high speed laser processing for monolithical series connection of silicon thin-film modules

Research

High Speed Laser Processing ForMonolithical Series Connection ofSilicon Thin-film ModulesStefan Haas*,y, Aad Gordijn and Helmut StiebigResearch Centre Julich, Institute of Photovoltaics (IEF-5), Julich, Germany

A detailed analysis of the monolithical series connection of silicon thin-film modules with metal back contact

fabricated by high-speed laser ablation will be presented. In this study, optically pumped solid-state lasers

with wavelengths of 1064 nm and 532 nm were used for the patterning process. The influence of various laser

parameters on the performance of amorphous and microcrystalline silicon modules will be discussed. In

particular, the line-scribing parameters for a TCO and Ag back contact system was analyzed in detail, since

it is the most critical patterning step. A detailed description of the back contact ablation process will be

presented and a criterion for flakeless patterning was defined. Finally the influence of the back contact

patterning on the electrical behavior of silicon single junction cells was studied. The dark current density

versus back-contact patterning line length was analyzed by means of a developed SPICE (simulation program

with integrated circuit emphasis) simulation model. Copyright # 2007 John Wiley & Sons, Ltd.

key words: silicon thin-film module; device simulation; monolithical series connection; laser processing

Received 6 June 2007; Revised 25 July 2007

INTRODUCTION

Silicon thin-film technology can play an important role

in the cost effective generation of photovoltaic energy.

Stable module efficiencies exceeding 10% are already

achieved for tandem cell structures based on amorphous

(a-Si:H) and microcrystalline silicon (mc-Si:H).1,2 In

comparison to the conventional crystallinewafer techno-

logy, it offers a higher cost reduction potential due to a

low material consumption, low process temperatures,

the possibility of large-area fabrication, and the mono-

lithically integrated series connection.3 The series

connection can be realized by a three step patterning

process selectively removing the individual layers—

TCO front contact, thin-film silicon layer stack, and

back contact system—of the solar cell. A high level of

automation can be achieved as all patterning steps can

be performed by laser scribing. Compared to other

patterning techniques, laser ablation also provides a

high throughput and small area losses. Although laser

scribing has been widely used in the silicon thin-film

module manufacturing for many years, there is a great

shortage of available information.4–8 In this study,

optically pumped solid-state lasers with wavelengths

of 1064 nm and 532 nm were used. The influence of

individual patterning steps on a-Si:H and mc-Si:Hsingle and tandem cell structures was analyzed and will

be discussed. The experimental part will be specified in

the next section. The most critical patterning step

during the module fabrication in superstrate configur-

ation is the back contact ablation,9 since metal flakes

can easily introduce local shunts at the laser lines.

Therefore, the ablation behavior of the back contact

was studied in detail. A criterion for a flakeless back

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS

Prog. Photovolt: Res. Appl. 2008; 16:195–203

Published online 19 November 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/pip.792

*Correspondence to: Stefan Haas, Research Centre Julich, Instituteof Photovoltaics (IEF-5), Julich, Germany.yE-mail: [email protected]

Copyright # 2007 John Wiley & Sons, Ltd.

contact laser-scribing process could be defined. The

derivation of the criterion will be discussed in

the section ‘Flake Formation Process’. For a detailed

analysis of the electrical losses in thin-film silicon

modules, SPICE simulations can be a useful tool.10

However, the influence of back contact laser scribing

on the module performance has not been investigated

before. Therefore, the module performance with

respect to the back contact patterning was analyzed

in detail in the section ‘Influence of the Back Contact

Patterning on the I/V-Characteristics’. A comparison

of experimental data and SPICE simulations will be

discussed.

