NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Conference Paper

NREL/CP-5K00-76842

June 2020

Advances in Coring Procedures of 

Silicon Photovoltaic Modules

Preprint

Helio Moutinho, Bobby To, Dana Sulas-Kern, ChunSheng Jiang, Mowafak Al-Jassim, and Steve Johnston

National Renewable Energy Laboratory

To be presented at the 47th IEEE Photovoltaic Specialists Conference

(PVSC 47)

June 15–August 21, 2020
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Conference Paper

NREL/CP-5K00-76842

June 2020

Advances in Coring Procedures of 

Silicon Photovoltaic Modules

Preprint

Helio Moutinho, Bobby To, Dana Sulas-Kern, ChunSheng Jiang, Mowafak Al-Jassim, and Steve Johnston

National Renewable Energy Laboratory

Suggested Citation

Moutinho, Helio, Bobby To, Dana Sulas-Kern, Chun-Sheng Jiang, Mowafak Al-Jassim, 

and Steve Johnston. 2020. Advances in Coring Procedures of Silicon Photovoltaic 

Modules: Preprint. Golden, CO: National Renewable Energy Laboratory. NREL/CP-5K00-

76842. https://www.nrel.gov/docs/fy20osti/76842.pdf.

 

 

 

 

 
NOTICE

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable 

Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding 

provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy 

Technologies Office. The views expressed herein do not necessarily represent the views of the DOE or the U.S. 

Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges 

that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce 

the published form of this work, or allow others to do so, for U.S. Government purposes.

This report is available at no cost from the National Renewable 

Energy Laboratory (NREL) at www.nrel.gov/publications.

U.S. Department of Energy (DOE) reports produced after 1991 

and a growing number of pre-1991 documents are available 

free via www.OSTI.gov.

Cover Photos by Dennis Schroeder: (clockwise, left to right) NREL 51934, NREL 45897, NREL 42160, NREL 45891, NREL 48097, 

NREL 46526.

NREL prints on paper that contains recycled content.
1 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

Advances in Coring Procedures of Silicon 

Photovoltaic Modules 

Helio Moutinho

National Renewable Energy 

Laboratory 

Golden, USA

helio.moutinho@nrel.gov 

Bobby To

National Renewable Energy 

Laboratory

Golden, USA

bobby.to@nrel.gov 

Dana Sulas-Kern

National Renewable Energy 

Laboratory

Golden, USA

dana.sulas@nrel.gov

Chun-Sheng Jiang

National Renewable Energy 

Laboratory

Golden, USA

chun.sheng.jiang@nrel.gov

Mowafak Al-Jassim

National Renewable Energy 

Laboratory

Golden, USA

mowafak.aljassim@nrel.gov

Steve Johnston

National Renewable Energy 

Laboratory

Golden, USA

steve.johnston@nrel.gov

Abstract—We describe advances on procedures used for the 

coring of silicon photovoltaic modules. These procedures involve 

coring specific locations, starting from the back of the modules, 

and removing selected parts of the cell structure, while preserving 

the front tempered glass. The standard coring procedures had to 

be modified from case to case due to the different cell structures

and measurement requirements. We provide examples of the 

characterization of the cored samples by scanning electron 

microscopy and energy-dispersive X-ray spectroscopy. Finally, we 

describe a simple process to create small openings at specific 

locations on the back of photovoltaic modules to allow for 

efficiency-related measurement of individual cells and groups of 

cells.

Keywords—coring, silicon, photovoltaic modules.

I. INTRODUCTION

The progress in solar cell fabrication in the last decades has

resulted in increased efficiencies, higher stability, and lower 

prices. Consequently, photovoltaic (PV) technology has gained 

significant ground in relation to other types of energyproduction methods. Nowadays, depending on the specific 

application, the use of PV modules may be economically viable. 

Furthermore, with the increase in the effects of global warming, 

PV technology has become a major source of alternative energy 

to reduce the emission of greenhouse gases.

Because of the exponential increase in the number of PV 

module installations, several areas are receiving increased 

interest, such as: batteries, needed for the storage of the energy 

generated by the PV modules in many applications; soiling

mitigation, to minimize the negative effects of weather on the 

performance of PV modules; and reliability, to guarantee that PV

modules will provide satisfactory performance for many decades. 

