Recent R&D Progress in Solar Frontier’s Small-sized

Cu(InGa)(SeS)2 Solar Cells

Motoshi Nakamura*, Nobutaka Yoneyama, Kyouhei Horiguchi, Yasuaki Iwata, Koji Yamaguchi,

Hiroki Sugimoto, and Takuya Kato

Energy Solution Business Center, Showa Shell Sekiyu K.K.,

Atsugi Research Center, Solar Frontier K.K.

123-1 Shimo-kawairi, Atsugi, Kanagawa 243-0206, Japan

Abstract — Our resent achievement of the record breaking

20.9% efficiency (independently confirmed by Fraunhofer ISE)

with small-sized CIS-based solar cell will be discussed in this

paper by means of comparison with our previous result of 19.7%.

The new record was mainly achieved by an improved Jsc arising

from modified depth profile of the absorber layer and the doping

concentration of the transparent conductive oxide (TCO) layer.

Combination of these two modifications drastically enhances

light absorption at a longer wavelength region, leading to Jsc

improvement of about 3 mA/cm2 without losing Voc nor FF.

This new 20.9% efficiency record resulted from a CIGS cell cut

from a 30 cm by 30 cm substrate produced using the same

method and materials we use in our factories, sputtering-

selenization formation method with Cd-free buffer layer.

Index Terms — Solar Frontier, Cu(In,Ga)(Se,S)2, Chalcopyrite,

CIS, 20.9%, Zn(O, S, OH)x buffer layer, EQE spectrum, band

gap, selenization

I. INTRODUCTION

CIS-based thin film solar cells are considered as one of the

most promising photovoltaic technologies due to its high

efficiency and cost reduction potential. At the end of last year,

the Zentrum für Sonnenenergie- und Wasserstoff-Forschung

Baden-Württemberg (Centre for Solar Energy and Hydrogen

Research, ZSW) had set a CIGS world record efficiency of

20.8%, which exceeds the champion efficiency of 20.4% for

multicrystalline silicon cells [1]. Several other groups also

have been reporting continuous efficiency improvement at the

range of over 20% [2]-[4]. All of the CIS-based photovoltaic

cells over 20% efficiency reported by those groups, however,

were prepared using an evaporation process, which may be

ideal for precise elemental control at laboratories but difficult

to reduce production cost at factories.

In this report, we will present achievement of a new world

record efficiency of 20.9% (independently confirmed by

Fraunhofer ISE) with small-sized CIS-based solar cell

produced using the same method and materials we use in our

factories, sputtering-selenization formation method, with Cd-

free buffer layer. The new champion cell was made from a 30

cm by 30 cm substrate with which absorber layer and window

layer were physically optimized to minimize optical loss.

II. EXPERIMENTAL CONDITION

The complete cell structure and the preparation method are

almost identical to the one we reported last year with 19.7%

efficiency [5]. Fig. 1 shows a schematic image of our standard

cell structure. First, Mo/Cu-Ga/In metal stack layer was

deposited by DC-sputtering onto 30 cm by 30 cm glass

substrates with an alkali barrier layer. The precursor layer was

first selenized with H2Se gas followed by sulfurization with

H2S gas in a furnace to form a p-type Cu(In,Ga)(Se,S)2

absorber layer. A Zn(O,S,OH)x buffer layer was deposited

onto the absorber layer by a chemical bath deposition (CBD)

with NH3 solution containing SC(NH2)2 and Zn(CH3COO)2.

Un-doped and B-doped ZnO layers were then deposited by a

metal-organic chemical vapor deposition (MOCVD) method

followed by Al contact electrode and MgF2 anti-reflective

layer depositions by electron beam evaporations.

The J-V and external quantum efficiency (EQE)

characteristics were independently measured by Fraunhofer

ISE CalLab PV Cells under the standard test conditions. The

elemental composition and the depth profile of the absorber

layers were measured using inductively coupled plasma

atomic emission spectroscopy (ICP-AES) and the glow

discharge optical emission spectrometry (GD-OES),

respectively.

Fig. 1. Schematic image of our device structure.

