Sample Preparation and Microstructural Analysis of Solar Cells

Table of Contents

Introduction
Common Anomalies
Sampling Techniques
Sample Mounting
Sample Grinding
Sample Polishing
Final Polishing
Metallographic Examination
Silicon Ingot Sample Preparation
Electron Backscatter Diffraction and Vibratory Polishing
Conclusion
About Buehler

Introduction

Development and commercialization of photovoltaic (PV) cells continues at a rapid pace. It is important for manufacturers to produce highly reliable and long lasting solar panels. Cross sectional microscopic analysis can play an important part in increasing cell or module dependability. Areas that can be assessed include material inspection,  interconnect verification and coating analysis. In this technical note, sample preparation techniques for good quality cross section of crystalline silicon solar cells are described to enable identification of soldering process quality and degradation mechanisms.

Common Anomalies

Conducting cross sectional analysis on wafers, microelectronic electric components, and raw materials show the parameters as listed below:

  • Joint cracks
  • Solder porosity & microvoids
  • Impurities
  • Poor solder wetting
  • Solder thickness and delamination
  • Unacceptable solder, silicon, and copper microstructure
  • Solder meniscus
  • IM-Phases, coarsening, etc.

Sampling Techniques

A bus bar, copper ribbon and a solder joint can be extracted from a cell to prepare micro-sections. As shown in Figure 1, solar cells are linked in series on the front and back sides with copper solder coated ribbons, which are soft soldered to the bus bar material. This paste-like bus bar material normally consists of silver and glass particles. The silver component provides electrical conductivity and solderability. As shown in Figure 2, the bus bar material is hardened in an oven to bond it to the cell before applying the solder-coated ribbons. Solder joints need to be of high quality to ensure extended service life and high electrical conductivity.

Figure 1. Solar cells connected in series with tabbing ribbons.

Figure 2. Interconnector on a soldered solar cell sample.

Solar cells are brittle and can be easily damaged during sample preparation. In order to avoid cracking and chipping during sectioning, a quick drying resin called SamplKwick is applied by a brush by just painting the bus lines on either sides of the cell as shown in Figure 3. Sectioning is conducted on polycrystalline or monocrystalline solar cells to scale-down the sample size and to the area of interest i.e. the solder joint (figure 4). This smaller size also facilitates mounting, grinding and polishing.

Figure 3. Front side, left (sunnyside side or emitter) and back side of a polycrystalline silicon solar cell.

Figure 4. Drawing of a solder interconnect of a solar cell bus line.

After curing of the acrylic resin covering the bus lines, the complete cell, 157mm x 157mm x approximately 0.2mm in size, is positioned in a solar cell camping device, specifically the Buehler #11-2706 holder used in the IsoMet® 4000 Linear Precision Saw as shown in Figure 5

Figure 5. Buehler solar cell holder for IsoMet® 4000 and 5000 Linear Precision Saws

Firstly, the entire bus bar is sectioned and then it is cut cross-wise into a size suitable for mounting i.e. about 1cm2 samples using an IsoMet diamond wafering blade LC-15, with a cutting speed of 3,750 rpm and a feed-rate of 5mm/minute as shown in Figure 6.

Figure 6. Sectioning the bus bar into samples appropriate for mounting. The first cut removes the bus bar trace. A second cut is made perpendicular to the bus bar section.

Sample Mounting

After sectioning, they are mounted in epoxy. A UniClip support clip as shown in Figure 7 is used to align the sample vertically in the SamplKup mounting cup. Mounting starts with the PV solar cell sample cast in an epoxy resin EpoxiCure as shown in Figure 8. In case, additional edge support is needed, the epoxy can be blended with flat edge filler as shown in Figure 9. Flat edge filler comprises globular oxide ceramic particles that increase the mount hardness and display proof of reduced shrinkage.

Figure 7. Solar cell with interconnector held perpendicular to the cross section with a UniClip Support Clip before epoxy mounting.

Figure 8. Solar cell interconnects mounted in EpoxiCure® epoxy.

Figure 9. EpoxiCure® mount solar cells interconnect with Flat Edge Filler (white layer) added for improved edge retention.

Sample Grinding

After mounting, the samples are mechanically ground and polished. Planar grinding is done using CarbiMet 2, 320 grit sized SiC grinding paper. After each grinding stage, the samples and machine parts need to be cleaned with tap water. This is important minimize the possibility of any abrasive being carried taken over to the next stage which could cause scratches forming on the sample.

Sample Polishing

Further refining of samples is done by polishing. With Si cells, four stages are conducted including polishing with diamond and alumina abrasives charged on a polishing cloth. In each stage a finer abrasive lubricated with MetaDi Fluid is used.

Final Polishing

The final step of the standard method uses an exceptionally fine abrasive (0.02 µm) silica for polishing to remove any fine scratches remaining from the previous step. Vibratory polishing used with a low nap polishing cloth and a colloidal silica suspension will remove very light scratches and remaining surface damage. Figures 10 and 11 show the results of vibratory polisher on an SAC 305 solder alloy. The entire sample preparation method and parameters are listed in Table 1.

Figure 10. Unetched SAC 305 Solder Alloy.

Figure 11. Unetched SAC 305 Solder Microphotograph after Vibratory Polishing.

Table 1. Sample Preparation of Polycrystalline Solar Cell Solder Interconnect

Step Surface Lubricant Abrasive Size/Type Time Min : Sec Force Lb (N) Base / Head rpm Head Direction
1. Surface Grind

CarbiMet® 2 SiC Paper

Water

320 (P400) grit

Untill plane

5 lb (22)

230/60

Contra

2.

