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

Characterization of 

Multicrystalline Silicon Modules 

with System Bias Voltage 

Applied in Damp Heat

P. Hacke, M. Kempe, K. Terwilliger, S. Glick,

N. Call, S. Johnston, and S. Kurtz 

National Renewable Energy Laboratory

I. Bennett and M. Kloos 

Energy Research Center of the Netherlands

Presented at the 25th European Photovoltaic Solar Energy 

Conference and Exhibition

Valencia, Spain

September 6 – 10, 2010

Conference Paper

NREL/CP-5200-49344

July 2011
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1 

Characterization of Multicrystalline Silicon Modules with System Bias Voltage Applied in Damp Heat

Peter Hacke, Michael Kempe, Kent Terwilliger, Steve Glick, Nathan Call, Steve Johnston, Sarah Kurtz

Ian Bennett1

, Mario Kloos1

National Renewable Energy Laboratory, 1617 Cole Blvd. Golden, CO 80401 USA 1

Energy Research Centre of the Netherlands, 1755 ZG Petten, NL

ABSTRACT: Because it is considered economically favorable to build arrays with high system voltage by serially 

connecting photovoltaic (PV) modules, it is necessary to explore the potential long-term degradation mechanisms that the 

modules may incur under such electrical potential. We performed accelerated lifetime testing of multicrystalline silicon 

PV modules in 85°C/85% relative humidity (RH) and 45°C/30% RH while placing the active layer in either positive or 

negative 600 V bias with respect to the grounded module frame. A negative bias applied to the active layer leads to more 

rapid and catastrophic module power degradation compared to a positive bias. This negative bias degradation is 

associated with significant shunting of individual cells as indicated by electroluminescence, thermal imaging, and I-V 

curves. Mass spectroscopy results support ion migration as one of the causes. Electrolytic corrosion is seen occurring 

with the silicon nitride antireflective coating and silver gridlines, and there is ionic transport of metallization at the 

encapsulant interface observed with damp heat and applied bias. Leakage current and module degradation are found to be 

highly dependent on the module construction, with factors such as encapsulant and front glass resistivity affecting 

performance. Measured leakage currents range from about the same as those seen in published reports of modules 

deployed in Florida (USA) to about 100 times higher when undergoing environmental chamber testing. 

Keywords: Multicrystalline Silicon, Module, Reliability

1 INTRODUCTION 

Serially connecting photovoltaic (PV) modules to 

build arrays with high system voltage is considered to be 

economically favorable based on reduced wiring and 

inverter costs [1]. However, this requires exploration of 

potential long-term degradation mechanisms that the 

modules may incur under such electrical potential. The 

Jet Propulsion Laboratory (JPL) investigated PV module 

leakage current under system bias in the 1980s [2]. They 

studied the electrolytic corrosion mechanisms associated 

with the current that flows from the active layer of the 

module through the encapsulant to ground via the 

module frame and the acceleration of the degradation 

that occurs as a function of temperature and humidity of 

the module environment.

The National Renewable Energy Laboratory (NREL) 

later performed indoor and outdoor testing to 

characterize the long-term effects of system voltage 

applied to active layers of solar cells. It was found that

current flows on the surface of the glass and through its 

bulk in humid conditions, but at lower humidity, current 

flows through the encapsulant/glass interface between 

the module frame and the active layer. Activation 

energies were also extrapolated for each process [3]. 

Indoor studies reviewed by Osterwald [4] indicated how 

corrosion occurs in the SnO2:F transparent conductor 

layers in a-Si and CdTe modules when the active layer is 

biased negative with respect to the grounded module 

frame. Such damage is reduced by lowering humidity, 

reducing voltage potential between the frame and the 

active layer, changing from soda-lime glass to 

borosilicate glass, and moving the module mount from 

the glass edge to adhesive bonding on the module rear.

Subsequent outdoor tests by BP Solar [5], the Florida 

Solar Energy Center (FSEC) [6,7], and others 

documented how system bias affects leakage current in 

an actual hot and humid environment. Thin-film 

modules displayed leakage current outdoors often in the 

range of 0.1 to 8 mA in wet conditions. FSEC has also 

published how ions may migrate in fielded modules [8]. 