EXPERIMENTAL

The laser-scribing set-up consists of several neodymium-

doped solid-state lasers from Rofin-Sinar Technologies

Inc. High power, Q-switched lasers with a wavelength

of 1064 nm (IR-lasers) or frequency doubled with a

wavelength of 532 nm (green lasers) were applied. All

lasers operate in TEM00 mode and offer pulse dura-

tions in the nanosecond range (<25 ns). A split-axis-

system allows patterning in x- and y-direction. Both

horizontal axes are driven by linear motors with

a maximum feed rate v of 1000mm/s, which is

consequently the maximum scan speed of the laser

spot. The accuracy of the laser spot position is in the

micrometer range. A 300mm-focusing lens focuses the

laser beams on the substrate resulting in a process with

high depth of field. The laser ablation processes are

carried out through the glass side. The pulsed laser

repetition frequency f can be varied over a wide range

(0 kHz< f< 100 kHz, depending on the used laser).

The laser power P is adjusted by the excitation current

of the pump source.

For the investigations, samples were prepared in

superstrate configuration with silicon film thicknesses

between 300 nm and 1200 nm. Smooth and textured

ZnO and rough SnO2 were used as front contact. The

textured ZnO shows a root-mean-square roughness

(dRMS) of around 125 nm and a crater-like structure

with lateral feature sizes>1mm while the SnO2 has a

dRMS of around 40 nm and lateral feature sizes of

around 400 nm. The back contact consists of a zinc

oxide/silver layer stack. More details regarding the

sputtering process and PECVD process used for the

preparation of the ZnO and the p-i-n layer sequences

are given elsewhere.11 The quality of the scribed laser

lines was assessed by optical microscopy, layer

thickness profile measurements, and sheet resistance

measurements. For the evaluation of the back-contact

patterning process also scanning electron microscopy

(SEM), atomic force microscopy (AFM), and I/V-

measurements of patterned cells and modules under

dark conditions or under AM 1.5 illumination were

carried out.

Figure 1a shows a sketch of the interconnection area

of a silicon thin-film module. By means of selective

ablation of the individual layer stacks, cell stripes were

defined. To achieve the series connection the front

contact and the back contact of two neighboring cell

stripes are connected in series. Depending on the

patterning technology applied, the interconnection

width is typically between 300mm and 1000mm, but

interconnection width of around 140mm are already

possible.12 A microscopic view of a typical inter-

connection area is shown in Figure 1b. Here the width

of the interconnection area is around 350mm. The

distance between the lines as well as the spot sizes are

not optimized with respect to a minimized inter-

connection width. The left line of Figure 1b represents

the TCO patterning line (P1), which is the first

patterning step in the fabrication of thin-film modules

Figure 1. Sketch of the interconnection region of a silicon thin-film module in superstrate configuration (a). Microscopic

view of an interconnection region with optimized laser parameters (b)

Copyright # 2007 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2008; 16:195–203

DOI: 10.1002/pip

196 S. HAAS, A. GORDIJN AND H. STIEBIG

in superstrate configuration. The middle line shows the

region where the silicon layer is selectively removed

(P2) and the right line is the back-contact patterning

line (P3). By means of a three-step process the

integrated series connection is realized.

For the ablation of different TCO types, a laser with

a wavelength of 1064 nm is used as also described in

the Reference [13]. The laser power is mainly absorbed

through free carrier absorption. In particular, the power

density of the laser pulse determines the quality of the

ablation process. When the energy exceeds a certain

threshold, a nearly complete removal of the TCO is

achieved. The applied laser peak power is in the range

between 10 kWand 11 kW. For two areas separated by

one laser scribed line with a length of 260mm a resis-

tance in theMV range for both types of transparent front

contact materials (SnO2, ZnO) is measured. The silicon

layer stack is patterned using a green laser with

a wavelength of 532 nm14 and laser peak powers

between 500Wand 1000W. This offers the possibility

of a selective removal of the silicon, since the

underlying TCO is highly transparent in this wave-

length range. Only for very high laser power densities,

cracks were found in the TCO layer.9 The damage of

the TCO is most probably caused by introduced heat

during the silicon ablation process. These cracks can

increase the series resistance which will lead to a

decrease of the fill factor of the I/V-characteristic.

