The formation of the MoSe<inf>2</inf> layer at Mo/CIGS interface and its effect on the CIGS device performance

Abstract

Mo thin films of thickness 0.6-0.7 μm were deposited on soda lime glass (SLG) substrates using direct-current (DC) planar magnetron sputtering. During Mo thin film growth, the average sputtering power density of 1.2 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was used with a discharge current of 1 A. To induce variation in the Mo thin films' morphology and microstructure, as well as to subsequently induce variations in the rate of Na out-diffusion from SLG substrate, the sputtering pressure was varied from 0.6 mT to 16 mT. This is to gain more understanding on the effect of Na out-diffused from SLG through Mo films on the formation of MoSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer at the Mo/CIGS interface during CIGS thin film growth, as well as to study the effect of MoSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> formation on the CIGS device performance. Subsequently, using a computer-controlled deposition system, CIGS thin films were deposited on the Mo-coated SLG substrates by physical vapor deposition using the 3-stage process. The device fabrication was completed by chemical bath deposition of about 500Å CdS followed by 500Å intrinsic ZnO and 3500Å Al-doped ZnO using RF sputtering. Grids of Ni/Al were applied as a front contact. Secondary ion mass spectrometry (SIMS) was applied to characterize the Na out-diffusion through the as deposited Mo/CIGS films as well as to depth profile the O and Se contents as a function of the Mo sputtering pressure. The MoSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer at the Mo/CIGS interface was examined using Transmission Electron Microscopy (TEM) after cross-sectional sample preparation by Focused Ion Beam technique (FIB). Moreover, the performance of the CIGS devices was evaluated under standard conditions of 1000 W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 25°C. The device external and internal quantum efficiencies were measured. Also, the saturation current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sub> , and the ideality factor A of the CIGS devices were evaluated from the light I-V curve.