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Measurement of Grain Boundary Properties in Cu(ln,Ga)Se 2 Thin Films


S. Rozeveld , 1 * C. Reinhardt , 1 E. Bykov , 2 and A. Wall 2 1 Dow Chemical Company , Analytical Science , 1897 Building , Midland , MI 48667 2 NuvoSun , 1565 Barber Lane , Milpitas , CA 95035


* SJRozeveld@dow.com


Abstract: Semiconductors CulnSe 2 (CIS) and alloys of Cu(ln,Ga) Se 2 (CIGS) are often used as the light absorbing layer in thin fi lm photovoltaic devices. These polycrystalline materials reach good conversion effi ciencies despite the presence of grain boundaries, which can degrade device performance. Grain properties such as size distribution and orientation can be characterized using electron backscatter diffraction (EBSD). The EBSD method has been used extensively to determine texture and recrystallization in metal forming processes but to a lesser extent for characterization of CIGS thin fi lm properties. This article describes measurements of grain properties for CIGS thin fi lms grown under different reaction conditions.


Keywords: electron backscatter diffraction (EBSD) , chalcopyrite semiconductors , thin fi lm photovoltaic devices , grain boundaries , grain size


Introduction


The demand for solar energy continues to climb with over 50 GW/year of photovoltaics installed globally in recent years [ 1 , 2 ]. The majority of solar cells are produced from crystalline silicon, but thin film materials such as CuInSe 2 (CIS) and alloys of Cu(In,Ga)Se 2 (CIGS) have also been shown to be good light absorbers for thin film photovoltaic devices with conversion efficiencies above 14% in commercial modules [ 1 – 3 ].


Polycrystalline CIGS thin fi lms have grain size on the order


of 0.3–2.0 µm depending on deposition and process conditions [ 4 – 8 ]. When the average grain size is smaller than the thickness of the CIGS fi lm, the generated current will pass through multiple grain boundaries, which are potential recombination centers for the charge carriers [ 3 – 5 ]. For many semiconductors materials such as Si or GaAs, the presence of grain boundaries will degrade the solar cell performance; however, polycrystalline Cu(In,Ga)Se 2 solar cells can reach good conversion effi ciencies despite the presence of a high grain boundary density [ 5 , 8 ]. Rau et al. modeled the influence of GBs on CIGS solar cell efficiencies and predicted a decrease in efficiency when the defect density at the grain boundary exceeded ~ 10 11 cm -2 [ 9 ]. This trend appeared over a wide range of defect energies, and their results predict a significant decrease in efficiency for CIGS films with a small grain size [ 9 ]. This article describes some experiments to better understand the effect of grain boundaries on CIGS thin film photovoltaic performance.


32


Materials and Methods Specimen preparation . Conventional scanning electron microscopy (SEM) images of thin film surfaces were recorded using a FEI Helios NanoLab G3 dual-beam focused-ion-beam SEM. Prior to the SEM analysis, the CIGS cells were etched in dilute (10% vol.%) HCl solution for several minutes to remove the top contact layers, which consisted of a transparent conductive oxide and CdS film. The CIGS thin films were fabricated at NuvoSun using a two-stage process in which a Cu-Ga-In alloy precursor was first partially selenized and then fully selenized at ~ 550–600 o C in a separate reactor roll-to-roll line. A focused- ion beam of 9.3 nA was used to ion-polish the sample surface at a glancing angle prior to electron backscatter diffraction (EBSD) experiments. The milled region was about 50 µm wide, which provided a relatively large area for EBSD. Atomic force microscopy (AFM) showed that ion milling removed about 100–200 nm of material. Electron backscatter diffraction . EBSD has been used extensively to determine grain properties such as texture and recrystallization, as well as failure analysis and strain mapping, but only to a limited extent for CIGS thin films [ 10 – 12 ]. In this study, EBSD patterns were collected for different thin film samples in order to determine the grain size distribution and orientation [ 10 – 12 ]. A typical instrument configuration is shown in Figure 1a , which shows a Helios FIB-SEM and EDAX EBSD detector. Figure 1b shows the orientation of the sample with respect to the EBSD detector. T e EBSD data were collected using an EDAX detector with OIM-Team soſt ware at 20 keV, 13 nA electron current,


Figure 1 : Instrumentation. (a) Helios G3 FIB-SEM with an EDAX EBSD detector. (b) Schematic of the EBSD setup showing sample orientation relative to the EBSD camera. The sample is tilted at an angle of 70 o toward the detector.


doi: 10.1017/S1551929518000457 www.microscopy-today.com • 2018 May


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