In high-efficiency, back-contact crystalline-silicon 

cells, a significant but readily reversible polarization 

affect was seen when a negative charge that developed 

over the cell surface because of leakage current (ion 

migration) through the glass face in positively biased 

strings led to annihilation of minority carriers in the ntype base. Specifying the positive terminal of the string 

as the grounded conductor has since mitigated this 

problem [9].

SOLON [10] and NREL [11] recently published 

studies detailing how system bias may also lead to 

shunting of multicrystalline solar cells. It is understood 

that negative-bias degradation involves the cells’ junction 

properties because SOLON’s work showed that widening 

of the depletion width through appropriate doping in the 

cell improved resistance to shunting. The NREL study 

indicates that dark I-V measurements show degradation 

or improvement in the diode-junction recombination 

parameters when negative or positive bias, respectively, 

are applied to the active layer. Ionic transport to the 

junction region was proposed as one of the possible

mechanisms. A number of methods to mitigate 

degradation caused by high voltage potential were 

proposed, ranging from variations in the cell’s silicon

nitride antireflective coating to increasing the resistance 

to leakage current in the package design. 

In this work, we further explore the degradation 

mechanisms such as the corrosion and the ionic motion 

that the stress of system bias exerts on PV modules. The 

leakage current and the path of that current—and how the 

module packaging contributes to them and the resulting 

degradation—are examined.

2 EXPERIMENT

Six commercially available Si module models in the 

160–230 Wp range were subjected to 85°C/85% relative 

humidity (RH) in positive or negative 600 V bias, DH(+)

or DH(-), respectively. The voltage bias was applied to 

the shorted module leads and the frame was grounded. 

The rated system voltage of these modules ranged from 
2 

600 to 1000 V. The current that flowed through each 

module under bias was monitored continuously over the 

course of the damp heat test. Modules were 

characterized by light and dark I-V measurements, 

imaged thermally, optically, and by electroluminescence. 

Two replicas of one of these module designs were 

stepped through a series of temperatures and RH stresses

to determine the activation energy of current flow. 

A designed experiment was carried out to test the 

effect of module packaging. The designs shown in 

Table 1 were constructed using 156-cm2 multicrystalline 

cells encapsulated in single-cell mini module 

configurations. Additionally, the cells in the mini

module group were divided into high lifetime (ingot 

center), low lifetime (ingot top or bottom), and mixed 

areas that include regions of low lifetime associated with 

the ingot edge that could be seen in electroluminescence. 

These frameless mini modules were placed in 45°C/30% 

RH with –1000 V applied to the shorted module leads 

with respect to the front glass surface, on which an antistatic carbon paste was applied to maintain the surface at 

ground potential. The samples were characterized for 

performance by Suns-Voc [12], thermal imaging, and 

electroluminescence.

Table 1. Constructions of one-cell mini modules. Six 

replicas per design were made. 

Design Glass Encapsulant

1 Quartz EVA

2 

Soda-lime 

with sodium 

barrier 

EVA

3 Soda-lime EVA

4 Soda-lime EVA with sodium acetate

5 Soda-lime Ionomer-thermoplastic

6-control Soda-lime EVA

3 RESULTS AND DISCUSSION

3.1 85°C/85% RH with positive bias—commercial 

modules

 In modules with the cells in the DH(+) configuration, 

corrosion of the silver grid fingers was apparent (Fig. 2). 

In aqueous solutions with bias applied to the cell, the 

reaction Ag  Ag+ + e- exists, whereby the Ag 

dissolution is proportional to the voltage applied [13]. 

The oxidation of the silver is a factor leading to increased 

series resistance of the module [11]. Leakage current has 

been reported to be greater with positive bias applied to 

the cell because of the comparative ease of ionization of 

the Ag anode, which provides the source of ionic 

migration leading to a leakage current through the wet 

encapsulant medium [2]. We however observed higher

current flow with positive bias to the active layer in 

model 1 only (Fig. 1). In most cases, the ionic motion 

was apparently limited in a place weakly influenced by 

the direction of the bias. 