However, damage of the TCO is not observed for an

optimized P2 process. The last and most critical step is

the back contact patterning. The back contact removal

with a green laser is obtained by the ablation of the

underlying silicon layer. However, remaining metal

flakes can easily introduce local shunts at the laser

lines, since they can connect neighbored cell stripes or

lead to a contact between front and back contact of a

cell stripe. Well-defined and flake-free ablation of the

back contact was achieved for a broad range of laser

power densities. An increase of the laser power also

leads to an increase of the diameter of the ablated

area because the threshold power density for silicon

ablation is exceeded over a larger area around the beam

center. A critical parameter is the quality of the laser

beam intensity profile. Profiles with higher edge

steepness result in a better line quality compared to

Gaussian intensity profiles. Therefore, an aperture

plate was applied to modify the intensity profile of

the laser beam before focusing. The most critical

parameter is the pulse overlap, which will be discussed

in the following.

FLAKE FORMATION PROCESS

An example of a patterned back contact using

optimized (a) and non-optimized (b) conditions with

respect to the flake-formation is given in Figure 2. Both

lines were produced with equal laser power and

repetition frequency but with different feed rates.

Consequently, different degrees of pulse overlaps exist

of successively ablated areas.

For a low pulse overlap between two successively

ablated areas, a nearly flake-free patterning can be

achieved (Figure 2a). The individual laser spots are

well defined and no imperfections occur at the laser

line edges. Only in the middle of every spot, a small

area occurs where the TCO is affected by the ablation

process. However, the sheet resistance of the TCO is

not significantly influenced for a broad laser power

range. With a high pulse overlap a huge amount of

failures can be observed, which are seen as the black

areas at the edges of the laser line (Figure 2b). These

black regions can be attributed to sloped metal flakes.

In the images, the flakes appear dark because the light

Figure 2. Optical micrographs of back contact laser lines on tin oxide substrates. The arrows show the moving direction of

the laser beam. In a well-defined process window a nearly flake-free patterning was observed (a). Using patterning

parameters outside of this process window leads to an increased flake formation (black regions) (b)


DOI: 10.1002/pip

HIGH SPEED LASER PROCESSING 197

emitted from the microscope illumination is reflected

to the side by the sloped metal. For processes with a

large pulse overlap, metal flakes were created nearly

independent of the other patterning parameters used.

Thus, the pulse overlap mainly determines the flake

formation process. To analyze this behavior in more

detail, samples prepared on smooth TCO were

investigated.

Figure 3 shows SEM images of the area where two

laser pulses overlapped. For the sample with a low

pulse overlap (a,c) a well-defined apex remains

between two laser pulses. The apex is slightly detached

from the substrate, but the silicon layer still remains

under the ZnO/Ag back contact. The sample with a

high pulse overlap (Figure 3b,d) shows again the

sloped metal flakes. This SEM picture also shows that

the back contact ablation is mainly determined by two

different tears. One of the tears corresponds to the

laser beam profile (arrow 1) while the other tear

starts at the edge that was produced by the former

ablated area (arrow 2). The second tear occurs with an

angle of 908 to the edge of the previously ablated area.The flake formation process is illustrated in detail by

Figure 4.

If the contact angle a is greater than 908 then tear

(2) strikes on tear (1), before tear (1) has reached the

edge of the previously ablated area (Figure 4a, gray

circle). Under these conditions the non-dashed area is

removed and a flake is created (dashed area). For

contact angels smaller than 908 the flake formation was

suppressed since tear (1) reaches the edge of the

previously ablated area before the competing tearing

process (2) can lead to a flake. Thus, for a flake-free

Figure 4. Mechanisms of the back contact ablation process

for contact angles greater than 908 (a) and for angles smaller

than 908 (b). The gray circle represents an already ablated

area and the dashed lines describe the ablation process of the

metal layer initialized by a laser pulse. The ablation is

determined by tear (1) and tear (2). The patterning process

can lead to a flake if the contact angle is greater than 908

Figure 3. Microscope views and SEM pictures from patterning lines produced with optimized (a,c) and non-optimized

conditions (b,c). For optimized conditions awell-defined apex occurs at the contact region between two successive laser spots

while for non-optimized conditions a flake formation can be observed. The arrows show the different tearing processes within

the ablation process


DOI: 10.1002/pip


laser-scribing process amust be in between 908 and 08.The contact angle a can be expressed by means of feed

rate v, laser pulse repetition frequency f, and diameter

of the ablated area d, as follows:

sin 90� � a

2

� �¼ v

d � f ð1Þ

With the prerequisite 0<a� 908 and Equation 1 we

can derive the following criteria for flakeless back

contact patterning:

