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Contract No. DE-AC36-08GO28308

Large-Area Material and 

Junction Damage in c-Si Solar 

Cells by Potential-Induced 

Degradation

Preprint

Chuanxiao Xiao, Chun-Sheng Jiang,

Steve P. Harvey, Jun Liu, Helio Moutinho,

Peter Hacke, Steve Johnston, 

and Mowafak Al-Jassim

National Renewable Energy Laboratory

Presented at the 2018 World Conference on 

Photovoltaic Energy Conversion (WCPEC-7)

Waikoloa, Hawaii

June 10–15, 2018

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Suggested Citation

Xiao, C., C.-S. Jiang, S.P. Harvey, J. Liu, H. Moutinho, P. Hacke, S. Johnston, 

and M. Al-Jassim. 2018. “Large Area Material and Junction Damage in c-Si Solar 

Cells by Potential-Induced Degradation.” Preprint. Golden, CO: National 

Renewable Energy Laboratory. NREL/CP-5K00-70819.

https://www.nrel.gov/docs/fy19osti/70819.pdf.

Conference Paper

NREL/CP-5K00-70819

October 2018
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Large-Area Material and Junction Damage in c-Si Solar Cells by 

Potential-Induced Degradation

Chuanxiao Xiao, Chun-Sheng Jiang, Steve P. Harvey, Jun Liu, Helio Moutinho, Peter Hacke, 

Steve Johnston, and Mowafak Al-Jassim

National Renewable Energy Laboratory, Golden, CO 80401 USA

Abstract — In this work, we discuss a new fundamental PID 

mechanism that has not been reported. We developed in-situ 

Kelvin probe force microscopy to monitor the potential evolution 

at nanometer scale under high-voltage stress. We observed largearea junction degradation during the stressing and junction 

recovery by heat treatment from the same location. Electronbeam induced current (EBIC) results support the large-area 

damage, which has a much lower collected current (dark region) 

and has an abrupt transition between the bright and dark areas, 

in addition to local shunts. Transmission electron microscopy does 

not find stacking faults in the dark-EBIC region. Furthermore, 

time-of-flight secondary-ion mass spectrometry indicates that the 

large-area damage correlates with more sodium content. The 

consistent results shed new light on PID mechanisms that are

essentially different from the widely reported local-junction 

shunts.

Index Terms — Potential-induced degradation, Si, large-area 

damage, sodium, microscopy.

I. INTRODUCTION

Potential-induced degradation (PID) of crystalline Si 

modules has been an important reliability issue [1]. It has been

widely reported that PID is related to sodium ions penetrate

into extended defects that penetrate p-n junction, thus causing

local electrical shunting [2]–[4]. In this work, we discuss a new 

fundamental PID mechanism that has not been reported. 

Sodium (Na), in addition to penetrate into stacking faults, 

likely penetrate widely into the bulk and causes large-area 

material and junction degradation by forming point defects. In 

the 2017 PVSC, we reported a large area of millimeter-scale 

material and p-n junction damage caused by PID, and the 

electrical transition is abrupt between good and damaged areas

[5]. In this paper, we developed in-situ Kelvin probe force 

microscopy (KPFM) to monitor the potential evolution at 

nanometer scale under high-voltage stress. We observed 

junction degradation during the PID stressing and junction 

recovery by heat treatment from the same location. Electronbeam induced current (EBIC) results support the large-area 

damage, which has a much lower collected current (dark 

region) and has an abrupt transition between the bright and dark 

areas, in addition to local shunts. Transmission electron 

microscopy (TEM) does not find stacking faults in the darkEBIC region. Furthermore, time-of-flight secondary-ion mass 

spectrometry (TOF-SIMS) indicates that the large-area damage 

correlates with more Na content. The consistent results shed 

new light on PID mechanisms that are essentially different 

from the widely reported local-junction shunts.

II. EXPERIMENT DETAILS

KPFM is an atomic force microscopy (AFM)-based 

technique that measures the electrostatic potential on a 

sample’s surface with a 30-nm spatial resolution and a 10-mV 

potential resolution [6]. We have developed in-situ highvoltage stressing capability on an AFM platform and 

successfully applied KPFM to investigate the PID issue [7].

The KPFM measurements on cross-sections of silicon solar 

cells allow direct imaging of the evolution of the electrical 

potential across the p-n junction during the degradation 

process.

Fig. 1. In-situ stressing capability with KPFM measurement

To simulate the PID process, the sample architecture is made 

to be identical to a real module: a piece of multicrystalline Si

cell is covered by pre-polymerized ethylene vinyl acetate 

(EVA) and a tempered solar glass. The cell studied in this work 

is the conventional aluminum back-surface-field (Al-BSF) ptype Si cell, with a single front junction formed by a 

phosphorous-doped emitter and SiNx on the top surface. 

Because the KPFM technique requires a flat surface, the crosssectional sample was chemical-mechanically polished to make 

the overall cross-section surface corrugation less than 20 nm.

