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Title

Changes in the current density‑voltage andexternal quantum efficiency characteristics of n‑
type single‑crystalline silicon photovoltaic
modules with a rear‑side emitter undergoing
potential‑induced degradation

Author(s)Yamaguchi, Seira; Masuda, Atsushi; Ohdaira,Keisuke

Citation

Solar Energy Materials and Solar Cells, 151: 113‑
119

Issue Date

2016‑03‑24

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/15343

RightsCopyright (C)2016, Elsevier. Licensed under theCreative Commons Attribution‑NonCommercial‑
NoDerivatives 4.0 International license (CC BY‑
NC‑ND 4.0).
[http://creativecommons.org/licenses/by‑nc‑
nd/4.0/] NOTICE: This is the author s version ofa work accepted for publication by Elsevier.
Changes resulting from the publishing process,including peer review, editing, corrections,structural formatting and other quality control
mechanisms, may not be reflected in thisdocument. Changes may have been made to this work
since it was submitted for publication. A
definitive version was subsequently published in
Seira Yamaguchi, Atsushi Masuda, Keisuke Ohdaira,Solar Energy Materials and Solar Cells, 151,
2016, 113‑119,
http://dx.doi.org/10.1016/j.solmat.2016.03.003

Description

Japan Advanced Institute of Science and Technology

Changes in the current density–voltage and external
quantum eﬃciency characteristics of n-typesingle-crystalline silicon photovoltaic modules with arear-side emitter undergoing potential-induced
degradation
Seira Yamaguchia,∗, Atsushi Masudab , Keisuke OhdairaaaJapan Advanced Institute of Science and Technology (JAIST), Nomi, Ishikawa 923-1292, Japanb
Research Center for Photovoltaics, National Institute of Advanced Industrial Science andTechnology (AIST), Tsukuba, Ibaraki 305-8568, Japan

Abstract
This study addresses the potential-induced degradation (PID) of n-type singlecrystalline silicon (sc-Si) photovoltaic (PV) modules with a rear-side emitter. The
n-type rear-emitter module configurations were fabricated using n-type bifacial
sc-Si solar cells by module lamination with the p+ emitter side down. After the
PID tests applying −1000 V, the modules show a rapid decrease in the open-circuit
voltage (Voc ), followed by relatively slower reductions in the fill factor and theshort-circuit current density (Jsc ). Their dark current density–voltage (J–V) dataand external quantum eﬃciencies (EQEs) indicate that the drop in Voc is causedby an increase in the saturation current density due to the enhanced surface recombination of minority carriers. In contrast, the modules exhibit slight degradation
under +1000 V, which is characterized by only slight decreases in Voc and Jsc . The
EQE measurement reveals that these decreases are also attributed to the enhanced
surface recombination of minority carriers. This behavior is almost identical tothat of the polarization eﬀect in n-type interdigitated back contact PV modules
reported in a previous study. By comparing the PID resistance with that of othertypes of modules, the n-type rear-emitter PV modules are relatively resistant to
PID. This may become an advantage of the n-type rear-emitter PV modules.
∗

Corresponding author
Email addresses: s-yamaguchi@jaist.ac.jp (Seira Yamaguchi),atsushi-masuda@aist.go.jp (Atsushi Masuda), ohdaira@jaist.ac.jp (Keisuke Ohdaira)Preprint submitted to Solar Energy Materials and Solar Cells

