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.

Contract No. DE-AC36-08GO28308

CIGS P1, P2, P3 Scribing 

Processes using a Pulse 

Programmable Industrial Fiber 

Laser

Preprint 

M. Rekow, R. Murison and T. Panarello

PyroPhotonics Lasers, Inc.

C. Dunsky 

Aeos Consulting, Inc.

C. Dinkel and S. Nikumb

NRC Canada

J. Pern and L. Mansfield

National Renewable Energy Laboratory

Presented at the 25th European PVSEC/WCPEC-5 

Valencia, Spain

September 6-10, 2010 

Conference Paper

NREL/CP-5200-49411

October 2010
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1 

CIGS P1, P2, P3 Scribing Processes using a Pulse Programmable Industrial Fiber Laser

M. Rekow1

, R. Murison1, C. Dunsky2, C. Dinkel3, J. Pern4, L. Mansfield4, T. Panarello1, and S. Nikumb3

1

PyroPhotonics Lasers, Inc. 275 Rue Kesmark, Dollard-des-Ormeaux, Québec H9B 3J1, Canada 2

Aeos Consulting, Inc. 72 Broadway, Los Gatos, CA 95030, USA 3

NRC Canada, Industrial Materials Institute-CAMM, 800 Collip Circle, London, Ontario NG6 4X8 Canada 4

National Center for Photovoltaics, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 

80401, USA

ABSTRACT: We describe a novel set of laser processes for the CIGS P1, P2 and P3 scribing steps, the development of 

which has been enabled by a unique pulse-programmable fiber laser. We find that the unique pulse control properties of this 

1064 nm wavelength laser have significant effects on the material removal dynamics of the various film layers in the CIGS 

material system. In the case of the P2 and P3 processes, the shaped pulses create new laser/material interaction effects that 

permit the material to be cleanly and precisely removed with zero Heat Affected Zone (HAZ) at the edges of the scribe. The 

new P2 and P3 processes we describe demonstrate the first use of infrared nanosecond laser pulses that eliminate the HAZ 

and the consequent localized compositional changes in the CIGS absorber material that result in poor shunt resistance. SEM 

micrographs and EDX compositional scans are presented. For the P1 scribe, we process the bi-layer molybdenum from the 

film side as well as through the glass substrate. Microscopic inspection and compositional analysis of the scribe lines are not 

sufficient to determine electrical and optical performance in working PV modules. Therefore, to demonstrate the 

applicability of the infrared pulse-programmable laser to all three scribing processes for thin-film CIGS, we fabricate smallsize multiple-cell monolithically interconnected mini-modules in partnership with the National Renewable Energy 

Laboratory (Golden, Colorado). A total of four mini-modules are produced, two utilizing all laser scribing, and two with the 

P2 and P3 steps mechanically scribed (by a third party) for reference. Mini-module performance data measured at NREL is 

presented, and we also discuss the commercialization potential of the new single-laser CIGS scribing process. Finally we 

present a phenomenological model to describe this physics underlying this novel ablation process.

Keywords: CIGS, Thin Film Solar Cell, Laser Scribing, P2, P3

1 Background

CIGS is rapidly gaining ground as preferred material 

system for thin-film photovoltaics (PV). Among other 

advantages commercial CIGS PV modules can boast 

13% [1] or more efficiency, are relatively non-toxic and 

environmentally benign, and have a very stable 

performance over time and environmental exposure. 

Given that the laboratory world record for CIGS 

efficiency is now 20.1% [2] the future for CIGS looks 

bright. Amid these obvious advantages, until now CIGSbased thin film PV modules have defied the traditional 

so-called P2 and P3 laser scribing processes [3] that have 

proven so valuable in creating the monolithic series 

interconnect structures required to achieve useful 

working voltage and current in large scale thin-film 

amorphous silicon and CdTe PV modules. With the 

failure of laser based processes, mechanical scribing with 

a force-controlled stylus has become the method of 

choice. This method can suffer, however, from poor 

edge quality (e.g., delamination, chipping, and variations 

over time due to mechanical wear). More importantly, 

the non-deterministic nature of the material removal 

mechanism yields wide, irregular scribe lines that 

necessitate large spacing between adjacent scribes. As a 

result, the CIGS modules suffer decreased efficiency [4]. 

Clearly a viable laser process would provide a tangible 

benefit to the mass production of CIGS thin film PV.

In addition to the P2 and P3 scribe processes, CIGS 

modules also require a P1 process. While there is a 

generally accepted laser process for the CIGS P1 scribe 

that utilizes 1064 nm wavelength and nanosecond pulses, 

until now the lasers used have proven completely 

ineffective for the P2 and P3 scribes. 

In this paper we first describe a novel set of laser 

processes for the CIGS P1, P2 and P3 scribing steps, the 

development of which has been enabled by a unique 

pulse-programmable fiber laser [5]. Secondly we 

document the construction and test of monolithically 

integrated mini-modules utilizing these processes. 

Finally we present a simple phenomenological model 

that describes the physics of this novel ablation process 

and discuss the commercialization potential of these 

processes.

 

2 Scribe Process

Thin film solar cell laser scribing processes allow 

division of large solar modules into an array of smaller 

series interconnected cells on one monolithic substrate. 

