Copper indium gallium selenide (CIGS) thin film photovoltaic devices are in the early stages of large-scale commercialization. Their high performance, uniformity, reliability, and a low carbon footprint make them an attractive alternative to standard silicon solar cells. Due to the complex processing required and the associated manufacturing costs, reliable in-line quality control technology is needed. By identifying defective cells early in production, faulty batches can be excluded from further processing, saving resources and costs. We show that micro-Raman spectroscopy (RS) and hyper-spectral imaging (HSI) are powerful tools for quality control and process improvement. Distinctive features in the Raman spectra allow the estimation of the copper to gallium plus indium (CGI) ratio, which is an important criterion for the cell’s efficiency. With HSI in the visible and near infrared range (VNIR) and the near-infrared spectral range (NIR) in combination with machine learning techniques, the layer thickness and CGI ratio are accurately predicted.
Novel thin-film solar cells based on Copper Indium Gallium Selenide (CIGS) are an alternative to standard crystalline silicon cells. This work tests whether two proposed optical methods: Micro-Raman spectroscopy (RS) and photoluminescence (PL) imaging, can measure quality parameters of CIGS PV plates during their manufacture. The investigation followed three steps. Step 1: semi-finished CIGS cells were deposited on a soda-lime glass carrier and measured with Raman and PL. The test cells consisted of a Molybdenum (Mo) back contact, a CIGS layer (varied in the absorber thickness), and a CdS layer. The measurements were used to train models for predictive quality monitoring. Step 2: the plates were finished by adding an iZnO buffer layer, ZnO:Al (AZO) front electrode and divided into 32 cells by scribing down to the Mo layer and electrically tested. I-V parameters such as the open circuit voltage VOC, shunt resistance Rsh, and EQE were measured. Step 3: the finished cells were again measured using the two proposed methods to estimate the composition, efficiency, and VOC of the thin-film cells. Our results show that the proposed methods can non-destructively predict the absorber composition and cell electrical parameters and can therefore be used to exclude samples with poor cell performance at an early production stage.
Copper Indium Gallium Selenide (CIGS) thin-film solar cells are a promising technology, but inline quality inspection systems are required for efficient high-volume production. Tests with two candidate methods: Raman spectroscopy and photo-luminescence imaging, are reported in this paper. The methods were used to estimate material compositions of CIGS samples that were varied in absorber thickness and the composition of the CIGS absorber layer. Our results indicate that both methods can be valuable for contact-free inline inspection during the manufacture of CIGS solar cells, both individually and in combination.
Two different THz applications in the semiconductor industry were explored and validated against established reference measurement techniques and simulations. The first application investigated the possibility of measuring mechanical deformation behaviour of silicon wafers. Time-domain THz tomography mapping scans were carried out to measure wafer thickness and flatness, both in the native state and under different external mechanical loads. These measurements were carried out for a variety of wafers, and the ensuing deformation maps used to validate newly developed numerical simulation models for wafer deformation, and vice versa. In the second part of this paper, carrier dynamics of optically injected charges were investigated by THz spectroscopy. THz pump/probe measurements were carried out in transmission and reflection arrangements on silicon wafers illuminated by a metal halide light source. The light source generates free charge carriers in the semiconductor material that affect the transmission and reflection properties of the semiconductor material. The results of the THz measurements are compared to established standard techniques, like microwave-detected photo-conductance decay (MWPCD) or quasi-steady-state photo conductance (QSSPC) measurements. The defective areas identified with the THz measurements are in good agreement with the defective areas identified by the reference methods. A common benefit of time-domain THz measurements is that the wafer thickness, which is an important measure for the interaction volume of the THz radiation with the semiconductor material, can be calculated from the time- domain signals. The results indicate that THz spectroscopy and imaging can be valuable tools for defect analysis and quality control of silicon wafers, especially since the measurement is fully contact-free and can determine mechanical and electrical properties within a single modality.
Terahertz (THz) time-domain spectroscopy has proven to be a promising technology for a wide range of applications, such as inspection of nished products or materials, quality control, biomedical imaging and diagnostics,
counterfeit detection and characterization of semiconductors. This paper investigates the applicability of THz
time-domain spectroscopy for the characterization of silicon solar cell properties such as: conductivity, charge
carrier mobility and density. Moreover, the possibilities for THz spectroscopy and imaging for the defect analysis
in semiconductor and photovoltaic materials are investigated. THz-pump/THz-probe measurements were carried
out on silicon wafers which were illuminated by a halogen light source to inject free charge carriers. Initial results
indicate that THz time-domain spectroscopy is a promising technique for the characterization of silicon wafers
for the photovoltaic industry.
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