March 3,2016

Keywords: Potential-induced degradation, n-type single-crystalline silicon
photovoltaic module, n-type rear-emitter photovoltaic module, Surface
recombination, Polarization eﬀect
1. Introduction
The n-type crystalline silicon (c-Si)-wafer-based solar cells attracted attention
owing to their high eﬃciency potential [1]. As for the p+ emitter formation in the
n-type c-Si solar cells, some diﬀerent techniques were successfully developed,
such as boron-diﬀused front emitters [2, 3], p-type/i-type amorphous silicon heterojunctions [4–6], and aluminum (Al)-alloyed rear emitters [7–9]. In particular,
the Al-alloyed rear emitters can reduce the production costs since the emitters canbe formed by the conventional fabrication process for back-surface fields, which
was already widely used in the production of the commercial p-type c-Si solar
cells. Therefore, Al-alloyed rear-side emitter solar cells are suitable for use inphotovoltaic (PV) power plants, such as very large-scale PV (VLS-PV) systems.
To ensure high reliability and long-term stability of PV systems, it is importantto understand the possible degradation behavior of the PV modules. In particular,
degradation associated with system bias voltage is identified as a central problem
in VLS-PV systems with remarkably high system voltage. This degradation is induced by high electrical potential diﬀerences between grounded frames and cells,
and it is generally referred to as potential-induced degradation (PID). Thereby,
modules deployed in the systems can show significant performance losses [10–12].
The PID of p-type c-Si PV modules was well documented by many researchers.
In p-type c-Si modules, significant degradation occurs only under negative potential diﬀerences from the grounded frames [10–12]. Such negative potential diﬀerences result in sodium cations (Na+ ) transfer from the cover glass toward the cells,
and the Na+ accumulates on the devices [12]. (There are controversial points ontrue Na sources. For instance, an experimental result suggests that Na originatesfrom contaminants on the cell surface [13].) In this situation, the Na+ ions caneasily pass even through SiN x passivation layers, which are known as good diffusion barriers for sodium [14], with the assistance of a strong electric field [15].
The cells contaminated with Na exhibit a reduction in parallel resistance (Rp ) [10]and enhancement of depletion-region recombination [16, 17]. This kind of PID
is generally referred to as the “PID of shunting type” (“PID-s”) [18]. With regard to these facts, Naumann et al. [19] discovered that Na accumulates into thesilicon nitride (SiN x )/Si interface of c-Si solar cells in PID-aﬀected regions. It
2was also reported that both PID shunts and Na accumulation locally occur at thesame spots [20, 21]. Moreover, stacking faults existing near the front surface of
c-Si were seen to be decorated by Na, and the Na-decorated stacking faults were
found to play a crucial role in PID [22]. Naumann et al. and coworkers proposeda physical model that can explain the shunting based on Na-decorated intrinsic
stacking faults [18, 22, 23]. Recently, Ziebarth et al. [24] discussed in detail thediﬀusion of Na in the intrinsic staking faults and the short-circuiting of p–n junctions by theoretical calculations based on density functional theory and reported
their theoretical findings to be consistent with the experimental results presented
by Naumann et al. [18, 22].
Regarding the PID of the n-type PV modules, there are several studies on interdigitated back contact (IBC) [25–27] and front-emitter [28] PV modules. It
was reported that n-type IBC PV modules degrade under positive bias and that
PID-aﬀected modules show decreased open-circuit voltage (Voc ) and short-circuitcurrent density (Jsc ) due to the enhanced surface recombination of minority carriers [25]. Naumann et al. showed that n-type IBC PV modules can also degrade
under negative bias [26]. In n-type IBC PV modules with a front-floating emitter, a similar degradation was observed under negative bias [27]. Hara et al. [28]revealed that the degradation of front-emitter PV modules occurs under negativebias. They found that the n-type front-emitter modules exhibit decreases in Voc andJsc , while maintaining fill factor (FF), and the degradation tends to occur easily
under low temperature or a low bias voltage compared with conventional p-typec-Si modules. By contrast, the PID of the n-type rear-emitter configurations has
not been investigated in detail so far.
Herein, using a PID acceleration test, we study in detail the PID behavior ofmodules with a rear-side emitter. To this end, we fabricated rear-emitter configurations by placing n-type single-crystalline Si (sc-Si) bifacial solar cells with therear side up. This investigated structure is exactly the same as the Al-alloyed rearside emitter cells except for the formation technique of the p+ emitters and therear-side electrode material and shape. Pingel et al. [29] already reported on the
PID of the n-type c-Si bifacial PV modules; however, they focused mainly on therecovery phenomena of the modules degraded by negative bias. In this contribution, we deal with in detail changes in the current density–voltage (J–V) and thespectral response characteristics of PID-aﬀected modules. Moreover, we focus on
the degradation of the modules under positive bias. The dark J–V data are analyzed in detail by fitting to an equation based on the two-diode model [30]. We