These processes are integral to the cell fabrication 

process and must be performed at specific points in the 

manufacturing process to successfully form the 

monolithic series interconnected end product [3]. In a 

typical CIGS process, a laser is used to segment the first 

conductive layer (P1), typically Molybdenum (Mo), into 

adjacent, electrically isolated strips (or bands). Next, the 

CIGS absorber, CdS, and intrinsic ZnO (iZO) layers are 

deposited and in the P2 step, scribed down to the first 

conductive layer (Mo) with a slight offset from the 

underlying P1 scribe. Finally, another transparent 

conductive layer is added and scribed again (P3) with an 

offset from the previous P2 scribe. The area between the 

P1 and the P3 scribe becomes effectively inactive for the 
2

purpose of generating electricity, and minimizing this so 

called “dead zone” is a primary motivation to consider a 

laser tool over a mechanical scribe tool. It should be 

noted that while it is only necessary to scribe the top 

conductive layer to make the P3 scribe, in practice the P3 

scribe is made down to the bottom conductor layer (Mo) 

making it nearly identical to the P2 scribe. 

2.1 Laser Source

The laser ultilized for this work was the 1064 nm 

PyroFlex-25 pulse programmable fiber laser from 

PyroPhotonics [5]. This innovative new laser technology 

allows pulse duration to be varied from approximately 2 

to several 100s of nanoseconds, independent of the laser 

repetition rate, which can be varied up to 500 kHz. In 

addition, each pulse can be arbitrarily programmed to 

generate a specific desired temporal profile of 

instantaneous laser power. Pulse trains comprised of 

these shaped pulses can be applied to the scribing process 

at high repetition rates. The same laser source and 

delivery optics was used to make all scribes. All process 

developement work and metrology was performed at 

NRC in London, Ontario and process development 

samples were provided by the National Renewable 

Energy Laboratory in Golden, Colorado.

2.2 P1 Process

The P1 process is generally considered a known 

process and many groups have reported successful and 

robust scribing processes utilizing Q-switched 1064nm 

lasers [6,7]. However it was discovered that the particular 

and unique bilayer structure employed at NREL for 

Molybdenum deposition process [8] resisted successful 

processing with previously published approaches. As a 

result substantial effort was invested in both glass-side 

and film-side process development.

In our experimental setup, the laser beam was 

expanded to the desired diameter, passed through a 

conventional variable attenuator device, and brought to 

focus on the test panel surface by a 100mm focal length 

lens. The sample was mounted on a 2-D linear scanning 

table to allow the laser beam to traverse its surface at 

variable speed. The scanning table was capable of 

reaching speeds of 800mm/s over a distance of ~6 inches. 

The lens was mounted on a vertical motion stage to 

enable rapid characterization of the depth-of-focus of the 

scribing process. The same basic configuration was 

utilized for all three (P1,P2, and P3) scribes.

For film side laser processing it was found that under 

no conditions could a single-pulse process remove the 

entire film cleanly. However it was determined that the 

first layer of this bilayer Mo structure could easily and 

cleanly be removed with a single 15μJ square-shaped 

10ns pulse as illustrated in Figure 1-left. However, 

increasing the pulse energy invariably caused eruption of 

the underlying layer leaving a high protruding ridge as 

shown in Figure 1-right.

Figure 1: Wyko interferometric images of ablation in 

bilayer Molybdenum. Left: Clean ablation of the top 

Mo layer only, by a single laser pulse. Right: Clean 

ablation of top Mo layer with “eruption” of bottom 

Mo layer.

It was observed that the top layer was removed in its 

solid form by some sort of brittle fracture mechanism 

driven by rapid stress build up, while the bottom layer 

would soften due to heating, releasing the residual builtup stress. For the bottom layer, the driving forces 

apparently switched from stress build-up to phase 

transition and gas pressure accumulation, with resulting 

plastic deformation and rupture. Based on these 

observations, we hypothesized that creating a double 

pulse shape with a pair of 15μJ, 10ns square pulses 

separated by a 275ns delay would permit sufficient 

cooling of the bottom layer between the pulses so that the 

stress build-up mechanism would again become 

dominant by the time the second pulse impinged. The 

result was clean brittle removal of both top and 

underlying layers. Figure 2 shows the resulting film side 

P1 groove and illustrates the laser double pulse 

configuration that achieved these results. More details of 

this process are given in another publication [9].

Figure 2: CIGS P1 film side process result utilizing a 

train of double pulses. Pulse 1 removes the first Mo 

layer and pulse 2 removes the second. The pulse pairs 

repeat at the trigger rate of the laser (PRF) and the 

trigger rate is set according to the sample motion 

speed such that adjacent laser “spots” have a slight 

overlap (30% typical) and form a continuous scribe 

line.

~ 60 μm
3

Development of a substrate-side (glass-side) process 

for this film presented different challenges. Initial results 

on a small sample look very good using single square 

10ns, 50μJ pulses, however processing larger samples 

revealed that the process often failed on large areas of the 

film. Further study revealed that there was a correlation 

between the orientation of cracks on the Mo film surface 

and the onset of failure of the process, as shown in Figure 

3. We speculate that the cracks indicate that the stress 

state of the film strongly influences the ablation process.

Figure 3: Orientation of crack pattern in area where 

initial P1 substrate-side ablation process failed. When 

crack pattern runs perpendicular to the scribe line the 

process begins to work. Where cracks have other 

orientations the ablation process fails.

We utilized the broad tunability of the PyroFlex-25

laser to survey the pulse parameter space in order to find 

a regime where the stress state of the film did not spoil 

the ablation process. To do this we first ran an 

experimental matrix of spot size versus energy and 

selected the most promising parameter zone. Next, the 

pulse duration was optimized and it was found that there 

was a very narrow window from about 8 to 12 ns over 

which the process produced a clean scribe using square 

pulses (Figure 4). Once again, the ablation mechanism 

was observed to be that of a brittle fracture.

