INSTITUTE OF PHYSICS PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY Semicond. Sci. Technol. 18 (2003) 312–318 PII: S0268-1242(03)55469-4 Efficient dye-sensitized photoelectrochemical cells made from nanocrystalline tin(IV) oxide–zinc oxide composite films G R R A Kumara 1 , K Tennakone 2 , I R M Kottegoda 2 , P K M Bandaranayake 2 , A Konno 1 , M Okuya 1 , S Kaneko 1 and K Murakami 1 1 Department of Material Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan 2 Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka E-mail: tenna@ifs.ac.lk Received 29 October 2002, in final form 7 January 2003 Published 26 February 2003 Online at stacks.iop.org/SST/18/312 Abstract Dye-sensitized photoelectrochemical cells based on nanocrystalline films of TiO 2 yield energy conversion efficiencies ∼10%. The efficiencies of similar cells with films of other oxide materials (SnO 2 , ZnO) are well below the above value. However, the cells made from SnO 2 –ZnO composite films give efficiencies comparable to TiO 2 cells. Two types of composite systems with SnO 2 and ZnO are possible. In the first type, SnO 2 crystallites are covered with an ultra-thin (<1 nm) outer shell of ZnO 2 and in the second type, the film comprises SnO 2 crystallites (∼10 nm) with a thin ZnO outer shell and larger ZnO particles (∼100 nm). The short-circuit photocurrent and efficiency of these cells are ∼17 mA cm −2 , 19 mA cm −2 and 7%, 8% respectively. This paper explains in detail how a thin shell of ZnO on SnO 2 could effectively counteract recombinations of electrons with acceptors in the electrolyte (e.g., I 3 − ) and increase the efficiency although SnO 2 and ZnO are individually not good materials for dye-sensitized photoelectrochemical cells. In the second type, larger ZnO crystallites reduce the rate of geminate recombinations, in addition to the effect of the outer shell. 1. Introduction Dye-sensitized (DS) photoelectrochemical cells (PECs) based on nanocrystalline films of TiO 2 , discovered nearly a decade ago [1, 2], continue to attract much attention as potential systems for conversion of solar energy [3, 4]. Equally important is the mechanism of charge injection transport and recombination in these systems. During the past few years, reports have appeared in the literature that describe the construction of DS PECs from other oxides [5–9] and composite nanocrystalline materials [10–17]. The authors of this paper and their collaborators have attempted to construct DS PECs from nanocrystalline films of SnO 2 and found that they are highly inefficient despite variations of crystallite size, film morphology, etc [11–14]. They have also observed that highly efficient DS PECs can be made from SnO 2 by compositation with other oxides [11–15]. Two types of composite oxide systems were tested. In the first type, crystallites in SnO 2 films were coated with a thin outer shell of an insulator [13, 14] (MgO or Al 2 O 3 ) and in the second type, the film consisted of a mixture of large size (200–300 nm) ZnO crystallites and much smaller SnO 2 crystallites [11, 12] (4–10 nm). We believe that in the first type the dye molecules anchored to the outer insulator shell, and on photoexcitation injected electrons to the conduction band (CB) of SnO 2 via tunnelling across the barrier [13]. When the electrons relax to the CB of SnO 2 , the barrier prevents leakage to the surface where they undergo recombinations [13]. The mechanism 0268-1242/03/040312+07$30.00 © 2003 IOP Publishing Ltd Printed in the UK 312