However, most studies on reliability are limited and mostly focus 

on the degradation of parameters related to efficiency and power 

output. Currently, there is very little study on the relation between 

these parameters and microscopic parameters such as 

composition or defect structure, which would provide insight on 

how to produce more efficient, reliable, and durable PV modules.

This task is very important for improving the performance and 

reliability of the next generation of photovoltaics. 

To make the connection between macroscopic and 

microscopic parameters, we must develop techniques to remove 

small specific areas of PV modules, without damaging them, 

that can be analyzed by techniques such as scanning electron 

microscopy (SEM), scanning probe microscopy (SPM), and 

more. Developing this technology is of prime importance for the 

task above. Unfortunately, there is very limited literature on this 

subject, and we had to develop several techniques to core small 

areas from PV modules without affecting the properties of the 

solar cell material. These techniques have been described in 

previous publications [1,2]. Furthermore, because of the 

different kinds of module structure and measurement 

requirements, no procedure exists that works for every module. 

In this work, we present advances on the coring techniques that 

allowed us to 1) avoid damaging the thin back-contact layer on

cored HIT Si modules, 2) core Si modules for the study of 

cracks, and 3) open connection areas for measuring individual 

cells and groups of cells in PV modules. 

II. EXPERIMENTAL PROCEDURE

We previously developed two different coring procedures: 

full and partial coring [1,2]. In full coring, small pieces of the 

module are removed from pre-selected areas using a special drill 

and diamond-embedded bits. Because most PV modules use 

tempered glass on the front, this procedure results in breakage 

of the whole module, and we want to avoid that whenever 

possible. In partial coring, we core from the back side of the 

module and stop just before we reach the tempered glass so as

to avoid breaking it. Here is a simplified description of the 

partial coring procedure for a standard Si module: 

• Select and mark areas to be cored.

• Mechanically remove the Tedlar polyester Tedlar

(TPT) and ethylene vinyl acetate (EVA) layers from 

the back of the Si module on areas to be cored using a 

Dremel tool, avoiding any significant increase in 

temperature. 
2 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

• Partially core the back of the module (until the front

EVA layer is reached) using a 1⁄2” or 3⁄4” coring bit and 

coolant/lubricant. 

• Attach metallic posts to the cored areas using acetonesoluble adhesive. 

• Wait several hours for the adhesive to cure.

• Remove the posts from the module using an open-end 

wrench. The solar cell separates from the module on 

the interface with the front EVA.

• Immerse posts with Si solar cell in acetone for about a day. 

• Remove Si samples from metallic posts.

For cross-section analysis, several samples were 

encapsulated between two pieces of high-conductivity silicon 

using Ag-conductive epoxy (EPO-TEK H20E) and polished in 

a MultiPrep mechanical polisher (Allied High Tech Products), 

using diamond-lapping-film pads with different grits. 

Samples produced in this work were analyzed by SEM and 

energy-dispersive X-ray spectroscopy (EDS) in a Hitachi 4800 

FEG SEM and Thermo Scientific UltraDry EDS detector, 

respectively.

III. RESULTS AND DISCUSSION

SI HIT MODULES

We applied the partial coring procedure on two HIT 

modules, one kept in dark and one deployed in the field for 10

years, to study the causes of degradation. The structure of the 

modules is seen in Fig. 1. The standard Si modules in which we 

previously applied our coring procedure have a relatively thick 

continuous Al layer as part of the back contact, and 

mechanically removing the back TPT and EVA is relatively 

easy. In contrast, the HIT modules use a very thin layer of 

indium tin oxide (ITO) and thin Ag fingers. Removing the TPT 

and EVA without affecting the back contact posed a challenge 

for coring. Furthermore, the degradation of these modules was 

possibly caused by deterioration of the ITO, which made it more 

important to preserve this layer during coring.

