Defect Study of CU2ZnSn(S,Se)4 Thin Film with Different Cu/Sn Ratio by Admittance Spectroscopy

Xianj ia Luo I ,Muhammad Monirul Islam I, Mohammad Abdul Halim I , Chong Xu I, Takeaki Sakurai I, Noriyuki Sakai2, Takuya Kato2, Hiroki Sugimoto2, Hitoshi Tampo3, Hajime Shibata3, Shigeru Niki3,

Katsuhiro Akimoto I

�Institute of A�plied �hysics, University of Tsukuba: 1-1-1 Tennodai, Tsukuba, Ibaraki,305-8573, Japan, Energy SolutIOn Busmess Center, Showa Shell SekIyu K.K., 2-3-2 Daiba, Minato-ku, Tokyo 135-8074,

Japan, 3National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan

Abstract - Defect properties of CU2ZnSn(S"Sel.,)4 (CZTSSe) were investigated by admittance spectroscopy (AS). Two defect states (labeled EAt and EA2) were observed in CZTSSe (x=O.15) with different CutSn ratio. When the CutSn ratio increased from 1.75 to 1.95, the activation energy of EAt and EA2 decreased and the defect densities increased. The capture cross sections of EAt and EA2 defects are in the order from 10.16 cm

2 to 10.18 cm

2,

indicating that these two defects possibly do not impact on device

performance.

Keywords-Cu2ZnSnS4, CU2ZnSn(S,Se)4, admittance spectroscopy, defects, capture cross section.

I. INTRODUCTION

CU2ZnSn(Sx,Se\.x)4 (CZTSSe) has been considered as a

sustainable alternative to Cu(In,Ga)Se2 (CIGS) thin film solar

cells and has attracted significant attention, due to its low

toxicity and abundance [1]. The CZTSSe is a semiconductor

with a direct band gap varying from 1.0 e V to 1.4 e V and high

absorption coefficient in the order of lO4 cm·1 [2,3]. It has

been reported that CZTSSe alloys with high selenium content

have higher efficiency than those with high sulfur content [3].

Sugimoto et al. have reported that its efficiency mainly

depends on Cu/Sn ratio rather than Zn!Sn ratio [4]. Recently

the highest efficiencies of CZTSSe solar cells have reached

toI2.6% [5]. However, knowledge about defects of these

materials and their impact on the device performance remains

unclear. A complete understanding of the defects properties of

these materials could improve the device performance.

In this work, in order to understand the defect properties in

CZTSSe (x=0.15), admittance measurements were performed

in CZTSSe solar cell structure with different Cu/Sn ratio. The

admittance spectroscopy (AS) shows two defect states in all

CZTSSe samples. With increasing Cu/Sn ratio, the activation

energy of the both defects decreased and the defect density

increased. The capture cross section for both defects has been

estimated and the effect on the device performance is

discussed.

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

II. EXPERIMENTAL

The CZTSSe thin film solar cells with different Cu/Sn ratio,

namely 1.65, 1.75 and 1.85, were fabricated in Energy

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

detailed fabrication conditions for CZTSSe solar cells have

been described elsewhere [4]. The S concentration was fixed

at S/(S+Se)� 15% for three samples.

For AS measurements, samples were mounted inside a Janis

closed-cycle cryostat and measurements were performed in

the temperature range from 20 k to 340 k using Agilent 4284

A LCR meter with the ac frequencies from 1 KHz to 1 MHz

and 25 mV AC modulation voltage. The activation energy of

defects and defect density has been estimated by AS

measurements [6].

III. RESULTS AND DISCUSSION

Fig. 1 shows the admittance (dC/dlnco) spectra of CZTSSe

(x=0.15) with the Cu/Sn ratio of 1.75, 1.85 and 1.95. Two

peaks were detected labeled as EAI and EA2. A small peak in

CZTSSe sample with the Cu/Sn ratio of 1.75 was observed in

the lower temperature range as shown in the inset of Fig. 1.

The peak position ofEAI and EA2 shifted to lower temperature

with higher Cu/Sn ratio. Therefore, these results suggest that

the activation energy of EAI and EA2 should decrease with

increasing Cu/Sn ratio. The activation energy of these defects,

EA, were obtained from the slope of the Arrhenius plot,

In(coo/T2) versus liT, using the following equation [7,8]:

(1)

(2)

2366

N = 2(2mn*kT)%

c,v h2

(3)

(4)

Where, COo is the characteristic frequency of capacitance,

�o is the temperature independent pre-exponential factor, Nc,v is the effective density of the band, Vth is the average thermal

velocity, (Jt is the capture cross section of defect, m* is the

hole effective mass, k is the Boltzmann constant, h is the

Plank constant and EA is the activation energy of the defect

The defect density Nt was calculated according to the

following equation [6]:

(5)

Where W, q, Vbi, and Eg are depletion width, elemental

charge, built-in-potential and band gap, respectively.