also briefly discuss possible measures to prevent the PID of the n-type rear-emitter
cell modules and investigate the degradation of the modules under positive bias.
32. Experimental
Commercial n-type bifacial sc-Si solar cells composed of silver (Ag) combshaped electrode/SiN x film/p+ emitter/n-type base/n+ back surface field/SiN x film/
Ag comb-shaped electrode were cleaved to 20 × 20 mm2 -sized pieces. Interconnector ribbons were soldered onto the busbars of the front and rear Ag electrodes.
The rear-emitter cell modules were fabricated by the lamination of the cells withthe p+ emitter side down. Prepared were stacks composed of conventional tempered cover glass/ethylene-vinyl acetate copolymer (EVA) encapsulant/n-type bifacial sc-Si cell with the p+ emitter side down/EVA encapsulant/typical backsheet[poly(vinyl fluoride) (PVF)/poly(ethylene terephthalate) (PET)/PVF]. The coverglass had a size of 45 × 45 mm2 and contained alkali metals such as Na. The
modules were fabricated from the stacks in a module laminator. The lamination
process mainly involves two steps: a degassing step that takes place for 5 min
and an adhesion step that takes place for 15 min. The stacks were placed withthe cover glass side down on a stage heated at 135 ◦ C during lamination. The
front-side configuration of the encapsulated cells is exactly the same as that of ntype Al-alloyed rear-emitter cells. The n-type rear-side emitter cell modules usedherein show an energy conversion eﬃciency of approximately 18%.
The PID tests were performed by applying a voltage of −1000 or +1000 V
to connected module interconnector ribbons with respect to an aluminum plate
placed on the module cover glass in a heating chamber maintained at 85 ◦ C.
Herein, we use the terms “negative bias” and “positive bias” for biases that produce the negative and positive potentials of cells with respect to the aluminum
plate, respectively. We performed the PID tests for three identical modules under each test condition. This test method and similar ones were widely used andestablished by many researchers as ways to easily produce PID-aﬀected modules
in a short time [17, 21, 28, 31–40]. We use the term “PID stress” here to referto such bias voltage and temperature stress. In this experiment, humidity in theheating chamber was not controlled during stressing; however, relative humidity
in a similar setup [32] is low (approximately 2% RH). We, therefore, disregarded
its influence on the performance degradation of the modules, such as the moisture
ingress into the modules and the corrosion of the metal parts. Moreover, at thepresent state, it is unknown for how long the PID test stress corresponds to theduration that generates PID in outdoor large-scale PV systems. We used this PID
test for intercomparison between the test samples.
To evaluate the degradation, J–V measurements were conducted under dark
and one-sun-illuminated conditions for the modules before and after the PID tests.
4Saturation current densities of the first and second diodes J01 and J02 , respectively,
ideality factors of the first and second diodes n1 and n2 , respectively, parallel resistance Rp , and series resistance Rs were determined by fitting the dark J–V data
to the following two-diode equation [30]:
[
(
)
]
[
(
)
]
q(V − JRs )q(V − JRs )V − JRs, (1)
J(V) = J01 exp− 1 + J02 exp
−1 +
n1 kT
n2 kT
Rp
where q is the elementary charge, k the Boltzmann constant, and T the absolutetemperature. J01 , J02 , Rp , and Rs were positive, n1 was restricted to one, andn2 was limited to greater than one. Additionally, Rs was restricted to ≥ Rs,initial ,where Rs,initial is the Rs of modules before the PID tests. The two-diode fitting
was performed using the statistical software Igor Pro 6 (WaveMetrics, Inc.). To
estimate the spectral response losses of the degraded modules, external quantum
eﬃciency (EQE) measurements were conducted before and after the PID tests.
3. Results
3.1. Degradation behavior under negative bias
It is so far reported that applying a high negative bias to cells leads to thedegradation of many types of PV modules, such as conventional p-type c-Si modules [10–12], n-type front-emitter c-Si modules [28], n-type IBC c-Si modules[26], n-type IBC c-Si modules with front-floating emitter [27], amorphous Si (aSi) modules [32, 41], Cu(In,Ga)Se2 cells [42] and modules [32, 41], and CdTe
modules [43]. Herein, we first clarify how the J–V characteristics and the EQEsof the n-type rear-emitter PV modules change by applying negative bias.
Fig. 1 presents the representative one-sun-illuminated J–V curves for the ntype rear-emitter modules before and after the PID tests when applying a voltageof −1000 V to the cell with respect to the front surface of the cover glass. The
modules exhibit a significant drop in Voc and a relatively small drop in Jsc , whichis consistent with the result of a previous study [29]. However, a decrease in FF isseen in this experiment, which was not observed in the previous studies [29].
Fig. 2 shows the dependence of Jsc /Jsc,0 , Voc /Voc,0 , FF/FF0 , and Pmax /Pmax,0of the n-type rear-emitter modules on PID-stress duration, where the subscript 0
indicates the initial value and Pmax is the maximum output power. The valuesplotted in Fig. 2 are calculated from the one-sun-illuminated J–V data of threeidentical modules, including the module whose J–V curves are shown in Fig. 1.
As mentioned above, all the parameters decrease with increasing PID-stress duration. In particular, Voc significantly decreases within the first hour, indicating that
5