Figure 4: Ablation process behavior as a function of 

pulse duration. Clean ablation is only achieved in a

narrow window from about 8ns to 12ns.

With further optimization of these parameters, excellent 

scribe quality was observed, but there remained a 

noticeable ‘disc’ marking of the glass by the laser which 

can be seen in Figure 4. Subsequent analysis revealed 

that this feature has no perceptible depth and there were 

no cracks or other overt defects left in the glass. We 

conclude that the “disc” marks the threshold at which 

there is a phase change in the Mo just before the onset of 

ablation. Furthermore the scribes showed excellent 

electrical isolation, beyond the limit of our measuring 

instruments. These optimized conditions, 250μJ 10ns 

square pulses (Figure 5), were later used to make the 

mini-modules.

Figure 5: 200X optical image of optimized scribe 

process result. Maximum scribe width is about 90 μm. 

The ring in the center was found to have no significant 

depth and no cracks in the glass substrate were 

observed.

2.3 P2 Process

Unlike the P1 process, there have not yet been any 

commercial implementations of a laser scribe for the P2 

or the P3 process. Those processes closest to being 

accepted by industry generally use picosecond pulses 

near 500nm wavelength [4]. However these processes 

still result in some melting of the CIGS and a significant 

reduction in shunt resistance which is detrimental, 

particularly on the P3 scribe. Figure 6 shows a typical P3 

scribe made with a ps laser.

Figure 6: Typical P3 CIGS scribe with picosecond laser at 

515 nm wavelength. (Reprinted with permission:

Jenoptik, “Structuring of thin film solar cells,” 2010.)

In contrast with the existing state-of-the-art laser scribe 

processes, the CIGS P2 process that was developed in 

this work results in no edge melting and very little 

residue in the bottom of the groove. Figure 7 below 

shows SEM images of a P2 scribe that was performed 

with the PyroFlex-25 pulse programmable fiber laser. 

10ns, 250 μJ

~90μm
4

Note the complete absence of melt at the edges of the 

scribe and the clean bottom of the scribe.

Figure 7: SEM images of edge quality for the PyroFlex 

CIGS P2 process. SEM images of PyroFlex CIGS P2 

scribes (a) top view showing cleanliness of groove and 

fractured edges, (b) close up of fractured edge 

indicating no melting of the CIGS absorber layer, and(c) 

angle view of edge scallop between adjacent laser 

pulses.

In Figure 7 it is clear that there is some residue left at 

the bottom of the scribe. As seen in Figure 7(c), the 

surface texture in the trench bottom is suggestive of that 

which occurs when a mass of solid material is lifted from 

an underlying solid surface, with a fluid gel or viscous 

liquid film sandwiched between the two solid layers. This 

is suggestive of the underlying processes responsible for 

this ablation to be discussed later.

For the P2 scribe, the material that remains at the 

bottom of the groove is of particular interest since the top 

contact layer (typically Aluminum doped ZnO or AZO

[8]) must make the contact to the bottom layer in this 

groove to make the cell-to-cell series interconnect. Any 

residue left in the groove could negatively affect the 

contact resistance and therefore the overall series 

resistance. With this in mind EDX scans (Figure 8) were 

performed with an emphasis on determining the 

composition of the residue at the bottom of the groove. 

Figure 8: EDX analysis reveals excess selenium in the 

scribe trench and at the trench edges.

The EDX analysis reveals that the trench bottom 

contains only trace amounts of Cu, In, and Ga. The Se 

levels, however, are comparable to those on the surface 

of the bulk CIGS film. While this analysis gives a good 

indication that the residue in the trench is primarily 

selenium it does not indicate what the electrical 

properties of the material may be. As will be discussed 

later in the section on the modules created with these 

scribe processes, there does not appear to be any 

significant increase in series resistance for laser scribe 

grooves vs mechanically scribed grooves.

Also of interest is the process window over which the 

process behaves well in terms of pulse duration, pulse 

energy and focus depth. As part of this work we 

characterized these aspects of the process window 

thoroughly. We determined that nominal pulse duration 

was less than 5 ns and that both the rise and fall time of 

the pulse were critical parameters for achieving a robust 

result with a wide process window. The results of the 

pulse shape study are published in another work [9]. 

Figure 9 shows the process window in terms of pulse 

energy for a spot size of about 50 μm in diameter. The 

limit for a stable process runs from about 10 μJ to about 

16 μJ which indicates a ± 25 % process window in 

energy. At lower energies the process simply fails to 

complete and at higher energies the amount of residue in 

the groove increases markedly and damage to the Mo 

layer in the form of pinholes and cracks appear. 

Figure 9: Robust process window with pulse energy. 

Robust process window extends from about 10 μJ to 16 

μJ. Nominal scribe width is about 50μm, laser pulse 

rate = 20 kHz, and stage speed is 800 mm/s.

In addition to the energy process window, a key 

parameter that indicates that a process will be robust in 

an industrial environment is the depth of focus (DOF). To 

characterize the DOF we configure the laser system for 

the nominal energy of about 12 μJ and adjust the focus 

position of the delivery lens until the process begins to 

degrade noticeably in both the positive and negative z 

directions. Figure 10 shows the laser scribes as the 

focusing lens is adjusted in 0.2 mm steps through focus. 

