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 reports investigations on the new In2S3 containing Cu and/or Na compounds, which are expected to be formed at the Cu(In, Ga)Se-2/In2S3 interface. The knowledge of these materials properties is very important in order to better understand the operation of the devices based on these junction partners.It has been observed that a solid solution Na(x)Cu(1-x)ln(5)S(8) exists from CuIn5S8 (x=0) to NaIn5S8 (x= 1) with a spinel-like structure. The single crystal structure determination shows that indium, copper and sodium atoms are statistically distributed on the tetrahedral sites.XPS investigations on the CuIn5S8, Na0.5Cu0.5In5S8 and NaIn5S8 compounds combined with the band gap changes reported in a previous work show that these variations are mainly due to valence band maximum shift; it is moved downward when x increases from 0 to 1. These observations are confirmed by the electron structure calculations based on the density functional theory, which additionally demonstrate that the pure sodium compound has direct gap whereas the copper-containing compounds have indirect gaps. (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.

The structural and optical properties of NaxCu1−xIn5S8 powders, with x=0, 0.25, 0.5, 0.75 and 1, are determined. It is shown that all of the samples have the same crystalline structure, which indicates the existence of a solid solution over the whole range of x (i.e. 0⩽x⩽1). The increase of the optical band gap of these compounds is found to be linear between x=0 (Eg=1.50eV) and x=0.75 (1.85eV), whereas for x=1 (i.e. NaIn5S8) Eg is found to be 2.40eV, which is much higher than the value expected from the linear slope. Such an evolution shows that the copper and the sodium weigh differently on the optical properties of the material. In thin film solar cells with indium sulfide buffer layer, NaxCu1−xIn5S8 compounds are formed at the Cu(In,Ga)Se2/In2S3 interface. The impact of their properties, here determined, on these thin film solar cells is discussed.

This paper is devoted to an X-ray photoelectron spectroscopy (XPS) study of Cu(In,Ga)Se2 (CIGSe)/In2S3 structures. The indium sulphide layers are grown by physical vapor deposition (PVD) in which indium and sulfur are evaporated on the substrates at a temperature Ts. This as-deposited thin films are then heated at 200 °C for 1 min. A 12.4% efficiency champion cell has been achieved using this process. The XPS study reveals that copper diffuses from the chalcopyrite absorber towards the indium sulphide layer during this synthesis process. The amount of copper strongly depends on Ts; the higher Ts, the more copper is diffused. This observation is then correlated with the solar cell performance to conclude that a significant Cu-diffusion inhibits the formation of a high-quality junction between the Cu(In,Ga)Se2 and the buffer layer.