40

2

J (mA/cm )30
20
initial
1h
24 h

10
0
-10
-20

-0.2

0.0

0.2

0.4

0.6

V (V)

Figure 1: Representative one-sun-illuminated J–V characteristics of the n-type rear-emitter PV
modules before and after the PID tests applying −1000 V.

the PID of n-type rear-emitter modules is characterized by a reduction in Voc . Voc
is then saturated, even after longer PID tests. Similar saturation behavior is also
observed in Jsc and FF.
Fig. 3 gives the representative dark J–V curves of the n-type rear-emitter cell
modules before and after the PID tests, and Fig. 4 shows the dependence of J01 ,J02 , n2 , and Rp on the PID stress duration. These parameters are obtained by fitting
the dark J–V data to the two-diode model [30]; thus, for instance, Rp does not generally correspond to that simply obtained from the slope of the dark J–V curves
under the reverse bias. Rs is considerably low and remain almost unchanged during the PID tests (not shown here). As observed in Fig. 3, the current density
significantly increases both under reverse and forward bias with PID-stress duration. The increase in the current density corresponds to the increases in J01 andJ02 displayed in Fig. 4, which implies that PID stress shortens the minority carrier
lifetime of the cells. J01 increases after the PID test for 1 h, indicating enhanced
recombination in the emitter and/or the base in the solar cells. n2 and 1000/Rp
slightly increase with PID-stress duration. However, unlike p-type c-Si modules,
n-type rear-emitter modules do not exhibit significant increases in 1000/Rp andn2 . This is probably because the p–n junction in rear-emitter configurations is lessinfluenced by PID stress, as compared with p-type c-Si cells with a front emitter.
The increases in 1000/Rp , and J02 and n2 may be due to the introduction of Na
through the unpassivated edge of the cells (Section 4.1).
Fig. 5 shows the EQE spectra of unaﬀected and degraded rear-side emitter PV
6

Jsc/Jsc,0

1.00
0.95
0.90
0.85

Voc/Voc,0

1.00
0.95
0.90
0.85

FF/FF0

1.00
0.95
0.90
0.85

Pmax/Pmax,0

1.00
0.95
0.90
0.85
0

5

10

15

20

25

PID-stress duration (h)Figure 2: Dependence of Jsc /Jsc,0 , Voc /Voc,0 , FF/FF0 , and Pmax /Pmax,0 of the n-type rear-emittermodules on negative-biasing PID-stress duration. Each data point shows the mean value for threemodules, and each error bar corresponds to the standard deviation of the mean. The solid lines are
guides to the eye.

7

1

10

0

2

J (mA/cm )

10

-1

10

-2

10

-3

10

initial
1h
24 h

-4

10

-5

10

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

V (V)Figure 3: Representative dark J–V characteristics of the rear-side emitter PV modules before andafter the PID tests applying −1000 V.