1ffiffiffi2

p � v

d � f < 1 ð2Þ

INFLUENCE OF THE BACKCONTACT PATTERNING ONTHE I/V-CHARACTERISTICS

Aspects of back contact patterning

on the dark I/V-characteristics

The influence of the back contact patterning on the

electrical characteristic of the module in the dark was

studied by means of a test structure based on single

a-Si:H and single mc-Si:H solar cells. The cells are

defined by the back contact and the size of all cells

is 1 cm2. Dark I/V-curves from cells patterned with

optimized laser conditions and a flake-free process are

shown in Figure 5.

The dark saturation current (I0) of the non-patterned

a-Si:H (Figure 5a) and mc-Si:H cell (Figure 5b) differs

by more than four orders of magnitude mainly caused

by the different band gap of the material and the defect

state distribution within the device. The influence of

the defect density on the I/V-characteristics was

discussed in more detail in References [15–18]. The

behavior of the non-patterned cells under reverse bias

is dominated by the influence of local effects.19

Therefore, the dark current of the a-Si:H and mc-Si:Hcell differs by only 1–2 orders of magnitude under

reverse bias (V<�1V). The laser line dependencewas

tested by laser lines scribed perpendicular to the back

contact edge of the 1� 1 cm2 large cells in several

steps. This test procedure is illustrated in Figure 6.

After each line-scribing step the dark I/V-behavior

of the diode was measured. Thus, the dark I/V-

characteristics as a function of the total laser line

length, the sum of the length of the single laser lines,

can be studied. An increase of the forward and reverse

bias current with increasing total laser line length was

observed for both, a-Si:H and mc-Si:H solar cells

(Figure 5). After the back contact patterning with

optimized conditions the behavior of both cell types is

comparable under reverse bias. Under forward bias, a

different behavior for a-Si:H and mc-Si:H solar cells is

still observed. In the case of a mc-Si:H cell the increase

of the current is more pronounced. For both types of

cells the increase of the dark current is probably

introduced by a heat affected zone in the vicinity close

to the patterned area. In the region near the laser line an

increased defect density, a recrystallization of the

material,20 or a diffusion of the dopands can explain

the observed dark I/V-behavior.

The dark current behavior of both a-Si:H and

mc-Si:H cells was fitted with SPICE to investigate the

influence of the line scribing on the electrical pro-

Figure 5. Influence of the back contact patterning on the dark I/V-behavior of: (a) a-Si:H single junction cell with total line

length of 0mm, 8.5mm, 25.6mm, 56.8mm, and 85.2mm and (b) mc-Si:H single junction cell with total line length of 0mm,

10.1mm, 30.4mm, 59.4mm, and 88.4mm.With increasing total line length the dark current increases for both types of cells.

Additionally, the short circuit current levels of the cells are indicated


DOI: 10.1002/pip


perties in more detail. The dark current behavior of the

a-Si:H and the mc-Si:H cell was fitted for both before

back contact patterning and after each patterning step.

The influence of the patterning could not be modeled

by a simple decrease of the shunt resistance. In order to

get a good fit to the experimental results additional

non-linear elements were added to the respective

equivalent circuit of the non-patterned cell. To describe

the I/V-characteristic under reverse bias, we added an

element with a non-linear (exponential) I/V-characteristic

in series connected with a resistor, Rreverse. In the

simulation the element with an exponential behavior

was represented by a diode, Dreverse, connected anti-

parallel to the elements of the non-patterned cell.