As can be clearly seen the quality of the scribe lines is 

very good and effectively unchanged over an entire 2 mm 

range of motion of the delivery lens. This is a good 

indicator that the process can likely be robust over very 

large panels where the panel surface flatness may be an 

issue [10]. This DOF also speaks to the excellent M2

value of the fiber laser, typically < 1.3. Furthermore the 

Cu In Ga

Se

Mo

Cu In Ga

Se

Mo

(c)

(a) (b)
5

large DOF implies that a system design may not require 

autofocus optics for a robust process.

Figure 10: DOF of PyroFlex CIGS P2 process. Note that 

the process remains approximately unchanged over a 

±1 mm focal depth with a 100 mm focal length delivery 

lens. 

Finally the optimized scribing process was applied to 

the P2 step in fabrication of the mini-modules in 

collaboration with NREL. The picture in Figure 11

shows the resulting P2 scribe (right) next to the P1 scribe 

(left). More details with regard to the structure, 

fabrication and test of the resulting modules can be found 

in ref. [8, 11]. Also, it should be noted that the large 

scribe spacing (~210 μm) was chosen for consistency 

with typical mechanical scribing systems and is not 

limited by the laser process.

Figure 11: Sample of P2 scribe with optimized process 

as it was applied to the NREL mini modules. Note the 

~90 um wide P1 scribe visible at left.

2.4 P3 Process

For the purposes of this work it was found that the 

scribing parameters for the P3 scribe were identical to the 

parameters for the P2 scribe. DOF, energy range, and 

pulse duration were all found to be approximately the 

same for both P2 and P3 so the data will not be repeated 

here. For P3, however, we also documented the process 

behavior as a function of the spot overlap. It was found 

that the process behaved well over a range of about 16 

kHz to 24 kHz (10 – 40% spot overlap) with the process 

becoming less stable above 50% overlap. Figure 12

illustrates the scribe quality as a function of pulse 

overlap.

Figure 12: P3 scribe appearance as a function of laser 

spot overlap. Process is robust up to 24 kHz (~ 40% 

overlap) but begins to degrade at higher spot overlaps.

As with the P2 step, the P3 optimized scribe was 

applied to the mini-modules fabricated by NREL. After 

the P2 scribe was completed at PyroPhotonics, the 

modules were returned to NREL and the AZO top contact 

layer was applied. The modules were then returned to 

PyroPhtonics to apply the P3 scribe. This top contact 

layer is responsible for the bluish appearance of the P2 

scribe as seen in Figure 13. The scribe spacing between 

the P2 and the P3 scribe is about 125 μm and again 

chosen to be comparable with existing mechanical scribe 

processes and is not limited by the laser process.

Figure 13: Sample of P3 (right) scribe as applied to the 

NREL mini-modules. Note the P1 scribe (left) and the P2 

scribe (middle). The P2 scribe now has a bluish 

appearance due to the presence of the AZO layer that 

was applied at NREL between the P2 and the P3 steps. 

2.5 P3 Rework, Edge Deletion and Defect Repair

After the P3 scribe, the same P3 process was used to 

perform edge deletion and to clear electrical contact pads 

so that the mini-modules could be tested. Initial test 

results however, indicated that the efficiency of the 

modules was only about 5% and that the series resistance 

was anomalously high. After investigation at NREL [8] it 

appeared that the initial AZO deposition had been too 

thin for the size of the cells in the mini-module. With this 

determination, the modules were returned to NREL after 

the P3 scribe for deposition of additional AZO material 

and then returned to PyroPhotonics for reworking the P3 

step. Because it was critical to minimize the “dead zone” 

between the P1 and the P3 scribe, this time we placed the 

~90 μm ~50 μm

~210 μm

~ 125 μm
6

new P3 scribe directly adjacent to the previous P3 scribe. 

As Figure 14 indicates, the spacing between the laser 

scribes can be made zero for all practical purposes and 

clearly demonstrates the potential of the laser process to 

reduce the dead space between the P1 and P3 scribe to 

near zero. After the additional AZO deposition and 

rework the module performance increased dramatically 

as discussed later.

Figure 14: Far right, second P3 scribe added after the 

deposition of additional AZO material to the module. 

Note that the scribe spacing is effectively zero. Again 

the width of the P3 scribe is about 50 μm. 

In addition to the P3 rework, the laser was used to delete 

the edges of the cells to ensure that there were no shorts 

between cells and that material non-uniformity near the 

edges did not negatively impact the performance of the 

modules. In addition several defects where the film was 

scratched in handling or otherwise damaged were 

identified. In places where these defects allowed shorting 

of the top and bottom conductor they were clearly 

detrimental to the performance of the cells and hence the 

entire module. Consequently the P3 process was used to 

isolate and/or remove these defects as shown in Figure 

15. Similar processes were used to delete the material 

from the edges of the module and clear the electrical 

contacts so that the module could be tested.

Figure 15: Example of how the laser was used to 

remove a shunting defect. Top: a scratch in the film 

before the AZO deposition resulted in a shunt between 

the front contact and the back contact on two cells. 

Bottom: many adjacent laser passes and a mask in the 

region of the defect are used to completely eliminate 

the defect. Defect area is approximately 1.5 mm 

square.