The silicon nitride antireflective (AR) coating of the 

cells was observed to thin relatively uniformly—more

rapidly in the case when there was greater moisture 

ingress, such as with delamination. Silicon nitride 

degradation was significantly more severe in model 1 

compared to others tested. Not only was silicon nitride 

thinning observed in model 1 with bias applied, but also 

to a small extent in those tested in 1000 h at 85°C/85%

Fig. 1. Coulombs transferred for six module models after 

1000 h of 85°C/85% relative humidity testing. Negative

coulombs are indicated when negative bias is applied to 

the active layer. The models, excepting model 1, exhibit a 

similar rate of charge flow for either polarity.

RH without bias. Silicon nitride films are found to react 

with water at high temperature and pressure to form 

hydrous silica [14].

Because the silicon nitride degradation occurred 

much faster with bias applied, the reaction appears to be 

accelerated electrolytically. It has been reported that the 

AR coating on a cell comes under attack in a 0.1 molar 

HCl solution when a bias of 9 V is applied; however, the 

nature of the AR coating was not specified in that work

[13]. 

Ionic transport from the bus ribbons to module edge

was the principal phenomenon that was studied by JPL

with the XTEST mini module sample geometry, whereby 

an electrode representing the grounded frame is 

positioned in the encapsulant at a set distance from a 

single cell [2]. Ion migration between the cell and 

electrode is precipitated with bias, temperature, and 

humidity. More recently, NREL has shown that when

conduction occurs along the front surface of glass, such 

as in high relative humidity, conduction through the glass

dominates with activation energy around 0.9 eV for 

crystalline-silicon modules, whereas the glass/ 

encapsulant interface conduction is more prevalent at 

lower relative humidity with activation energies of 0.4— 

Fig. 2. Region of a module (model 1) after 2000 h in 

damp heat with + 600 V applied to the active layer with 

respect to the grounded module frame. There is evidence 

of grid-finger corrosion and metal-ion transport from the 

bussing, especially in regions of de-adhesion in the model 

package seen on the bottom right of the image. 
3 

0.6 eV [3]. In these outdoor tests, the humidity inside the 

encapsulant is likely to become equilibrated, so the 

measured humidity in this case is that of the package 

exterior. The humidity increases the conduction of the 

face of the glass and bleeds current to ground.

Two replicas of module model 1 were subjected to 

various temperatures (Fig. 3) and relative humidities

(Fig. 4) in the positive bias configuration. At a constant 

RH of 30%, the activation energy of these modules for 

current flow was 0.86 eV, which corresponds closely to 

the activation energy of current flow through glass 

(0.9 eV) [3]. This suggests current flow through the 

module face as the rate-limiting path for leakage, even at 

modest RH. Resistivity of the polymeric materials used 

in PV modules is orders of magnitude higher than sodalime glass [15]. If the interfaces were to degrade in 

humidity or because of ultraviolet light, the current flow 

can revert through the interfaces. This was observed in 

model 1 placed at 85°C/85% RH with +600 V applied to 

the active layer with respect to the grounded module 

frame for 2000 h (Fig. 2). Current flow in this model

was significantly elevated, as can be seen in Fig. 1.

This module design was also placed at 85°C with 

humidity stepped up to 85%. In the 65% to 70% range, 

there appears to be additional current flow mechanisms 

leading to a much higher current leakage. This is in 

contrast to the work of JPL, which found that leakage 

Fig. 3. Plot of leakage current of a module (model 1) at 

five temperatures (25°, 40°, 55°, 70°, and 85°C) and 

30% RH. The activation energy for the current flow was 

0.86 eV, which is similar to that reported for soda-lime 

glass. The voltage applied to the active layer was 

+600 V.

Fig. 4. Median leakage current as a function of RH at 

85°C for two replicas of model 1. In the range of 60%–

85% relative humidity, leakage current increases 

significantly. The voltage applied to the active layer was 

+600 V.

current saturates at higher humidity levels, but in parallel 

to the work of BP Solar, which found that the interfacial 

adhesion rate drops precipitously at 85% RH compared to 

60% RH [5]. Considering that we observe interfacial ion 

migration in this model design in 85°C/85% RH (Fig. 2), 

there is remarkable consistency between the high leakage 

current, reduced interface strength, and visible ionic 

motion in model 1. In the framework of the results of del 

Cueto [3], we may consider that with weakened 

interfaces for reasons such as higher humidities than seen 

in outdoor tests, the leakage current can revert from 

through-glass to the encapsulant/glass interface.