modules as a function of PID-stress duration. A slight decrease in EQE is seen
only at a wavelength of 400–600 nm after the PID test for 1 h. The EQE spectrum
after the PID test for 24 h in total is considerably similar to that after the PID testfor 1 h, corresponding to the saturation behavior observed in Jsc given in Fig. 2.
The reduction in EQE suggests an enhanced surface recombination of minority
carriers due to PID stress. As reported by Hara et al. [28], the EQE of the n-type
front-emitter PV modules is also reduced at a short-wavelength region after a PID
test. The EQE reduction of the rear-emitter cell modules is, however, more slightthan that of the n-type front-emitter solar cells.
3.2. Degradation behavior under positive bias
Prior work revealed that n-type IBC PV modules exhibit a significant loss
in output power accompanied by reduction of Jsc and Voc under positive voltage
stress [25]. This phenomenon is generally referred to as the “polarization effect” [25]. For the n-type rear-emitter cell modules, a configuration near the front
surface involves Ag electrode/SiN x passivation layer/n+ front surface field/n-typebase. This structure is very similar to that of the n-type IBC solar cells except for
the existence of the front Ag electrode. The n-type rear-emitter cell modules may,therefore, degrade due to positive bias. To confirm this, we conduct the PID tests
applying +1000 V to the cell with respect to the front surface of the cover glass.
Fig. 6 shows the representative one-sun-illuminated and dark J–V curves of
the n-type rear-emitter cell modules before and after the PID tests by applying
8

-5

J02

10

-6

10

J01
-12

n2

10

1000/Rp (Ω-1 cm-2)-7

10J02 (A/cm2)

J01 (A/cm2)

-11

10

-8

10

4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.20
0.15
0.10
0.05
0.00
0

5

10

15

20

25

PID-stress duration (h)Figure 4: Dependence of J01 , J02 , n2 , and 1000/Rp of the n-type rear-emitter modules on negativebiasing PID-stress duration. Each data point shows the mean value for three modules, and eacherror bar corresponds to the standard deviation of the mean. The solid lines are guides to the eye.

9

1.0

EQE

0.8
0.6
0.4
initial
1h
24 h

0.2
0.0

400

600

800

1000

1200

Wavelength, λ (nm)Figure 5: EQE spectra of the rear-emitter PV modules before and after the PID tests applying−1000 V for 1 and 24 h.

+1000 V. The J–V curves exhibit slight decreases in Jsc , Voc , and FF after the PID
tests. As shown in Fig. 6b, the current density increases both under reverse andforward bias after the PID test for 24 h. The two-diode fitting of the dark J–V
curves clarifies that J02 increases from 1.9 × 10−8 A/cm2 to 2.5 × 10−7 A/cm2 , andn2 increases from 2.0 to 2.5, which may be the reason behind the reduced FF. By
contrast, J01 is almost unchanged probably since the drop in Voc is considerably
small.
Fig. 7 exhibits the EQE spectra of the n-type rear-emitter cell modules before
and after the PID test for 24 h. A slight decrease in EQE is seen at wavelengths of
400–500 nm, indicating that the degradation is attributed to the enhanced surface
recombination of minority carriers at the front. The above results imply that ntype rear-emitter cell modules may undergo the polarization eﬀect under positivebias.
3.3. Eﬀect of the insertion of an ionomer thin film
To oﬀer suﬃcient performance stability, it is important to propose possible
techniques for the prevention of the PID. So far, reports suggest that in p-type c-Si
cell modules, the PID can be prevented by the chemical composition modificationof SiN x surface passivation layer [11, 33, 44], by the use of quartz cover glass [16],alminosilicate-chemically strengthened glass [34], an acrylic front cover film [35],or cover glass with a TiO2 -coated inner surface [36], by using electric resistanceencapsulant [16, 37] instead of the conventional EVA encapsulant, by inserting a
10

40

(a)
2

J (mA/cm )

30
20
10

Initial
24 h

0
-10
-20

-0.2

0.0

0.2

0.4

0.6

V (V)
1

10

(b)

0

2

J (mA/cm )

10

-1

10

-2

10

-3

10

-4

10

Initial
24 h

-5

10

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

V (V)Figure 6: Representative (a) illuminated and (b) dark J–V curves of the n-type rear-emitter PV
modules before and after the PID test applying +1000 V for 24 h.