3 Mini-Module 

3.1 Mini-Module Fabrication

As already described in the process development 

portion of this paper, the CIGS P1, P2 and P3 processes 

were applied to NREL material to create two fully 

functional mini-modules for testing. Details of the 

material structure and fabrication are found in the 

referenced works [8,11], and the process steps are 

outlined below:

a) Mo layer deposited on 75mm square glass

substrates at NREL

b) P1 scribe applied at PyroPhotonics

c) Absorber layers applied at NREL

d) P2 scribe applied at PyroPhotonics

e) Top contact AZO layer applied at NREL

f) P3 scribe applied at PyroPhotonics, edges 

isolated, anode and cathode contacts cleared and 

simple electrical tests performed.

g) Module characterization at NREL and additional 

AZO deposition

h) P3 rework at PyroPhotonics.

i) Module characterization at NREL

As a control, a third sample was not laser scribed but was 

shipped between NREL and PyroPhotonics along with 

the scribed modules. That sample was later processed 

per NREL’s standard cell processing procedure to serve 

as a reference. 

In addition to the two laser scribed modules, two 

additional modules were fabricated. However, this 

second pair of modules was sent to Jenoptik for the P2 

and P3 scribes that were applied using a stylus scribing 

system. Other than the fact that these materials were 

mechanically scribed for P2 and P3, they were identical 

to the laser scribed modules. Figure 16 gives an 
7

overview of the physical series interconnect structure that 

was formed by the P1, P2 and P3 scribes, depicts how 

successive P3 scribes at each edge of the module were 

used to created the anode and cathode contacts, and lastly 

shows the physical appearance of the all laser scribed 

functional mini-modules.

Figure 16: (Top) Electrical / Mechanical structure of 

mini-module cell interconnects. The cathode and anode 

contacts were formed by using the PyroFlex P3 process 

with hundreds of successive scribes placed adjacent to 

one another. (Bottom) Appearance of actual minimodules after all laser processing, including edge 

deletion and defect deletion.

3.2 Mini-Module Test

Once the mini-modules were fabricated they were 

tested in direct sunlight by PyroPhotonics. In order to 

estimate the solar flux a laser power meter was pointed 

toward the sun and the power reading was divided by the 

absorber surface area in order to calculate the solar flux. 

To characterize the module performance both open 

circuit voltage (Voc) and short circuit current (Isc) were 

measured. Next, various load resisters were applied to 

each module and the cell efficiency was calculated as the 

power dissipated in the load resistor divided by the 

estimated solar flux on the module. The initial 

measurement taken after the first AZO deposition and 

first P3 scribe indicated that the maximum module 

efficiency was only about 5%, results that were later 

confirmed at NREL. With some investigation we 

suspected that the AZO layer was too thin for the chosen 

width of each cell in the module (6.5 mm). This resulted 

in an excessively high series resistance and larger internal 

power loss in the module. Additional AZO was added by 

NREL, and the modules were then P3 scribed a second 

time (Figure 14). Again the modules were tested at 

PyroPhotonics and at NREL. The initial testing at 

PyroPhotonics indicated about a factor of two 

improvement in the module efficiency as shown in Figure 

17(top). Figure 17 (bottom) shows the result of 

subsequent testing at NREL under standard test 

conditions (STC). The difference between the two results 

is attributable to the non-standard illumination conditions 

used at PyroPhotonics, but they are generally consistent.

Figure 17: (Top) Laser scribed and mechanically scribed 

mini-module efficiency as a function of load resistance

under 790 W/m^2 estimated solar flux. (Bottom) I-V 

parameters as measured at NREL under Standard Test 

Conditions (STC).

As Figure 17(top) clearly indicates, the efficiencies of 

the laser scribed modules and the mechanically scribed 

modules are comparable. The best module, just over 

10% (8.3% NREL STC) efficiency, is a laser scribed 

module where as the second best module at about 9.8% 

(7.5% NREL STC) is one of the mechanically scribed 

modules. The control sample that was processed with 

NRELs standard process indicated that the material was 

capable of reaching 16% (STC) efficiency [8], suggesting 

that there were some deficiencies in the mini-module 

structures that limited the performance. The fact that 

there are no systematic differences in performance 

between the laser scribed modules and the mechanically 

scribed modules implies that laser scribing results in no 

sacrifice in performance compared with mechanical 

scribing. NREL is currently characterizing these modules 

further to determine what issues are limiting the 

performance of these modules [8].

3.3 Mini-Module Conclusions

With the results from the mini-modules we conclude 

that the laser P2 and P3 scribe processes produce modules 

that are of at least equal performance to a mechanical P2 

and P3 scribes if not better. We also note that there is no 

intrinsic limit to the spacing of the laser scribes and that 

they can be placed directly adjacent to one another, being 

limited only by the process of mechanical registration. This 

ability to place the scribes very close together should allow 

reduction of the resulting “dead space” between the P1 and 

P3 scribe that in turn should allow reduction of the overall 

“dead zone” and a concomitant increase in efficiency. 

4 Ablation Process Physics

In another published work we advance a 

phenomenological model that describes the physical 

processes underlying this novel ablation process [11]. 

Here, we briefly recap our conceptual model for this 

Mini-Module Total AZO Voc Jsc FF Eff. Rs Rsh Scribing

Meas. ID (μm) (V) (mA/cm2

) (%) (%) (ohm-cm) (ohm-cm) Method

C2681-1 0.12 5.742 26.97 34.30 5.31 113.6 503.4 All Laser

C2681-2 0.50 5.846 26.74 46.32 7.24 81.9 697.2 10 cells

C2684-1 0.12 4.979 27.61 29.65 4.08 128.0 280.5 All Laser

C2684-2 0.50 5.975 26.76 51.93 8.30 48.1 726.0 10 cells

Laser + Mechanical

10 cells

Laser + Mechanical

only 9 cells

7.48 49.2 999.8

6.63 50.3 761.1

C2682-2

C2683-2

0.50 5.811 25.16 51.18

0.50 5.458 25.19 48.22

6.5mm
8

“brittle fracture” process with some modifications that 

reflect some further insight we have gained through some 

simple numerical modeling.