3.2 85°C/85% RH with negative bias—commercial 

modules

Commercial modules were tested in 85°C/85% RH 

with –600 V applied to the active layer. Fill factor was 

the major source of power loss, and the root cause was 

cell shunting, which has been demonstrated previously 

for modules stressed in such conditions [10,11]. 

Significant differences in the performance of the modules 

can be seen after 1000 h, at which time some modules 

retained all their power, and some modules lost all their 

power (Fig. 6). Also of note was that the cells used in the 

construction of the modules that exhibited some of these 

extremes were reported to be from the same cell maker, 

pointing to the packaging as being an important 

modulator of this shunt degradation.

As with the positive bias case discussed above, 

silicon-nitride thinning occurred with increased moisture 

ingress, but much more drastically. Negative potential on 

the active layer can pull Na from the soda-lime glass 

toward the cell surface [16]. Studies of decomposition of 

silicon nitride have shown that sodium assists in the 

oxidation of silicon nitride at high temperature to form 

sodium silicate [17]. Although the chemistry of this 

system requires further study, the application of negative 

bias to the active layer in the sodium and-water-saturated 

Fig. 6. Performance of five commercial modules in 85°C/ 

85% RH, -600 V applied to the active layer for 1000 h 

rounds (left), and the detailed module test results for 

model 1 (right). 
4 

Fig. 7. Complete removal of the silicon nitride cell 

coating seen after 1000 h in 85°C/ 85% RH with –600 V 

applied to the active layer. Silicon nitride remains, 

however, in the center of the cell where water 

concentration is expected to be lower.

environment results in electrolytically assisted stripping 

of silicon nitride from the cell in a faster, more localized 

way than in the DH (+) configuration (Fig. 7). 

Although migration of Na ions under bias has been 

studied in thin-film modules, the effects have not been 

well established in c-Si modules. SIMS analysis was 

carried out, comparing the silicon surface of a module 

exposed for 1000 h in 85°C/85% RH to a control that had

never been encapsulated. The stressed sample was 

removed from an area of a module that exhibited 

delamination for ease of removal from the encapsulant. 

Although sample preparation was enabled because of the 

delamination, shunting in these regions tended to be 

lower according to thermal imaging; therefore, 

concentration of species causing shunting may be higher 

elsewhere. Also, one should note that the textured 

multicrystalline surface leads to inaccuracies in the depth 

resolution. With these provisos, it can be seen that a 

very high Na concentration exists in the vicinity of the 

surface of the stressed sample (Fig. 8). Cu, a species 

known to shunt junctions very effectively [18], can be 

seen in concentrations that are as much as one order of 

magnitude higher in the stressed sample compared to the 

control. The concentration of Fe was not found to be 

elevated in the stressed sample compared to the control, 

evidence that elevated Fe concentration is not a cause of 

the observed shunting degradation in the DH (-) 

configuration. 

Fig. 8. SIMS analysis of a cell in a module that has 

undergone 1000 h of 85°C/85% RH with –600 V applied 

to the active layer. Significant Na accumulation and up 

to an order of magnitude higher Cu concentration is seen 

near the surface. Fe concentration was not significantly 

changed.

JPL attempted to make correlations between 

coulombs leaked per unit length of edge of the module 

frame (ground electrode) and the power degradation [19]. 

They suggested that precipitous (>50%) degradation 

occurred between 1 and 10 coulombs per centimeter 

(C/cm) of charge transfer. These results were obtained 

for stress conditions in the range of 70°–80°C, RH 

between 0% and 100%, and voltage bias between 30 and 

500 V; however, the polarity of the bias was not 

specified. We examined this relationship for the six 

commercial module designs. The results shown in Fig. 9 

(top) for +600 V bias applied to the active layer trend 

consistently with JPL results. Further, there is a 

noticeable and reasonably reproducible degradation of

about 5% with charge transfer in the 0.1–1.0 C/cm range

for the various models tested. After examining the 

degradation with negative bias to the active layer in

Fig. 9 (bottom), one can see that the power is 

significantly degraded with less than 1.0 C/cm charge 

transfer—always more than 20% and usually more than 

70% degraded. These results, however, offer little 

resolution in the 0.01 to 0.1 C/cm range. Bracketing the 

degradation over a decade, the failure occurs somewhere 

in the range of 0.02 to 0.2 C/cm.