11

1.0

EQE

0.8
0.6
0.4
0.2
0.0

initial
24 h

400

600

800

1000

1200

Wavelength, λ (nm)Figure 7: EQE spectra of the rear-emitter PV modules before and after the PID test applying+1000 V for 24 h.

polyethylene [38] or an ionomer [45] thin film between the EVA encapsulant andfront cover glass or cells, or by increasing the cross-linkage level or the thicknessof the encapsulant [39]. Here, we investigate the eﬀect of the insertion of anionomer film between the cover glass and EVA encapsulant on the PID undernegative bias.
Shown in Fig. 8 are the illuminated J–V curves of the n-type rear-emitter cell
module with a 30-µm-thick ionomer film, which is inserted between the coverglass and the EVA encapsulant, before and after the PID test applying −1000 V
to the cell with respect to the front surface of the cover glass at 85 ◦ C for 24 h.
The module with the ionomer film degrades after the PID test for 24 h. The Pmax
then relatively decreases to 0.94. This implies that the PID of the n-type rearemitter modules under negative bias cannot be completely prevented by insertingthe 30-µm-thick ionomer film.
4. Discussion
4.1. Degradation behavior under negative bias
The degradation of the n-type rear-emitter modules occurs under negative bias
and is characterized by a relatively large drop in Voc accompanied by decreases
in Jsc and FF, as shown in Figs. 1 and 2. The Voc drop is probably attributed tothe enhanced surface recombination of minority carriers, since the PID-aﬀected
modules exhibit an increase in J01 and a decrease in EQE at the short-wavelength
12

40

2

J (mA/cm )

30
20
10

Initial
24 h

0
-10
-20

-0.2

0.0

0.2

0.4

0.6

V (V)

Figure 8: J–V curves of the n-type rear-emitter module with the 30-µm-thick ionomer film before
and after the PID test for 24 h.

region, as indicated in Figs. 4 and 5, respectively. The decrease in Jsc corresponds
to the reduction in EQE at the short-wavelength region. The negative bias might
results in the polarization of the surface passivation layer of cells and Na introduction into the cells. In the n-type rear-emitter modules, the former may not be
detrimental to the surface passivation, because expected polarization produces apositively charged region at the n-type surface and repels the minority carriers,
holes. The enhanced surface recombination of minority carriers may therefore becaused by the introduction of Na into the surface region of the cells. For p-typec-Si modules, stacking faults are decorated by such Na. If such Na-decorated
stacking faults penetrate the p–n junctions, they behave as two-dimensional electrical conductive layers [18, 22]. Contrarily, n-type rear-emitter cell modules havethe p–n junction at the rear side. Because of this, the short-circuiting of the p–njunctions does not take place. Instead, the Na-decorated stacking faults may behave as recombination centers at the front n-type region, whereby J01 is increased.
Naumann et al. [26] suggested that, in n-type IBC c-Si PV cells, Na-decorated
stacking faults increase the surface recombination velocity of minority carriers.
The root cause of the PID of n-type rear-emitter c-Si PV modules may be identical to that of the PID of n-type IBC c-Si PV modules.
The degradation behavior is similar to that of the n-type front-emitter c-Si
modules [28], whereas Voc and Jsc decreases of the n-type rear-emitter modules
are smaller than those of the n-type front-emitter PV modules. The degradationof both modules is probably attributed to the surface recombination of the minor13