We started by applying the standard procedure, and there was 

not enough precision to remove the TPT and EVA while 

preserving the Ag lines, as was visually clear. Further EDS 

analysis (Fig. 2) showed that the thin ITO layer was also removed 

during this process, as well as part of the Si on the back. Preserving 

a thin layer of about 100 nm is very difficult, and most of our 

attempts—such as applying temperature to soften the EVA layer—

failed because they require applying hundreds of degrees to the 

solar cell material, which could induce changes, such as diffusion 

of impurities. After many experiments, we were able to develop 

two new coring methods that preserved the back ITO layer.

The first method required designing and fabricating much 

thinner metallic stubs that could fit into the chamber of 

analytical equipment, such as an SEM. Initially, we cored the 

module until we just touched the back EVA, on a larger area 

than desired for the final cored sample (Fig. 3); then we applied 

a large-diameter stub (1”) with adhesive directly onto the TPT. 

Fig. 1. Structure of the cored HIT modules.

Backsheet

ITO (~100 nm)

n-type a-Si:H (~10 nm)

Intrinsic a-Si:H (~10 nm)

n-type c-Si:H (~160 μm)

Intrinsic a-Si:H (~10 nm)

p-type a-Si:H (~10 nm)

EVA

ITO (~100 nm)

EVA

Glass

Fig. 2. EDS analysis of the back of the cored cell showing that the 

ITO layer was removed during the mechanical removal of TPT 

and EVA. The inset (top right) shows the In peaks of the ITO 

layer on the front of the cored cell.

Fig. 3. Back of a cored HIT module showing the initial larger 

cored area, with the back EVA still in the module; the cored area, 

with the front EVA at the center; and the cored sample.
3 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

With an open-ended wrench, we were able to remove the TPT,

leaving just the back EVA on the module. After that, we cored 

inside that area with a 3⁄4”-diameter drill until we reached the 

glass, and then we applied the thin metallic stub with adhesive. 

After curing, we used the open-end wrench to remove the cored 

sample from the module, as observed in Fig. 3. SEM and EDS

analyses showed that the whole sample was cored, but this 

method produces samples that cannot be easily removed from 

the metallic stub. This is not an issue for analytical techniques 

that can examine deep into the material, such as secondary-ion 

mass spectrometry, which has a depth resolution of up to ~2 nm

[3]. However, it does not allow for the analysis of the cross 

section of the sample, unless special methods such as beveling 

are used for sample preparation. 

The second coring method allowed for the easy analysis of 

cross sections and was also developed for analytical techniques 

that require access to the back contact, such as several different

SPM measurements. In this case, we noticed that for this type of 

module, we were able to carefully peel off the TPT and back

EVA using tweezers. This is a delicate process, but after 

practicing for some time, we were able to consistently remove 

the back material of the HIT modules while preserving the Ag 

fingers and ITO, as demonstrated in SEM and EDS analyses. 

After this, the module was cored using the standard procedure 

with excellent results.

EDS and SEM analyses on the back of these samples 

confirmed that both the back Al gridlines and ITO were still in 

the sample after coring using this method. 

We were able to core samples from both modules, which are 

currently being analyzed by many different analytical 

techniques. Several of these samples were used in one paper that 

will be presented in this meeting [4]. 

INVESTIGATION OF SOLDER IN SI PV MODULES

In this study, our interest was in investigating the solder in 

Si modules deployed in the field for several years. These 

modules showed strong fill factor degradation that was 

associated with an increase in series resistance [5]. The 

objective was to investigate the existence of cracks in the solder 

and/or the formation of different compounds on the Ag contact.

Coring of the solder region presented a particular challenge 

because of the difference in height between the Al back contact and 

the ribbon on the back of the PV module. In the standard 

procedure, when we need to core an area around a ribbon, we 

simply remove it to make the area flat. Applying the standard 

procedure without removing the ribbon did not work because after 

coring, there was a gap between the bottom of the metallic stub and 

the back contact that was too large to be filled by the adhesive, as 

observed in Figs. 4(a) and 4(c). The solution for this problem was 

to design a special metallic stub (Fig. 4b) with a groove to 

accommodate the ribbon and eliminate the empty space (Fig. 4d). 

Coring was performed following the standard procedure. 

