Luminescent imaging

The basic principle of luminous imaging is to excite the surface of a sample to make it glow, and then capture the luminous image with a camera.

Two imaging methods are commonly used in crystalline silicon solar cells

PL, Photo-luminescence, photoluminescence, excitation by incident photons. The light source uniformly illuminates the entire sample, so the resulting image is a true luminescence profile of the sample, not affected by uneven excitation or local string resistance. The local luminous intensity of the image is determined by the carrier density and lifetime of the region. The stronger the light in an area, the higher the pixel value. Conversely, the less luminous the region, the lower its pixel value. There are several factors that can make the glow decrease:

• Low MCLT
• The doping density of MCLT is low
• Local defects (forming a carrier recombination center and reducing local carrier lifetime). Local defects include elemental impurities, physical defects of silicon crystals (such as hidden cracks, dislocations), and grain boundaries of polysilicon.

EL, electroluminescence, excited by an electric current. In the process of generating an EL image, an excited current is injected into the busbar of the solar cell. The series resistance of the solar cell itself causes the excitation voltage to drop gradually, so that the excitation degree outside the busbar is gradually weakened. Therefore, the imaging also reflects the excitation heterogeneity on the basis of the real luminescence distribution. In addition, the local series resistance distribution also changes the degree of drop in excitation voltage, which further weakens the luminescence.

PL vs EL analysis and comparison

• EL image contains series resistance effect + local defect effect (compound loss)
• PL image does not reflect the effect of series resistance, mainly affected by local defects (compound loss)
• PL imaging is normal and EL imaging area is dark, indicating that resistance loss is the main loss factor
• PL imaging darkens and EL imaging darkens, indicating that defects (compound loss) are the main cause

The luminescence intensity was further analyzed

• Generation of excess minority carriers → Recombination of minority carriers
• SRH recombination, poor luminous intensity
• Auger recombination, poor luminous intensity
• Radiative recombination, maximum luminous intensity (no defect state)

PL imaging interpretation

The PL intensity is positively correlated with the proportion of spontaneous radiation excitation

• PL can be tested with both steady state (SS) and quasi steady state (QSS) modes
• PL tests for MCLT, independent of space charge region
• Output MCLT distribution diagram
• Can be converted to iVoc
• Higher brightness → means more radiation recombination
• Darker brightness → less radiation recombination → lower np value → lower MCLT