ity carriers. Furthermore, the degradation of the n-type front-emitter PV modules
might originate from the surface polarization of the cells [28]. However, there
is a possibility that Na atoms are introduced into the p+ front emitter and activate recombination there. We now assume that such Na atoms introduced into Si
also contribute to the PID of the n-type front-emitter cell modules. Based on thishypothesis, the diﬀerence may be due to the combination of the two eﬀects, thesurface polarization and Na introduction. In other words, the n-type rear-emittermodules may be aﬀected only by the Na introduction, whereas the front-emittermodules may degrade both by the eﬀects of the surface polarization and Na introduction. Furthermore, this hypothesis can consistently explain the diﬀerence inthe n-type front-emitter and n-type rear-emitter modules in terms of a diﬀerence inthe capture cross-sections between electrons and holes. Na impurities generally
produce donor-like defects in Si [46]. Namely, the Na impurities are positively
charged in Si; thereby, a relatively large capture cross-section of electrons can beprovided. Hence, when Na atoms are introduced in the p+ emitters of the n-type
front-emitter cell modules, they eﬃciently capture minority carriers, electrons.
This leads to the significant shortening of the minority carrier lifetime in a p-type
region, and results in significant reduction in Voc and Jsc of solar cells. Therefore,n-type front-emitter modules can be significantly degraded by Na introduction.
However, Na impurities are less eﬃcient in the front n-type region for n-type rearemitter modules, because Na atoms less eﬃciently capture minority carriers ofn-type semiconductors, holes. Therefore, the Voc and Jsc losses of n-type rearemitter modules are not significant, compared with the n-type front-emitter cell
modules.
In Figs. 1 and 2, we observe the saturation behavior of drops in Jsc , Voc , and
FF. This kind of saturation behavior is not observed in the conventional p-typec-Si PV modules. This saturation behavior may be an advantage of the n-typerear-emitter configurations, because, at least, the saturated eﬃciency is ensured as
power generation performance in PV systems. At present, it is unclear why thesaturation eﬀect occurs.
As shown in Figs. 1 and 2, FF decreases by PID-stress. This was not observedin the previous study [29]. Generally, FF is mainly related to Rs , Rp , and J02 andn2 . In our experiment, Rs was considerably low and almost unchanged during PID
tests. The decrease in FF can therefore be attributed to the increases in 1000/Rp
and/or J02 and n2 . However, Rp was suﬃciently high (approximately 10 kΩ·cm2
after the PID tests for 24 h in total), and such a high Rp does not influence FF.
Hence, the drop in FF is caused by the increases in J02 and n2 . J02 and n2 generally indicate the degree of recombination currents, and large J02 and n2 imply
14

