Micro crack detection of multi‐crystalline silicon solar wafer using machine vision techniques

Abstract

Purpose -The detection of invisible micro cracks (m-cracks) in multi-crystalline silicon (mc-si) solar wafers is difficult because of the wafers' heterogeneously textured backgrounds. The difficulty is twofold. First, invisible m-cracks must be visualized to imaging devices. Second, an image processing sequence capable of extracting m-cracks from the captured images must be developed. The purpose of this paper is to reveal invisible m-cracks that lie beneath the surface of mc-si solar wafers. Design/methodology/approach -To solve the problems, the authors first set up a near infrared (NIR) imaging system to capture images of interior m-cracks. After being able to see the invisible m-cracks, a region-growing flaw detection algorithm was then developed to extract m-cracks from the captured images. Findings -The experimental results showed that the proposed m-cracks inspection system is effective in detecting m-cracks. In addition, the system can also be used for the inspection of silicon solar wafers for stain, pinhole, inclusion and macro cracks. The overall accuracy of the defect detection system is 99.85 percent. Research limitations/implications -At present, the developed prototype system can detect m-crack down to 13.4 mm. The inspection resolution is high but the speed is low. However, the limitation on inspection speed can easily be lifted by choosing a higher resolution NIR camera. Practical implications -Generally, this paper is a great reference for researchers who are interested in developing automatic optical inspection systems for inspecting solar wafer for invisible m-cracks. Originality/value -The research described in this paper makes a step toward developing an effective while low-cost approach for revealing invisible m-crack of mc-si solar wafers. The advantages provided by the proposed system include excellent crack detection sensitivity, capability of detecting hidden subsurface m-cracks, and low cost.

Review of Microcrack Detection Techniques for Silicon Solar Cells

“…In this paper, we have classified cracks as either macro-or microcracks (μ-cracks) according to the crack width sizes. The crack with size smaller than 30 μm in width is usually referred to as a μ-crack [39]. The cracks are further classified according to their position as either facial or subfacial cracks.…”

“…They did not report the minimum microcrack size that can be detected using this approach. The PL imaging system proposed by Yih-Chih et al [39] has been used to detect invisible subfacial microcracks down to 13.4 μm. Image processing was used to extract the microcracks.…”

Defect detection in multi-crystal solar cells using clustering with uniformity measures

“…It can detect only the specific saw-mark defects. Chiou and Liu [16] used a near infrared imaging system to highlight micro-cracks in solar wafers. The method works only when the sensed microcrack is significantly darker than the crystal grains.…”

IR Reflectance Imaging for Crystalline Si Solar Cell Crack Detection

“…A great deal of effort has been directed to this end, where a variety of techniques including resonance ultrasonic vibration analysis [7], electroluminescence [8], and photoluminescence imaging [9] have been developed and demonstrated. The use of nIR backlighting and front side imaging has also been suggested as a viable screening method [10]. Furthermore, several infrared thermography-based approaches have been proposed: radiometric pulse and thermal imaging under various cell voltage and light bias conditions [11], lock-in thermography incorporating temperature cycling and voltage biasing [12], and a thermography technique aimed to assess edge leakage currents from periodic surface temperature oscillations following laser scribing and cleavage [13].…”