The present contribution deals with indium sulfide buffer layers grown by thermal cc-evaporation of elemental indium and sulfur. It has been necessary to deposit these buffer layers at low substrate temperatures in order to reach V-oc values similar to those with (CBD)CdS. However, such deposition conditions lead to the formation of a highly recombinative Cu(In,Ga)Se-2/indium sulfide interface. This behaviour may be associated to the presence of sodium carbonates/oxides at the interface even though the Cu(In,Ga)Se, surface was cleaned in NH3 (1 M, room temperature) prior to the indium sulfide deposition. An explanation is that, despite the chemical etch, sodium carbonates/oxides remain in the air exposed Cu(In,Ga)Se, grain boundaries and can migrate towards the surface when the Cu(In,Ga)Se, is heated under vacuum. These polluted interface areas act as recombination zones and thus inferior devices. A possibility to improve the device performance (i.e. improve the interface quality) is to sulfurize the remaining sodium carbonates/oxides. The resulting Na2S can then leave the interface by formation of a solid solution with the indium sulfide. By adapting the buffer layer deposition process, 13.3% efficiency devices with co-evaporated indium sulfide are realized, performance which is close to that reached with (CBD)CdS. (C) 2006 Elsevier B.V. All rights reserved.

The present work studies the influence of the Ga content (x-Ga / (Ga+In)) in the absorber on the solar cell performance for devices using (PVD)In2S3-based buffers. Input to the hypothesis of the relative conduction band positions can be found in the evolution of the device parameters with x. For experiments with x between 0 and 0.5 devices using (PVD)In-2 S-3-based buffers are compared to reference devices using (CBD)CdS. Both buffers give similar cell characteristics for narrow band gap absorbers, typically E-gCIGSe < 1. 1 eV However, the parameters of the cells buffered with (PVD)In2S3 are degraded when the absorber gap is widened whereas (CBD)CdS reference devices are only slightly affected. Consequently, the solar cell efficiency is similar for both buffer layers at the lower x values and increases with x only in the case of (CBD)CdS. These evolutions are coherent with the existence of a conduction band cliff at the CIGSe/(PVD)In2S3 interface. (C) 2006 Elsevier B.V All rights reserved.