Fig. 9. Fraction of module power remaining vs. 

accumulated charge transfer for commercially available 

crystalline-Si modules with +600 V bias to active layer 

(top) and –600 V bias to active layer (bottom).

3.3 45°C/30% RH with negative bias—one-cell mini

modules

One-cell mini modules with various constructions 

were exposed to –1000 V bias applied to shorted leads of 

the module in a 45°C/30% RH chamber. These chamber 

conditions were chosen to achieve an average activity for 

water that a module encapsulant may see in Florida 

(USA). An anti-static carbon-containing film was 

applied to the module faces over the cells with the goal of 

keeping the front surface at ground potential, as rain, 

mist, or condensation on the module face would provide, 

but in a constant and uniform manner.
5 

Fig. 10. Loss of pseudo fill factor (FF) and open-circuit 

voltage (Voc) after 140 h at 45°C/30% RH with –1000 V 

bias applied to the active layer for five mini module 

constructions (plus controls). Cells were made with 

wafers from various positions in the ingot. Control 

modules were not stressed. 

The effect of this stress on mini module performance 

is shown in Fig. 10. The performance can be divided 

into three categories. (1) Samples with a quartz face or 

soda-lime glass in combination with an ionomer 

encapsulant did not degrade. (2) Soda-lime glass with an 

added SiO2 barrier, as is used by the thin-film PV 

industry to mitigate Na migration, performed in an 

intermediate manner, but still showed significant 

degradation. (3) Conventional ethylene vinyl acetate 

(EVA) or EVA with added sodium acetate using sodalime glass performed the least favorably. Based on these 

tests, there is no relationship between the degradation 

and the various impurities such as Fe and O that exist in 

higher concentrations in the silicon wafer at the ingot 

edges. Ingot edge regions that appeared dark due to 

lower lifetime in electroluminescence had no correlation

with shunted regions after bias according to thermal 

imaging.

We have previously speculated that glass AR

coatings may improve the performance of modules 

undergoing system voltage stress [11]. The present 

results further support this hypothesis because the sodalime glass with an added SiO2 barrier yields an 

improvement over the samples without the barrier (Fig. 

10). Manufacturers such as SunPower have indicated 

that such films used as AR coatings are of higher purity

than the glass itself and thus resist ion leaching [20]. 

The mini module data are applied in Fig. 11 to 

further understand the relationship between power 

degradation and coulombs per centimeter of module edge

when there is a negative bias on the active layer. It is 

confirmed that less than 1x10-5 C/cm has no measureable 

effect on performance. Marked degradation occurs in the 

0.001–0.01 C/cm range. We could not, however, resolve

the effect of the SiO2 film on glass with regard to

reducing current flow, despite its ability to slow the 

voltage-bias degradation.

Whether it is because of the mini module design or 

the anti-static C paste that is used to transfer charge to 

Fig. 11. Loss in power (as estimated by Suns-Voc 

technique) for five mini module constructions, 

superimposed over those results of commercial modules.

the cell more efficiently, the mini module degradation 

rate is much faster than that found in the damp-heat 

chamber-loaded commercial modules on a C/cm basis 

(Fig. 11). We therefore chose to exclude the mini

module data in our estimate of 0.02–0.2 C/cm as the 

threshold for degradation of real modules in the DH(-) 

configuration that we extracted from the commercial fullsize module data in Fig. 9 (bottom). It must, however, be 

considered that some differences in this threshold may be 

expected depending on current paths and nature of the 

materials used.

To put the lower limit of this range, 0.02 C/cm, in 

perspective, model 1 is analyzed further. Considering a

leakage current through the glass at the IEC 61215 ed. 2 

section 10.15 wet-leakage current limit, the area is

1.2 m2

, the resistance limit 33.2 MΩ, and the current 3 x 

10-5 A at 1000 V. With a perimeter of 475 cm, 9.5 C 

would flow to reach this 0.02 C/cm threshold, and it 

would reach this threshold in 88 h. This is on the same 

order of the results previously reported for a module to 

degrade under system bias [10]. Instantaneously 

measured leakage currents of model 1 and the calculated 

time to reach 0.02 C/cm are given in Table 2 for other 

environmental conditions. The simplifying assumption 

that current is stable over time is made, which increases 

the uncertainty. Examples of mini module leakage 

Table 2. Estimated time to failure for a 165 W mc-Si 

module in –1000 V bias (0.02 C per cm module frame 

threshold). Except for the current defined by the IEC 

61215 ed. 2 limit, the leakage current was measured 

under the conditions indicated.

