Field modulation in Na-incorporated Cu(In,Ga)Se2 (CIGS) polycrystalline films influenced by alloy-hardening and pair-annihilation probabilities

Article (PDF Available)inNanoscale Research Letters 6(1):581 · November 2011with52 Reads
DOI: 10.1186/1556-276X-6-581 · Source: PubMed
  • 24.5 · Gwangju Institute of Science and Technology
  • 15.56 · Korea Institute of Industrial Technology
  • 30.61 · German Aerospace Center (DLR)
  • 29.19 · Korea Atomic Energy Research Institute (KAERI)
Abstract
The influence of Na on Cu(In,Ga)Se2 (CIGS) solar cells was investigated. A gradient profile of the Na in the CIGS absorber layer can induce an electric field modulation and significantly strengthen the back surface field effect. This field modulation originates from a grain growth model introduced by a combination of alloy-hardening and pair-annihilation probabilities, wherein the Cu supply and Na diffusion together screen the driving force of the grain boundary motion (GBM) by alloy hardening, which indicates a specific GBM pinning by Cu and Na. The pair annihilation between the ubiquitously evolving GBMs has a coincident probability with the alloy-hardening event. PACS: 88. 40. H-, 81. 10. Aj, 81. 40. Cd

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NANO EXPRESS Open Access
Field modulation in Na-incorporated Cu(In,Ga)Se
2
(CIGS) polycrystalline films influenced by alloy-
hardening and pair-annihilation probabilities
Yonkil Jeong
1
, Chae-Woong Kim
2
, Dong-Won Park
1
, Seung Chul Jung
2
, Jongjin Lee
1
and Hee-Sang Shim
1*
Abstract
The influence of Na on Cu(In,Ga)Se
2
(CIGS) solar cells was investigated. A gradient profile of the Na in the CIGS
absorber layer can induce an electric field modulation and significantly strengthen the back surface field effect. This
field modulation originates from a grain growth model introduced by a combination of alloy-hardening and pair-
annihilation probabilities, wherein the Cu supply and Na diffusion together screen the driving force of the grain
boundary motion (GBM) by alloy hardening, which indicates a specific GBM pinning by Cu and Na. The pair
annihilation between the ubiquitously evolving GBMs has a coincident probability with the alloy-hardening event.
PACS: 88. 40. H-, 81. 10. Aj, 81. 40. Cd,
Keywords: Cu(In,Ga)Se
2
, solar cells, grain growth model, alloy hardening, pair-annihilation
Introduction
Thin film solar cells are promising candidates for po wer
generation and other integrated photovoltaic applica-
tions, as part of an effort to develop new renewable
energy technologies [1,2]. Specifically, chalcopyrite semi-
conductor systems, such as Cu(In,Ga)Se
2
(CIGS), have
attracted a great deal of interest as potential absorber
materials for thin film solar cells. In recent years, the
CIGS solar cells have demonstrated efficiencies of
greater than 20% using three-stage co-evaporation meth-
ods [3]. One of the common methods for improving the
performance o f CIGS solar cells uses soda-lime glass
(SLG) substrates, in which the amount of Na incorpo-
rated into the CIGS absorber layer is on the order of 0.1
at.% [4,5]. Several models have been proposed that
explain the effect of Na on device performance. Wang
et al. reported that the carrier concentration in the
CIGS absorber layer increases due to a reduction in the
amount of compensating (In,Ga)
Cu
defects due to the
substitution with Na
Cu
[6-8]. In contrast, Herberholz et
al. suggest that the existence range of a-CuInSe
2
widens
due to the incorporation of 0.1 at.% of Na, which sup-
presses the formation of the b-phase [9]. Rockett sug-
gests that Na incorporation leads to an increase in the
grain size and the lowest energy surfaces, such as Se-ter-
minated (112) surfaces, due to an in crease in the atomic
mobility during CIGS growth and at grain boundaries
[10,11]. Based on ab initio calculations, Persson and
Zunger demonstrate that Na
Cu
defects or NaInSe
2
phases at grain boundaries decrease t he valence-band
(VB) maximum due to a lack of Na d-electron states,
which is similar to the case of (2V
Cu
+In
Cu
)neutral
defect complexes at grain boundaries [12,13]. However,
the dominant phenomenon is an increase in the o utput
voltage from the perspective of CIGS device physics,
and an in crease in the grain size from a crystallographic
perspect ive. These increases are being recognized as far
more acceptable explanations for the improvement in
performance.
