Electroluminescence for PV Cells

Electroluminescence Inspection of Photovoltaics with SWIR Imaging

High quality, high yield and maximum power output are crucial objectives for solar cell manufacturers trying to displace fossil fuels from energy generation.  Solar cell inspection by machine vision with InGaAs short-wave infrared (SWIR) cameras reveals voids in silicon boules before slicing them into wafers to produce mono-crystalline solar cells.  Inspection of the resulting wafers with SWIR permits detecting defects, hidden cracks or saw marks inside or on the opposite side of the wafer due to silicon’s transparency at SWIR wavelengths beyond 1.1 µm.   Beyond finding physical defects, SWIR solar cell inspection of electroluminescence (EL) and/or photoluminescence (PL) permits actively finding problems that will hurt cell or system power output.  Below, a video scanning a SWIR camera across a panel of solar cells shows a large variation in EL emission, both within individual cells and across the array of cells, finding cracks, dead spots, weak areas and weak cells.  The intensity of the glow directly correlates with the solar collection efficiency of the panel and of each cell’s contribution.   As weak cells or regions end up dissipating some of the power generated by the more efficient cells, it is of vital importance for system integrators, panel manufacturers and cell fabricators to perform solar inspection in their products with electroluminescence imaging in order to properly match cells of similar performance.

Read our article on NIR Trends: Maximizing Solar Cell Yield and Efficiency with Machine Vision.

The image gallery below demonstrates the use of EL for solar cell inspection to find non-uniformities, cracks, defects, mismatched cell efficiencies, and the limited usefulness of thermal imaging for these applications.

SWIR video scanning over the length of a commercial 36 cell panel while forward biased with 18 V.

 

Video of electroluminescence from polycrystalline solar cell induced by 1 Hz squarewave. Recorded in room light by SU320KTSX at 60 Hz.

 

Left: Commercial 36 cell solar panel imaged with a Sensors Unlimited SWIR camera while forward biased with 18 V.  Right: Close-up view of two cells with defects visible in the image on the left, revealing a semi-circular crack in the lower cell, dead sections, and a scatter of defects in the upper cell.

 

All types of photovoltaic solar cells, such as mono and poly-crystalline, thin-film ribbon, CIS and CIGS, can be tested for uniformity and defects by simply forward biasing the cell until it glows as a result of electroluminescence, as seen in the figures below.

The samples were purchased off the open market for components, and show considerable variation, both within a single device and between several samples of a device type.  The Sensors Unlimited SU640KTSX camera and SUI Image Analysis software was used to acquire the videos at 30 fps.

This 30 fps video shows the non-uniformity in electroluminescence emissions from three poly-crystalline solar cells that were grown with the ribbon method as the bias voltage is ramped from 2 to 3.8 V.  The brighter areas are near the connection wires to the cells, which were wired in series.  Note that the back-side contacts (small rectangles along the busbars) are clearly visible through the silicon.

The following images illustrate how non-uniformity can be found with SWIR solar cell inspection, both within an individual cell (note the non-uniformity across individual cells) and among several cells assembled into a panel.

The left-hand image was taken with the Sensors Unlimited SU320KTS camera, which collects the photoemission from within the thickness of the cell.  The middle image taken in the visible shows color differences due to differences in the top layer thickness, but it can be seen that those variations have little to do with the photodiode efficiency or uniformity.  The right-hand image was taken with a thermal microbolometer, which exposes little or no detail.

 

InGaAs cameras are ideal for use by the solar industry for cell inspection due to their high Quantum Efficiency for the wavelengths that the silicon and other solar cell material luminesce as illustrated by the graph below:

Both Sensors Unlimited linescan and area cameras can be used for solar cell inspection. The high-sensitivity area cameras provide convenient still images while the digital high-speed, high-resolution line cameras are ideal for providing higher resolution at lower cost with continuous production of cells or moving inspection stages.

Contact Sensors Unlimited today to find how easy it is to improve your product quality while improving your bottom line and helping your customers reduce their dependence on fossil fuels.

For additional information on SWIR solar cell inspection, please visit our pages on:

Photoluminescence for PV Cells

Electroluminescence of Multi-Junction PV Cells

SWIR...Solar...Synergy

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