Spectroscopy In The Production Of Solar Panels

INTRODUCTION

The photovoltaic (PV) industry is keenly interested in incorporating process-monitoring technology in solar cell manufacturing lines as a means of improving solar cell yield and thereby lowering manufacturing cost. A typical solar cell line involves a host of process steps, which include sawing, texturing, deposition of antireflection coating(s), junction formation, and metallizations. Because a solar cell facility processes a large volume of wafers, typically 50,000 to 100,000 a day, the measurement technique must be rapid. Unfortunately, the techniques developed for the microelectronics industry are not suitable for process monitoring in solar cell manufacturing. Hence, there is a need for fast, real-time, low-cost techniques that can measure suitable parameters for monitoring solar cell processing. In an ideal case, it is best to have one approach that can work on many process steps. This minimizes the cost of equipment, training, maintenance, and spare parts.

Advantages of Using Spectroscopy:

• Non-Destructive Testing: One of the primary advantages of spectroscopy is its non-destructive nature. This means that materials and components can be analyzed without altering or damaging them, preserving the integrity of the sample.
• High Sensitivity and Specificity: Spectroscopy offers high sensitivity, allowing the detection of minute quantities of impurities or defects that could significantly impact the performance of solar panels.
• Real-Time Monitoring Capabilities: Real-time data allows for immediate adjustments during manufacturing, ensuring optimal conditions and preventing defects from occurring.
• Automation and Integration into Production Lines: Spectroscopy can be easily integrated into automated production lines, facilitating continuous monitoring and quality control without interrupting the manufacturing process.

METHODS

Reflectance spectroscopy is a very versatile technique for measuring the physical parameters of semiconductor wafers and devices. A broadband reflectance spectrum of a semiconductor device (or any stratified structure consisting of a semiconductor and dielectric films) can provide information on such parameters as thickness and interface roughness of each layer. Combining spectroscopy with scatterometry can further enhance the ability of reflectance spectroscopy. This combination can be used for characterizing non-conformal structures on a wafer/solar cell.

From the measured reflectance spectra, its application potential is huge: monitoring the solar cell manufacturing process (measuring the thickness of each coating layer), analyzing anti-reflection coatings, performance testing,...

PV modules experience reflection losses of ~4% at the front glass surface. This loss can be mitigated by the use of anti-reflection coatings, which now cover over 90% of commercial modules. To evaluate the reflectance of coatings on solar cells, the reflectance spectrum is a prerequisite.

Typical porous SiO₂ AR coatings reduce reflection by between 2% and 3%. Assuming there is no absorption in the AR coating material, this reduction in reflection will result in a direct increase in light transmittance, up to 94–95%, taking the uncoated back surface into account. Fig. 1 shows the reflectance and transmittance for a typical single layer AR-coated PV module cover glass (at the front surface only, in this case the uncoated back surface reflectance is suppressed). The reflectance minimum (and corresponding transmittance maximum) is clearly seen, with increasing values on either side of this. For this reason, single layer AR coatings are often called ‘V’ coatings. Ideally, for an AR coating, the sum of the reflectance and transmittance values at each wavelength would total 100%, as absorption losses should be avoided in optical coating materials.

Figure 1: Reflectance and transmittance of a typical single layer AR-coated glass.

Reflectance measurements after each deposition step allow for the determination of thickness for each layer throughout the whole process. spectrometers and the number of interference fringes measured by the spectrometer are automatically analyzed to determine the film thickness. Figure 2 is the copper indium gallium selenide (CIGS) based thin-film solar cell production processes.

Figure 2: Layer of CIGS-based solar cell.

While the TCO and CdS film thicknesses can be detected with a spectrometer operating in the visible to near-infrared spectral range (500 - 1000 nm), a determination of the film thickness of the absorber requires an infrared reflectance measurement, as these materials absorb the visible light. The accuracy of the measurement is in the range of 1 % for absorber layers, and 2 % for TCO layers, respectively. The determination of the CdS layers, however, is more challenging, as these layers are thin and grow directly on the rough absorber layer. For CdS layers below 50 nm it is very important that the absorber layer reflectance spectra are taken before CdS deposition in the same process.

Figure 3: Reflectance measurement after each deposition step leads to complete control of the entire PV-layer deposition process.

BUILD SYSTEM

INTINS can provide a complete system for these applications. The TS series is a flexible and configurable thin film measurement system that uses spectral reflectance measurement to accurately determine optical and non-optical thin film thickness. This series is suitable for a variety of semiconductor, medical and industrial applications. The TS series is easy to operate with many outstanding features: multi-layer measurement, optional custom wavelength range selection according to needs, easy online and on-site thickness measurement with flexible optical probe clusters,...

The systems of the TM series measure anti-reflective coatings, anti-scratch coatings and rough layers on substrates such as steel, aluminum, brass, copper, ceramics and plastics. The thin film reflectometer system of the TM series can analyze the thickness of optical layers from 1nm to 250μm. A single thickness can be observed with 0.1 nm resolution, and single- or multi-layer films can be analyzed in less than a second.