Conditions Leakage 

current (A)

Time to 0.02 

C/cm (h)

IEC 61215 ed. 2 wetleakage current limit 3 x 10-5 88

85°C/30% RH 

(chamber) 9 x10-6 293

55°C/30% RH

(chamber) 1 x 10-7 26,400

55°C/~30% RH

anti-static glass surface 

(outdoors)

2.2 x 10-4 12 

33°C/~30% RH

wet glass surface 

(outdoors)

4 x 10-8 66,000
6 

current vs. time relationships have been previously 

published [2]. The time to reach the 0.02 C/cm threshold 

is seen to strongly depend on the environmental and 

surface condition. The leakage current was measured to 

be linear with applied voltage up to –1000 V. 

Under hot conditions such as 85°C, which is shown 

to occur on roof-mounted modules with marginal air gap, 

and with moderate humidity, the degradation is predicted 

to be quite rapid. The wet-module condition at 33°C led 

to relatively slow predicted degradation; however, it 

would be expected that with higher temperature and 

increased conductivity in the water film (such as with 

salt mist or ion-containing pollution), the degradation 

would move toward the more rapid anti-static film-onglass condition.

The current flowing from the active layer to ground 

has been also measured at NREL (Golden, Colorado, 

USA) [3], and the Florida Solar Energy Center (FSEC)

[7]. At these locations, maximum leakage current of 

2x10-6 A and 8x10-6 A, respectively, are reported under 

high humidity conditions. Outdoor-mounted mc-Si 

modules at NREL have recently been reported not to 

degrade under system voltage bias [21]; however, such

modules at the more hot and humid FSEC location have 

degraded 4%–10% in power over several years [22]. 

Both studies, however, apply bias day and night, which 

accelerates the degradation through the humid (but 

cooler) night conditions. More work is therefore 

necessary to understand the degradation rate associated 

with high system voltage bias in fielded modules.

4.0 SUMMARY AND CONCLUSION

Differing degradation mechanisms are observed 

under positive and negative system bias voltage in damp 

heat and the rate of degradation is strongly differentiated 

by the polarity of the bias. A negative system bias 

applied to the active layer leads to more rapid and 

catastrophic module power degradation within 1000 h. 

This is associated with significant shunting of individual 

cells. A positive system bias is associated with Ag gridfinger corrosion and ion migration from the cell or 

interconnect metallization to the grounded module frame, 

especially in areas of high moisture ingress and 

weakened encapsulant. Based on the measured 

activation energy for module leakage current of 0.86 eV, 

ion transport through glass is the dominant mechanism at 

30% RH (25°–85°C), and the magnitude of the leakage 

current is generally independent of bias direction except 

in a module design with high moisture ingress and

weakened encapsulant adhesion. The threshold for 

severe degradation of 1–10 C/cm is still applicable per 

the work of the Jet Propulsion Laboratory (1985) for (+) 

bias to active layer, but we additionally find that 0.1–1.0 

C/cm leads to ~5% degradation. A 0.02–0.2 C/cm 

threshold is proposed for (-) bias to active layer based on 

measurements of six commercial crystalline-silicon 

modules. Some types of degradation observed, such as 

electrolytic corrosion, will happen after many years in 

hot/humid environments, but others, such as ion 

migration, happen more quickly and are of pressing 

concern. Higher electrical resistance packaging will help 

reduce electrolytic and ion migration-related degradation, 

which can be controlled by choices including the glass 

and its coatings, the encapsulant, and the frame.

ACKNOWLEDGEMENTS

We thank David Trudell and Ed Gelak for help with 

the accelerated lifetime testing; Steve Rummel, Allan 

Anderberg, and Keith Emery for module measurements; 

Yu Wang, Kaitlyn VanSant, Ron Sinton, Michael 

Quintana, and Prof. Govindasamy Tamizhmani for 

helpful discussions. This work was supported by the U.S. 

Department of Energy under Contract No. DE-AC36-

08GO28308 with the National Renewable Energy 

Laboratory.

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