An organic solar cell (at left), and a transistor-based sensor designed by PolyU researchers. Credit: the Hong Kong Polytechnic University (PolyU)
Silicone-based solar cells have become ubiquitous, but their high production cost, and lack of flexibility, present limitations.
‘Plastic’ photovoltaics based on organic polymers — or organic photovoltaics (OPV) — are a promising alternative with potential for cheap fabrication as thin flexible films, says Gang Li, who is based at the Hong Kong Polytechnic University (PolyU).
In the early 2000s when interest in organic photovoltaics was starting to pick up, the field was essentially 50 years behind silicon.
Pioneering breakthroughs
“Silicon is of course a great success,” says Li, who previously studied condensed matter physics at Iowa State University in the United States, before moving to University of California Los Angeles to study OPVs. “But OPVs were always interesting because of their versatility — they can be transparent, flexible and portable, and so could be used in many different applications, such as on windows and in buildings.”
When Li started research on OPVs, silicon-based cells had already reached energy conversion efficiencies of 20% or more, while researchers in the OPV field were still grappling with basic polymer morphology and fabrication challenges at efficiencies of just a few percent.
“It was then that we made the first major breakthrough, developing a replicable structure and fabrication process, and setting the standard for performance testing and efficiency, hitting a record 4.4% at the time,” he says.
Li’s landmark 2005 paper published in Nature Materials1 enshrined OPVs as a standalone field of research and became the go-to reference and benchmark for others to follow, remaining in the journal’s top-10 cited papers for more than a decade.
Building on that research, Li led a startup aiming to scale-up to commercial production of these ‘first-generation’ OPVs. “There is a very tight interplay between science and application, and the fabrication side is very important from a scalability perspective,” says Li.
“However, we remained puzzled as to why the energy-conversion efficiency was so low. We could make a sub-micrometre-thin OPV film using inexpensive solution-based processing methods, but without higher efficiency, the technology was never going to be competitive.”
Competition with silicon
Gang Li, chair professor of Energy Conversion Technology and professor in the department of Electronic and Information Engineering at PolyU. Credit: the Hong Kong Polytechnic University (PolyU)
Li then began work on the next generation of OPVs, exploring mixtures of different organic polymers as ‘co-polymer’ systems, which required a completely different molecular design and fabrication approach.
“With this approach we were eventually able to start breaking efficiency records for OPVs on a regular basis,” he says. “The next major step change came with the development of ‘non-fullerene acceptor’ technology, which has finally lifted the performance of OPVs to levels competitive with silicon.”
Fullerenes, soccer-ball-shaped molecules built of carbon atoms, had been part of the OPV landscape since the beginning, being the only viable ‘acceptor’ molecules that could collect the light-excited electrons to create an electrical current. However, fullerenes only work over a very limited range of electron energies, which imposed tight restrictions on the efficiency of OPV cells.
The discovery of non-fullerene molecular systems that can be tuned over a wide range of energies has now opened the door to a new world of material options and has brought renewed acceleration in efficiency increases.
“Our review paper in Nature Photonics2 on OPVs using non-fullerene acceptors has become the second highest cited paper published in the journal since 2018, and we have just recently reported a new efficiency record for a binary system of 19.3% in Nature Communication3,” says Li. “By further evolving the materials and reducing losses via device engineering, we believe we are well on the way to 25%, which will put the efficiency on par with silicon, and really support the commercialization of OPVs.”
Transistor-based sensors
Feng Yan, chair professor of Organic Electronics and professor in the department of Applied Physics at PolyU. Credit: the Hong Kong Polytechnic University (PolyU)
Feng Yan joined Hong Kong Polytechnic University (PolyU) in 2006 — and along with colleague Gang Li — has been instrumental in establishing PolyU as an emergent leader in advanced materials. This is particularly true in the fields of polymer- and perovskite–based solar cell technology and transistor-based sensors, with a focus on practical devices and applications.
Yan, previously of the University of Cambridge, in the United Kingdom, is a global leader in organic electronics and the developer of highly sensitive transistor-based sensors for light, molecule and biomarker detections.
Yan’s novel research on advanced materials, including organic semiconductors and perovskite materials, has greatly advanced biosensors, optoelectronic devices such as photodetectors and solar cells, and other technologies.
“Quantum dots are really interesting because they are highly responsive to light and make for highly sensitive photodetectors,” says Yan. “But for them to work, they need to be fixed to a conductive channel of a transistor. We developed a field-effect transistor using graphene as a channel and modified quantum dots with short molecular connections to create a high-performance, photo-detector system that has now been further developed for a range of industrial applications.”
Yan and his team went on to extend that work to organic or two-dimensional, metal-organic framework –based transistors that can be combined with commercial biomolecular probes — molecules designed to bond to proteins and other biomolecules of interest — to create ultra-high-sensitivity and low-cost biosensors. Their device consists of an array of transistors on a chip that, when modified with the right probe, can detect various types of biomolecules at very low concentrations.
In his recent study, two-dimensional conjugated metal organic frameworks are proven to be excellent semiconductor materials for high-performance electrochemical transistors (ECTs) with promising applications in flexible and wearable electronics. ECTs have shown broad application in bioelectronics and neuromorphic devices due to their high transconductance, low working voltage and versatile device design4.
“We developed a bioelectronics device for detecting Sars-CoV-2, the virus responsible for COVID-19, as an ultrafast, sensitive and portable diagnostic tool4,” says Yan. “We continue to develop this biosensor system, because it can be used non-invasively with saliva to detect a range of useful biomarkers.”
Yan’s team is also working on perovskites — a class of inorganic crystalline materials with photoelectric properties — as another alternative to silicon-based solar cells. Mirroring Li’s success in OPVs, Yan has made significant breakthroughs that improve the efficiency and stability of perovskite solar cells in an ambient atmosphere5 and also by using tin to replace lead, which is conventionally used in perovskites — providing a less toxic alternative.
This global leadership of Li and Yan in advanced materials epitomises the focus of researchers at PolyU on science that contributes to society.
“Scientific research is a journey, with ups and downs, and happy and hard times,” says Li. “But passion and perseverance can lead to amazing success, and to the reward of seeing your research being linked to applications.”