SUPPORTING INFORMATION
Minimizing metastabilities in
Cu(In,Ga)Se2/(CBD)Zn(S,O,OH)/i-ZnO based solar cells
M. Buffière1,2*, N. Barreau1, L. Arzel1, P. Zabierowski3, J. Kessler1,2
1
Institut des Matériaux Jean Rouxel (IMN)-UMR 6502, Université de Nantes, CNRS, 2 rue de
la Houssinière, BP 32229, 44322 Nantes Cedex 3, France
2
44solar, 14 rue Kepler, 44240 La-Chapelle-sur-Erdre, France
3
Faculty of Physics, Warsaw University of Technology, 00 662 Warszawa, Poland
* Corresponding author :
Current affiliation : imec, Kapeldreef 75, 3001 Heverlee, Belgium
E-mai l: buffiere@imec.be
Phone : 0032 16 28 12 05
A. Characterization of the standard devices
The standard device consists of a CIGSe absorber with Cu content corresponding to y =
0.9 (y2), a (CBD)Zn(S,O,OH) buffer layer with thickness of t = 30 nm (t1) and i-ZnO with
resistivity of ρ = 2.102 Ω.cm (ρb) (see Fig. 1). The typical J(V) curves of such standard
devices for different soaking times under white light for short-circuit conditions (WLSS) are
shown in Fig. 8. It can be seen that the metastable behavior of the device mainly affects the
fill factor (FF) and the Jsc of the cell. In contrast to the light soaking (LS) effect reported in the
literature for such devices [27, 28], the Voc of the cell is only weakly affected by light
soaking, although the metastable effects similarly relax in darkness. The plots in Fig. 8 also
show that the improvement of Jsc and FF is not monotonic against soaking time; the level of
changes appears decreasing with the LS duration. In order to follow the time dependence of
the metastable behavior of the solar cells investigated in the present study, the evolution of the
current density JF at VF = (Voc-150 mV) as a function of LS time is used as criterion.
1
Fig. 8 Evolution of the electrical characteristics under AM1.5 at short circuit conditions for a
standard CIGSe solar cell with 30 nm-thick Zn(S,O,OH) buffer layer. The evolution of the
current density JF at VF=Voc-150 mV as a function of WLSS time will be further used to study
the metastable behavior of different samples.
For the reference sample, the effect of the nature of the light source on the metastable
behavior is clearly demonstrated in Fig.9. The J(V) curves were recorded at 25 °C under red
light (i.e. the photons are absorbed neither by the buffer nor by the window layers), white
light (AM1.5G spectrum) or Xenon lamp (UV-rich). Whatever be the soaking time under red
light, JF remains constant; which denotes that the FF does not change. Under white light, JF
reaches its highest value after the device has been soaked for 20 min, meaning the FF
improves through long term exposure. When the device is soaked with the Xenon lamp, J F
reaches its highest level after only 1 min. Such spectrum-dependent metastable behavior
emphasizes the benefits of the UV radiation for the cell to quickly reach its optimal level of
energy conversion. This observation confirms that the barrier at the origin of the FF loss is
located where the high energy photons are absorbed, that being the CIGSe/buffer/window
region. This phenomenon seems rather similar to the red/blue effect observed for CdSbuffered devices [39].
2
JF (mA/cm²) @ Voc-150 mV
25
20
Xe light
AM1.5
Red light
15
10
5
0
1
2
3
4
5
6
7
8
9
10
Light Soaking time (min)
Fig. 9 Evolution of JF as a function of the LS time for a 30 nm-thick Zn(S,O,OH)-buffered
CIGSe solar cell under different illumination sources.
The effect of the complete cells annealing at 200 °C in air for 10 min on their
metastable behavior has also been investigated; this treatment is often used to improve the
performance of Zn(S,O,OH)-buffered cells [21, 22]. Figure 10a shows the evolution of JF
versus the WLSS time measured on a cell before and after annealing; it is observed that the
time required to reach the highest JF value is reduced after annealing. In order to better
locate the origin of this change, we have compared the impact of air annealing performed at
different steps of the device fabrication, namely either after (CBD)Zn(S,O,OH) deposition
(1) or after the device is completed (2). In contrast to the beneficial effect of treating the
complete cell, the annealing after the buffer growth yields devices showing similar
JF (mA/cm²) @ Voc-150 mV
metastable behavior to untreated ones.
25
20
15
10
Before annealing
o
Annealed (200 C, air, 10 min)
5
0
1
10
Light Soaking time (min)
(a)
3
1.0
EQE
0.8
0.6
0.4
Annealing (1)
Annealing (1) + Light bias
Annealing (2)
0.2
0.0
400
600
800
 (nm)
1000
(b)
Fig. 10
a. Semi-logarithmic plot of the evolution of JF as a function of the LS time for the reference
sample under AM1.5G before and after thermal annealing of the cell (10 min at 200 °C in air).
b. EQE spectra of the reference cell which underwent the annealing treatment either after the
buffer layer deposition (1) or once the cell was completed (2).
