The industry of conventional silicon solar cells is growing extremely rapidly with a global market of 36.9 GW in 2013. The asian market has seen the fastest growth the last years with a market growth of above 100% last year. In some European countries like Germany and Italy solar cells provide 5-10% of the annual total electricity consumption. From a technology view-point the market is very much dominated by silicon solar cells. The price of silicon solar cells has decreased much more rapid than expected a few years ago and grid-parity, the point when solar cell generated electricity becomes competitive with the retail rate of grid power, is within reach in some countries with the support of governmental subsidies. To compete with conventional silicon solar cells is challenging and requires low-cost, environmental-friendly and abundant materials, and large-scale production methods. As there is a large interest to
integrate solar cells in buildings, design opportunities for example in terms of different colors, shapes and transparencies would make a competitive edge for a new technology.
The technology platform of mesoscopic solar cells (MSC) fulfills all of the desired properties mentioned above. The major breakthrough of MSC was done here at EPFL with Prof. Michael Grätzel and co-workers, in 1991 with the introduction of nanostructured dye-sensitized solar cells showing that a high-surface area electrode could efficiently separate and transport the different charge carriers. A mesoporous wide-bandgap oxide semiconductor (TiO2) is used as an electrode substrate on which dye molecules are anchored, sensitizing the TiO2 so that sunlight can be absorbed in the visible region of the spectrum. Photoexcitation of the dye-sensitizer initializes a charge separation process that typically occurs through injection of the electron in the excited state of the dye into the semiconductor conduction band. The oxidized sensitizer is then regenerated by the redox electrolyte. This photoelectrochemical cell is complete with a counter electrode as cathode.
Since 1991 dye-sensitized solar cells (DSSC) have developed to become a promising option for low-cost photovoltaics. Record efficiencies of 13 % have been obtained as well as accelerated life-time tests passed with high durability efficiencies. Over the years, the DSSC concept has developed into a plethora of mesoscopic systems for conversion of solar energy to electricity and recently also to water splitting devices. The MSC family now includes traditional liquid photoelectrochemical devices, solid-state DSSC using organic/polymeric hole transporter materials, semiconductor quantum-dot sensitized solar cells, energy transfer devices and perovskite solar cells (PSC). For the latter, impressive efficiencies above 20% have been achieved in short time. In addition, a rapidly growing interest is emerging for the use of the DSSC platform to develop solar fuel devices (DSSF, dye-sensitized solar fuel) with molecular light absorbers and catalysts to split water into hydrogen and oxygen or to reduce CO2.
At LSPM we combine basic and applied research aiming to improve our fundamental understanding of the materials, interfaces and devices to improve device performances in terms of efficiency, stability and processability. Our three main directions of research are (i) DSSC with new combinations of dyes and electrolytes to improve efficiencies and to provide design opportunities for building integration, (ii) PSC materials and devices for fundamental studies with the long-term goal of achieving improved efficiency and durability with a minimum of materials cost and up-scalabe device configurations, and (iii) DSSF devices for water splitting systems.
