Since the seminal Nature paper by O’Regan and Grätzel in 1991 [1] the highest efficiencies of dye-sensitized solar cells (DSC) have been achieved using the iodide/tri-iodide redox system. A disadvantage of this mediator is the large internal losses caused by the fact that it is a two-electron redox couple. In 2010 we made a breakthrough by using one-electron transfer redox systems such as cobalt-complexes, in combination with a new generation of organic dyes, which efficiently prevents recombination losses [2]. This discovery was quickly embraced by Grätzel and co-workers, and the new world record for DSC is at present 13.0% by using a Co-complex redox couple and a porphyrin [3]. Our focus now is to develop high efficiency DSC utilizing different colors such as blue, green, yellow and red aiming for aesthetically attractive applications in for example building integration.

Besides liquid DSC we develop solid-state DSC (ssDSC). In one configuration, we prepare a conducting polymer by in situ photopolymerization of the monomers in a photoelectrochemical cell. ssDSCs based on an organic dye, D35, gives together with PEDOT or PEDOP as hole transporting material (HTM) efficiencies up to 7%. Recently [4], we showed that copper phenanthroline complexes in the solid phase can act as an efficient HTM. We prepared ssDSCs with the organic dye LEG4 and copper(I/II)-phenantroline as redox system and achieved power conversion efficiencies of more than 8%, with open-circuit potentials of more than 1.0 V.

The phenomenal breakthrough of the so called perovskite solar cells (PSC) originates from the ideas of replacing the dye layer adsorbed on the mesoporous oxide surface with an ultrathin inorganic perovskite layer and replacing the liquid electrolyte with a solid-state hole conductor [5, 6]. We will report on our latest work on optimizing the solar cell efficiency that at present is above 20% in our laboratories. We have developed new hole conductor materials that reach efficiencies similar to the conventional hole conductor spiro-OMeTAD but with the advantage of being more easily synthesized.  

References

[1]  B. O’Regan, M. Grätzel, Nature, 353 (1991) 737.

[2]  Feldt et al., J. Am. Chem. Soc., 132 (2010) 16714.

[3]  Mathew et al. Nature Chemistry 6, 242–247 (2014)

[4] Freitag et al., Energy & Envir. Sci., DOI: 10.1039/C5EE1204J.

[5]  Kim et al. Sci Rep-Uk, 2 (2012) 591.[6]  Lee et al. Science, 338 (2012) 643.