Figure 10b plots the external quantum efficiency (EQE) of devices annealed either
before (annealing “1”) or after (annealing “2”) the window deposition (i.e. CIGSe/buffer or
completed cell respectively); one should note that the measurements were performed at
room temperature after the cell had been stored in darkness for several hours. Considering
the device annealed before the window deposition, the EQE suggests that the Jsc loss is due
to low collection length, which can be attributed to either a very low diffusion length
(material related) or a very narrow space charge region (junction related). Nevertheless,
EQE measurements under light bias (i.e. with an additional white light source at 0.3 sun
intensity) lead to high improvement in the long wavelengths range, which supports the
hypothesis of poor collection induced by a narrow space charge region. One should note that
the long wavelengths gain is not persistent and quenches when the light bias source is
switched off. Similar results have been observed for different CIGSe/buffer annealing
durations. In contrast, as expected from the J(V) characteristics, the EQE of the device
annealed after the TCO deposition shows optimal carrier collection throughout the whole
wavelengths range even without light bias. The comparison of these two devices annealed
either before or after the window deposition suggests that the annealing rather impacts the
buffer/window interface and/or window properties than those of the buffer or CIGSe/buffer
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interface. Furthermore, according to the EQE results, it seems that these annealing-induced
changes in the buffer/window area imply changes of the near surface absorber region
properties.
All of the conclusions resulting from these investigations on the so-called standard
solar cell structure suggest that the origins of the dominating optoelectronic metastabilities
dwell within the absorber near surface/buffer/window region. Consequently, the properties
of these stacked layers have been sequentially varied in order to determine the influence of
each of them on the metastable behavior of the solar cell.
B. Raman analysis corresponding the CIGSe absorbers with 3 different Cu concentrations
The micro-Raman spectroscopy analyses performed on the CIGSe samples relative to
the series (1) are shown in Fig. 11. The 514 nm laser used for these measurements allowed a
surface sensitive analysis of the CIGSe layer (about 100 nm of penetration depth). For the
sample containing the lowest amount of Cu (i.e. CIGSe (3)), the Raman spectrum exhibits an
additional peak located at 158 cm-1, and a shoulder at 196 cm-1; these contributions can be
assigned to Cu(In,Ga)3Se5 phase [40]. For the sample with the highest Cu content (CIGSe (1))
these peaks are not detected, whereas they have very low intensities for the CIGSe with
intermediate Cu content (CIGSe (2)). This analysis reveals that decreasing the Cu content of
the absorber leads to the formation of Cu(In,Ga)3Se5 phase, which is located at the surface of
the absorber according to the EDS measurements. The presence of this phase at the interface
with the buffer layer should be then taken into account in the interpretation of the results
regarding the structure of the p-n junction of the devices.
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Fig. 11 Micro-Raman spectra corresponding to the three CIGSe absorbers with different Cu
contents.
C. Electrical parameters corresponding to the part III.A and III.B
TAB. I. Photovoltaic parameters of the solar cells with different absorber Cu contents after
WLSS for both CdS and Zn(S,O,OH) buffer layers;  is the duration needed by the cell to
reach its highest level of performance.
Sample
Voc (mV)
Jsc (mA/cm2)
FF (%)
Efficiency (%)
(min)
CIGSe(1)/CdS
CIGSe(1)/Zn(S,O,OH)
620
550
31.2
31.7
74.4
69.7
14.4
12.2
4
CIGSe(2)/CdS
CIGSe(2)/Zn(S,O,OH)
618
544
31.4
31.5
74.0
67.7
14.3
11.6
10
CIGSe(3)/CdS
CIGSe(3)/Zn(S,O,OH)
610
541
31.2
31.4
72.7
65.1
13.8
11.1
60
6
TAB. II. Photovoltaic parameters of the solar cells with different Zn(S,O,OH) buffer layer
thickness before WLSS (t0) and after WLSS (tmax).
Sample
Zn(S,O,OH)
WLSS time
1
( ~ 30 nm )
Zn(S,O,OH) 2
( ~ 5 0 nm )
Zn(S,O,OH) 3
( ~ 10 0 nm )
Voc (mV)
Jsc (mA/cm²)
FF (%)
Effici ency (%)
t0
550
15.1
22.0
1.82
tmax~ 20 min
565
31.1
67.7
11.9
t0
570
28.1
49.6
7.94
tmax~ 10 min
570
30.9
68.1
12.0
t0
575
30.7
58.3
10.3
tmax~ 2 min
575
30.8
68.1
12.1
The Cu-content of the CIGSe film strongly influences the metastable behavior of the
devices based on Zn(S,O,OH) buffer layers; moreover it also influences the highest FF
reached by the cells after WLSS. The limited gain in Jsc for these devices is mainly due to the
change in positions and amplitudes of the optical losses (interferences) on the EQE spectra,
since no anti-reflective coating has been used for these cells. When the CdS-based devices are
considered, the efficiency is rather similar for the different CIGSe absorbers. The thickness of
the Zn(S,O,OH) buffer layer also influences the metastable behavior of the devices; however,
after LS, the efficiency of the different devices is very similar.
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