4.1 Physical Process Overview

We have summarized our understanding of the 

mechanisms underpinning this ablation process as a 

competition between two processes, one that drives the 

brittle fracture process to completion and another that 

spoils the brittle fracture process. These two mechanistic 

paths are proposed in Figure 18.

Figure 18: Conceptualization of two competing 

processes proposed for the laser-material interaction. 

The process in the left column moves toward spoiling 

the brittle fracture ablation process where as the one 

on the right tends toward completing the brittle 

fracture process.

Referring to Figure 18 we can make the following 

generalizations about the process:

1. Thermal conduction from the Mo into the CIGS 

puts an upper time limitation on the duration of 

the laser pulse.

2. Slow rise time on the laser pulse will limit the 

total peak energy that can reach the Mo layer, 

meaning that the CIGS gets too hot and 

becomes mechanically plastic before the Se 

vapor pressure reaches a level that can drive an 

ablation process

3. A long fall time for the pulse will only 

contribute additional heating to the CIGS layer 

and spoil the brittle fracture process which we 

believe evolves over a period of several 

nanoseconds.

4. Excess peak power causes non-linear optical 

absorption in the CIGS.

As mentioned at the beginning of this paper, until now 

there has been no report of successful CIGS P2 and P3 

scribing utilizing a nanosecond class 1064 nm pulse laser. 

Furthermore, the process parameters that were found to 

be appropriate for this work have since proven to be also 

appropriate for nearly a dozen different P2 and P3 

material samples from many different suppliers. This 

then begs the question of why this process has not 

already been reported in the literature and why the 

PyroFlex-25 pulse programmable fiber laser is uniquely 

capable of consistently generating this ablation 

phenomenon over a relatively wide range of process 

conditions. We postulate that the answer lies in 

examination of the points listed above.

To examine the first point we constructed a very 

simple thermal diffusion model using real CIGS thermal 

properties to determine the time scale on which thermal 

diffusion became important. There are a number of ways 

that such a thermal diffusion problem can be modeled

[12,13], however we chose to model the CIGS as a 

cylinder with a heat source inside (Figure 19 top). The 

resulting 1-D thermal diffusion model quite interestingly 

predicted that the time constant for thermal diffusion 

from the Mo into the CIGS is about 5 ns (Figure 19), 

consistent with our observation that the process begins to 

fail at pulse durations longer than about 5 ns.

Figure 19: One dimensional thermal diffusion model. 

(Top) Heat is deposited at the Mo-CIGS interface by the 

laser pulse and diffuses upward. (Bottom) Plot showing 

the solution as a function of time for the 1-D diffusion 

equation at the mid plane between the Mo layer and 

the top surface (T1μm).

To examine the second point we also constructed a 

very simple linear absorption model and simulated the 

expected transmission of the laser pulse to the Mo layer. 

The results of this simple modeling exercise are shown in 

Figure 20 and show the peak intensity of the laser pulse 

that develops at the Mo layer for a pulse of fast rise time 

and a pulse of comparatively low rise time. The plot 

clearly shows that a pulse of slower rise time results in 

proportionally lower peak intensity on the Mo layer than 

would be predicted just based on the relative peak 

intensity of the starting pulses. This can simply be 

understood as the fact that heating due to the leading 

edge increases the CIGS absorption coefficient before the 

peak of the pulse can arrive. In our laboratory 

experiments we do indeed find that increasing the rise 

0 5 10 15 20 0

100

200

300

Square Pulse

CIGS Temperature 1 um from Moly Interface

Time (ns)

Temperature (C)

~2μm CIGS 

thickness

Mo

CIGS

Heat 

Introduction 

Zone

Temp = T1μm

50 μm
9

time has a negative impact on the ablation process and 

narrows the process window in energy.

Figure 20: Solid Curves: Laser pulse intensity reaching 

the Mo surface for two different pulses. Dotted Curves: 

(r), a 5ns duration rectangular pulse (fast rise and fall 

time); (b), a 5ns FWHM Q-switched pulse (slow rise and 

fall time). Both shapes contain equal pulse energy but 

due to the long rise time on (b) the peak intensity 

reaching the Mo interface is proportionally lower.

To address the third point, how pulse fall time affects 

the brittle fracture ablation process, we consider the 

evolution in time of the mechanical properties of the 

CIGS film. The brittle fracture process has been so 

described because the film is literally blown off of the 

Mo surface while still in the solid state. The edges left 

behind are mechanically broken and completely free of 

melt. Thermal conduction from the Mo layer into the 

CIGS layer, and/or the absorption of additional energy 

within the CIGS layer after it has reached opacity, heats it 

further. As CIGS approaches its melting point there will 

be a transition from brittle ceramic-like behavior to a 

more ductile-type behavior. Referring to Figure 21, in 

general the energy required to fracture can be considered 

as proportional to the area under the stress-strain curve 

for the material [14]. Figure 21 shows how an idealized 

stress strain curve may look for a brittle material when it 

is cold (blue) and when it is hot (red). From the figure it 

is clear that the material at higher temperature will absorb 

much more energy before finally undergoing fracture (red 

“x” in the figure) than the cooler material. Consequently 

if the material is allowed to become too hot, the energy 

being released by the explosive expansion of selenium at 

the Mo interface is consumed in the plastic deformation 

of the material rather than fracture and ejection of the 

debris. The end result is melted mass of CIGS rather than 

clean, brittle material ejection.

