Tel: 0046 18 471 3635, Fax: 0046 18 50 85 42
e-mail: gerrit.boschloo@fki.uu.se
Last updated April 25, 2000
Uppsala University, Department of Physical Chemistry
Box 532, 751 21 Uppsala, Sweden
About 10 years ago, the first efficient dye-sensitized solar cells were reported [1]. In current research, essentially the same materials are used as back then, i.e. ruthenium complexes as sensitizers and nanostructured TiO2 as the semiconducting substrate. Other types of dyes and semiconductors (SnO2, ZnO) did not improve solar cell efficiencies. Therefore, we started to work on the cathode of the dye-sensitized solar cell, which currently is a Pt-coated conducting glass electrode. Replacement of this electrode with a dye-sensitized photocathode will give a photoelectrochemical dye-sensitized tandem cell. In theory, a significant increase of the cell efficiency can be achieved using a tandem cell. NiO was chosen as the material for the photocathode, as this is one of the few simple metal oxides which are stable and p-type semiconducting.
Transparent or translucent nanostructured NiO electrodes were prepared by doctor-blading a hydrous Ni-containing paste onto conducting glass followed by annealing at a temperature in the range of 300 to 450*C. The particle size was in 5 to 10 nm as determined by X-ray diffraction and scanning electron microscopy. The thickness of the porous films was 0.5 to 1.5 mm.
Stable cathodic photocurrents are readily found in dye-sensitized NiO in combination with an electrolyte containing a I-/I3- redox couple and a platinized conducting glass counter electrode [2]. Dyes that have been investigated include erythrosin B and tetra(4-carboxyphenyl)porphyrin. The observed photocurrents are disappointingly low: the maximum IPCE value is about 3%. About 50% of the light is absorbed, hence the quantum efficiency is only about 6%. By comparison, quantum efficiencies close to 100% are found for dye-sensitized TiO2.
Femtosecond pump-probe experiments of erythrosin B adsorbed to nanostructured NiO showed a ultrafast bleach of the ground state (<400 fs), followed by a recovery with t1/2 ª1 ps. By comparison, t1/2 ª40 ps was found for erythrosin B on Al2O3 (presumably an inert substrate). The rapid deactivation of the excited (or possibly oxidized or reduced) state of erythrosin B adsorbed to NiO is possibly caused by energy transfer or a double electron transfer from the dye to the NiO. Localized 3d electronic states of the Ni-atoms may be involved in these reactions. It is noted in this respect that NiII-complexes are efficient quenchers of the excited state of organic dyes.
Concluding, nanostructured NiO has been tested as a dye-sensitized photocathode with limited succes. It is, however, an interesting material for other applications such as electrochromic devices and supercapacitors.
References
1. OíRegan, B.; Grätzel, M. Nature 1991, 353, 737-740.
2. He, J; Lindström, H.; Hagfeldt, A., Lindquist, S.-E. J. Phys. Chem. 1999, 103,
8940-8943.
Acknowledgment
This work has been supported by the Swedish Foundation for Strategic Environmental
Research (MISTRA), Swedish National Board for Industrial and Technical Development
(NUTEK), the Swedish National Energy Administration and Swedish Foundation for Strategic
Research (SSF).
Tel: 0046 18 471 3635, Fax: 0046 18 50 85 42
e-mail: gerrit.boschloo@fki.uu.se