In this paper, we discuss how an increase in the fill
factor is caused by the back surface field (BSF) effect
from the perspective of CIGS device physics, and a
structural change in the grain size from a crystallo-
graphic approach using a combination of alloy-harden-
ing and pair-annihilation events.
* Correspondence: thanks21@gist.ac.kr
Contributed equally
1
Research Institute for Solar and Sustainable Energies (RISE), Gwangju
Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu,
Gwangju 500-712, South Korea
Full list of author information is available at the end of the article
Jeong et al. Nanoscale Research Letters 2011, 6:581
http://www.nanoscalereslett.com/content/6/1/581
© 2011 Jeong et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution
License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Experimental
Fabrication of CIGS absorber film and device
CIGS solar cells were fabricated on Corning g lass (CG)
and SLG substrates. The basic properties, including the
coefficient of therma l expansion (CTE), are summarized
in Table 1 for the SLG and the CG [14] (http://www.
abrisatechnologies.com). The polycrystalline CIGS films
were deposited on Mo-coated SLG and CG using a typi-
cal three-stage co-evaporation process, as described in
other studies [15,16]. In the first stage, indiu m (In), gal-
lium (Ga), and selenium (Se) sources were evaporated at
a growth temperature of 350°C to form a (In,Ga)
2
Se
3
layer with a thickness of 1 μm. In the second stage, cop-
per (Cu) and Se were evaporated and reacted with the
(In,Ga)
2
Se
3
layer at a temperature of 550°C to form the
Cu-richphase.Inthethirdstage,In,Ga,andSewere
evaporated to form the Cu-poor phase, while maintain-
ing the substr ate tem perature. Then, CdS buffer and i-
ZnO/Al:ZnO window layers were sequentially depo sited.
Thecellareawasdefinedas0.49cm
2
using a simple
mechanical scribing tool.
Characterization
The microstructure of the CIGS absorber layers was
investigated using transmission electron microscopy
(TEM, JEOL, Tokyo, Japan) operated at an acceleration
voltage of 200 keV. The samples were prepared using a
dual focu sed ion beam (FI B) s yste m. Depth profiling of
the chemical composition in both device structures was
performed with a secondary ion mass spectrometer
(SIMS, in a Cameca IMS 4f system, CAMECA SAS,
Gennevilliers Cedex, France) using an impact energy of
7.5 keV and a 20 0 nA O
2
+
beam and detecting MCs +
complexes (M =
63
Cu,
115
In,
69
Ga,
80
Se,
23
Na,
and
98
Mo). The solar cell effic iencies were measured and
recorded using a Keithley 4300 source meter under 100-
mW/cm
2
irradiation (Oriel
®
Sol3A , 450-W solar
simulator equipped with an AM 1.5-G filter; Oriel
Instr uments, Irvine, CA, USA) a nd an incident-photon-
to-electron conversion efficiency measurement system
for the wavelength range of 300 to 1,200 nm (QEX7, PV
Measurements Inc., Boulder, Colorado, USA).
Results and discussion
Microstructure and photovoltaic performances
Figure 1 shows the cross-sectional TEM (XTEM) images
of the interfaces in the CIGS solar cells on the CG
(Figure 1a,c) and SLG substrates (Figure 1b,d). The solar
cells fabricated on the CG and the SLG substrates will
be referred to as Na-restricted and Na-incorporated
devices, respectively. The interfa ces in Figure 1c,d have
a quasi-ohmic MoSe
2
layer formed by the inter-diffusion
of Se and Mo atoms [17,18]. In Figure 1a,b, the grain
configurations, such as t he size and crystallographic
orientation, seem to be similar to one another, while the
bottom regions of the CIGS absorber layers in Figure
1c,d clearly show different features. The interface con-
figuration in Figure 1c might be that of t he elongated
grain boundary motion (GBM) between the CIGS poly-
crystalline grain blocks, moving toward the Mo surface.
Figure 2 shows the illuminated J-V and external quan-
tum efficiency (EQE) curves of the Na-restricted and
Na-incorporated devices. The open-circuit voltage (V
oc
),
short-c ircuit current density (J
sc
), fill factor (FF), and
photo-conversion efficiency (Eff) are summarized in
Table 2. The EQE curves exhibit similar absorption
band-edges, which represent a very small dif ference in
the output voltage, as shown in Figure 2b. From the
EQE in the wavelength range of 400 nm to approxi-
mately 550 nm and 600 nm to approximate ly 850 nm in
Figure 2b, the slightly higher quantum efficiency of the
Na-incorporated device leads an improvement in the J
sc
.
