Direct comparison of atomic layer deposition and sputtering of In2O3:H used as transparent conductive oxide layer in CuIn1−xGaxSe2 thin film solar cells

Abstract

In this study thin films of hydrogenated In2O3 (IOH) were fabricated by physical vapor deposition (PVD) with and without a post-annealing step, and by atomic layer deposition (ALD). The electro-optical properties on glass as well as the performance as a transparent conductive oxide (TCO) layer in CuIn1−xGaxSe2 (CIGSe)-based solar cells are compared and related to a ZnO:Al (AZO) baseline TCO. Corresponding TCO film thicknesses were adjusted to a resulting sheet resistance of about Rsh = 20 Ω/sq for all samples. Structural investigations were conducted by X-ray diffraction (XRD) and transmission electron microscopy (TEM), while Hall and optical absorption measurements were performed to analyze the electrical and optical quality of the window layers. It is shown that the fully crystallized IOH layers processed by ALD and PVD show similar microstructural and electro-optical properties, which are superior to the AZO baseline. The finalized solar cells were characterized by current-voltage and reflectance-corrected quantum efficiency measurements. While there is no significant gain in short circuit current density (Jsc) for as-deposited PVD In2O3 layers, the application of crystalline In2O3 TCOs leads to an improvement of more than 2mA/cm2 due to an increase in “optical” band gap energy and less free charge carrier absorption (FCA). The open circuit voltage (Voc) of the best cells is 10–15mV higher as compared to the AZO reference, independent of the crystallinity and process of the In2O3 films. The results indicate that the gain in Voc is due to inherent material properties of the IOH films and does not originate from less sputter damage or an affected i-ZnO/TCO interface. Device simulations show that the higher electron affinity χ of the IOH can explain an increased Voc if the Fermi level (EF) is pinned at the CIGSe/CdS interface and why it might not be possible to see the gain when alternative buffer layers are applied.