Record-breaking efficiency for organic photovoltaics achieved
The best-performing organic photovoltaics developed by a team at City University of Hong Kong have achieved power conversion efficiency of higher than 19%, topping any existing level recorded. The team expects to exceed 20% very soon.
A solar cell technology based on organic semiconductors, organic photovoltaics are a promising candidate for clean energy because of their low material toxicity and vast molecular tunability in photoactive materials. Currently, most high-performance organic photovoltaics adopt a “bulk-heterojunction” structure, consisting of electron donor and acceptor materials intermixed throughout the active layer of the device.
An organic photovoltaic cell along with its architecture and schematic diagrams that show the difference between the inter-mixed bulk-heterojunction and the rather de-mixed planar-mixed heterojunction. Credit: City University of Hong Kong
When converting sunlight into electricity in the technology, 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. 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 so-called low-energy “spin-triplet exciton”, 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 power conversion efficiency of organic photovoltaics.
A research team led by Professor Alex Jen Kwan-yue invented a novel device-engineering strategy to suppress T1 formation and minimise the associated recombination loss, which led to the record-breaking efficiency. By replacing the traditional architecture inside the solar cell with a rather de-mixed “planar-mixed heterojunction” to reduce the donor-acceptor interface inside the active layer, the team managed to alleviate the energy conversion loss by suppressing the recombination of the charge carriers. This discovery maximized the photocurrent of organic photovoltaics, resulting in devices with a high PCE of over 19%. Their findings were published in the scientific journal Nature Energy.
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 is applying for a patent for the discovery, and believes that it 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.