Filbert J. Bartoli, Chandler Weaver Chair and professor of electrical and computer engineering at Lehigh has been studying ways to improve the performance of solar cells. For the journal Advanced Materials, he and his team (former student Prof. Qiaoqiang Gan of the University of Buffalo and Lehigh adjunct professor Zakya H. Kafafi) reviewed the state of research into a new breed of cell, the plasmonic-enhanced organic photovoltaic (OPV) devices.
Photovoltaic, or PV, technology shows promise as a clean source of renewable energy that may help meet the world’s energy needs far into the future. Made up of semiconductor materials that convert solar energy into electricity, PV could prove the alternative to fossil fuels.
The success of PVs, however, require signiﬁcant advances in materials research and cell design to increase efﬁciency and stability, as well as reduce the manufacturing, installation and operational cost. Right now, PVs rely heavily on two expensive options: thick silicon wafers or extremely thin-ﬁlm solar cells made up of inorganic materials like silicon. That’s where Bartoli’s research comes in.
He and researchers worldwide are pursuing the development of efﬁcient OPVs made up of polymers and small organic molecules that, if designed correctly, can be fabricated in bulk, potentially becoming as inexpensive as making paint. OPVs made from most molecular and polymeric materials are required to be very thin, due to the short range over which excitons can diffuse in these materials, and the low mobility of charge carriers which must reach the electrodes to generate current. However, at these thicknesses, the active light-harvesting layer of the cell is so thin it leads to poor solar light absorption and low power conversion efﬁciency (PCE).
The hard truth is that until power conversion is improved to at least 10 percent or more, cheap, organic PVs will not become a competitor in the solar energy marketplace. However, Bartoli and others believe incorporating metal nanoparticles and/or patterned plasmonic nanostructures within an OPV cell, as well as integrating them with the electrodes, will highly increase solar light absorption, current generation and cell efficiency without altering the active light-harvesting layer thickness.
“Incorporation of plasmonic nanostructures for light trapping in these devices offers an attractive solution for realizing higher-efficiency without altering the active layer thickness,” said Bartoli. “Recent advances in organic photovoltaics have shown 12 percent power conversion efficiency since the submission of the Advanced Materials article, which put them in direct competition with silicon-based PVs,” said Kafafi.
How are you designing the next-generation solar cell?
Bartoli: In the past decade, researchers have explored various strategies to enhance sunlight absorption in the active light-harvesting layer of PV devices, including surface texturing and the use of photonic crystals. However, many of these strategies were intended for active light-harvesting layers greater than the wavelength of visible and near-infrared sunlight. This approach is well-suited for thick, conventional inorganic semiconducting solar cells but not for thin-ﬁlm OPVs which are promising cheap alternatives. Consequently, researchers have been working hard to develop new light-trapping strategies that can be easily adopted in thin organic and polymeric ﬁlms.
What’s the key difference between present solar cells in the market place and those now reviewed in this article?
Bartoli: Conventional wafer-based silicon solar cells (the current market leaders) typically employ a layer of silicon (the active light-harvesting material) as thick as 180–300 microns in order to achieve efﬁcient light absorption and electric current generation. (The period at the end of this sentence is roughly 600 microns in diameter). That’s thick and expensive.
In contrast, OPVs inherently use a very thin active layer on the order of 50 to 100 nanometers in order to collect the photo-generated electron carriers efﬁciently. That causes a bottleneck, a trade-off between the amount of light absorption and photogenerated carriers, and their collection at the electrodes. Effective light trapping schemes must be built into the OPVs to allow them to fully absorb sunlight.
What are researchers doing to make OPVs more efficient?
Bartoli: A significant number of material science researchers are using various plasmonic nanostructures for light trapping and absorption enhancement. Plasmonic metal nanostructures can be easily stimulated by light, like tiny nano-antennas, permitting them to strongly concentrate the light intensity, or scatter it, making solar light more strongly absorbed.
In recent years, surface plasmons have been extensively investigated to improve the efficiency of solar cells and trap more light. Surface plasmons are electromagnetic waves and free electrons oscillating back and forth across the interface of a metal and a semiconductor material. In theory, incorporating nanoparticles, nanostructures and nanohole arrays on the solar cells should make them more efficient without making them thicker. Through the proper design and engineering of these tiny structures, light can be concentrated in the thin, active light-harvesting layer of an OPV, thereby enhancing its absorption and hopefully its current density. Incorporation of plasmonic nanostructures in thin-ﬁlm OPVs offers great promise for achieving energy conversion surpassing the present 12 percent barrier.
How do nanoparticles and nanostructures help make a thin-film solar cell more efficient?
Bartoli: Placing metallic nanoparticles outside the active layer of a solar cell causes a strong, localized plasmon field and increased light scattering.
Placing metallic nanoparticles inside the active layer of the solar cell causes more efficient light scattering within the active layers. Small metallic nanoparticles act as sub-wavelength antennas in which the enhanced near-field is coupled to the absorbing OPV layer, increasing its effective absorption. Large nanoparticles can be used as effective sub-wavelength scattering elements that significantly increase the optical path length of the sunlight within the active layers.
It might seem that creating plasmonic metal nanoparticles is simple, but it is actually quite challenging to control their size, shape, and dispersion along a surface. Patterned metal nanostructures, however, offer another way to enhance light absorption. By properly designing plasmonic nanostructures, light can be effectively coupled to the metal surface.
More recent studies have explored the dual use of metallic nanostructures as electrodes as well as for light trapping in order to achieve electrical and optical functionalities simultaneously.