However, t he main source of efficiency improvement,
which increases significa ntly f rom 10.9% to 14.6%, is
caused by the enhancement in the FF of greater tha n
10%. This result could b e attributed to the BSF induced
at the bottom region of the CIGS absorber layer. The
BSF effect might not be from the energy band-gap tun-
ing by the I n and Ga profile modulation but instead
from the en ergy-level pinn ing ca used by a structural
change in the Na-incorporated CIGS absorber layer.
Compositional profile
Figure 3 shows the SIMS profile plotted on a logarithm
and linear scale for the Na-incorporated device as a
function of the sputter-etch time (the SIMS profile of
the Na-restricted device is not shown in here). The N a
is distributed from the substrate t o the surface: the Na
intensity is higher than that of Cu and Se in the CIGS
absorber layer, whereas it is lower than that of In and
Ga (Figure 3a). In particular, the gradient of the Na pro-
file changes abruptly at the inter face between CIGS and
Mo. This finding implies two possible diffusion mechan-
isms of elemental Na. First, the Na continuously diffuses
Table 1 CTE and chemical composition of soda-lime glass and Corning glass substrates
Type CTE (×10
-6
/°C) Chemical contents (%)
SiO
2
Na
2
O Others
Soda-lime glass 8.4 (in the range of 25°C to approximately 513°C) 72.6 13.9 13.5
Corning glass 4.2 (in the range of 25°C to approximately 671°C) 69.0 1.0 30.0
Jeong et al. Nanoscale Research Letters 2011, 6:581
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Page 2 of 6
through the Mo from the SLG during the CIGS grain
growth, in which Na diffusion is gradually restricted by
the compact CIGS grain blocks, and consequently, Na
accumulation is more prominent in the bottom region
of the CIGS absorber layer. Second, the Na diffusion
should alre ady be in progress during the deposition of
the (In,Ga)
2
Se
3
precursor film, and in the beginning of
the CIGS grain growth, Na s hould begin to spread out
upward and downward, as shown in Figure 3b. At the
same time, the continuo us Na diffusion from the sub-
strate results in a prominent Na profile in the bottom
region. In practice, such a Na profile could improve the
device performance, as reported in the literature [19].
As far as the Na diffusion mechanism is concerned, the
second mechanism seems to be a far more acceptable
explanation. Herein, we suggest a model describing a
structural change in the CIGS polycrystalline film.
Grain growth model
Figure 4 provides a detailed schematic of the struc-
tural change that occurs during the growth process of
the CIGS film for the Na-restricted device ( Figure a,b,
Figure 2 J-V and EQE curves. (a) Illuminated J-V and (b) EQE curves of Na-restricted and Na-incorporated devices.
Figure 1 XTEM images of each interface. (a) the interface between the CdS buffer; (b) th e CIGS absorber layers for the Na-restricted device
and the Na-incorporated device; the interface between the CIGS absorber layers and the Mo back contact layers for (c) the Na-restricted device;
(d) the Na-incorporated device.
Jeong et al. Nanoscale Research Letters 2011, 6:581
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Page 3 of 6
c) and the Na-incorporated device (Figure d,e,f). The
(In
x
Ga
1-x
)
2
Se
3
precursorfilmisformedinthefirst
deposition stage of the CIGS film. For the second
stage in which Cu and Se are introduced, the grain
growth of the CIGS starts from the surface of the
(In
x
Ga
1-x
)
2
Se
3
film (Figure 4a,d). The grain growth
and volume expansion of the CIGS c rystal would pro-
gress simultaneously u pward and downward, leading
the GBM. The upward grain growth from the initially
formed surface grain results from alloy-hardening and
pair-annihilation events in both devices [20-22]. The
constant Cu supply to the surface leads to a lower
activation energy for the GBM [23,24] because the
supplied Cu atoms induce an alloy hardening in the
CIGS film and make the upward propagation of the
grain boundary more energetically unfavorable. In
addition, Cu could act as a driving force screen for
the GBM, which indicates that the GBM is partially
pinned by the Cu supply. Subsequently, the grain
boundaries come together in the same growth direc-
tion and are eventually pair-annihilated. The grain size
at the t op region of t he CIGS films, as shown in Fig-
ure1a,b,canbeexplainedbytheupwardstructural
evolution. However, the downward structural evolu-
tion occurs in a different manner in the Na-restricted
and the Na-incorporated devices. In the case of the
Na-restricted film, the downward grain growth is
accompanied by a volume expansion that weakens the
grain size effect because the Cu diffusion is gradually
restricted due t o the compactly preformed CIGS crys-
tal grains. Finally, the polycrystalline CIGS film is
formed as shown in Figure 4c, which is attributed to
the structural configuration of the bottom region in
Figure 1c. In the Na-incorporated film, the downward
grain growth should be the same as that of the Na-
restricted device. However, there is sufficient Na diffu-
sion from the SLG such that the Cu vacancies are
compensated due to the limited diffusion rate of Cu
and it maintains grain growth by alloy hardening.
