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

Understanding Light-Induced 

Degradation of c-Si Solar Cells

Preprint

Bhushan Sopori

National Renewable Energy Laboratory

Prakash Basnyat, Sudhakar Shet, 

and Vishal Mehta

National Renewable Energy Laboratory 

and New Jersey Institute of Technology

Jeff Binns and Jesse Appel

MEMC Electronic Materials

Presented at the 2012 IEEE Photovoltaic Specialists Conference

Austin, Texas

June 3–8, 2012

Conference Paper

NREL/CP-5200-54200

June 2012
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1

Understanding Light-Induced Degradation of c-Si Solar Cells

Bhushan Sopori,

1 Prakash Basnyat,1,2 Srinivas Devayajanam,1,2 Sudhakar Shet,1,2 Vishal Mehta,1,2

Jeff Binns,

3 and Jesse Appel4

1

National Renewable Energy Laboratory, Golden, Colorado, USA

2

New Jersey Institute of Technology, Newark, New Jersey, USA

3

MEMC Electronic Materials, Portland, Oregon, USA

4

MEMC Electronic Materials, St. Peters, Missouri, USA

Abstract — We discuss results of our investigations toward

understanding bulk and surface components of light-induced 

degradation (LID) in low-Fe c-Si solar cells. The bulk effects,

arising from boron-oxygen defects, are determined by comparing 

degradation of cell parameters and their thermal recovery, with 

that of the minority-carrier lifetime (τ) in sister wafers. We found 

that the recovery of τ in wafers takes a much longer annealing 

time compared to that of the cell. We also show that cells having 

SiN:H coating experience a surface degradation (ascribed to 

surface recombination). The surface LID is seen as an increase in 

the q/2kT component of the dark saturation current (J02). The 

surface LID does not recover fully upon annealing and is 

attributed to degradation of the SiN:H-Si interface. This 

behavior is also exhibited by mc-Si cells that have very low 

oxygen content and do not show any bulk degradation.

Index Terms — crystalline silicon, solar cells, light-induced 

degradation, minority-carrier lifetime, surface, bulk, annealing.

I. INTRODUCTION

It is now well recognized that c-Si solar cells fabricated on 

Czochralski (CZ) wafers exhibit light-induced degradation 

(LID) of the cell performance [1,2]. This effect is generally 

ascribed to boron-oxygen (B-O) defects in the wafer itself — a 

defect complex that is formed by prolonged exposure of CZ 

wafers to light (via a carrier-induced recombination 

mechanism) and is accompanied by a reduction in the 

minority-carrier lifetime in the bulk of the wafer [3]. This 

degradation in lifetime of the wafer is thought to be fully 

recoverable upon annealing, which causes dissociation of the 

B-O complex [4].

The LID in solar cells has been studied by a rather small 

number of research groups, using relatively small sets of 

samples [5,6]. Consequently, there is a large diversity in the 

results reported in the literature, and the details of cell 

degradation and recovery mechanisms are sketchy. For 

example: i) Reported degradation in the efficiency of cells 

fabricated on the wafers of about the same resistivity ranges 

between 0.5 and 1.5% absolute [6]; ii) It is believed that LID 

can be fully recovered upon annealing [1,7]; iii) The 

temperature/time for recovery by thermal annealing, used by 

many researchers, is 200°C for 30 minutes.

One issue that has not been addressed to date is that a 

finished solar cell has a multilayer structure, and that it is also 

possible to have some degradation at the interfaces upon 

illumination. In particular, it is known that the SiN:H/Si 

interface has defects and a strong positive charge 

accumulation that can be affected by light [8]. Hence, LID is 

expected to have an interface/surface component, if ionization 

of this charge occurs or if there is an increase in the density of 

interface states, Dit.

We have investigated LID effects of solar cells fabricated on 

low-Fe substrates with the intention of separating bulk and 

surface effects. To observe the bulk effects, we have light

soaked finished solar cells and their unprocessed sister wafers.

