An international group of researchers has developed and tested a new generation of photovoltaic cells in Lausanne, Switzerland. For the first time, dye-sensitized solar cell technology incorporates two different dyes, making the cells capable of reacting to a larger range of the light spectrum and thus more efficient. The results could revolutionize solar cell technology.
The discovery is “a new paradigm in how we capture light and transform it into electricity,” declares EPFL Professor Michael Grätzel, inventor of the dye-sensitized solar cell technology and a lead member of the research team, which includes scientists from EPFL, Stanford University, University of California Berkeley and the Georgia Institute of Technology.
Dye-sensitized solar cells, known as Grätzel cells in recognition of their inventor, date to the 1990s. The EPFL professor developed a system based on dyes that, like naturally occurring chlorophyll in plants, are stimulated by light and generate electric charges. With this technique it is possible to produce solar cells that function in weak or indirect light and that are relatively cheap – an important consideration that compensates for the cells’ inferior efficiency when compared to traditional silicon-based solar cells.
The phthalocyanine dyes used in Grätzel cells are only sensitive to a restricted range of the solar spectrum. The new research allows that spectral sensitivity to extend to the red, green and blue ranges of visible light, by incorporating a second, perylene dye. With a larger range of light sensitivity, the cells’ efficiency is thus improved. Although the perylenes do not directly generate an electric charge, they react to the blue and green parts of the visible light spectrum. They transfer their energy to the phthalocyanines, which in turn transmit the electric charge. Without the new dyes, the phthalocyanines can react only to the red part of the spectrum. “It’s not possible for a single dye to be sensitive to the entire spectrum of visible light,” explains Khaja Nazeeruddin, a researcher in Grätzel’s group at EPFL. “Which is why we incorporated a second dye. This is the first time this has been done.”
Nature’s own technology
This form of indirect energy transmission is inspired by nature. In plant photosynthesis, certain chlorophyll molecules emit signals which other chlorophyll molecules receive before they set in motion the process of electric charge transfer. “It’s what we call energy transfer by dipolar interaction,” explains Grätzel. “Until now, the dyes in our cells had the unique role of directly generating the electric charge.” The model was tested in the EPFL laboratory by Grätzel and Stanford University professor Brian Hardin. Results are more than encouraging. The charge transfer improved by 26% over cells with phthaolcyanine alone. “We have many areas to explore to improve the model in the future,” adds Nazeeruddin. “We can play with the sensitive parts of the light spectrum, or design a system with three or even four dyes.”
This discovery is the first result of a recent partnership that was created between EPFL, Stanford, Berkeley and Georgia Tech. Led by Stanford, the project, known as CAMP, aims to improve the efficiency and longevity of molecular solar cells, and to develop low-cost solar cell production techniques.