Figure 22 depicts the process window in terms of 

DOF, for a pulse of 8 μJ, for pulses with a) short rise and 

fall time, b) long rise time, and c)long fall time. As our 

model predicts rise and fall time appear to be critical 

parameters to establish a robust process window.

Finally, we address our fourth point. Given that ours 

is the first work in the field to address these problems 

with a pulse-shaped laser, we conclude that the only 

previous work to have utilized short rise time and short 

fall-time pulses have involved picosecond lasers. 

According to our arguments above, these should have 

been sufficient to drive the brittle fracture process. 

However, they have not done so, and appear only to have 

succeeded in producing a regular vaporization-based 

ablation mechanism with melted CIGS at the side walls. 

We therefore postulate the following factors that have 

prevented ps lasers from achieving the brittle fracture 

result:

1. Increased non-linear absorption results in a 

larger fraction of energy absorption in the 

CIGS rather than Mo layer.

2. Pulse energies are too low.

3. PRF is too high in mode locked systems to 

allow single pulse delivery.

4. CW background levels are too high. 

(From our own work we have determined that 

any CW background also spoils the process.)

In several references examined [15,16,17], we see 

that at least one or more of the above points is at work 

and would have prevented the success of the brittle 

fracture mechanism. Most relevant is the potential 

increase in absorption in the CIGS layer due to non-linear 

effects caused by high peak power. This mandates that 

the ideal pulse delivers the certain net energy required 

within a pulse being substantially shorter than 5ns, with 

fast rise and fall times, and with a limitation of the peak 

power. These conditions require an essentially square 

pulse shape. 

Figure 21: Hypothetical stress strain curve for a 

material at two different temperatures. The red “x” 

represents the breaking stress. 

Figure 22: Impact of laser pulse rise and fall time on 

process window with pulse energy = to 8 μJ. Z-mm

(vertical axis) represents the process DOF in mm.

0 5 10 15 20 0

1

2

3

0

1

2

3

Fast Rise and Fall Time Pulse - at Mo interface

Slow Rise and Fall Time Pulse - at Mo interface

Fast Rise and Fall Laser Pulse

Slow Rise and Fall Laser Pulse

Laser Intensity Reaching Moly Layer

Time - ns

Intensity Reaching Mo (kW)

Starting Laser Pulse Intensity (kW)

r

b
10

4.5 Process window and the laser pulse

While this is by no means a complete or rigorous 

analysis of the mechanisms at work in this ablation 

process, the correlation of physical results and the trends 

that our model predicts indicate that this model provides 

a good starting point for understanding of this ablation 

process. It also gives an indication of why existing lasers 

have not been able to exploit this ablation mechanism for 

CIGS as the PyroFlex-25 has. Namely, Q-switched lasers 

and fiber lasers available until now have not provided the 

combination of fast rise time, fast fall time, short overall 

pulse duration and sufficient energy content per pulse 

that is required to drive this process forward at 1064 nm. 

In terms of establishing a robust manufacturing 

process we have established that the pulse properties of 

pulse programmable fiber laser allow the laser to operate 

comfortably inside of this ablation process window. The 

resulting process for P2 and P3 have excellent DOF, 

comfortable tolerance for energy fluctuations, and the 

PyroFlex-25 has sufficient pulse energy available so that 

many scribes can be made in parallel. We find the added 

advantage that the exact same laser can be used also for 

the P1 scribe with the same optical configuration. This 

opens the door to a single process machine for all three 

P1, P2, and P3 processes. Even edge isolation and repair 

of large defects are possibilities.

5 Summary

We have outlined the development of CIGS P1, P2, 

and P3 scribe processes with the PyroFlex-25 pulse 

programmable fiber laser. The P1 process developed was 

found to be comparable to other commercial P1 processes 

with excellent groove quality. The P2 and P3 processes 

were found to remove the CIGS material in a “brittle 

fracture” manner with no melting and little residue left in 

the trench bottom. The process window was found to be 

broad in terms of DOF and pulse energy and 5ns was 

established as an upper limit on pulse duration for 

establishing the best process. The processes developed 

were put to work in cooperation with NREL to fabricate 

mini-modules to test the viability of this all-laser scribing 

process for the production of monolithic series 

interconnected solar modules. For comparison minimodules were also fabricated utilizing mechanical 

scribing for the P2 and P3 steps. The resulting 

performance of the laser scribed and mechanically 

scribed cells was found to be comparable, leading to the 

conclusion that the laser scribe process is capable of 

similar if not better performance than the mechanically 

scribed process. More work and study shall enable us to 

fine-tune the scribing conditions and to improve the 

performance of mini-modules. Finally we put forward a 

model for the ablation mechanism that involves a 

competition between the desired brittle fracture ablation 

process and a competing parasitic process. We offered a 

qualitative interpretation of some modeled and some 

empirical data that give a framework for understanding 

which pulse parameters are important for favoring the 

“brittle fracture” process over the parasitic process. This 

interpretation also gives insight as to how the pulse 

characteristic of the PyroFlex-25 pulse programmable 

fiber laser allow it to operate in this “brittle fracture” 

process window where as other lasers to date have failed.

