First reported in 1991 by Prof. Michael Graetzel from EPFL, Lausanne (Switzerland), Dye sensitised Solar Cells (DSSC) have known important developments in the late 1990s ans early 2000s. Then the field began to falter, the community having massively preferred turning to organic and then Perovskite solar cells. Here I present how DSSCs are working and with the example of an article from Anders Hagfeldt and co-workers published in Nature last June, show that the topic is still evolving.
How do DSSCs work?
The behavior of DSSC is mostly an electrochemical behaviour. The current generation is based on the absorption of light by a dye molecule. Just after the absorption of photons, charge carriers move to a mesoporous semiconductor, generally TiO2 nanoparticles, where electrons and holes are separated and collected at hole-conducting or electron-conducting electrodes. A good engineering of the energy levels is necessary, to confine recombinations and enhance the photoelectric effect.
In original DSSCs, the system needs an electrolyte solution, typically Iodide/iodine, to enhance the conductivity to the cathode. Nowadays, most of the DSSCs are composed of a solid electrolyte.
Perovskite solar cells have been, at first, in 2009 used in dye sensitzed solar cells, as the dye molecule. But rapidly it has been shown that the dye molecule could be used alone, thanks to its exceptional semiconducting properties. For this reason, all the research that had been carried out on solid state DSSC (ssDSC) had been transferred to the field of perovskite solar cells, allowing its unprecedented growth.
The latest evolutions of DSSCs have been focused on the stability and the efficiency of solar cells, particularly in indoor conditions. In 2005, Fukuzumi et al. reported that copper-based electrolytes worked well in indoor (low-light) conditions. Later on, in 2012, Cao and co-workers have shown that ruthenium-based dyes used with iodide-based electrolytes perform well in ambient conditions. That is what I want to share.
What efforts are done to improve their efficiency?
Nowadays, only a few articles each year are dealing with the field of DSSCs, because it is admitted in the community that the field is mature enough. Most of the research is carried out by Prof. Graetzel or Prof. Nazeeruddin teams at EPFL in Switzerland. That is where Marina Freitag carried out research reported in his Nature Article: “Dye-sensitised solar cells for efficient power generation under ambient lighting”.
The authors show that a proper design of DSSCs can result in outstanding results in term of power onversion efficiency, and particularly in indoors applications. They introduce a DSSC design that uses a copper organic complex as a redox shuttle, with D35 and XY1 sensitisers. These materials are based on the one previously known to perform well in indoor applications.
In the article, many reasons are discussed to explain the good performance of the produced cell. The most important, to me, is that the good engineering of the cell is responsible of its behaviour. The binding of the dye to the TiO₂ has been well controlled, as well as the position of the arylamine donor group, tho that the energy loss is minimized during the electronic process. The great matching of the potentials are of utter importance for the application with DSSCs, as well as the good choice of sensitizers.
This example shows that DSSCs developments are still going towards more stable and more efficient cells, particularly in indoor applications. I think that the good engineering of materials must be done in other solar cells applications, from Organic Solar Cells to Perovskite, that also present tremendous performance in indoor conditions.