Under forward bias the increase of the dark current can

be described by only one additional diode, Dforward,

connected parallel to the elements of the non-patterned

cell. Figure 7 shows the modified equivalent circuit

including the additional components.

Based on the presented equivalent circuit (Figure 7)

of patterned a-Si:H and mc-Si:H cells, a good agree-

ment was achieved between experimental data and

the simulations. Examples for different total laser line

length are given in Figure 8.

The parameters of the additional elements as a

function of the total laser line length are summarized in

Figure 9. The simulations have shown that the value of

the additional resistors Rreverse behaves inversely

proportional to the total laser line length. Therefore,

Figure 6. Illustration of the back contact-patterning process

of a 1� 1 cm2 thin-film silicon solar cell. The back contacts

of the cells are patterned in several steps and cells are

characterized after each patterning step to investigate the

influence of line scribing on the dark I/V-characteristic for

different total laser line length

Figure 7. Equivalent circuit of a silicon thin-film solar cell

with patterned back contact

Figure 8. Comparison between measured (lines) and simulated (squares) dark I/V-curves of patterned (a) a-Si:H cell with

total laser line length of 0mm, 8.5mm, 25.6mm, and 85.2mm and (b) mc-Si:H cell with total laser line length of 0mm,

10.1mm, 30.4mm, and 88.4mm


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the conductance G (G¼ 1/Rreverse) that depends

linearly on the total laser line length is plotted in

Figure 9. The dark saturation current I0 of both

additional diodes Dreverse and Dforward behaves almost

linearly with the total laser line length independent

from the type of cell investigated. The diode quality

factor is nearly independent from the total laser line

length. Since the diode quality factor is significantly

larger than 2, the additional diodes are only used to fit

the measured dark I/V-behavior.

The behavior of the forward-biased diode can be

well explained under the assumption that each laser

pulse leads to an additional current contribution in the

vicinity of the patterned area, described by a diode

Dforward,i where i denotes the respective laser pulse.

After m pulses the additional current under forward

bias can be described by Equation (3):

IFW ¼Xmi¼1

If;i � expU

ni � UT

� �� 1

� �ð3Þ

whereUT is the thermal voltage, which is equal to kT/e.

ni denotes the diode quality factor of the i-th diode and

If,i describes the individual dark saturation current of

each forward-biased diode. Since thermographic

images of lines with a good quality show no indication

for local effects, the patterning step has a homogenous

influence on the electrical properties at the laser line.

Thus, the local diode saturation currents and the diode

quality factors of all local diodes are also equal and

Equation (3) simplifies to:

I ¼ m � If � expU

n � UT

� �� 1

� �ð4Þ

So the influence of the laser lines under forward

bias can be described by only one diode, which is

characterized by the dark saturation current I0 equal to

m�If and a diode quality factor n. This explains the

linear dependence of the dark saturation current on

the total line length (Figure 9b,d), as the line length is

linear proportional to m.

Similarly, we can describe the additional current

under reverse bias through diodes Dreverse,i in series

with resistors Rr,i, which are added by the laser pulses.

Under the assumption of a homogenous influence on

the electrical properties under reverse bias Equation

(4) can be modified to include the currents for Dreverse,i

and Rr,i. We obtain for the whole patterned area:

IREV ¼ m � Ir � expU � Rr � I=m

n � UT

� �� 1

� �ð5Þ

To describe the reverse bias behavior with only one

diode in series connected with one resistor, the dark

Figure 9. Parameters of the additional equivalent circuit elements versus total laser line length for a-Si:H (a), (b), and

mc-Si:H (c), (d) silicon solar cells


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saturation current I0 of this diode must be equal to m�Irand the value of the resistor must be equal to Rr/m.

Therefore, the dark saturation current behaves pro-

portional and the resistor inverse proportional to the

total laser line length. This result explains the para-

meter behavior as shown in Figure 9. The simulations

and measurements show that the back contact pattern-

ing with a green laser (l¼ 532 nm) has a very uniform

influence on the device performance. Like mentioned

above, the change of the dark current behavior is

probably introduced by an increased defect density, a

recrystallization of the material,20 or a diffusion of the

dopands.

