Steered by the advantages over conventional silicon solar cells, expectations are that the demand for organic solar cells would increase by over 20% from 2017 to 2020.
Organic solar cells can be increasingly produced at a large scale by means of roll-to-roll processing. The materials containing them can be discovered without difficulty in the soil and could be used for solar cells through green chemistry. Since they are semitransparent, they are less visually intrusive—that is, they can be placed on screens or windows and are suitable for mobile devices. They are ultra-flexible, can expand, and can be ultra-lightweight.
However, in contrast to silicon solar cells, organic cells are highly susceptible to moisture, sunlight, and oxygen. Advanced remediation entails cell encapsulation, which increases unit weight and production cost and decreasing efficiency at the same time.
Scientists at the New York University (NYU) Tandon School of Engineering have found an extraordinary way of producing more strong organic solar panels, including conferring resistance to water, oxygen, and light by going the other way: removal, and not addition, of material.
The scientists, under the guidance of André Taylor, professor of chemical and biomolecular engineering at the NYU Tandon School of Engineering, together with Jaemin Kong, a postdoctoral researcher at NYU, and scientists at Yale University’s Transformative Materials and Devices laboratory, conducted the molecular equivalent of hair removal by means of waxing.
They removed the electron-accepting molecules—the conjugated fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM)—from the topmost surface of the photoactive layer of the solar cell by using an adhesive tape, keeping only the non-reactive organic polymers exposed.
The oxidation of these fullerene derivatives is one of the major perpetrators in device degradation. The removal of PCBM from the exposed film surface decreases the likelihood of contacts with the sources of oxidation like water and oxygen molecules, the former particularly causing damage to PCBM.
In a cover story, titled “Underwater Organic Solar Cells via Selective Removal of Electron Acceptors Near the Top Electrode,” in volume 4, issue 5 of ACS Energy Letters, the researchers examined an organic cell with a mixture of PCBM and the more resilient conjugated polymer, poly(3-hexylthiophene) (P3HT) as an active layer. Following the application of the adhesive tape to the photoactive layer surface of the film, the cell was exposed to heat and pressure, and once the film had returned to ambient temperature, the researchers gently removed the tape from the surface of the film.
According to the researchers, later, only 6% of the PCBM acceptor components remained, creating a surface rich in polymer. They explained that this reduced the contact of the fullerene electron acceptors with water and oxygen molecules and markedly increased the adhesion between the top metal electrode and the photoactive layer, which occurs to avoid another problem that comes with flexion: electrode delamination.
Our results finally demonstrate that the selective removal of electron acceptors near the top electrode leads to highly durable organic solar cells that can even function underwater without encapsulation.
André Taylor, Professor of Chemical and Biomolecular Engineering, New York University Tandon School of Engineering.
We demonstrated how much longer the cell lasts under exposure to water without significant efficiency loss. Moreover, using our tape stripping technique we can control the compositional distribution in a vertical direction of the photoactive layer, which consequently leads to better charge extraction out of the solar cells.
Jaemin Kong, Postdoctoral Researcher, New York University.
According to Taylor, post-procedure stress tests included treating the solar units to 10,000 cycles of bending to show that the technique is robust. He also explained that this technique can confer water resistance to organic solar cells, a blessing for products like solar-powered diving watches.
But if you look at the obvious use case for solar panels, you have to make sure organic photovoltaics can compete against silicon on rooftops, in rain and snow. This is where organic solar cells simply have not been able to compete for a long time. We are showing a pathway to making this possible.
André Taylor, New York University Tandon School of Engineering.
This study was supported by a grant from the National Science Foundation and an NSF Presidential Early Career Award for Scientists and Engineers.