Grid electrode (Al)

Transparent conductive layer (ZnO:B)Intrinsic buffer layer (i-ZnO)Buffer layer (Zn(O,S,OH))

p-type absorber (CIGSSe)

Back electrode (Mo) Barrier layer (SiO2)

Glass substrate

Buffer layer (Mo(S,Se)2 )

Anti-refractive layer (MgF2)

978-1-4799-4398-2/14/$31.00 ©2014 IEEE 0107

III. RESULTS AND DISCUSSIONS

A. The new champion CIGS cell

The J-V and EQE characteristics of the new champion device

measured at Fraunhofer ISE CalLab PV Cells are shown in

Fig. 2. The characteristics of our previous champion device

are also shown in the same figure for comparison. The

conversion efficiency of the new record cell has reached to

20.9% with Voc, Jsc and FF are 0.686 mV, 39.9 mA/cm2, and

76.4%, respectively. Comparing with our previous record of

19.7% cell, it can be seen that the Jsc increased drastically,

while Voc and FF remained almost the same. The EQE

spectrum revealed that the higher Jsc arises from less EQE loss

at longer wavelength.

Fig. 2 (a) J-V and (b) EQE characteristic comparison between our

previous record cell (0.50 cm2) and the new champion cell (0.52 cm2).

The measurements were independently conducted under the standard

test condition by third parties (AIST and Fraunhofer ISE for 19.7%

and 20.9% cells, respectively). The absolute values of the EQE

curves were adjusted so that the integral of EQE become the same

value as Jsc of the J-V measurement.

B. Jsc Loss Analysis

The Voc, Jsc, and FF of our previous champion cell with

19.7% efficiency were 683 mV, 37.1 mA/cm2, and 77.8%,

respectively. When we compared these values with the ones

reported as Si cell champion (706 mV, 42.7 mA/cm2, 82.8%),

it was apparent that the Jsc of our 19.7% cell had plenty of

room to be improved. Since its EQE spectrum under short-

circuited and reverse-biased condition showed almost identical

shape, we assumed that charge collection through the absorber

layer is highly efficient and therefore the origins of the Jsc loss

should be limited to the following four elements; shadow loss

due to the Al grid, reflection loss at interfaces of each layers,

non-transmittance loss at the buffer and window layers, and

non-absorption loss at the absorber layer.

We quantified the absolute value of each loss element. The

shadow loss was simply identified by measuring Al grid area

with optical microscope. The reflection loss, non-

transmittance loss, and non-absorption loss were calculated

based on the matrix method [6],

�� = � cos δ� i sin δ��/η��η� sin δ� cos δ�

� ;

δ� = 2πη�d� cos θ /λ ;

η� = n� − ik� ;

where dk, nk, and kk are the thickness, refractive index, and

extinction coefficient of each layers, respectively. All of these

parameters of each component layers were measured using in-

house ellipsometry.

The calculated results of optical losses are summarized in Fig.

3. The values of shadow loss, reflection loss, non-absorption

loss, and non-transmittance loss were estimated as 1.3

mA/cm2, 1.5 mA/cm

2, 1.8 mA/cm

2, and 2.8 mA/cm

2,

respectively. According to this simulation, it is suggested that

transparency of the ZnO:B layer and absorption of the CIGS

layer at the longer wavelength region must be improved to

achieve higher efficiency.

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Voltage/cell (mV)Voltage/cell (mV)Voltage/cell (mV)Voltage/cell (mV)

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Eff(%) 19.7 20.92Voc(mV) 683 686Jsc(mA) 37.1 39.9

FF 0.778 0.764

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Wavelength (nm)Wavelength (nm)Wavelength (nm)Wavelength (nm)

2013(19.7%)2013(19.7%)2013(19.7%)2013(19.7%)

2014(20.9%)2014(20.9%)2014(20.9%)2014(20.9%)

978-1-4799-4398-2/14/$31.00 ©2014 IEEE 0108

Fig. 3. Summary of optical loss calculations

C. Optimization of ZnO:B Window Layer

It is well known that a higher doping concentration reduces

the light transmittance of TCO layers due to free-carrier

absorption. Fig. 4 shows transmittance spectra of ZnO:B

layers as a function of doping concentrations. As expected,

transparency especially at longer wavelength decreases as

doping concentration increases. If we compare this result with

the optical loss simulation in Fig. 3, reducing doping

concentration of ZnO:B layer seems to be a promising

approach to improve Jsc. In this sense, we established a new

base-line ZnO:B layer and grid shape which was specifically

optimized for our small-sized CIGS cell.

Fig. 4 Transmittance spectra of B-doped ZnO layer with different

doping concentration deposited on glass substrates. Antireflection

layer was not applied for all samples.