Such a structural feature of the bottom region (Figure
4f) is in good agreement with the TEM image, as
shown in Figure 1d. It should be noted that the Na
diffusion from the SLG occurs during the second
stageduetoahigherCTEoftheSLGsubstrate,while
the Corning glass substrate restricts the Na diffusion
due to its lower CTE and lower Na content. From the
SIMS profile, we observe that the Na is prominently
located at the interface between the CIGS absorber
and the Mo back contact, in which the Na might
occupy the Cu vacancies or replace the Cu atoms and
form Na(In,Ga)Se
2
phases, which produces a decrease
in the valence-band maximum at the grain boundaries
[12,13]. In other words, the electric field is modulated
because the valence-band maximum appro aches the
Fermi-energy level. Specifically, the prominent Na
profile in the bottom region of the CIGS absorber
layer is likely to cause energy-level pinning, which
could strengthen the BSF.
Conclusions
We fabricated Na-restricted and Na-incorporated CIGS
solar cells to investigate the influence of Na on the
Table 2 Parameters obtained from illuminated J-V curves.
Device V
oc
(V) J
sc
(mA cm
-2
) FF (%) Eff (%)
Na restricted 0.626 30.4 57.1 10.9
Na incorporated 0.641 32.0 71.1 14.6
Figure 3 SIMS depth profile. Plotted on (a) a logarithm scale and (b) a linear scale for the Na-incorporated devices.
Jeong et al. Nanoscale Research Letters 2011, 6:581
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device performances. The enhancement in the output
voltage and photocurrent density for the Na-incorpo-
rated device was negligible, while the FF revealed a
remarkable increase. This finding could be attributed to
the strengthening of the BSF by the energy-level pinning
in the bottom region of the CIGS ab sorber layer. The
energy-level pinning originates from the proposed grai n
growth model wherein the Cu s upply and the Na diffu-
sion both contribute to the grain growth by a combina-
tion of alloy-hardening and pair-annihilation events that
occur between grain boundaries.
Abbreviations
CIGS: Cu(In,Ga)Se
2
; GBM: grain boundary motion; SLG: soda-lime glass; CG:
Corning glass; VB: valence band; BSF: back surface field; CTE: coefficient of
thermal expansion; TEM: transmission electron microscopy; FIB: focused ion
beam; SIMS: secondary ion mass spectrometer; XTEM: cross-sectional TEM;
EQE: external quantum efficiency; V
oc
: open-circuit voltage; J
sc
: short-circuit
current; FF: fill factor; Eff: photo-conversion efficiency.
Acknowledgements
This work was supported by the Core Technolo gy Development Program for
the Next-generation Solar Cells of Research Institute for Solar and
Sustainable Energies (RISE), GIST.
Author details
1
Research Institute for Solar and Sustainable Energies (RISE), Gwangju
Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu,
Gwangju 500-712, South Korea
2
Korea Institute of Industrial Technology,
1110-9 Oryong-dong, Buk-gu, Gwangju 500-757, South Korea
Authors contributions
YKJ, CWK and HSS designed and drafted the study. CWK fabricated the CIGS
absorber films and devices using three-stage co-evaporation technique. YKJ
and HSS carried out the characterization of the CIGS devices. DWP and JJL
participated in the establishment of the grain growth mechanism for CIGS
absorber film during the three-stage evaporation. All authors read and
approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 12 August 2011 Accepted: 7 November 2011
Published: 7 November 2011
Figure 4 The mechanism of the crystal growth model for CIGS films.In(a,b,c) Na-restricted and (d,e,f) Na-incorporated devices.
Jeong et al. Nanoscale Research Letters 2011, 6:581
http://www.nanoscalereslett.com/content/6/1/581
Page 5 of 6
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doi:10.1186/1556-276X-6-581
Cite this article as: Jeong et al.: Field modulation in Na-incorporated Cu
(In,Ga)Se
2
(CIGS) polycrystalline films influenced by alloy-hardening and
pair-annihilation probabilities. Nanoscale Research Letters 2011 6:581.