Aspects of back contact patterning on

the I/V-characteristics under illumination

The back contact patterning also influences the cell

characteristic under illumination, since the I/V char-

acteristic of solar cells under illumination is a

superposition of the dark current behavior and the

voltage-dependent photo current. Figure 5 shows that

the dark current increases with total laser line length

while the short circuit current and fill factor measured

under AM 1.5 illumination conditions slightly decre-

ases (not shown). The decrease in Isc corresponds to

the removed cell area by line scribing. Thus, Isc of the

non-patterned area remains nearly unchanged. Recom-

bination losses of the photo-generated carriers by

the defects introduced by line scribing must be more

pronounced at maximum power point rather than at

short circuit conditions. However, an analysis of the

I/V-characteristics under illumination, taking the

principle of superposition into account, has shown

that the decrease of the fill factor originates from the

increase of the dark current with longer laser line

length. Due to the low diffusion length of the carriers

(<1mm)15 and the device structure used—the thin

absorber layer is embedded between the p- and n-type

doped layers—the photocurrent is not significantly

affected by the defects at the laser lines using

optimized patterning conditions.

Consequently, the ablation process can significantly

deteriorate the low illumination behavior of solar cells

and modules. The influence of the back contact

patterning on the solar cell and module behavior under

illumination strongly depends on the ratio between the

active cell area and the total laser line length per cell

because the active area determines the short circuit

current while the total laser line length mainly deter-

mines the dark current at smaller bias. For mono-

lithically series-connected thin-filmmodules the active

area of each cell stripe is equal to the product of width

and length of the cell stripe, whereas the effective laser

line length corresponds to the length of the cell stripe.

Thus, the ratio of the cell area and laser line length only

depends on the width of a cell stripe. Consequently,

the influence of the back contact patterning on the

module characteristic is independent of the cell length

of the module.

This is an important result for up-scaling of this

technology. Under standard test condition the per-

formance loss through the dark current increase

introduced by the line-scribing process is negligible

for typical cell stripe width of 0.7–1.0 cm. Using an

optimized process for laser-scribing thin-film modules

based on single and tandem structures were success-

fully produced. For small area modules with an

aperture area of 64 cm2, an initial efficiency of 9.5%

for a-Si:H and 8.9% for mc-Si:H modules was

achieved. An a-Si:H/mc-Si:H tandem module of the

same size was realized with an initial efficiency of

11.1%. The tandem cell module consists of 8 cells

connected in series and the a-Si:H and mc-Si:Hmodules based on 16 cells. The corresponding

I/V-curves are shown in Figure 10.

CONCLUSION

The influence of the monolithical series connection of

silicon thin-film modules fabricated by high-speed

laser ablation on the dark and illuminated I/V-

Figure 10. Silicon thin-film modules based on single or

tandem structures fabricated with the presented patterning

steps. The aperture area of all modules was 64 cm2


DOI: 10.1002/pip


characteristic was analyzed. For the front contact

patterning and the local removal of the silicon layer

only the laser power has a significant influence on the

quality of the laser-scribed lines. For the patterning of

the back contact especially the contact angle between

two successively ablated areas determines the pattern-

ing quality. A detailed description of the back contact

ablation process was introduced and a criterion for

flakeless patterning was formulated.

The influence of the back contact patterning on the

solar cell behavior was analyzed by means of a SPICE

model. The SPICE model accurately describes the

observed uniform change of the dark I/V-characteristic

with increasing laser line length. The dark I/V-

measurements in combination with SPICE simulations

represent a powerful tool to evaluate the back-contact

patterning process.

Acknowledgements

The authors like to thankW. Appenzeller, J. Kirchhoff,

G. Schope, H. Siekmann, C. Zahren, and B. Zwaygardt

for their technical support and T. Repmann and B.

Rech for helpful discussions.

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DOI: 10.1002/pip


high speed laser processing for monolithical series connection of silicon thin-film modules

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