Binary organic solar cell achieves 19,31% efficiency

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Research efforts in non-fullerene acceptor (NFA) materials have led to a tremendous improvement of power conversion efficiency (PCE) of OSCs in recent years, with 18.2% certified record on the US National Renewable Energy Laboratory (NREL) efficiency chart. However, further advancements are needed in non-radiative recombination loss suppression and performance boosting to help accelerate the practical application of OSCs.

Now, researchers at the Hong Kong Polytechnic University have achieved a breakthrough PCE of 19.31% for binary OSCs, which have one donor and one acceptor in the photoactive layer. The research team invented a novel OSC morphology-regulating technique to boost cell efficiency and stability by using 1,3,5-trichlorobenzene as a crystallisation regulator.

They developed a non-monotonic intermediated state manipulation (ISM) strategy to manipulate the bulk-heterojunction (BHJ) OSC morphology and simultaneously optimize the crystallization dynamics and energy loss of non-fullerene OSCs.

Unlike the strategy of using traditional solvent additives, which is based on excessive molecular aggregation in films, the ISM strategy promotes the formation of more ordered molecular stacking and favorable molecular aggregation, the researchers explain. As a result, the PCE was considerably increased, and the undesirable non-radiative recombination loss, which lowers the light generation efficiency and increases the heat loss, was reduced.

The research team took about two years to devise the non-monotonic ISM strategy.

“Challenges in research came from the existing additive-based benchmark morphology control methods, which suffer from non-radiative recombination loss, thus lowering the open-circuit voltage due to excessive aggregation,” says Li Gang, chair professor of energy conversion technology at the Hong Kong Polytechnic University.

According to Li, the new finding will likely create “tremendous opportunities” in applications like portable electronics and building-integrated photovoltaics. With the OSC technology still in the early stages, further application opportunities are expected when low-cost single-junction OSCs cross the 20% PCE threshold and achieve more stable performance to complement their unique advantages such as flexibility, transparency, stretchability, low weight, and tunable color.

“The latest study shows a record low non-radiative recombination loss of 0.168 eV in a binary OSC with a PCE of over 19%,” Li says. “This is a very encouraging result for the long-standing research on OSCs that I have conducted over the past two decades.”

The researchers described their findings in “19.3% Binary Organic Solar Cell and Low Non-Radiative Recombination Enabled by Non-Monotonic Intermediate State Transition,” which was recently published in Nature Communications.

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