ALD-Zn(x)Ti(y)O as Window Layer in Cu(In,Ga)Se(2) Solar Cells

[Image: see text] We report on the application of Zn(x)Ti(y)O deposited by atomic layer deposition (ALD) as buffer layer in thin film Cu(In,Ga)Se(2) (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(x)Ti(y)O 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(x)Ti(y)O 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(x)Ti(y)O 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(x)Ti(y)O are discussed based on X-ray photoelectron spectroscopy (XPS) measurements.