Journal of Photochemistry and Photobiology A: Chemistry 148 (2002) 57–64 Nanostructured ZnO electrodes for dye-sensitized solar cell applications K. Keis a , C. Bauer a , G. Boschloo a , A. Hagfeldt a,∗ , K. Westermark b , H. Rensmo b , H. Siegbahn b a Department of Physical Chemistry, Uppsala University, P.O. Box 532, S-75121 Uppsala, Sweden b Department of Physics, Uppsala University, P.O. Box 530, S-75121 Uppsala, Sweden Received 24 July 2001; received in revised form 1 October 2001; accepted 1 October 2001 Abstract Dye-sensitized photoelectrochemical solar cells constitute a promising candidate in the search for cost-effective and environment-friendly solar cells. The most extensively studied, and to date the most efficient systems are based on titanium dioxide. In this paper, the possi- bilities to use nanostructured ZnO electrodes in photoelectrochemical solar cells are investigated. Various experimental techniques (e.g. infrared, photoelectron, femtosecond and nanosecond laser spectroscopies, laser flash induced photocurrent transient measurements, two- and three-electrode photoelectrochemical measurements) show that the thermodynamics, kinetics and charge transport properties are com- parable for ZnO and TiO 2 . The preparation techniques of ZnO provide more possibilities of varying the particle size and shape compared to TiO 2 . However, the dye-sensitization process is more complex in case of ZnO and care needs to be taken to achieve an optimal performance of the solar cell. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Zinc oxide; Nanostructured; Dye-adsorption; Solar cells 1. Introduction Solar cells based on dye-sensitized nanostructured metal oxides are promising for low-cost solar energy conversion and are intensively investigated nowadays. Systems based on the cis-bis(isothiocyanato)bis(2,2 ′ -bipyridyl-4,4 ′ -dicarboxy- lato)-ruthenium(II) (or Ru(dcbpyH 2 ) 2 (NCS) 2 ) sensitizer adsorbed onto nanostructured TiO 2 films are the most ex- tensively studied and, at present, the most efficient for photoelectrochemical (PEC) solar cells [1,2]. One ad- vantage with dye-sensitized PEC solar cells is the flex- ibility in the choice of material, in which all included components can be varied and their properties may be fine-tuned. This is especially true for nanostructured elec- trodes where beside the band gap, band gap position and surface structure, the particle size and shape, doping den- sity, porosity, film thickness are important parameters to optimize. Several attempts have been made to use ZnO in PEC so- lar cells. Dye-sensitized semiconducting single-crystal ZnO electrodes were studied by Gerischer and Tributsch [3,4] as early as 1969. In 1980, a 2.5% energy conversion effi- ∗ Corresponding author. Tel.: +46-18-471-3642; fax: +46-18-508-542. E-mail address: anders.hagfeldt@fki.uu.se (A. Hagfeldt). ciency at 562 nm using sintered porous disks of ZnO was reported by Matsumura et al. [5]. In the following years many different dyes were tried for the photosensitization of ZnO electrodes, e.g. chlorophyll [6], phthalocyanines [7], rhodamine B [8], coumarins [9], rose-bengal [5,10], Cu(I)- [11,12] and Ru(II)-complexes [13]. In 1994 Red- mond et al. [14] achieved an incident photon-to-current conversion efficiency (IPCE) of 13% at 520 nm when using the Ru(dcbpyH 2 ) 2 (NCS) 2 dye. In our laboratory, the initial tests of ZnO in PEC solar cells were promising: IPCE val- ues of 50–60% at 540 nm and an overall solar energy con- version efficiency of 2% was obtained under 56 mW/cm 2 illumination with a solar simulator [15]. Recently, for the mercurochrome-sensitized ZnO PEC solar cell a solar en- ergy conversion efficiency under 99 mW/cm 2 illumination reaching 2.5% has been reported [16]. However, the effi- ciency is still moderate compared to solar cells based on TiO 2 . In this paper, the possibility to use nanostructured ZnO electrodes in PEC solar cells is discussed. Summarizing the work performed in our laboratory as well as published in the literature by other researchers, a comparison between solar cells based on ZnO and TiO 2 is made and discussed in terms of energy level matching, light harvesting efficiency, charge carrier injection and electron transport properties. 1010-6030/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S1010-6030(02)00039-4