Variable Light Soaking Effect of Cu(In,Ga)Se

Solar Cells with Conduction Band Offset

Control of Window/Cu(In,Ga)Se

Layers

Takashi Minemoto

, Yasuhiro Hashimoto

, Takuya Satoh

, Takayuki Negami

, and Hideyuki

Takakura

Photonics, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan

Matsushita Electric Ind. Co., Ltd., 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto, 612-0237,

Japan

ABSTRACT

The impact of the conduction band offset (CBO) between window and Cu(In,Ga)Se

(CIGS) layers on the light soaking (LS) effect in CIGS solar cells has been clarified with

continuous CBO control using a (Zn,Mg)O (ZMO) window layer. Two types of CIGS solar cells

with different window/buffer/absorber layers configurations were fabricated, i.e., ZMO/CIGS

(without buffer layer) and ZMO/CdS/CIGS structures. Current-voltage (J-V) characteristics of

the solar cells revealed that positive CBO values, where the conduction band of ZMO is higher

than that of CIGS, higher than 0.16 eV induces J-V curve distortion, i.e., LS effect, and all the J-

V characteristics stabilized in 30 min. The degrees of the LS effect were dominated by the CBO

value between ZMO and CIGS layers in the both structures regardless of the existence of CdS

buffer layers. These results indicate that the LS effect is dominated by the highest barrier for

photo-generated electrons in the conduction band diagram, i.e., the CBO between ZMO and

CIGS layers, and quantitatively the LS effect emerges from the CBO value higher than 0.16 eV.

INTRODUCTION

Cu(In,Ga)Se

(CIGS) solar cells are one of the most promising candidate for high-

efficiency and low-cost thin-film solar cells. While a conversion efficiency of the cells has

approaching 20% [1], there still remain unclear current transportation mechanisms and device

operations. One of the unclear operation mechanisms is light soaking (LS) effect [2-6]. The

efficiency of CIGS solar cells are observed to improve after exposure to white light for typically

30 min, generally due to increases in fill factor (FF) and open-circuit voltage (V

). The degree

of the LS effect, i.e., a difference between initial to stabilized values, seems to be greater in

CIGS solar cells with buffer/window layers such as ZnS [4], InS(O,OH) [5] and In(OH)

:Zn

[6]

other than CdS. This would be due to the difference in band alignments of the devices, especially

in the conduction band offset (CBO) between window and CIGS layers. In previous reports, we

proposed a new window layer of (Zn,Mg)O (ZMO) which can continuously control the CBO

value in the CIGS solar cells [7]. Also, we clarified the effect of the CBO value on the device

performance and demonstrated the high efficiency CIGS solar cells without a CdS layer [8-11].

In the series of operation characterizations of these devices, we have found that there is clear

difference in the LS effect in CIGS solar cells with different CBO values. In this paper, we report

on the variable degree of the LS effect on the CIGS solar cells with the controlled CBO using the

ZMO window layer.

EXPERIMENT

Two types of CIGS solar cells with different window/buffer/absorber layers

configurations were fabricated. To investigate the direct effect of the CBO value between

window and CIGS layers, ZMO/CIGS solar cells without buffer layer were fabricated; however

the interface between ZMO/CIGS layer should have sputtering damages which may reduce solar

cell performances. ZMO/CdS/CIGS solar cells were fabricated to avoid the interface damage and

to obtain high quality junction properties. In both structures, the other components of the solar

cells, i.e., substrate, back contact, transparent electrode, and front contact, are same. Fabrication

processes of the cells are as follows. After covering clean soda-lime glass (SLG) substrates with

back electrode of 0.8 µm-thick Mo films by sputtering, 1.5~1.8 µm-thick CIGS layers with

Ga/(Ga+In) compositional ratio of 0.24~0.32 were deposited using physical vapor deposition

[12]. Then, the surfaces of the CIGS films were modified by the solution that contains 0.005M

InCl

and 0.1M CH

CSNH

(thioacetamide). The pH of the aqueous solution was adjusted to

1.95 adding HCl. The solution was heated up to 70°C in the water bath for treatment. The CIGS

films were soaked in the solution for 10 seconds to form extremely thin surface sulfurized layer

[13]. Until this surface modification, the processes are common for both solar cells. For

ZMO/CIGS solar cells, the CIGS films were dipped in a solution containing Cd

ion, which is a

typical CdS bath [14] without S source of thiourea, to prepare Cd-doped n-type CIGS surface

layer and to form buried junction that reduce the interface recombination velocity between

window/CIGS layers. For ZMO/CdS/CIGS solar cells, 60 nm-thick CdS buffer layers were

deposited by chemical bath deposition (CBD). The solution for the CBD-CdS contains 0.001M

Cd(CH

COO)