We compared degradation of solar cell parameters with that of 

the minority-carrier lifetime measured on sister wafers of the 

solar cells. In this paper, we show the typical time-dependent 

behavior of solar cell parameters during light soaking, and the 

effect of thermal annealing. The surface effect is confirmed by 

comparing degradation between similarly processed monoand multicrystalline cells. We are able to distinguish between 

fully recoverable and non-recoverable components of dark 

current. These data suggest that degradation related to B-O 

defects is fully recoverable in a cell (about 0.4% absolute in 

cell efficiency), whereas surface degradation (about 0.1% of 

cell efficiency) is not recovered. Interestingly, a partial 

recovery of the surface-phenomenon-related effect occurs if 

the light soaking is not complete.

II. EXPERIMENTAL DETAILS

We carried out the experiments in three steps:

1. Studies on monocrystalline cells to determine kinetics of 

LID upon light soaking and its (partial) recovery upon 

annealing.

2. Minority-carrier lifetime studies to determine lifetime 

changes associated with the bulk of the wafer/cell.

3. Light-soaking effects on low-oxygen mc-Si cells to 

determine interface effects and compare them with 

monocrystalline cells.

To investigate the bulk and the surface effects in a 

systematic and statistically meaningful manner, we have 

performed LID studies on a large number (> 25) of 156 mm x 

156 mm monocrystalline solar cells. The cells were fabricated 

on high-quality (minority-carrier lifetime between 300 and

350 μs) wafers, which have very low Fe content to minimize 

B-Fe effects. The resistivity of the p-type monocrystalline 

wafers was 1 to 2 Ω-cm. Cells were tested and soaked under 
2

1-sun (at 25°C) for different periods of time and tested during 

the light exposure. Cell measurements included: illuminated 

and dark current-voltage (I-V) plots, spectral response, and 

light-beam-induced current (LBIC) mapping. Illuminated I-V 

measurements were done in two modes: in one case, the I-V 

measurements were done during the light soaking itself using 

a DC testing system (XT-10); in the other case, cells were 

removed from the light-soaking station and tested under a 

(Sinton) pulse tester. Dark I-V data were analyzed by 

software we have developed to determine J01 (q/kT 

component), J02 (q/2kT component), series resistance (Rs), 

and shunt resistance (Rsh). Cells were annealed, and in some 

cases, they were cycled through light soaking and annealing.

Annealing was done at 200°C for 30 minutes following the 

procedure described in Ref. [9]. All the measurements were 

done to promote reliable determination of the small changes in 

the cell parameters that occur as a result of LID. In 

monocrystalline Si cells, the Fe concentration is typically quite 

low (<1012), whereas the oxygen content is high (in CZ). 

Hence, the main effect of light is expected to be related to 

B-O.

In the following section, we will show our results of: 

1. Degradation of cell parameters as a function of time under 

1-sun illumination at 25°C.

2. Changes in the illuminated and dark I-V characteristics 

and spectral response with light soaking and with 

subsequent annealing.

3. Results of dissecting dark I-V characteristics to determine 

J0 components, Rs, and Rsh.

4. A comparison of cell degradation and recovery with 

degradation in the minority-carrier lifetime in wafers and 

their recovery upon annealing.

III. RESULTS

To begin with, Fig. 1(a) shows the time-dependent 

degradation behavior of open-circuit voltage (Voc) and shortcircuit current density (Jsc) typical of our cells. Figure 2(b) 

shows the corresponding changes in the fill factor (FF) and

cell efficiency.

For the first hour, the cell was taken out every 5 minutes and 

the I-V data measured with a pulse tester. The inset in Fig. 