However, many of the front ribbons detached from the cored 

samples during removal from the module, as observed in Fig. 

5(a). Probably, the thick, relatively low-speed coring drill did not 

completely cut the ductile Cu ribbon in the front of the module, 

and the Cu ribbon on the cored sample was pulled out by the part 

of the ribbon that stayed on the module during the sample 

removal step. Because the module uses tempered glass, a single 

cut will result in the whole module breaking in thousands of small 

pieces [6]. To prevent this from happening, we stop coring as 

soon as the tempered glass is reached— which, in this case, was 

not enough to completely cut the front Cu ribbon.

The solution for this problem was to use a small, thin, highspeed rotating saw to cut the module from the top of the back 

Cu ribbon down to the front Cu ribbon on the exact locations 

where the coring drill was going to cut the ribbons. The rotating 

saw was stopped as soon as the front EVA was reached and 

before it could cut the glass. These cuts are observed in Fig. 

5(b) from the back of the module. The ribbon is soldered to the 

back of the module in three places only, and part of it detached 

from the module after the cuts, as observed in the figure. With 

the front ribbon cut at the coring position, we resolved the 

delamination issue. 

After these adjustments, and the new metallic stub, we were 

able to successfully and repeatedly core the module, as 

observed in Fig. 5(c), which shows a cored area on the module, 

with the front EVA, and the cored solar cell, with the front 

ribbon still attached to it.

Fig. 4. (a)(b) Standard sample holder and new design, respectively; 

(c)(d) Configuration of the standard and new setup for stub 

attachment to the sample, respectively.

Fig. 5. (a) Cored cell with detached Cu ribbon after removal from 

the module; (b) Back of the module with cuts at locations to be 

cored; (c) Back of the module after coring and removal of cored 

sample. 

(c)

(b) (a)
4 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

As mentioned before, the objective of coring this module

was to analyze the cross section in the region around the solder 

location. For this purpose, we needed to protect the solder 

regions, including the ribbons, during sample preparation. This 

was accomplished by cutting the cored samples with pliers in 

the region close to the ribbon and encapsulating and polishing 

them. The polishing was stopped as soon as the ribbon and 

solder region were reached, as revealed by an optical 

microscope. The structure of such a sample is shown in Fig. 6, 

where we observe the region from where the sample was 

prepared (a) and the cross section showing the different layers.

The cracks observed in the Si cell may or may not have been 

introduced during coring and are under investigation. The EDS 

analysis showed a significant amount of Sn in the Ag back and 

front contacts, compatible with the formation of Ag3Sn, which 

has been shown to increase the series resistance of the module

[7]. The initial findings of this work have been presented

elsewhere [8]. 

CONTACTS FOR EFFICIENCY-BASED MEASUREMENTS

Finally, we describe a procedure to place small openings at 

different locations directly over the bus bars, to allow for the 

measurement of the cell parameters of individual cells on PV 

modules. For this purpose, we use a special bit installed on a 

Dremel tool that cuts down into the back of the PV module. 

Initially, we tried to control the size and depth of the cut by 

simply holding the Dremel tool, but it proved to be unreliable 

and often the bit would go deeper than desired. To avoid this 

problem, we use a special device to hold the drill exactly over 

the place where the opening will be made. At the same time, the 

device limits the depth that can be reached by the drill, so there 

is no damage of the module. The schematic of this procedure is 

shown in Fig. 7. Several openings were cut on many different 

locations of individual solar cells in the module described in the 

previous section. This allowed us to measure efficiency-related 

parameters of individual cells, as well as groups of cells, and to 

study how the power output varied at different locations of the 

module and how the degradation was distributed. These 

measurements can identify areas to be cored and allow for the

correlation between microscopic and macroscopic properties 

measured on different locations of the module. 

CONCLUSIONS

Due to differences in PV module design and analysis 

requirements, standard coring procedures must be constantly 

modified. In this work, we describe examples of modifications

in our standard coring procedure that allowed us to core different 

types of modules. For instance, we were able to core HIT 

modules while preserving the thin ITO back contact, as well as 

areas with bus bars and Cu ribbons on standard Si modules to 

study the structure and composition of the solder regions. 