, 0.4M NH

, 0.01M CH

COONH

, 0.005M, NH

CSNH

[14]. The beaker

containing the solution was soaked in the water bath kept 85°C and the solution was heated up

from the room temperature to occur reaction. The film thickness was controlled by soaking time

in the bath and a typical time was 15 min. Then, for both solar cells, 0.1 µm-thick ZMO window

layers were deposited with cosputtering of ZnO and MgO targets. The CBO value between ZMO

and CIGS layers was controlled to -0.15~0.25 eV by changing the Mg content to 0~22% in the

ZMO layers resulting in the band gap of ZMO of 3.28~3.71 eV [7]. Here, plus and minus signs

of the CBO values indicate the conduction band minimums of ZMO above and below that of

CIGS, respectively. The CBO value was determined from the band gap energy of CIGS and

ZMO layers with the assumption that the valence band offset between CIGS and ZMO layers is

the fixed value of 2.30 eV in this composition range of both layers [9]. Finally, 0.1 µm-thick ITO

layers were deposited by sputtering and Au/NiCr front grids were deposited by vacuum

evaporation. Antireflective coatings such as MgF

are not deposited in this study. An active area

of the cells was 0.96 cm

or 0.24 cm

(if some part of the cell shunted, the cell was scribed again

to avoid the shunting, resulting in the small area of 0.24 cm

). The change in the current-voltage

(J-V) characteristics of the CIGS solar cells were measured under AM1.5G 100 mW/cm

illumination at 25

C with the exposure durations of the illumination of 0 (without prior light

exposure), 1, 2, 3, 5, 10, 15, 20, 25 and 30 min.

RESULTS AND DISCUSSION

Before discussing the LS effect, the stabilized performances of the solar cells are

presented to demonstrate successful device performances of the CIGS solar cells with the ZMO

window layer. Table I and II shows the stabilized solar cell parameters of ZMO/CIGS and

ZMO/CdS/CIGS solar cells with different CBO values between ZMO and CIGS layers. Also in

ZMO/CdS/CIGS solar cells, the CBO value between ZMO and CIGS layers are used as a

parameter to discuss the LS effect in both structures in the same manner. In ZMO/CIGS solar

cells, the stabilized efficiencies for the cells with the CBO values higher than 0.16 eV (cell b, c

and d) were 10.8~11.3%. A low efficiency in the cell a with the CBO value of 0.00 eV is

attributed to a shunting due to an inappropriate CBO of window/CIGS layers [9]. On the other

hand, ZMO/CdS/CIGS solar cells show relatively high efficiency compared to that of

ZMO/CIGS solar cells because of superior junction properties.

Table I. Stabilized solar cell parameters of ZMO/CIGS solar cells with different CBO values.

cell ID CBO (eV) Mg/(Mg+Zn) Effi. (%) J

(mA/cm

) V

(V) FF

a 0.00 0.10 5.22 32.0 0.363 0.450

b 0.16 0.18 11.2 32.8 0.534 0.639

c 0.20 0.20 10.8 31.2 0.560 0.617

d 0.25 0.22 11.3 30.7 0.556 0.662

Table II. Stabilized solar cell parameters of ZMO/CdS/CIGS solar cells with different CBO

values.

cell ID CBO (eV) Mg/(Mg+Zn) Effi. (%) J

(mA/cm

) V

(V) FF

e -0.15 0.00 13.9 31.6 0.601 0.731

f 0.03 0.10 10.0 30.6 0.577 0.564

g 0.10 0.14 13.0 30.3 0.583 0.734

h 0.17 0.17 12.1 30.5 0.579 0.686

i 0.24 0.20 14.0 32.5 0.614 0.701

Figure 1 shows the LS rate of solar cell parameters of ZMO/CIGS and ZMO/CdS/CIGS

solar cells as a function of the CBO values between ZMO and CIGS layers. Here, the LS rate is

defined as initial solar cell parameters (no prior white light exposure) divided by stabilized

values measured after 30 min white light exposure. The LS rate indicates the degree of the LS

effect; the lower LS rate means the more severe LS behavior. The LS effect was not observed in

a negative CBO. In contrast, the LS effect emerged with increasing the CBO value. In both

structures, small or negligible degrees of LS effects in short-circuit current density (J

) and V

were observed. The small LS effects in ZMO/CIGS solar cells would be attributed to that the

recombination velocity for photo-generated electrons at the ZMO/CIGS interface caused by the

CBO decreased with white light exposure, which may be due to a surface n-type inversion by Cu

migration [2] or an increase in hole carrier density in the CIGS layer [15, 16]. For