Figure 1 shows a schematic of the in-situ characterization. A

constant bias voltage of -1,500 V was applied to the shorted

cell-piece with glass and EVA at a temperature of 25°C for 

over 40 days. Note that this timeframe is much longer than in 

regular PID accelerated life testing (~two weeks) [4]. The 

potential profiles were monitored by KPFM. In the 

measurement, we applied different small bias voltages across 

the cell Vb = -1.5 ~ +1 V. The longer stress time is usually 

necessary in our case. PID may randomly cause highlynonuniform defective areas even though a uniform high 

voltage is applied. 
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Fig. 2. a)–d) AFM images indicating the same characterized location under stressing and recovery; e)–h) KPFM mapping of potential evolution 

across the p-n junction for over 40 days, corresponding to a–d.

We cannot control exactly how long time we need to apply 

the high voltage, to make the area involving the cross-section 

get significantly degraded. However, we usually observed the 

degradation after certain stressing time by monitoring the 

change in the potential image.

In parallel, a sample from the same Si cell went through 

similar PID stressing and was characterized by dark lock-in 

thermography (DLIT), EBIC, TEM, and TOF-SIMS. Note that 

after the laboratory PID stressing, the cell was extracted 

without mechanical damage to avoid possible measurement 

artifacts. This is a multiscale approach: 1) DLIT images large 

scale at hundreds μμμμ to roughly determine the degraded areas; 

2) higher-magnification EBIC mapping was done at the same 

degraded location; 3) TOF-SIMS analyzed the chemical 

contamination on the same area as EBIC, and 4) TEM 

investigated the dark EBIC area.

III. RESULTS AND DISCUSSION

We used the in-situ stressing capability in KPFM 

measurements and monitored the potential evolution across the 

p-n junction along with stressing voltage of -1,500 V for over 

40 days. We found that the potential evolution is uniform on a 

long range over hundreds of μm: PID stressing shifts the 

potential-drop location to the cell front-surface and the 

potential amplitude decreases gradually to ~0. The potential 

drop at the junction area eventually smears out. Heat treatment 

partially recovers the degraded area. The results are shown in 

Fig. 2. AFM images (Figs. 2a–2d) show that the measurements 

were taken in the same location. The Si/EVA interface is 

indicated by the black dashed line. KPFM potential images 

(Figs. 2e–2h) were taken at reverse-bias voltage of -1.5 V.

Initially, a fresh cell is measured in Figs. 2a and 2e. In Fig. 2e,

the applied voltage drops the most (the potential change

between bright and dark areas) at ~500 nm to the Si/EVA 

interface, indicating a normal p-n junction. After 20 days of 

PID stressing, the potential-drop position shifted ~200 nm

toward the Si front surface (Fig. 2f). As the stressing continued

for 40 days, the potential drop was no longer measured (Fig. 

2g), suggesting a full collapse of junction potential. After 

thermal annealing at 300°C for 30 min, we observed a junction 

recovery. However, the recovery is not fully healed back to a 

normal junction (Fig. 2e), and potential-drop position and 

amplitude are similar to Fig. 2f, suggesting a partial recovery. 

Note that Figs. 2c, 2g, 2d, and 2h were combined by two 

consecutive scans, because we scan a very large area of ~350-

μm total length along the junction every time period with every 

scan of 5×5 μm2

. These results are similar to what we observed 

in our ex-situ KPFM characterization [5].

Another piece of plan-view sample was cut out from the 

same fresh cell as used in the KPFM imaging. This piece went 

through PID stressing with -1500 V stressing for 60 days at 

room temperature (RT). Then, this piece was studied by DLIT 

and EBIC, with results shown in Fig. 3. Figure 3a is the optical 

image of the sample, and Fig. 3b is the DLIT phase image taken 

at -1 V reverse bias, which showed several degraded locations. 

But due to the relatively low spatial resolution of the technique, 

we cannot determine whether they are point shunts or largearea degradation. We used EBIC to zoom into the area 

indicated by the purple rectangle. We could not distinguish a 

crack at the location of the sharp change of dark and bright 

regions by SEM (Fig. 3c). However, figure 3d clearly showed 

a large area of weak current collection (dark region), and the 

transition between dark and bright region is sharp. Note that in 

both the dark and bright regions, there are dark points induced 

by the effect of surface roughness. The EBIC line profile along

the orange line in Fig. 3d (blue curve) is overlapped in the 

image. The large dark region can be caused by poorer qualities 

of both/either the underlying Si material and/or junction, such 
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as shorter minority carrier lifetime (SHR recombination) and 

larger diode quality factor (junction recombination) than the 

brighter area. However, it should not be caused by the effect of 

point shunts nearby, because of the abrupt change of EBIC 

signal. The abrupt EBIC signal change may be due to different 

PID accessibility in different orientations of the two grains but 

should not be due to the original lifetime before PID, because 

of the higher Na content of the dark EBIC area than the bright 

area, as shown later. Investigation of grain orientation and 

structure by TEM is under way.

Further, the EBIC areas were analyzed by TOF-SIMS to 

investigate the chemical impurity, especially searching for Na+. 