ACKNOWLEDGEMENT

The authors would like to acknowledge the contribution 

of JENOPTIK Automatisierungstechnik GmbH to this 

paper, and in particular to thank Dr Gabriele Eberhardt 

and Dr Torsten Reichl for their support. 

References

[1.] Q-Cells SE. Q-Cells sets a new 13.0 % efficiency 

record for mass-produced CIGS thin-film modules . 

Q.cells. [Online] June 9, 2010. [Cited: July 21, 2010.] 

http://www.qcells.com/en/company/press/corporate_news/index.html.

[2.] Claudia Brusdeylins. Press Releases. Zentrum für 

Sonnenenergieund und Wasserstoff-Forschung BadenWürttemberg (ZSW). [Online] April 28th , 2010. [Cited: 

July 21, 2010.] http://www.zswbw.de/fileadmin/ZSW_files/Infoportal/Presseinformation

en/docs/pi05-2010-ZSW-Worldrecord-TF-CIGS.pdf.

[3.] Jha, A. R. Solar Cell Technology and Applications. 

Boca Raton : Auerbach Publications, 2009. p. 21.

[4.] Structuring of thin film solar cells. Eberhardt, 

Gabriele, et al. San Jose : SPIE, 2010. Proceedings Vol. 

7585.

[5.] Murison, Richard, et al. 7,742,511 USA, 2008.

[6.] Thin film removal mechanisms in ns-laser processing 

of photovoltaic materials. Bovatsek, J. s.l. : Thin Solid 

Films, 2010. Vol. 518, pp. 2897-2904.

[7.] Large-Area CIGS Modules: Processes and 

Properties. Powalla, M., Hariskos, D. and Lotter, E et 

al. s.l. : Thin Solid Films, 2003. 523-533.

[8.] ALL-LASER SCRIBING FOR THIN-FILM 

CuInGaSe2 SOLAR CELLS. F.J. Pern, et al. s.l. : IEEE, 

2010. IEEE PVSC.

[9.] Application of a pulse programmable fiber laser to a 

broad range of Micro-Processing Applications. Rekow, 

Mathew and R., Murison. Anaheim : LIA, 2010.

[10.] Patel, Rajesh and Bovatsek, James. Why M2 

matters. http://www.optoiq.com/. [Online] May 2010. 

[11.] CIGS P1, P2, AND P3 LASER SCRIBING WITH 

AN INNOVATIVE FIBER LASER. Richard Murison, et 

al. s.l. : IEEE, 2010. IEEE PVSC.

[12.] Elżanowski, Marek.

http://www.mth.pdx.edu/~marek/mth510pde/notes%202.

pdf. http://www.mth.pdx.edu. [Online] 

[13.] Laser Flash Method for Estimating the Thermal 

Diffusivities of Thin Films. Peichun, YANG, Hou, Li 

and Lichang, Qi. Beijing : KTK Scientific Publishers, 

1990.

[14.] The effect of strain rate on the stress–strain curve of 

oriented polymers. . Hall, I. H. 4, s.l. : Journal of Applied 

Polymer Science, 1968, Vol. 12.

[15.] Picosecond-Laser Structuring of Thin Films for 

CIGS Solar Cells. Gediminas, Račiukaitis and Paulius, 

Gečys. Vilnius : LAMP2009, 2009.

[16.] ps-laser scribing of CIGS films at different 

wavelengths. Gečys, P., et al. Berlin / Heidelberg : 

Springer, 2010-06-18. 0947-8396.

[17.] Compaan, Alvin. photovoltaics-laser-scribingcreates-monolithic-thin-film-arrays.html. 

http://www.optoiq.com/. [Online] Jan 01, 2000. [Cited: 9 

2, 2010.]
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CIGS P1, P2, P3 Scribing Processes using a Pulse Programmable 

Industrial Fiber Laser: Preprint

5a. CONTRACT NUMBER

DE-AC36-08GO28308

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6. AUTHOR(S)

M. Rekow, R. Murison and T. Panarello (PyroPhotonics Lasers, Inc.) 

C. Dunsky (Aeos Consulting, Inc.)

C. Dinkel and S. Nikumb (NRC Canada)

J. Pern and L. Mansfield (NREL)

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14. ABSTRACT (Maximum 200 Words)

We describe a novel set of laser processes for the CIGS P1, P2 and P3 scribing steps, the development of which has bee 

enabled by a unique pulse-programmable fiber laser. We find that the unique pulse control properties of this 1064 nm 

wavelength laser have significant effects on the material removal dynamics of the various film layers in the CIGS material 

system. In the case of the P2 and P3 processes, the shaped pulses create new laser/material interaction effects that 

permit the material to be cleanly and precisely removed with zero Heat Affected Zone (HAZ) at the edges of the scribe. 

The new P2 and P3 processes we describe demonstrate the first use of infrared nanosecond laser pulses that eliminate 

the HAZ and the consequent localized compositional changes in the CIGS absorber material that result in poor shunt 

resistance. SEM micrographs and EDX compositional scans are presented. For the P1 scribe, we process the bi-layer 

molybdenum from the film side as well as through the glass substrate. Microscopic inspection and compositional analysis 

of the scribe lines are not sufficient to determine electrical and optical performance in working PV modules. 

15. SUBJECT TERMS

CIGS, Thin Film Solar Cell, Laser Scribing, P2, P3

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