Organic photovoltaics (OPVs) are a promising, economical, next-generation solar cell technology for scalable clean energy and wearable electronics. But the energy conversion loss due to the recom­bination of photo­generated charge carriers in OPVs has hindered further enhancement of their power conversion efficiency (PCE). Recently, researchers from City University of Hong Kong (CityU) overcame this obstacle by inventing a novel device-engi­neering strategy to success­fully suppress the energy conversion loss, resulting in record-breaking efficiency. The best-performing OPVs developed by the CityU team have achieved PCE of over 19 %, and the team expects to exceed 20 % very soon. The discovery is promising for the commerciali­zation of OPVs.

Currently, most high-performance organic photovoltaics adopt a bulk-hetero­junction (BHJ) structure, consisting of electron donor and acceptor materials intermixed throughout the active layer of the device. When converting sunlight into elec­tricity in OPVs, energy from sunlight creates excitons, which then dissociate into free electrons and holes at the nanoscale donor-acceptor interface, generating charge carriers and hence electricity. However, if these charge carriers are not collected by the electrodes and encounter each other again at the donor-acceptor interface, they may recombine to form a low-energy spin-triplet exciton (T₁), which conse­cutively relaxes back to ground state, causing energy loss in the form of heat and hence photo­current loss. This irreversible process strongly limits the maximum achievable PCE of OPVs.

The research team led by Alex Jen Kwan-yue and Lee Shau Kee overcame this obstacle by inventing a novel device-engineering strategy to suppress T₁ formation and mini­mize the associated recombination loss, which led to the record-breaking effi­ciency of OPVs. “We are the first team that managed to modulate T₁ formation through device engineering without changing the properties of the photoactive materials and to reveal the funda­mental mechanism,” said Jen. “Using this strategy, we have expanded it to 14 other material systems to show the universal appli­cability of this study.”

By replacing the tradi­tional highly intermixed bulk-heterojunction architecture inside the solar cell with a rather de-mixed planar-mixed hetero­junction (PMHJ) to reduce the donor-acceptor interface inside the active layer of OPVs, the team managed to alleviate the energy conversion loss in OPVs by suppressing the recom­bination of the charge carriers. This discovery maximized the photo­current of OPVs, resulting in devices with a high PCE of over 19 %. “Compared to the traditional inter-mixed bulk-heterojunction architecture, our rather de-mixed planar-mixed hetero­junction strategy is capable of suppressing the loss pathway mediated by charge-transfer states at the donor-acceptor interface,” Jen explained. “We revealed that having fewer donor-acceptor contacts in planar-mixed hetero­junction minimizes the chance of recombination and results in reduced T₁concen­tration. This funda­mentally changes researchers’ previous impression of OPVs – that the more donor-acceptor contacts, the higher the OPV performance.”

“The achieved optimum photo­voltage-photocurrent trade-off resulting from our strategy enables OPVs with compe­titive efficiency comparable to that of inorganic photo­voltaics,” added Francis Lin, postdoc in the Department of Chemistry, who also took part in the study. He explained that organic photovoltaic cells have several advantages over inorganic counter­parts, such as being lightweight and flexible, like a thin plastic film, and allowing cost-effective fabri­cation, using roll-to-roll printing production. The team believes that its latest discovery provides a comprehensive basis for future organic photo­voltaics to reach their full promise and stimulate a new wave of studies on the versatile photo­physical processes in organic semiconductors.

They are applying for a patent for the discovery. “We hope to further boost the performance of OPVs following our novel discovery of modulating the photo­physical processes. This redefines the maximum potential of OPVs to facilitate their commerciali­zation,” said Jen. (Source. CityU Hong Kong)

Reference: K. Jiang et al.: Suppressed recombination loss in organic photovoltaics adopting a planar–mixed heterojunction architecture, Nat. Energy 7, 1076 (2022); DOI: 10.1038/s41560-022-01138-y

Link: Dept. of Materials Science and Engineering, City University of Hong Kong, Kowloon, Hong Kong