Record 19.31% efficiency with organic solar cells achieved by PolyU researchers

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

The PCE (Power-conversion efficiency), 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 constitutes a record for binary OSCs, which have one donor and one acceptor in the photo-active layer.

Led by Prof. LI Gang, Chair Professor of Energy Conversion Technology and Sir Sze-Yen Chung Endowed Professor in Renewable Energy at PolyU, the research team invented a novel OSC morphology-regulating technique by using 1,3,5-trichlorobenzene as a crystallisation regulator. This new technique boosts OSC efficiency and stability.

The team developed a non-monotonic intermediated state manipulation (ISM) strategy to manipulate the bulk-heterojunction (BHJ) OSC morphology, which simultaneously optimises 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 considerably 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 are described in the study “19.3% Binary Organic Solar Cell and Low Non-Radiative Recombination Enabled by Non-Monotonic Intermediate State Transition” 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.

Non-fullerene acceptors based organic solar cells represent the frontier of research in the field of organic photovoltaics due to both the materials and morphology manipulation innovations. Nevertheless, non-radiative recombination loss suppress and performance boosting are in the centre of organic cell research. 

Prof. Li said, “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.” The research team took about two years to devise a non-monotonic ISM strategy for increasing the OSC efficiency and lowering the non-radiative recombination loss. The publication of the study promises to galvanise OSC research.

Prof. Li said, “The new finding will make OSC research an exciting field, and this will likely create tremendous opportunities in applications like portable electronics and building-integrated PVs.” The new door will open when low cost single-junction OSCs can achieve a PCE of over 20%, along with more stable performance and other unique advantages such as flexibility, transparency, stretchability, low weight and tuneable colour.

Prof. Li has been recognised as a Highly Cited Researcher 9 years in a row since 2014, which testifies to his significant impact on global research. His pioneering contributions to research on polymer solar cells since 2005 have brought sustainable influence on printable solar energy development with global recognition.

Underpinning the research on OPV field, Prof LI’s study titled, "High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends," was published on Nature Materials in 2005. This represented OPV’s first generation research breakthrough which has fuelled solar technology from this frontier study. 

In 2010, Prof LI’s study titled "For the Bright Future—Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%" was published on Advanced Materials. The study brought remarkable influence on the second generation of research on OPV. 

Prof. Li said, “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 long-standing 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.”

References: 
Nature Materials 4, 864-868 (2005). High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. 
Advanced Materials Vol. 22, Issue 20 (2010). For the Bright Future—Bulk Heterojunction Polymer Solar Cells with Power Conversion Efficiency of 7.4%.