D. Optimization of the band structure

We calculate the bandgap, Eg, depth profile of our 19.7%

cell using an equation proposed by M. Bar et. al. [7],

Eg = (1.00 + 0.13x2 + 0.08x

2y + 0.13xy + 0.55x + 0.54y) eV,

where x and y are atomic ratio of Ga/(Ga+In) and S/(S+Se),

respectively. We obtained the elemental depth profile by

means of a GD-OES measurament (result not shown here).

The calculated band profile of our 19.7% cell is illustrated in

Fig. 5. Since a steep downward and upward Eg slope is formed

by S incorporation in the vicinity of the front side and Ga

accumulation toward the back side, respectively, the band

profile of our 19.7% cell has a “V” like shape. This result

implies that the lack of absorption at the longer wavelength

calculated by the optical simulation may arise from the steep

notch, as the light with a longer wavelength can only be

absorbed at this part of the absorber layer.

In order to know the effect of the steep notch, we ran a

SCAPS simulation [8]. Fig. 6 shows the simulated EQE

spectrums with varing notch width. As was predicted, EQE

response at higher wavelength region was enhanced as the

notch length was increased. We therefore refined our baseline

recipe of our metal precursor and selenization-sulferization

process to fabricate an absober with a wider Eg notch.

Fig. 5. Schematic image of the calculated band profile of our

19.7% cell with SCAPS.

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Measured QEEff=19.7 %Eg=1.08 eV

978-1-4799-4398-2/14/$31.00 ©2014 IEEE 0109

Fig. 6. SCAPS Simulation result of EQE spectrums with varying

notch width

IV. CONCLUSION

We have been able to obtain certified 20.9% conversion

efficiency with a small-sized CIGS single cell extracted from

a 30 x 30 cm2-sized sub-module. The new record was mainly

brought by an improved Jsc arising from modified depth

profile of the absorber layer and the doping concentration of

the TCO layer. It should be worth to emphasize that the new

champion cell was resulted from a CIGS cell produced using

the same method we use in our factories; sputtering-

selenization formation method with Cd-free buffer layer.

As a leading company of CIS-based solar module, we will

keep exploring the potential of CIS-based solar cell both on

commercial and lab scales.

ACKNOWLEDGEMENT

This study was consigned from the New Energy and

Industrial Technology Development Organization (NEDO)

PV R&D programs. We would like to acknowledge NEDO.

REFERENCES

[1] P. Jackson, D. Hariskos, R. Wuerz, W. Wischmann, and M. Powalla, “Compositional investigation of potassium doped Cu(In,Ga)Se2 solar cells with efficiencies up to 20.8%”, Physica Status Solidi (RRL) - Rapid Research Letters, Volume 8, Issue 3, p 219–222, 2014

[2] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, and Michael Powalla, “New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%”, Prog. Photovolt: Res. Appl., vol. 19, pp. 894-897, 2011.

[3] A. Chiril, P. Reinhard, F. Pianezzi, P. Bloesch, A. R. Uhl, C. Fella, L. Kranz, D. Keller, C. Gretener, H. Hagendorfer, D. Jaeger, R. Erni, S. Nishiwaki, S. Buecheler and A. N. Tiwari, “Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells”, Nat. Mat., vol. 12, 1107, 2013

[4] I. Repins, S. Glynn, J. Duenow, T.J. Coutts, W. Metzger, and M.A. Contreras, “Required Materials Properties for High- Efficiency CIGS Modules”, SPIE 2009 Solar Energy + Technology Conference, 2009

[5] M. Nakamura, K. Yamaguchi, Y. Chiba, H. Hakuma, T. Kobayashi, and T. Nakada, Proceeding of the 39th IEEE PVSC (IEEE, Tampa, 2013)

[6] H. A. Macleod, “Thin-Film Optical Filters, 3rd Ed.” IoP Publishing (Bristol and Philadelphia), Chap. 2., 2001

[7] M. Bär, W. Bohne, J. Röhrich, E. Strub, S. Lindner, M. C. Lux-Steiner, Ch.-H. Fischer, T. P. Niesen, and F. Karg, “Determination of the band gap depth profile of the penternary chalcopyrite from its composition gradient”, J. Appl. Phys. 96, 3857, 2004

[8] M. Burgelman, P. Nollet and S. Degrave, "Modelling polycrystalline semiconductor solar cells", Thin Solid Films, 361-362, 527-532, 2000

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978-1-4799-4398-2/14/$31.00 ©2014 IEEE 0110