Development of direct current magnetron sputtered TiO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si75.svg" display="inline" id="d1e1649"><mml:msub><mml:mrow/><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math> thin films as buffer layers for copper indium gallium diselenide based solar cells

We describe the development and characterization of direct current magnetron sputtered titanium dioxide thin films from ceramic targets mixed with metallic titanium particles. The aim of this article is to assess their suitability for the application as buffer layer in copper indium gallium diselenide (CIGSe) based solar cells. The absorber material is produced in a semi-industrial, roll-to-roll hybrid sputter co-evaporation process. A potential strategy to modify electro-optical properties of TiO2 thin films by controlling the concentration of defects, such as oxygen vacancies, is investigated by X-ray photoelectron spectroscopy (XPS), transmission/reflection spectroscopy, spectroscopic ellipsometry, and cathodoluminescence. The presence of oxygen in the plasma during the sputter deposition process has a crucial impact on the electron transport mechanism in the studied devices. The source of the changed device characteristic can be found in modified band discontinuities at the absorber-buffer interface and in an oxidized CIGSe surface, indicated by the presence of cationic selenium, as detected by XPS. We sketch a tentative band alignment diagram and outline the range of possible modifications. The harmful effect of oxygen in the plasma forces the omission of oxygen in the process and restricts the possibilities to modify the material properties of the TiO2 layers, and consequently the prospects for an application on CIGSe. Nevertheless, the results of this work depict the principal eligibility, albeit optimization on both sides of the absorber-buffer interface are inevitable for an improved cell performance.