emission in solid organic materials.
Validating this photophysical mechanism required experimental and computational techniques.
Photoluminescence quantum yield measurements verified that the guest molecules retain strong solid-state emission when blended into the host.
Active layer solutions dissolved in chloroform were spin-coated onto PEDOT:PSS layers or 2PACZ-coated substrates. Active layers underwent methanol pretreatment and thermal annealing at 100 degrees Celsius for 10 minutes to remove residual solvent and control morphology before trifluoroethanol solutions of cathode interlayers were deposited.