that the recombination currents are more dominant in the total currents in p–njunctions. In this context, the PID stress from the front cover glass side shouldaﬀect the rear-side p–n junction of the n-type rear-emitter solar cells. In termsof the configuration of the cells, Na introduced through the front surface of cells
cannot probably reach the rear-side p–n junction through the thicker n-type baseregion. A reason for the increased J02 and n2 might be the Na introduction into therear-side p–n junction through the edge surface. The edge surface is unpassivated,
and Na can therefore be easily introduced through the cell edge. Such Na atoms
may cause the enhancement of depletion region recombination. Herein, we used
20 × 20 mm2 -sized small-area cells. This may explain why the drop in FF is seen
only in our experiment, since such small-area cells are generally aﬀected by theedge region.
4.2. Degradation behavior under positive bias
As mentioned above, n-type rear-emitter cells have very similar front structurewith n-type IBC solar cells. Because of this, the polarization eﬀect reported by
Swanson et al. [25] may also occur in n-type rear-emitter cell modules. As shownin Fig. 6a, the n-type rear-emitter module is slightly degraded by the applied positive bias, and the degradation is characterized by slight decrease in Jsc and Voc .
As observed in Fig. 7, the EQE is decreased at the short-wavelength region after
the PID test, implying that the decreases in Jsc and Voc are caused by the enhancement of the surface recombination of minority carriers. The degradation behavior
under positive bias is considerably similar to that of the n-type IBC solar cells[25]. These probably demonstrate that n-type rear-emitter modules are subjectedto the polarization eﬀect. However, the modules may tend to be less aﬀected by
the polarization eﬀect compared to n-type IBC PV modules. Note that the root
cause of the polarization eﬀect has not been clarified so far. Therefore much additional work is required to elucidate the degradation phenomenon occurring underpositive bias.
We now briefly consider the high stability of the n-type rear-emitter modules
under positive bias. As explained below, the presence of the front electrode might
be related to the higher stability under positive bias. Applying a positive voltage stress on the cells with respect to the frame, non-zero leakage currents areobserved between the frame and the cells [25]. Such currents can lead to the accumulation of negative charges on the surface passivation layer, and these negative
charges polarize the passivation layer [25]. For rear-emitter cell modules, the negative charges can also reach the front surface of the cells. However, in the presenceof the front electrode, some of the negative charges are collected by them and may
15then neutralized on the positively charged electrode surfaces. Moreover, negative
charges reaching the passivation layer might also move to the electrode throughthe surface of the layer.
4.3. Eﬀect of the insertion of an ionomer thin film
It was reported that the PID of p-type c-Si PV modules can be completely prevented by inserting a polymer film, such as polyethylene [38] or ionomer [45]. As
indicated in Fig. 8, however, the insertion of the 30-µm-thick ionomer film does
not prevent the PID of the n-type rear-emitter module. Such an inserted ionomer
film significantly reduces Na ion migration rate [45]. However, ionomer encapsulants cannot suppress the migration of Na atoms to cells at all [40]. Na atoms
may thus reach the cell surface even in the presence of an ionomer film, whichmight be assisted by a negative bias. The n-type rear-emitter modules might tend
to degrade by fewer doses of Na atoms. On the basis of a model proposed by
Naumann et al. [47], PID suppression eﬀects by high-electric-resistance encapsulant are caused by increased resistance. The above result may suggest that higher
resistance is required for preventing the PID. The increased resistance of encapsulation makes voltages across the SiN x layers low, implying that the PID of n-typerear-emitter c-Si PV modules tends to occur under a relatively small bias voltage.
To determine possible measures to prevent the PID, we must further investigate
the eﬀect of module components on the PID behavior of the n-type rear-emittermodules.
4.4. Comparing PID resistance with that of other types
In terms of installation in VLS-PV systems, it is important to compare the
PID resistance of the n-type rear-emitter modules with that of other types of modules. We reported the test results of multi-crystalline Si (mc-Si), a-Si, and CIGS
modules [32]. After the tests within 1 day under the same temperature and voltage conditions, Pmax /Pmax,0 of the mc-Si, a-Si, and CIGS decrease to 0.04–0.4,
0.23, and 0.95, respectively. By contrast, Pmax /Pmax,0 of the n-type rear-emittermodules drops to 0.93, and the PID-resistance is comparable to that of the CIGS
module. These indicate that n-type rear-emitter modules have relatively high PID
resistance. This may become an advantage of the n-type rear-emitter modules.
Note that the PID resistance is largely influenced, for instance, by the fabrication conditions and by the test conditions. Our PV modules are placed in a dry
environment (typically under a humidity of 2% RH [32]) during our PID test, andthe influence of humidity was neglected. However, humidity may strongly aﬀect
PID. For instance, CIGS PV modules have higher resistance to our PID tests [32],16whereas they undergo significant degradation under “biased damp heat” conditions, where modules are subjected to intense humidity as well as high temperature and voltage stress [43]. The n-type rear-emitter modules may also show
diﬀerent degradation behavior under diﬀerent test conditions. We therefore must
perform the PID tests of the n-type rear-emitter modules under various conditions.
5. Conclusions
We studied in detail the PID behavior of the n-type rear-emitter modules, using
a PID acceleration test. After the PID tests applying −1000 V, the modules show
decreases in Voc accompanied by FF and slight Jsc reduction. However, the FF
reduction may be due to Na introduction through the edge surfaces; in particular,
our small-area-cell modules may tend to be more strongly influenced by the properties of the edge region. These indicate that a drop in Voc mainly characterizes
the PID of n-type rear-emitter modules. The dark J–V data of the PID-aﬀected
modules exhibit increased J01 . Furthermore, the EQE spectra of the PID-aﬀected
modules are reduced at the short-wavelength region of 400–600 nm. These imply that the decreases in Voc and Jsc are due to the enhancement of the surface
recombination of minority carriers.
The n-type rear-emitter modules show slight degradation under +1000 V; however, the modules are less influenced by positive bias compared to negative bias.
The degradation by positive bias is characterized by decreases in Voc and Jsc due toenhanced surface recombination of minority carriers, and is almost identical to thepolarization eﬀect observed in n-type IBC PV modules. The high stability underpositive bias compared to n-type IBC cell modules might be due to the presenceof the front electrode.
Compared to other types of modules such as mc-Si, a-Si, and CIGS PV, theresistance of the n-type rear-emitter modules to our PID test is much higher thanthose of the mc-Si and a-Si modules, and is comparable to that of the CIGS module. This suggests that n-type rear-emitter modules have higher stability than thoseof the conventional Si modules and are more suitable for use in VLS-PV systems.
Acknowledgements
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO). The authors would like to thank Yukiko Haraand Sachiko Jonai of AIST for their assistance in module lamination.

17

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