UltraPad™

MetaDi®

9-μm MetaDi® Supreme Diamond

5:00

5 lb (22)

130/60

Contra

3.

TexMet™

MetaDi®

3-μm MetaDi® Supreme Diamond

4:00

5 lb (22)

130/60

Contra

4.

MicroCloth®

MetaDi®

0.05-μm MasterPrep® Alumina Suspension

3:00

5 lb (22)

130/60

Contra

5.

ChemoMet®

MetaDi®

0.02-μm MasterMet® 2 Colloidal Silica Suspension

2:00

5 lb (22)

150/60

Contra

6. Optional

VibroMet® 2 Vibratory Polisher; MicroCloth®

N/A

0.06-μm MasterMet® Colloidal Silica Suspension

≥60

N/A

N/A

N/A

Metallographic Examination

To ensure quality during solar modular production, the examination of soft solder joints is required. Solder interconnect quality is important to extend the solar cell life. A cross section of the ribbon interconnect is displayed in Figure 12. In Figure 13, the solder encapsulated copper ribbon shows insufficient solder at each end. The other solder layers and bus bar need to be measured for accurate thickness.

Figure 12. Microstructure of Sn62-Pb36-Ag02 solder, unetched solar cell copper ribbon.

Figure 13. The entire solar cells interconnect shows both the top and bottom soldered seams. The solder is not uniform and lacking at each end of the bottom seam.

Soldering of the solder coated ribbon to the bus bar is done by an alloy process known as wetting. Proper wetting is required for strong stable connections. Figure 14 shows an acceptable solder wetting angle between the coated copper ribbon and the bus bar. Figure 15 shows an improper wetting angle having voids in the solder.

Figure 14. Proper wetting angle of soldered copper ribbon.

Figure 15. Both solder interconnects show an insufficient wetting angle and several voids in the solder layer.

Accelerated aging of a solder interconnect is done on a bismuth lead-free solder. The SEM images in Figure 16 shows intermetallic phase growth of the solder above the bus bar before and after the aging process. The microstructure and their mechanical properties will constantly form on exposure to accelerated aging. It is important to understand the behavior and effects on the solder joint to enable a better knowledge of how to extend the life of these interconnects.

Figure 16. SEM image showing the intermetallic phase growth after accelerated aging. From the top, Silicon cell, (a.) Bismuth containing solder on (b.) bus bar.

Silicon Ingot Sample Preparation

Microstructural analysis on amorphous silicon ingots used for the fabrication of cell wafers is critical to maintaining better performance. Characterization of materials for quality purposes can reveal structure, impurities, grain size, annealing twins and provide a record of its process for future reference.

Firstly material removal is done for the ingot. Sectioning needs to be done with much care to minimize any mechanical and thermal damage.

Next the sample is compression mounted using a SimpliMet 1000 automated mounting press. EpoMet F molding compound is used for the mount media since it has high hardness and good edge support. After mounting the sample is prepared using the steps listed in Table 2. Figures 17 and 18 show light optical images of high purity Si ingot material etched with NaOH.

Table 2. Preparation Method for Pure Silicon

Surface Abrasive Size/Type Force lb. (N) Speed rpm/direction Time (minutes)
CarbiMet® 2

240-grit SiC (water cooled)

5 (22)

240 Contra

Until Planar

CarbiMet® 2

320-grit SiC (water cooled)

5 (22)

240 Contra

1

UltraPol® silk

9-μm Diamond (MetaDi® Fluid)

5 (22)

150 Contra

10

TriDent®

3-μm Diamond (MetaDi® Fluid)

5 (22)

150 Contra

8

TriDent®

1-μm Diamond (MetaDi® Fluid)

5 (22)

150 Contra

5

MicroCloth®

0.05-μm MasterMet® Colloidal Silica

5 (22)

150 Contra

5

MicroCloth®

0.06-μm MasterMet® Colloidal Silica

VibroMet® 2 Vibratory Polisher

≥60

Figure 17. High purity, defect free, polycrystalline silicon viewed with Nomarski DIC (etched with 100mL water, 75 g NaOH).

Figure 18. Polycrystalline silicon viewed with Nomarski DIC (etched: 100mL water, 75 g NaOH)

Electron Backscatter Diffraction and Vibratory Polishing

Electron backscatter diffraction analysis (EBSD) is performed with a scanning electron microscope to obtain analytical data, for orientation of imaging grain size measurements and phase identification. Vibratory polishing with a standard preparation will improve diffraction patterns by increasing the confidence index and band contrast. Minor scratches are eliminated using VibroMet 2 vibratory polisher with MasterMet® colloidal silica suspension on MicroCloth Polishing Cloth. Figures 19 and 20 show EBSD patterns and band contrast for pure silicon, before and after vibratory polishing.

Figure 19. Standard Sample Preparation of Pure Single-Crystal, Band Contrast 205.8.

Figure 20. Standard Sample Preparation Plus Vibratory Polishing of Pure Single-Crystal, Band Contrast 233.

A vibratory polishing video is available for viewing below

Conclusion

Sample preparation techniques of superior quality cross sections of crystalline silicon solar cells are explained to enable identification of soldering process quality and degradation mechanisms.

About Buehler

Buehler is the world's premier manufacturer of scientific equipment and supplies for use in materials analysis.

Buehler products are used throughout the world in manufacturing facilities, quality laboratories, and universities to analyze all types of materials.

Source: Buehler, Solar Cell Microstructural Analysis by D. Maier, P. Schmitt, P. Voos and R Wagner.

For more information on this source please visit Buehler

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