When it comes to graphene and photovoltaics, for the most part it’s only been a story about replacing the indium tin oxide (ITO) used as the transparent electrodes of organic solar cells.
But last year Spanish researchers in collaboration with teams from the Massachusetts Institute of Technology and Max Planck Institute for Polymer Research in Germany started to change the game and took graphene into the conversion and conduction layers of a photovoltaic cell.
Now, Spanish scientists at the Universitat Jaume I in collaboration this time with researchers from Oxford University have developed a photovoltaic system in which graphene and titanium dioxide combine to serve as the charge collector while perovskite acts as the sunlight absorber.
The mineral perovskite is enjoying a period of rapid improvements for its use in solar cells where its particular crystal structure offers an inexpensive solution for creating photovoltaics with high charge-carrier mobility and long diffusion lengths. These properties make it possible for the photo-generated electrons and holes to travel long distances without energy loss. In real world terms this means that the electrons in perovskite-based photovoltaics can travel through thicker solar cells, which absorbs more light and thereby generates more electricity than thinner cells.
This latest combination of perovskite with graphene, which is described in the journal Nano Letters (“Low-Temperature Processed Electron Collection Layers of Graphene/TiO2 Nanocomposites in Thin Film Perovskite Solar Cells”), offers a way to produce perovskite solar cells more cheaply and with a high efficiency.
Previous perovskite cells needed a 500-degrees C sintering process to build the electron collection layer. So that pretty much rules out making solar cells on inexpensive polymer substrates as well as creating multi-junction device architectures.
But the graphene and the titanium dioxide electron collection layers can be produced at temperatures that never rise above 150 degrees C.
The researchers also report that not only can they produce the perovskite solar cells in a low-cost process but also that the energy conversion efficiency reached 15.6 percent, just slightly above the 15 percent achieved by the highestper-forming perovskite cells manufactured with the sintering process. The conversion efficiency also surpasses levels reached when silicon and graphene are combined.
Based on this latest research, It would seem that when graphene and perovskite are a winning combination in photovoltaics.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.