1(a) shows the changes in the cell Voc and Jsc during the first 

60 minutes. Following the first hour of soaking, the cell was 

exposed for one hour at a time and taken out for 

measurements. The inset of Fig. 1(a) suggests that there is a 

sort of recovery of the Voc for the first one hour of light 

soaking of the cell. This type of recovery was not observed on 

Jsc values during the same time. Results from other cells (of 

this batch and others) show similar effects of partial recovery 

in the initial phase of light soaking. It is important to point out 

that this happens only if the cell is withdrawn from the lightsoaking source. We feel that this is a manifestation of interface

activity and that defects are either generated at the interface

and/or there is charge redistribution during light soaking. The 

result is a higher effective surface recombination (this

conclusion is also attested by the changes in spectral response

Fig. 1. (a) Variation of Voc and Jsc, (b) FF and efficiency of one of 

the cells plotted as a function of light-soaking time. Inset of Fig. 1(a) 

shows the decrease in Voc and Jsc for the first hour of light soaking.

seen later in Fig. 3). Figures 1(a) and 1(b) also confirm that 

there appears to be an initial fast decay followed by a slower 

decay of cell parameters, as observed by Bothe and Schmidt 

[10].

Figure 2(a) shows illuminated I-V plots of a typical cell 

before and after light soaking for 72 hours, followed by 

annealing. The cell parameters before/after light soaking and 

after annealing are shown in Table 1. It is seen that all cell 

parameters (Voc, Jsc, and FF) degrade with light soaking and 

then recover: in this case, the Voc recovered fully, whereas Jsc

and FF recovered only partially.

Figure 2(b) shows the corresponding dark log I-V plots of 

the cell before and after light soaking and after annealing. It is 

seen that both J01 and J02 increase after light soaking. An 

interesting feature is that upon annealing, J01 decreases back to 

its original value, but J02 does not recover. This is a clear 

indication of surface instability and is believed to be related to 

the SiN:H/Si interface effects. We point out that degradation 

34.95

35

35.05

35.1

35.15

35.2

35.25

35.3

35.35

617

618

619

620

621

622

623

624

625

0 20 40 60 80

Jsc (mA/cm2)

Voc (mV)

Light soaking time (Hours)

Voc

Jsc

(a)

16.7

16.8

16.9

17

17.1

17.2

17.3

77.55

77.65

77.75

77.85

77.95

78.05

78.15

0 20 40 60 80

Efficiency (%)

FF (%)

Light soaking time (Hours)

FF

Efficiency

(b)
3

of J01 with light soaking and its full recovery upon annealing 

has been observed and reported by Herguth et al. [11]. The 

values of J01, J02, Rs, and Rsh are also shown in Table 1.

Fig. 2. (a) Light and dark I-V characteristics of a cell plotted 

together under different conditions. (b) Log plots of dark I-V of the 

same cell under different conditions.

Table 1. Cell parameters of one sc-Si cell before degradation, 

after degradation, and after annealing

Parameters Before light 

soaking

After light 

soaking

(72 h, 1-sun, 

25°C)

After 

annealing 

(200°C, 

30 min)

Voc (mV) 624 619.3 624.2

Jsc (mA/cm2) 35.616 35.033 35.542

FF (%) 78.58 78.16 78.13

Efficiency (%) 17.45 16.96 17.33

J01 (mA/cm2) 7.88x10-10 9.36x10-10 7.05x10-10

J02 (mA/cm2) 2.49x10-5 3.62x10-5 3.3x10-5

Rs (mΩ) 1.45 1.65 1.65

Rsh (kΩ) 135 32 35

The degradation at the SiN:H/Si interface and within SiN:H 

has been reported previously and is thought to be due to an 

increase in Dit and in the defect density within SiN:H film

[12,13]. It has been shown that exposure to light will cause 

an increase in the interface state density Dit in both p- and ntype Si. This effect is also expected to cause the observed 

changes in the J02 [14].

Figure 3 shows spectral response plots before and after light 

soaking. It is seen that although the response in most of the 

wavelength range is reduced, the reduction in response at the 

shortest wavelength is relatively large. This supports the 

interface effect described above.

