The photovoltaic industry expects to regain its stable growth in and beyond the year of 2010. This growth is likely to be accompanied by demands for a significant increase in the quality of PV products as well as a simultaneous reduction of manufacturing costs. To achieve these goals it is imperative to improve production yields of solar cells and panels. The automated visual inspection of PV cells by electroluminescent imaging and quality control could become a vital factor.

Electroluminescent (EL) measurements are essentially the inverse of the photovoltaic (PV) effect whereby a solar cell or panel does not absorb light but rather emits it upon application of electric current. A conductive cell subjected to 3-10 A constant current will experience a number of radiative recombinations between electrons and holes leading to weak emission of light. Defective areas, however, will remain dark.

Advantage: EL Measurement

• Quality inspection based on electroluminescent measurements offer several considerable advantages:
• Analysis and detection of a large and varied number of structural defects during cells and modules production cycle,
• spatial-resolved detection of defects and feedback loops for corrective measures in the production processes,
• high reproducibility of measurement results,
• fast detection and analysis at timescales of 1 s and below
• modularity of the approach which can be applied at many stages of the production process allowing for flexible reshuffling of resources depending on current requirements,
• fast returns on the investment.

All these advantages have strong potential of reducing various downtimes caused by methods of analysis slower and less flexible than EL. However, despite electroluminescent inspection being discussed within PV industries already for a number of years so far only a few PV equipment providers offer products for in-line inspection. To date, many suppliers are utilizing the current economic downturn to improve their positioning in this segment.
A variety of typical production defects can be detected by the use of EL inspection: Electrode and contact faults typically stem from blemishes in screen-printing masks, cracks in the silicon wafer or are due to problems during electrode soldering. Such spots as well as shunts can be visualized as low-light areas on the cell.
Micro-cracks within a cell are of particular interest since they are often missed by superficial inspection. These defects may lead to more widespread damage especially when a cell is subjected to mechanical or thermal stress. Consequences of this can be damning, resulting in substantial material losses and prolonged process downtimes.

Straightforward Set-up, High Camera Requirements
The measurement of the EL phenomenon is straightforward and requires only a power supply, the PV cell itself and a dark box with a digital camera connected to a PC for the analyses of the camera images. The choice of the camera, however, is essential for the automated analysis of EL.
The typical spectral response of the EL measurement on a PV production line is in the near infrared (NIR) part of the electromagnetic spectrum and can range from approx. 900 nm up to 1,450 nm. This spectral range is not only outside the visible spectrum but also poses severe sensitivity challenges for most Si-based CCD cameras.
The use of InGaAs sensors for EL imaging with their higher sensitivity in the NIR has been a recurring topic in the PV industry. InGaAs cameras can indeed deliver millisecond integration times, however, the cost of these systems, the high system noise, low spatial resolution and strict export license regulations make them less than ideal for deployment in the PV industry.
An alternative solution to these limitations are highly sensitive, deep-cooled and low-noise Si-CCD cameras which come with various sensor formats and offer much needed flexibility for EL-based inspection. For EL inspection, four parameters are of special significance: quantum efficiency (QE), read-noise, dynamic range and spatial resolution (see table).
Cameras based on interline sensors (e.g. on ICX285 from Sony) have been often used for EL measurements thanks to their enhanced NIR mode. However, 3 s required to obtain images for automated quality control analysis are not fast enough for cell and module inspection.

Promising New Avenues
EMCCD (Electron Multiplying CCD, www.emccd.com) sensors offer virtually negligible readout noise thanks to integrated signal enhancement electronics. This feature makes it possible to achieve outstanding signal-to-noise ratio as well as higher dynamic range of measurement. An EMCCD sensor used in the Andor Luca R camera has better NIR response characteristics than most interline sensors used in cameras for EL measurements which makes EMCCD a viable choice for PV production line inspection.
Deep depletion CCDs are particularly well suited for measurements in the NIR range and their high quantum efficiency sets them clearly apart from all other Si-CCD cameras. With cameras like the Andor iKon M BR-DD, EL images are characterized by very low noise which can facilitate substantially PV inspection processes. Exposure times needed to produce an image are in the range of 0.2 s making these cameras especially well positioned for high throughput systems like cell sorters and stringers.
The advent of sCMOS sensors (www.scmos.com) opens up many new and promising avenues with cameras combining advantages of both classic CMOS and CCD sensors. Cameras with these chip types will become available from 2010 and thanks to their very high resolution will be of special interest for module inspection in the PV industry.

Literature
[1] Osborne M. (2008): Photovoltaic Fab Managers' Review. Photovoltaics International, 2/08
[2] Fuyuki T. et al. (2005): Photographic surveying of minority carrier diffusion length in polycrystalline silicon solar cells by electroluminescence. Appl. Phys. Lett. 86, 262108
[3] Bothe K. et al. (2006): Electroluminescence imaging as an in-line characterisation tool for solar cell production. 21st European Photovoltaic Solar Energy Conference, 2006, Dresden, Germany
[4] Krauter S. and Grunow P. (2008): Wafer, cell and module quality requirements. Photovoltaics International, 2/08
[5] Köntges M. and Bothe K. (2008): Elektrolumineszenzmessung an PV-Modulen. Photovoltaik Aktuell 7/8 2008
[6] Gabor A. M. et al. (2006): Soldering induced damage to thin Si solar cells and detection of cracked cells in modules. 21st European Photovoltaic Solar Energy Conference, 2006, Dresden, Germany