Researchers at Karlstad University in Sweden have used light beam-induced current (LBIC) phase mapping to investigate the local electrical behavior of half-cell solar modules and have found that the technique shows potential for assessing electrical contact quality and detecting localized defects with high spatial resolution.
LBIC mapping is a non-destructive, spatially resolved optoelectronic characterization technique used to investigate the electrical properties and performance of photovoltaic cells and modules. It involves scanning a focused light beam across a solar cell or module while measuring the resulting photocurrent to identify spatial variations in electrical performance.
Unlike conventional LBIC mapping, which primarily measures photocurrent amplitude, LBIC phase mapping uses modulated illumination to measure the phase shift of the photocurrent response, providing additional insights into local electrical resistance and contact quality.
“Our experiments demonstrated that LBIC phase mapping can detect variations in contact quality, including regions with good contacts and regions with intentionally reduced contact area and discontinuous wire contact,” corresponding author Markus Rinio told pv magazine. “The results demonstrate the potential of LBIC phase mapping as a sensitive local diagnostic technique for identifying variations in interconnection quality.”
“A central contribution of our work is the development of an analytical equivalent-circuit model and the derivation of an equation describing the relationship between the measured LBIC phase shift and the electrical parameters of the solar cell,” said co-author Majid Salari. “In particular, the equation establishes how changes in local series resistance (Rs) influence the phase response. This provides a physical and analytical basis for interpreting LBIC phase maps in terms of local electrical properties, rather than relying solely on empirical correlations between phase shifts and defects.”
For their experiments, the scientists used an 828 nm laser with a power of 22 μW, a spatial resolution of 6 μm, and a typical penetration depth of 15 μm in silicon. The laser was modulated at 640 Hz to generate an alternating photocurrent, which was measured using lock-in amplifiers to minimize noise and improve precision.
They explained that although the photocurrent oscillates at the same frequency as the laser illumination, a phase shift occurs between the two signals. This shift reflects the electrical impedances encountered as photogenerated charge carriers travel through the cell and are collected at its contacts. Analyzing these phase shifts provides additional information about local electrical properties, including series resistance and contact quality.
The team used 156.75 mm × 156.75 mm monocrystalline bifacial silicon solar cells with five busbars, supplied by Chinese manufacturer Lightway Solar. The cells were cut in half using laser scribing followed by mechanical cleaving, and their internal quantum efficiency (IQE) and phase shifts were mapped using LBIC measurements.
The half-cells were then laminated using The Tape Solution (TTS), an interconnection method developed by Sweden-based Sticky Solar Power. The process involves attaching low-temperature solder-coated wires to both sides of the cells using transparent tapes, followed by simultaneous lamination and soldering.
To assess interconnection quality, the researchers deliberately reduced the contact area between wires and busbars in selected regions by applying less pressure. They then compared LBIC phase maps recorded before and after lamination to identify changes in electrical contact quality. They noted that although the five-busbar M2 cells are no longer representative of current industrial standards, they provided a simplified platform for validating a methodology that could also be applied to larger solar cells.
The experiments showed that phase shifts became more negative after lamination, consistent with increased local Rs associated with the interconnection wires. Regions with high-quality contacts showed average phase differences of around 150 millidegrees, compared with approximately 250 millidegrees in areas with deliberately weakened contacts. Optical microscopy and scanning electron microscopy (SEM) supported these findings, revealing nearly continuous connections between solder and silver pads in well-connected regions and visible gaps in areas with poorer contacts.
The researchers also compared LBIC phase mapping with electroluminescence (EL) imaging, finding that the latter could not easily distinguish the localized resistance variations detected by LBIC. They said the results highlight the technique’s sensitivity to interconnection defects and its potential for diagnosing electrical performance losses in laminated solar modules.
Moreover, the researchers attributed the increased LBIC phase shifts after lamination primarily to higher local Rs, based on the proposed equivalent-circuit model. By measuring the same cells under identical conditions before and after lamination, they minimized variations arising from the measurement setup. Regions with deliberately weakened contacts showed significantly larger phase shifts than those with better connections, a finding also supported by cross-sectional microscopy.
“We acknowledge that possible changes in parallel capacitance or other effects cannot be completely excluded,” said Rinio. “However, our analysis indicates that the changes in local Rs are the primary explanation for the observed behavior.”
The researchers also found that LBIC phase mapping could distinguish between well-connected contacts and those with reduced contact areas or discontinuous wire connections, with the latter showing larger phase shifts associated with higher local series resistance. Completely disconnected contacts would be expected to produce even stronger electrical disturbances, provided other contacts remain functional. However, the technique has not yet been systematically tested across the full range of contact conditions.
“Therefore, we would not claim that the current study establishes a definitive distinction between partially soldered and completely disconnected contacts,” Salari explained.
The research findings are presented in the study “Probing interconnection defects in solar modules via LBIC phase mapping,” published in Solar Energy Materials and Solar Cells.
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