ALD-Zn_xTi_yO as Window Layer in Cu(In,Ga)Se₂ Solar Cells

We report on the application of Zn _xTi _yO deposited by atomic layer deposition (ALD) as buffer layer in thin film Cu(In,Ga)Se₂ (CIGS) solar cells to improve the photovoltaic device performance. State-of-the-art CIGS devices employ a CdS/ZnO layer stack sandwiched between the absorber layer and the front contact. Replacing the sputter deposited ZnO with ALD-Zn _xTi _yO allowed a reduction of the CdS layer thickness without adversely affecting open-circuit voltage ( V_OC). This leads to an increased photocurrent density with a device efficiency of up to 20.8% by minimizing the parasitic absorption losses commonly observed for CdS. ALD was chosen as method to deposit homogeneous layers of Zn _xTi _yO with varying Ti content with a [Ti]/([Ti] + [Zn]) atomic fraction up to ∼0.35 at a relatively low temperature of 373 K. The Ti content influenced the absorption behavior of the Zn _xTi _yO layer by increasing the optical bandgap >3.5 eV in the investigated range. Temperature-dependent current-voltage ( I- V) measurements of solar cells were performed to investigate the photocurrent blocking behavior observed for high Ti content. Possible conduction band discontinuities introduced by Zn _xTi _yO are discussed based on X-ray photoelectron spectroscopy (XPS) measurements.