Fig. 3. Quantum efficiency of one of the cells before and after 

light soaking. Inset shows magnified short wavelength region.

The above results clearly suggest that light soaking also has 

a surface effect, which we believe is a result of increase in Dit

at the SiN:H/Si interface. Because multicrystalline Si solar 

cells also use SiN:H as an antireflection coating, a similar 

behavior of mc-Si solar cells under light soaking is expected.

IV. MINORITY-CARRIER DEGRADATION VS.

DEGRADATION IN THE CELL PERFORMANCE

It is instructive to examine if the LID in the cells is fully 

controlled by the degradation in τ. To accomplish this, we 

measured changes in the minority-carrier lifetime with light 

soaking of the similar wafers (prepared by the same 

crystal/wafer technology) corresponding to various cells. The 

wafers were prepared by a procedure described in Ref. [15]

and their lifetime measured by quasi steady-state 

photoconductive decay (QSS-PCD) (in “generalized mode”) 

using iodine-ethanol passivation. After performing lifetime 

measurement, the wafers were then cleaned by the same 

procedure and soaked for different periods of time under 1-sun 

at 25°C. Following the light soaking, lifetime of each wafer 

was measured again. Figure 4 shows the effect of light soaking 

on the lifetime.

From Fig. 4, one can see that the behavior of the time 

dependence of τ degradation is about the same as for a cell 

(made on the adjacent wafer), as seen in Fig. 1. The lifetime 

decays rapidly in the first hour, followed by a slower decrease. 

0

5

10

15

20

25

30

35

40

0 0.2 0.4 0.6 0.8

Jsc (mA/cm2)

Voltage (V)

Before light soaking

After light soaking

After annealing

(a)

0.00001

0.0001

0.001

0.01

0.1

1

10

100

0 0.2 0.4 0.6 0.8

Log [Jsc (mA/cm2)]

Voltage (V)

Before light soaking

After light soaking

After annealing

(b)

0

10

20

30

40

50

60

70

80

90

100

350 550 750 950 1,150

QE (%)

Wavelenght (nm)

Before light soaking

After light soaking
4

Interestingly, the decay shows a partial recovery behavior 

similar to that in Fig. 1(a).

Figure 4 also shows the effect of 30-min anneal (at 200°C) 

on the lifetime of each of the wafers that were light soaked. It

is seen that the recovery is quite incomplete. Our 

measurements (not shown here) indicate that it takes more 

than 4 hours to fully recover the lifetime.

Fig. 4. Measured changes in the minority-carrier lifetime as a 

function of light soaking on a sc-Si wafer.

Thus, the recovery of lifetime takes a significantly longer 

annealing time than 30 minutes reported (by other references) 

for solar cells. Although we believe that the full recovery of 

the cell should take the same time as the recovery of the 

lifetime, one could reconcile this fact by the following 

argument. Because the lifetime of the wafers is large (> 100 μs 

even after degradation) and that the cell efficiency is only 

17.5%, it is possible that there is an apparent recovery of the 

cell parameters. Indeed, a solar cell simulation shows that a 

lifetime of 150 μs is quite sufficient to yield the cell parameter 

shown in Fig. 1.

V. STUDY OF LID IN MC-SI SOLAR CELLS

Multicrystalline silicon has a low concentration of oxygen 

(typically ∼1017 cm-3

) and is not expected to display 

significant B-O defect formation. Hence, solar cells made 

from cast material are not expected to have LID. Yet, some 

researchers have reported degradation in the cell performance 

upon exposure to light [16,17].

We have tested many (>20) mc-Si solar cells to investigate 

if the mechanism of observed LID is also related to the surface 

effect that we have observed in CZ cells. Figures 5(a) and (b)

show the measured variations in cell parameters with light 

soaking under 1-sun at 25°C. The cell parameters before and 

after light soaking are given in Table 2. It can be seen that 

there is a very small change in the cell parameters. However, it 

is interesting to note that Figs. 5(a) and (b) have recovery 

features similar to that of Figs. 1 and 2.