Figure. 4a shows the 3D render of Na+ on the 500 ×500 μm2

; 

the Na+ impurity map is correlated to the EBIC dark region. 

There is significantly more localized Na+ that has been 

attributed to Na-decorated extended defects [4] on the lower 

right region than the upper left region. In Fig. 4b, we take the 

depth profiles in areas away from the localized highly Nacontaminated area but still in the region with dark EBIC signal. 

The red and blue points indicate the location for taking the Na+

profiles. The dark-EBIC region has a Na+ concentration of mid 

1019 cm-3

, whereas Na is 2–5 times less in the bright-EBIC 

region. Also, we did TEM on a random area in the dark-EBIC 

region, and no extended defects were found in this random liftout region (15 μm long x 100 nm wide ×15 μm deep). We 

note that this is a small sampling area compared with the EBIC 

and SIMS imaged area, should not involve a loalized highlyNa decorated region. However, in this area, Na content should 

approximately be in mid-1019/cm3 (the red curve in Fig. 4b) and 

higher than the bright-EBIC area (blue curve). 

Fig. 3. a) optical image of a stressed Si piece; b) DLIT phase image 

of the Si corresponding to a); c) SEM image of EBIC area; d) EBIC 

mapping of the purple rectangle area indicated in b).

Besides the localized regions of high Na concentration going 

into the depth of the Si, the common heavier Na+

concentration in the dark-EBIC region suggests a higher

Fig. 4. a) TOF-SIMS 3D render of Na+; b) Na depth profiles of EBIC 

dark and bright area; inset is EBIC image, and the red and blue spots 

indicate the location of Na profiles taken.

concentration of point defects in the heavily PID-degraded 

area. Point defects formed by Na+ in Si have been reported to 

induce six gap states, two of which can be detrimental for PV,

with one at 0.27 eV above the valence band maximum (VBM)

(deep level) and the other being donor-like at 88 mV above the 

VBM [8]. These two defects can degrade the solar cell

performance by different mechanisms of Shockley-Read-Hall 

(SRH) recombination and altering doping around the junction 

and in the Si bulk. More importantly, Na+ concentration in the 

PID area is high, on the order of up to 1019/cm-3 (for reference, 

the average Na+ concentration in Si bulk in ref. [8] is 1013/cm3

); so this level can cause massive SRH recombination and 

massive lifetime degradation. If a p-n junction is formed by Si 

material with such a poor quality, the junction cannot be well 

formed with very leaky carrier transport through the gap states, 

causing a large J0 and small equivalent resistance (Rj) of the 

junction. KPFM measures the voltage drop by measuring the 

potential change induced by applying a bias voltage to the 

device. If Rj is significantly smaller than the series resistance 

of other parts than the junction (including the electrical 

contacts and conduction lines), then the voltage drop will not 

be at the junction and KPFM shows no or a small voltage drop 

at the junction location. As for the effect on EBIC, if the 

lifetime of Si material is much degraded by SRH

recombination, then minority carriers cannot be collected by 

the junction before they recombine, making a small EBIC 

current. On the other hand, the donor-like state is just above the 

VBM, lower than all other acceptor-like states; so this level 

with the high Na concentration can largely alter the p-doping 

of Si on the order of ~1016/cm-3

—making the p-n doping 

boundary and the bulk p-doping nonuniform, and also making 

the doping of both n and p around the junction much smaller 

than the original p-n junction, and thus, widening the depletion 

region. This potential profile is consistent with the poor and 

irregular p-n interface with much lower carrier concentrations 

at both the p and n sides. To increase confidence in the analysis, 

macroscopic regions that show high Na+ point defects without 

localized regions of high Na concentration going into the depth 

of the Si (from 2D or 3D defects) remain to be isolated. 

Further, models for why Na may penetrate certain grains as 

point defects and not others, and by what mechanism Na may 

be introduced into the Si bulk remain to be clarified. On another 
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hand, the localized highly Na-decorated extended defects may 

couple with relatively high-Na content nearby, and both the 

extended defect and the surrounding area may all contribute to 

the PID with different mechanisms. 

IV. CONCLUSION

We found a new PID mechanism of silicon modules that is 

different from and complementary to the current knowledge 

that PID shunts are localized in small and separate spots. We 

found that degradation not only occurs in stacking faults, but 

also, can cause materials and junction damage across large 

areas. In-situ KPFM results showed uniform potential 

evolution across the p-n junction; DLIT, EBIC, and TEM 

results are consistent with the large-area degradation. And 

TOF-SIMS indicated that Na is also correlated to the PID in a 

large area. A large amount of Na possibly causes point defects 

in Si. This study sheds new light on the c-Si PID mechanism.

ACKNOWLEDGEMENTS

This work was supported by the U.S. Department of Energy 

under Contract No. DE-AC36-08GO28308 with the National 

Renewable Energy Laboratory. The U.S. Government retains 

and the publisher, by accepting the article for publication, 

acknowledges that the U.S. Government retains a 

nonexclusive, paid up, irrevocable, worldwide license to 

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

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

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