[Image above] Proposed photovoltaic cell structure. Top left: Bird’s-eye view of a solar cell array with hemispherical-shell-shaped active layers. Bottom left: Unit cell of the dome-shaped device. Bottom right: Cross-section view of the dome-shaped device. Credit: Hah, Journal of Photonics for Energy (CC BY 4.0)
By Laurel Sheppard
Even as the cost of traditional solar technology continues to fall, researchers are investigating new materials to make photovoltaic modules even more efficient and affordable. Of these alternatives, organic photovoltaic cells are receiving attention because of several major advantages: they consist of abundant materials, can be manufactured at low cost, and allow flexible substrates. They also are lightweight, and the cell’s absorption spectrum can be customized to better match the solar spectrum.
Power conversion efficiencies of organic photovoltaic cells are approaching conventional solar technologies, with reports of nearly 20% efficiency at the small-cell level. To improve these efficiencies, researchers are investigating ways to better manage how light interacts with the cell through coupling and retention strategies.
Angular coverage, or the range of angles from which the cell can absorb incident light beams, is critical to improving efficiency, especially in wearable device applications. Strategies to improve this property include surface nanotexturing, embedded light scatterers, and antireflection coatings incorporating nanostructures.
Scientists have also looked at active layers made with different shapes, including spherical nano-shells and fiber arrays. In an open-access article published in February 2024, Abdullah Gül University assistant professor Dooyoung Hah took a different approach by analyzing the absorption efficiency of hemispherical-shaped, or “domed” cells.
Hah previously investigated solar cell structures based on shell-shaped organic active layers with semicircular and triangular cross sections. These cells, which featured a circular shape in two dimensions, showed improvements in adsorption and angular coverage compared to a rectangular, flat shape. However, because these cells still appeared rectangular in the xz plane, Hah believed that completing the circle in all directions (i.e., making a hemisphere) would improve the absorption performance.
His proposed hemispherical-shell-shaped cell consists of an organic polymer called P3HT:ICBA as the active layer placed on a layer of aluminum and a substrate of the synthetic polymer PMMA. The cell is capped off with a transparent protective layer of indium tin oxide.
These layers can be conformally coated on the hemisphere-shaped polymer substrate using soft lithography methods, such as microcontact printing, which maintains the shape. Other thin film layers can be deposited using conventional deposition methods, such as sputtering.
For the flexible polymer substrate, an array of hemispheres at the front surface can be fabricated using thermal reflow of resist, a method for transferring lens-shaped structures into the substrate by heating the material above the glass transition temperature. Other potential ways to make the hemispheres include grayscale lithography or nanosphere lithography followed by two steps of casting and reactive ion etching.
Hah modeled his proposed hemispherical-shell-shaped cell on the computer and investigated its potential using 3D finite element analysis. The results showed
- Absorption improvements of 66% and 36% when incoming light is transverse electric (TE) and transverse magnetic-polarized (TM), respectively, compared to a flat-structured device (electric and magnetic fields are perpendicular to the direction of propagation).
- Absorption improvement is as high as 13% (TE) and 21% (TM) when compared to the semicylindrical shell structure, depending on the polarization of the light.
- Angular coverage reached 81 degrees (TE) and 82 degrees (TM), allowing light to enter from a wider range of directions than a flat surface.
“With the improved absorption and omnidirectionality characteristics, the proposed hemispherical-shell-shaped active layers will be found beneficial in various application areas of organic solar cells, such as biomedical devices, as well as applications such as power-generation windows and greenhouses, internet-of-things, and so on,” Hah concludes .
He also noted that other organic active materials could be used in the proposed structure.
The open-access paper, published in Journal of Photonics for Energy, is “Hemispherical-shell-shaped organic photovoltaic cells for absorption enhancement and improved angular coverage” (DOI: 10.1117/1.JPE.14.018501).
Further readings
“Overview of the current state of flexible solar panels and photovoltaic materials,” Dallaev et al., Materials (2023)
“Large-area organic photovoltaic modules with 14.5% certified world record efficiency,” Basu et al., Joule (2024)