Photoelectron Spectroscopy Studies of Ru(dcbpyH 2 ) 2 (NCS) 2 /CuI and Ru(dcbpyH 2 ) 2 (NCS) 2 / CuSCN Interfaces for Solar Cell Applications Boriss Mahrov, †,‡ Gerrit Boschloo, † Anders Hagfeldt, † Hans Siegbahn, ‡ and Ha ˚ kan Rensmo* ,‡ Department of Physical Chemistry, Uppsala UniVersity, Box 579, SE-751 23 Uppsala, Sweden, and Department of Physics, Uppsala UniVersity, Box 530, SE-751 21 Uppsala, Sweden ReceiVed: December 17, 2003; In Final Form: April 2, 2004 In this work, the electronic structure of the wide band gap hole conductors CuI and CuSCN in contact with an organic dye (Ru(dcbpyH 2 ) 2 (NCS) 2 , cis-bis(4,4′-dicarboxy-2,2′-bipyridine)bis(isothiocyanato)ruthenium- (II)) were investigated by means of photoelectron spectroscopy. The experiments show specific interaction between the NCS groups of the dye molecules and the CuI and CuSCN surfaces. For CuI there are strong indications that the dye molecules interact with CuI through both NCS ligands. Also, one of the carboxylic groups is affected by the surface adsorption on the CuI substrate. For the CuSCN surface the results indicate that about half of the molecules interact with the surface through both NCS ligands, that about half of the molecules interact with the surface through one NCS ligand, and that there is no specific interaction with the carboxylic groups. The measurements also reveal changes in the electronic structure of the dye molecule when adsorbed onto the substrates. In particular, changes in the upper valence electronic structure, important for the function of this material combination in a solar cell device, are discussed. 1. Introduction Dye-sensitized nanostructured solar cells (DNSC) have become a promising alternative in the search for cost-effective and environment-friendly light to electrical energy converting systems. 1-4 In such solar cells light absorbed by dye molecules (e.g., Ru(dcbpyH 2 ) 2 (NCS) 2 , cis-bis(4,4′-dicarboxy-2,2′-bipy- ridine)bis(isothiocyanato)ruthenium(II), Chart 1) generates a photovoltaic output. In the most efficient DNSC the key process takes place at an interface between a dye-sensitized nanostruc- tured semiconductor film and a liquid electrolyte. One advantage in combining the dye-sensitized nanostructured film and a liquid is that the liquid easily penetrates the nanonetwork, resulting in a huge interface active in the conversion process. Encapsulation of the liquid electrolyte in these cells leads, however, to several technological problems such as solvent evaporation, degradation, and seal imperfection. These problems have encouraged studies in which the liquid redox electrolyte in dye-sensitized solar cells is replaced with solid-state equiva- lents, such as polymer-gel electrolytes, 5 conducting organic polymers, 6 ionic conductive polymer electrolytes, 7 organic hole conductors, 8 and inorganic semiconductors. 9 CuI and CuSCN are p-type semiconductors that are suitable as hole conductors in dye-sensitized solar cells, and a number of attempts in the preparation of devices have been made. 9-12 These materials are transparent, have good conductivity, and possess favorable valence band positions, which allow hole injection from oxidized or excited dye molecules. Furthermore they can be applied onto dye-sensitized TiO 2 electrodes using low-temperature methods. For solar cells based on TiO 2 sensitized with Ru(dcbpyH 2 ) 2 - (NCS) 2 efficiencies on the order of 2% are achieved with CuSCN 13 and up to 3.75% with CuI as the hole conductor. 14 The photoconversion yields of these heterojunctions depend largely on the electron-transfer processes between the dye molecule and the semiconductor surface. This in turn is related to the geometrical and electronic structure of the dye in the heterojunction environment. Photoelectron spectroscopy (PES) is a route to the understanding of such properties and is particularly useful for surface analysis. In PES effects on the core level binding energy can be used to study element-specific interactions between the materials, while valence level measure- ments can be used to investigate energy matching and the changes in density of states in the frontier electronic structure. We have previously performed such studies on the dye/TiO 2 interface. 15-17 In the current study we focus on the specific dye adsorption from solution of Ru(dcbpyH 2 ) 2 (NCS) 2 at CuI and CuSCN substrates and on possible changes in the upper valence electronic structure (e.g., in the highest occupied molecular orbital, HOMO), when the dye interacts with the substrate. 2. Experimental Section CuI and CuSCN sample preparation was similar to the procedure reported previously. 9,10 The CuI and CuSCN films * To whom correspondence should be addressed. E-mail: hakan.rensmo@fysik.uu.se. † Department of Physical Chemistry. ‡ Department of Physics. CHART 1: Ru(dcbpyH 2 ) 2 (NCS) 2 11604 J. Phys. Chem. B 2004, 108, 11604-11610 10.1021/jp037905u CCC: $27.50 © 2004 American Chemical Society Published on Web 07/10/2004