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    • "The graded bandgap-induced back-surface field effect, which arises because of the composition dependence of the Ga/(Ga + In) ratio, leads to an increase in the open-circuit voltage (V OC ), fill factor, and short-circuit current density (J SC ) [25, 26]. Na incorporation in CIGS films affects their grain growth and preferred orientation, resulting in small grain size and an increase in V OC [27]. Thus, the current level of understanding of CIGS films is fairly high. "
    [Show abstract] [Hide abstract] ABSTRACT: Cu(2)ZnSnS(4) (CZTS) thin films were prepared by the sequential sulfurization of a co-sputtered precursor with a multitarget (Cu, ZnS, and SnS(2)) sputtering system. In order to investigate the crystallization behaviour of the thin films, the precursors were sulfurized in a tube furnace at different temperatures for different time durations. The Raman spectra of the sulfurized thin films showed that their crystallinity gradually improved with an increase in the sulfurization temperature and duration. However, transmission electron microscopy revealed an unexpected result-the precursor thin films were not completely transformed to the CZTS phase and showed the presence of uncrystallized material when sulfurized at 250-400 °C for 60 min and at 500 °C for 30 min. Thus, the crystallization of the co-sputtered precursor thin films showed a strong dependence on the sulfurization temperature and duration. The crystallization mechanism of the precursor thin films was understood on the basis of these results and has been described in this paper. The understanding of this mechanism may improve the standard preparation method for high-quality CZTS absorber layers.
    Full-text · Article · Feb 2013
    • "In addition, the capability to measure light elements such as Na by LIBS is a benefit in the analysis of thin films on glass substrate [14,15] because the diffusion of intrinsic Na from the glass to the thin films can occur and often becomes important for soda-lime glass (SLG) substrates6789 . For example, the photoconversion efficiency of CIGS solar cells is critically influenced by Na concentration6789 . These advantages of LIBS as well as the short measurement time and high sensitivity could be highly useful in applications such as real-time monitoring of CIGS composition and thus for quality control during solar cell manufacturing. "
    [Show abstract] [Hide abstract] ABSTRACT: The results for laser-induced breakdown spectroscopy (LIBS) measurement of thin Cu films (1 μm) on soda-lime glass (SLG) substrates with and without a supporting thin Mo layer (1 μm) are reported. The ablation was carried out using a nanosecond Q-switched Nd:YAG laser (λ=1064 nm, τ=4 ns, spot diameter=50  μm, top-hat profile) in the laser fluence range of 19.16-65.97 J/cm(2). It was found that, under the same laser irradiance conditions, the depth and morphology of ablation craters produced with and without the Mo layer were completely different. The electron number densities of the plasma from the two samples calculated from the measured LIBS spectra differed by a factor of 4 as 4.1×10(17) cm(-3) (Cu/Mo/SLG) and 17.7×10(17) cm(-3) (Cu/SLG), which was attributed to the matrix effects resulting from ionization of Na atoms diffused into the Mo layer. It is demonstrated that a nanosecond-laser-based LIBS is applicable for the characterization and composition analysis of thin film layers of a few micrometer thickness on glass substrates, especially for the measurement of Na contents of each layer.
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  • [Show abstract] [Hide abstract] ABSTRACT: This paper reports that selective removal of a CuIn 1−x Ga x Se 2 (CIGS) thin film on a Mo-coated glass substrate can be achieved with no edge melting or damage of the Mo layer using a nanosecond Nd : YAG laser with a wavelength of 1064 nm. It is shown that the two crucial parameters that determine the possibility of clean removal of only the CIGS layer are Ga concentration of the CIGS film and laser fluence. For CIGS films with Ga/(Ga+In) ratio greater than about 0.2 for which the band gap energy is close to or over the photon energy (1.17 eV), laser-induced thermal expansion proved to be the mechanism of film removal that drives an initial bulging of the film and then fracture into tens of micrometre sized fragments as observed in in situ shadowgraph images. The fracture-type removal of CIGS films was further verified by scanning electron micrographs of the craters showing that the original shapes of the CIGS polycrystals remain intact along the crater rim. A numerical simulation of film temperature under the irradiation conditions of selective removal was carried out to show that the magnitude of induced thermal stress within the film closely agreed to the yield strength of the CIGS thin film. The results confirmed that a nanosecond laser could be a better choice for P2 and P3 scribing of CIGS thin films if process conditions are properly determined. (Some figures may appear in colour only in the online journal)
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