Swedish researchers show LBIC phase mapping detects solar interconnection defects

Researchers at Karlstad University in Sweden have demonstrated a new approach to detecting interconnection defects in half-cell solar modules.

Markus Rinio, the corresponding author of the study, stated that their experiments showed the method can detect variations in contact quality.

Measurement Methodology

The research team employed laser-based beam induced current (LBIC) phase mapping to analyze the modules. This technique relies on modulated illumination, which allows researchers to measure the photocurrent phase shift generated by the solar cells.

The experimental setup utilized a laser operating at a wavelength of 828 nm with a power output of 22 μW. The system achieved a spatial resolution of 6 μm, enabling precise targeting of specific interconnection areas. The laser light penetrated the material to a depth of 15 μm, ensuring interaction with the relevant layers of the half-cell module. To capture the dynamic response of the photocurrent, the illumination was modulated at a frequency of 640 Hz.

Majid Salari, a co-author on the paper, noted that a central contribution of the work is the development of an analytical equivalent-circuit model. This model provides the necessary mathematical basis for translating the raw phase shift measurements into meaningful indicators of contact quality, thereby supporting the detection of interconnection defects identified in the experiments.

Test Sample Specifications

• The experimental samples consisted of monocrystalline bifacial silicon solar cells measuring 156.75 mm × 156.75 mm.
• Each cell featured a five-busbar design.
• The cells were supplied by Lightway Solar.
• Interconnections were established using The Tape Solution (TTS) manufactured by Sticky Solar Power.

Results and Limitations

The experiments revealed distinct differences in electrical response between healthy and compromised connections. High-quality contacts exhibited a phase shift of 150 millidegrees, whereas weakened contacts registered 250 millidegrees. The phase shift also became more negative following the lamination process. This change is attributed to an increase in local series resistance within the module structure after the encapsulation layers were applied.

When compared to electroluminescence (EL) imaging, LBIC phase mapping demonstrated an ability to detect resistance variations that EL imaging could not easily distinguish.

However, the study acknowledges certain limitations in isolating the exact physical mechanisms behind the observed signals. While increased series resistance explains the post-lamination phase shift, the researchers note that possible changes in parallel capacitance or other effects were not completely excluded.

Cell Preparation and Microscopy Validation

Researchers at Karlstad University fabricated the test specimens by bisecting Lightway Solar cells via laser scribing and subsequent mechanical cleaving. Prior to lamination, the internal quantum efficiency (IQE) and phase shifts of these half-cells were mapped. To evaluate the quality of interconnections, the team intentionally diminished the contact area between wires and busbars in specific zones by applying reduced pressure during the TTS process.

LBIC findings were corroborated by optical microscopy and scanning electron microscopy (SEM). These imaging methods showed nearly unbroken links between solder and silver pads in areas with good connections, whereas distinct gaps were visible in regions with inferior contacts. The investigators observed that although five-busbar M2 cells no longer reflect current industrial norms, they offered a simplified framework for verifying a method applicable to larger solar cells.

By testing the identical cells under consistent conditions both before and after lamination, the team reduced variability stemming from the measurement setup. Rinio recognized that while the data suggests local series resistance changes are the main cause of the observed phenomena, potential shifts in parallel capacitance or other factors cannot be entirely ruled out. The research further indicated that fully severed contacts would likely generate more pronounced electrical disturbances, assuming the remaining contacts stay operational.