ZMO/CdS/CIGS solar cells, negligible LS effects on J

and V

were observed. This would be

due to that the high quality of the CdS/CIGS interface did not cause a high recombination

velocity in the first place and/or the effective CBO value between CdS/CIGS layers during white

light exposure is not high because of accepter-like deep states in the CdS layer [3] or the

intermixing of the CdS/CIGS interface. In contrast to the behavior of J

and V

, the consistent

decrease in the LS rate of FF with increasing the CBO value were observed in the both structure,

indicating that the CBO values higher than 0.16 eV induced the LS effect. Also, the degree of the

LS effect on FF was the strongest in solar cell parameters so that the LS effect on efficiency

(Effi.) was dominated by the change in FF. Figure 2 shows the effect of white light exposure

duration (0~30 min) on the J-V curve shapes for ZMO/CIGS solar cells with the CBO value of

(a) 0.16 eV and (b) 0.25 eV and ZMO/CdS/CIGS solar cells with the CBO value of (c) 0.17 eV

and (d) 0.24eV. Clearly as shown by the comparison of Fig.2 (a) and (c), and Fig. 2(b) and (d),

the changes in the J-V curve shapes for almost same CBO values are quite similar although

ZMO/CdS/CIGS solar cells have CdS buffer layers. This is probably due to the above mentioned

reasons of the low enough effective CBO value of CdS/CIGS during white light exposure. These

results indicate that the degree of the LS effect is dominated by the highest barrier for photo-

generated electrons in the conduction band diagram, i.e., the CBO between ZMO and CIGS

layers, and quantitatively the LS effect emerges the CBO value higher than 0.16 eV.

Figure 1. LS rate (initial/stabilized) of solar cell parameters of ZMO/CIGS and ZMO/CdS/CIGS

solar cells as a function of the CBO values between ZMO and CIGS layers.

CBO (eV)

-0.2 -0.1 0.0 0.1 0.2 0.3

LS rate for J

(initial/stablized)

0.2

0.4

0.6

0.8

1.0

1.2

ZMO/CIGS

ZMO/CdS/CIGS

CBO (eV)

-0.2 -0.1 0.0 0.1 0.2 0.3

LS rate for V

(initial/stablized)

0.2

0.4

0.6

0.8

1.0

1.2

CBO (eV)

-0.2 -0.1 0.0 0.1 0.2 0.3

LS rate for FF (initial/stablized)

0.2

0.4

0.6

0.8

1.0

1.2

CBO (eV)

-0.2 -0.1 0.0 0.1 0.2 0.3

LS rate for Effi. (initial/stablized)

0.2

0.4

0.6

0.8

1.0

1.2

Figure 2. Effect of white light exposure duration (0~30 min) on the J-V curve shapes for

ZMO/CIGS solar cells with the CBO value of (a) 0.16 eV and (b) 0.25 eV, and ZMO/CdS/CIGS

solar cells with (c) the CBO value of (c) 0.17 eV and (d) 0.24 eV.

CONCLUSIONS

The impact of the CBO value between window and CIGS layers on the LS effect in CIGS

solar cells has been clarified with continuous CBO control using a ZMO window layer. Two

types of CIGS solar cells with different window/buffer/absorber layers configurations were

fabricated, i.e., ZMO/CIGS (without buffer layer) and ZMO/CdS/CIGS structures. The CBO

values between ZMO and CIGS layers were controlled to -0.15~0.25 eV. J-V characteristics of

the solar cells with different white light exposure durations revealed that a positive CBO value

higher than 0.16 eV induced the LS effect and all the J-V characteristics stabilized in 30 min.

Also, the degree of the LS effect was almost dominated by the CBO value between ZMO and

CIGS layers in the both structures regardless of the existence of CdS buffer layers. These results

indicate that the degree of the LS effect is dominated by the highest barrier for photo-generated

Voltage (V)

0.00.10.20.30.40.50.60.7

Current density (mA/cm

)

30min

0min

0,1,2,3,5,10,15,30min

Voltage (V)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Current density (mA/cm

)

30min

0min

0,1,2,3,5,10,15,30min

Voltage (V)

0.00.10.20.30.40.50.60.7

Current density (mA/cm

)

0min

30min

0,1,2,3,5,10,15,30min

Voltage (V)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Current density (mA/cm

)

0 min

30 min

0,1,2,3,5,10,15,30min

(a) ZMO/CIGS (CBO:0.16eV) (b) ZMO/CIGS (CBO:0.25eV)

(d) ZMO/CdS/CIGS (CBO:0.24eV)(c) ZMO/CdS/CIGS (CBO:0.17eV)

Cell b Cell d

Cell h Cell i

electrons in the conduction band diagram, i.e., the CBO between ZMO and CIGS layers, and

quantitatively the LS effect emerges the CBO value higher than 0.16 eV.

ACKNOWLEDGMENTS

This work is consigned from the New Energy and Industrial Technology Development

Organization. The authors would like to thank Prof. T. Wada of Ryukoku University for useful

discussion.

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