Finally, we described a simple procedure to allow for measuring

efficiency-related parameters on individual cells and groups of 

cells in PV modules.

ACKNOWLEDGMENT

This work was authored by the Alliance for Sustainable 

Energy, LLC, the manager and operator of the National 

Renewable Energy Laboratory. The views expressed in the 

article do not necessarily represent the views of the DOE or 

the U.S. Government. The U.S. Government retains and the 

publisher, by accepting the article for publication, 

acknowledges that the U.S. Government retains a 

nonexclusive, paid-up, irrevocable, worldwide license to 

Fig. 6. (a) Silicon sample after coring. (b) Optical microscopy

image showing the several layers of the cross section of the cored 

sample. (c) SEM image of the cross section showing the location 

of the Ag and solder layers. Cracks extending from the bottom to 

the top of the Si solar cell are clearly visible.

Fig. 7. (a) Configuration of the procedure to make openings on 

the back of PV modules. (b) Higher magnification showing the 

removed material. (c) Openings on the module and the 

measurement schematic.
5 

This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.

publish or reproduce the published form of this work, or allow 

others to do so, for U.S. Government purposes.

REFERENCES

[1] H.R. Moutinho, S. Johnston, B. To, C.-S. Jiang, C. Xiao, P. 

Hacke, J. Moseley, J. Tynan, N.G. Dhere, and M.M. Al-Jassim, 

“Development of coring procedures applied to Si, CdTe, and 

CIGS solar panels,” Solar Energy 161 (2018) 235–241.

[2] H.R. Moutinho, B. To, C.-S. Jiang, C. Xiao, C.P. Muzzillo, P. 

Hacke, J. Moseley, J. Tynan, N.G. Dhere, L. Mansfield, M.M. AlJassim, and S. Johnston, Artifact-free coring procedures for 

removing samples from photovoltaic modules for microscopic 

analysis, Proc. World Conf. Photov. En. Conv. (Waikoloa, HI, 

2018) p. 1313.

[3] P. Chakraborty, “Ultra-high depth resolution SIMS for the

interfcace analysis of complex low-dimensional structure,” Nucl. 

Instrum. Meth. Phys. Res. B 266 (2008) 1858.

[4] S.M.F. Zhang, D.B. Sulas-Kern, R. (Rob) L. Chin, A. Mahboubi 

Soufiani, D.C. Jordan, H.R. Moutinho, I. Perez-Wurfl, and Z. Hameiri, “Investigation of HIT module degradation: A postmortem approach,” To be presented at the 47th IEEE Photov. 

Spec. Conf. (June 15-August 21). Virtual meeting.

[5] D.B. Sulas-Kern, S. Johnston, and J. Meydbray, “Fill factor loss 

in fielded photovoltaic modules due to metallization failures, 

characterized by luminescence and thermal imaging,” 2019 IEEE 

46th Photovoltaic Specialists Conference (PVSC), Chicago, IL, 

USA, 2019, pp. 2008-2012, doi: 

10.1109/PVSC40753.2019.8980840.

[6] https://www.scienceabc.com/eyeopeners/tempered-laminatedcar-windshield-glass-why-break-such-small-pieces.html

[7] T.L. Yang, K.Y. Huang S. Yang, H.H. Hsieh, and C.R. Kao, 

“Growth kinetics of Ag3Sn in silicon solar cells with a sintered 

Ag metallization layer,” Sol. En. Mater. Sol. Cells 123 (2014) 

139.

[8] D.B. Sulas-Kern, S. Johnston, D.C. Jordan, H. Moutinho, M. 

Young, J. Meydbray, C.S. Jiang, M. Al-Jassim, A. Norman, M.A. 

Alam, R. Asadpour, M. Owen-Bellini, K. Terwilliger, A. Sinha, 

L. Schelhas, R. Bhoopathy, S. Zhang, and Z. Hamieri 

“Degradation mechanisms in fielded modules: Imaging for 

identification of resistance effects,” presented at the Photovoltaic 

Reliability Workshop (PVRW), Lakewood, CO, USA, 2020.