Cheap, flexible organic solar cells capable of being incorporated into clothing, applied on windows and wrapped over rooftops and buildings like wallpaper has moved a step closer to commercialization thanks to research conducted at the University of Alberta.
Conventional silicon-based inorganic solar panels are costly to make and inflexible – they tend to be composed of thick, rigid sheets that require fixed installation points.
On the other hand, organic photovoltaics that incorporate carbon into their construction are capable of being inexpensively manufactured in rolls resembling a layer of plastic. Able to be produced in any colour or to be transparent, they can be spread over nonlinear objects and blended seamlessly into their environment, making them suitable for many more applications.
But until now, organic photovoltaics have proven to be less efficient than conventional silicon-based panels, prompting research efforts around the world to crack the nut on increasing device efficiency and lifetime, potentially making them ubiquitous and changing the energy paradigm.
Cheapest energy source
Singlet fission – converting a molecular singlet state into two triplet states – offers a realistic route to advance the efficiencies of organic solar cells beyond that of conventional systems, according to the University of Alberta researchers.
“Solar energy is by far the cheapest of energy sources. But to do it effectively, the costs have to come down,” said Rik Tykwinski, professor in the University of Alberta’s Department of Chemistry. “Singlet fission is basically like a two-for-one energy store, doubling the amount of charge you can generate. Increasing the efficiency of organic materials in solar energy capture will be a huge advance. It’s a key challenge that has to be solved.”
With Tykwinski’s new molecules, an interdisciplinary effort between Canada, the United States, and Germany is providing groundbreaking mechanistic aspects of singlet fission allowing for the use of organic materials in solar energy capture. The group’s new discovery is just the latest in an increasing line-up of solar energy advances coming out of the University of Alberta Department of Chemistry.
How it works
While conversion rates quoted may sound small, in the solar power world they are comparatively high. Tykwinski explains how singlet fission can lead to those high rates, focusing on the use of singlet fission to increase efficiency beyond that achievable by conventional solar cells.
“In conventional solar cells, one photon ideally generates one electron as the carrier of the current, and this design faces a theoretical limit (the so-called Shockley-Queisser limit), that prohibits conversion efficiencies of more than about 33 per cent.”
“Using the molecules we have designed (dimers that contain two light absorbing parts called chromophores), one absorbed photon creates two “excited states.” These electrons can, in turn, be used to generate current (electricity) – i.e., the process can create more current per every absorbed photon. In the optimal case, a significant increase in the performance of solar cells could be achieved, perhaps as high as 50 per cent in a photovoltaic in which the sensitizer is capable of quantitative singlet fission.”
The work on singlet fission is, however, still fundamental in many ways, since there is a need to better understand the process to effectively control it, Tykwinski told JWN. “Singlet fission occurs very rapidly (picoseconds) and require specialized spectrometers to adequately analyze all the steps, hence our collaborations with groups in Erlangen Germany and Northwestern University,” he said.
“Thus, the potential for commercialization is some years off. But already in the past three years, huge strides have been made, from the first reports of singlet fission in organic dimers to the very recently reported first examples of solar cells based singlet fission from these dimers,” Tykwinski added, pointing to work at other universities.
Record efficiency levels
For example, in April the University of Michigan announced the demonstration of organic solar cells that can achieve 15 per cent efficiency – a new record for organic cells and in the range of many solar panels currently on the market – reaching what is considered a benchmark for commercialization.
"Organic photovoltaics can potentially cut way down on the total solar energy system cost, making solar a truly ubiquitous clean energy source," Stephen Forrest, the Peter A. Franken Distinguished University Professor of Engineering and Paul G. Goebel Professor of Engineering, who led the work, said in a statement.
At 15 per cent efficiency and given a 20-year lifetime, researchers estimate organic solar cells could produce electricity at a cost of less than 7 cents per kilowatt-hour, the university said. In comparison, the average cost of electricity in the U.S. was 10.5 cents per kilowatt-hour in 2017, according to the U.S. Energy Information Administration.
University of Michigan researchers, who combined multiple advancements in design and process to bolster cell efficiency, believe further gains are imminent. “We can improve the light absorption to increase electric current, and minimize the energy loss to increase voltage,” said Xiaozhou Che, a University of Michigan doctoral candidate in the Applied Physics Program. “Based on calculations, an 18 per cent efficiency is expected in the near future for this type of multijunction device.”
Molecules from UAlberta researchers are arguably at the forefront of the field for deciphering singlet fission, officials said. In addition to Tykwinski’s work with singlet fission, a number of UAlberta chemistry colleagues are pushing the boundaries of knowledge of the fundamental understanding of solar energy capture to fully harness the power of the sun.
Vladmir Michaelis is working with hybrid perovskites to develop non-traditional and more efficient solar cells. Jillian Buriak, Canada Research Chair in Nanomaterials for Energy, is working with increasing efficiency and decreasing the size of inorganic silicon-type solar cells.
“It speaks to our societal need for solar energy,” said Tykwinski of he and his colleagues work. “Solar energy is still, in many ways, an area of fundamental research.
“We are not all necessarily trying to solve the same chemistry problem, but we are all taking our own approach to finding a way to move the field forward. And we have outstanding infrastructure and students, which are allowing us to do this. The potential for discovery here is great, so we are willing to tackle challenging problems in competitive areas.”
Tykwinski’s work on singlet fission is being supported by NSERC and the John R. Evans Leadership Fund. Evidence for Charge-Transfer Mediation in the Primary Events of Singlet Fission in a Weakly Coupled Pentacene Dimer appeared in a recent issue of Chem, a peer-reviewed Cell Press journal.