New technique boosts PCE of organic solar cells to 19.31%

Researchers from the Hong Kong Polytechnic University (PolyU) have achieved a power-conversion efficiency (PCE) of 19.31% with organic solar cells (OSCs), also known as polymer solar cells. This binary OSC efficiency could help enhance applications of these advanced solar devices.

The PCE, a measure of the power generated from a given solar irradiation, is considered a significant benchmark for the performance of photovoltaics (PVs) or solar panels in power generation. The improved efficiency of over 19% that was achieved by the PolyU researchers is reportedly a record for binary OSCs, which have one donor and one acceptor in the photoactive layer.

Led by Professor LI, Chair Professor of Energy Conversion Technology and Sir Sze-Yen Chung Endowed Professor in Renewable Energy at PolyU, the researchers developed a novel OSC morphology-regulating technique by using 1,3,5- trichlorobenzene as a crystallisation regulator to boost OSC efficiency and stability.

The team developed a non-monotonic intermediated state manipulation (ISM) strategy to manipulate the bulk-heterojunction (BHJ) OS morphology and simultaneously optimise the crystallisation 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 favourable molecular aggregation. As a result, the PCE was increased and the undesirable non-radiative recombination loss was reduced. Notably, non-radiative recombination lowers the light generation efficiency and increases the heat loss.

The research team’s findings were published in Nature Communications. The conversion of solar energy to electricity is an essential technology for achieving a sustainable environment. Although OSCs are promising devices that harness solar energy cost-effectively, their efficiency must be improved if they are to be used widely in practical applications.

According to Li, 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. It took the researchers two years to devise a non-monotonic ISM strategy for increasing OSC efficiency and lowering the non-radiative recombination loss. Professor Li added that the new finding will create more opportunities in applications like portable electronics and building-integrated PVs. This will likely occur when low-cost, single-junction OSCs can achieve a PCE of over 20%, along with more stable performance and other advantages such as flexibility, transparency, stretchability, low weight and tuneable colour.

“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%. This is a very encouraging result for the longstanding research on OSCs that I have conducted over the past two decades. We have already achieved better OSC efficiency, and this will subsequently help accelerate the applications of solar energy,” Li said.

Image caption: Prof. Li Gang invented a novel technique to achieve a breakthrough efficiency with organic solar cell. Image credit: The Hong Kong Polytechnic University.