Solar Energy Materials 11 (1984) 57-74 57 North-Holland, Amsterdam TERNARY CHALCOGENIDE-BASED PHOTOELECTROCHEMICAL CELLS III. n-Culns Ss / aqueous polysulfide Geulah DAGAN, Saburo ENDO *, Gary HODES, George SAWATZKY * and David CAHEN The Weizmann Institute of Science, Rehooot 76 100, Israel Received 24 April 1984 The photoelectrochemistry of the spinel phase CulnsS s in polysulfide electrolyte was studied. A sequence of annealing in S and in vacuum was found to give optimal material for this purpose. The performance of such material could be improved by acid etching and photoelectrochemical etching; both of these treatments deplete the top layer of Cu. Notwithstanding obvious kinetic limitations in polysulfide, the title system shows a negative temperature dependence. The output stability which is inferior to that of n-CuInS 2 systems in polysulfide, is limited by increasing dark currents. These may be due to the formation of CuS on the surface. While, for all examples studied, the indirect bandgap around 1.3 eV was observed, the direct gap around 1.5 eV was seen clearly only for photoclectrochemically etched electrodes. It is suggested that the poor photovoltaic performance and mediocre output stability behaviour of CuInsS s photoanodes is related to the inability of the material to tolerate significant Cu depletion and to the absence of a homogeneous indium oxide surface layer. This latter finding may be connected with the fact that most of the indium is octahedrally, rather than tetrahedrally coordinated, in contrast to what is the case for the chalcopyrite type disulfide. 1. Introduction In their study of the phase relations in the system Cu2S-In2S3, Binsma et al. [1] found the cubic spinel CuInsS 8 to be the only true chemical compound besides the well-known chalcopyrite CuInS 2 [2]. Our interest in ternary chalcogenide semicon- ductors for photovoltaic applications led us to study this Cu-poor semiconductor of which no analogue selenide seems to exist [3]. The Cu-In-chalcogenides are of particular importance in photoelectrochemistry, because of the impressive performance of cells, containing CuInS 2 [4,5] and, espe- cially, CuInSe 2 [5-7], in terms of output stability. This extraordinary stability has, to date, not been explained satisfactorily, and we will give here only some of the possible hypotheses. Experimental [2] and theoretical [8] indications exist for signifi- cant Cu d-orbital participation in the upper valence band (VB) of CuInX 2 (X = S or Se). Because of this the upper VB will have mixed anion-cation (Cu) character, * Dept. of Electr. Engn., Science University of Tokyo, Tokyo 162, Japan. * Lab. of Physical Chemistry, University of Groningen, Groningen, The Netherlands. 0165-1633/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)