Table 2. Cell parameters of one mc-Si cell before degradation, 

after degradation, and after annealing

Parameters Before light 

soaking

After light 

soaking 

(86 hours, 

1-sun, 25°C)

After 

annealing 

(200°C, 

30 min)

Voc (mV) 616.4 616.5 616.3

Jsc (mA/cm2) 33.846 33.603 33.530

FF (%) 78.20 78.08 78.18

Efficiency (%) 16.31 16.18 16.16

J01 (mA/cm2) 12.0 x10-10 10.5x10-10 9.8x10-10

J02 (mA/cm2) 1.61x10-5 2.88x10-5 3.41x10-5

Rs (mΩ) 2.3771 2.2335 2.0179

Rsh (kΩ) 50 35 55

Fig. 5. Measured variations in the (a)Voc and Jsc (b) FF and cell 

efficiency in a mc-Si solar cell as a function of light soaking. 

Figures 6(a) and (b) show the illuminated and dark I-V plots 

before and after light soaking, and after annealing. The 

0

50

100

150

200

250

300

350

400

0 20 40 60 80

Lifetime (us)

Light soaking time (hrs)

after light sokaing

after 30 min annealing

33.55

33.6

33.65

33.7

33.75

33.8

33.85

0

100

200

300

400

500

600

700

0 50 100

Jsc (mA/cm2)

Voc (mV)

Light soaking time (Hours)

Voc

Jsc

16.1

16.12

16.14

16.16

16.18

16.2

16.22

16.24

77.75

77.8

77.85

77.9

77.95

78

78.05

0 50 100

Efficiency (%)

FF (%)

Light soaking time (Hours)

FF

Efficiency

(a)

(b)
5

degradation in efficiency is only 0.1%. Again, it is seen that 

the J02 component of the dark current does not recover upon 

annealing. Table 2 also includes J01, J02, Rs, and Rsh values.

Fig. 6. (a) Illuminated and dark I-V plots before and after light 

soaking, and after annealing. (b) Changes in the log plot of the dark IV characteristics of the same mc-Si solar cell as a function of light 

soaking.

VI. CONCLUSIONS

We have carried out detailed analyses of LID of c-Si and 

mc-Si solar cells to further understand the bulk and surface 

components. Results to date have shown that degradation 

mechanisms are bulk and surface related. The bulk effect is 

primarily B-O related and follows a behavior similar to that of 

the changes in the minority-carrier lifetime. The annealing 

results in almost full recovery. The surface effect appears to be 

related to SiN:H/Si interface. Although their manifestation is 

very little in the illuminated cell characteristics, it can be 

easily seen in the J02 component of the dark plot. Likewise, it 

appears as a slightly lower short-wavelength response in the 

spectral response after light soaking.

The other salient characteristics of LID that we have seen 

are the following:

1. As is already known, there is an initial rapid decay of all 

cell parameters (within a few minutes), followed by a 

slower degradation. In general, all cell parameters 

experience a reduction.

2. Under 1-sun at 25°C, it takes about 72 hours for complete 

light-induced degradation.

3. The total effect of LID is about 0.5% absolute (we have 

not observed > 0.5% degradation).

4. In the initial course of light soaking, there is evidence of 

partial recovery of cell parameters if the cell is taken out 

of illumination for I-V measurement.

We are carrying out measurements to directly determine 

changes in Dit at the SiN:H/Si surface of the cell as a result of 

light soaking and annealing. We will present these results 

along with LBIC studies (which can detect smaller spatial 

changes) in forthcoming publications.

ACKNOWLEDGEMENTS

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

under Contract No. DE-AC36-08GO28308 with NREL.

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0

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(a)

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