Recent developments in amorphous sputterred ITO thin films acting as transparent front contact layer of CIGS solar cells for energy autonomous wireless microsystems

In this paper, we report on the study of electrical, optical and structural properties of RF sputtered Indium Tin Oxide (ITO) thin films at room temperature. These films are dedicated to act as front electrode of CIGS solar microcells and shall so compel with the electrical and optical criteria that are required for such an application. It is well-known that the main drawback of the sputtering deposition technique deals with the inherent generation of highly energetic particles that causes bombardment onto the sample. The developed deposition process targets to be damage free onto the underlying layer since, in the case of CIGS solar cells, it is crucial to preserve the surface and the properties of the absorber layer on which these films will be deposited. At room temperature, it can be considered that amorphous ITO films are only obtained when this energetic bombardment does not occur. This can be obtained if the kinetic energy of the particles is fully dissipated by collisions within the deposition plasma [1-3]. The deposition process is developed in a conventional magnetron sputtering system without external heating, in such a way that films shall be amorphous. Furthermore, film internal stress is kept very low. Optical studies show a transparency over 80% in the visible range and a high transparency in the infrared region. The lowest obtained sheet resistance is 12.6 Ω/□ (~ 300nm film thickness) with a carrier concentration of 2.4 × 10²⁰ cm^-3 and a carrier mobility of 45.1 cm²/V.s. As we can deposit a dual ITO layer structure, with a different resistivity level being attributed to each layer, we suggest our amorphous ITO thin films can be deposited directly above the absorbing CIGS material to act as both highly resistive (HR) and electrode layer.