-50 o 50 100 150 200 250 300 350 400

Temperatrue (K)

Fig. 1. Temperature-dependent admittance spectra of CZTSSe with the Cu/Sn ration of 1.75, 1.85 and 1.95 at 10

45 Hz. The inset is the expanded admittance spectrum of CZTSSe with 1.75 Cu/Sn ratio.

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

The activation energy for all samples obtained by AS is

listed in Table I, which shows two kinds of defect in CZTSSe.

The activation energy ofEAI defect in CZTSSe with the Cu/Sn

ratio of l.75, l.85 and l.95 are around 270 meV, 220 meV

and 90 meV, and the values ofEA2 defect are around 150 meV,

70 meV and 60 meV, respectively. These results indicate that

high CuiSn ratio results in low activation energy. Recently

Sugimoto et al. have found the activation energy of defect

decreased with increasing Cu/Sn ratio by photoluminescence

and external quantum efficiency [4]. They also reported that

the valence band maximum of high Cu/Sn ratio sample is

higher than that of low Cu/Sn ratio sample, which maybe

leads to the shift of activation energy of defect in high Cu/Sn

ratio. The origin of this shift remains unclear and needs further

studied.

a CufSn=1.75

0.2

b CufSn=1.B5

';" lE1B N...,=1.40xl0'· (cm·3) > Q)

M � lE17

� .� lE16 Q)

'0 Q.

0.3 0.4 0.5 0.6

� lE15 ������������

'? E !:!. lE1B � 'iii c: � lE17 Q. �

I- 1 E16 L-'l..........J...L.I....�-'--�-'-�--'-�---'_'-----' 0.0 0.1 0.2 0.3 0.4 0.5 0.6

Activation Energy (eV)

Fig. 2.Defect spectra derived from admittance measurements in

CZTSSe with the Cu/Sn ratio of 1.75 (a), 1.85 (b) and 1.95 (c).

2367

Table I

The activation energy, defect density and capture cross section of defects in CZTSSe grown with different Cu/Sn ratio

EAt EA(meV)

Cu/Sn=1.75 270

Cu/Sn=1.85 220

Cu/Sn=1.95 90

EA2 Cu/Sn=1.75 150

Cu/Sn=1.85 70

Cu/Sn=1.95 60

The two kinds of defect levels in CZTSSe were also found

by Kask et al. [9]. They have assumed that the EAl and EA2

state belong to an interface state and acceptor defect of CUzn antisite, respectively, on the basis of their AS measurement

and photoluminescence. However, these assignments need

further evidence. Similar defect levels in CIGS have been

found in our previous study [ 10, 11]. However, the shallower

acceptor lever of V Cu, as the main p-type source in CIGS, was

not observed in this experiment. Yang et al. found only one

defect level in CZTSSe by AS measurements [3]. Such

difference may be due to the growth process of CZTSSe.

Fig. 2 shows defect spectra of CZTSSe with the Cu/Sn ratio

of 1.75, 1.85 and 1.95 derived from AS. Defect density

obtained by AS for all samples are shown in Table I. The

Gaussian fit was utilized to calculate the defect density, as

shown by dashed line. Due to the small activation energy

difference between two defects in CZTSSe with 1.95 Cu/Sn

ratio, only one defect density peak was observed from the

spectra, as shown in Fig. 2 (c). The Gaussian fit shows that the

defect density of EAl and EA2 are in the order of 1016 cm-3 and

the density increased with increasing Cu/Sn ratio. These

results reveal that the high Cu/Sn ratio results in high defect

density.

The thermal capture cross sections of the defect were

calculated from the equation (2), (3) and (4) and the results are

shown in Table I. The hole effective masses of CZTSe have

been calculated by Liu et al. using first-principles [ 12]. Capture cross sections of EAl and EA2 defect states are in the

range from 10-18 cm2

to 10-16 cm2

depending on Cu/Sn ratio.

The obtained value of the cross section is relatively small,

therefore, these defects possibly do not affect the device

performance.

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

Nt (cm-J) (ft (cml)

3.1x1016 6.3xl0-17

3.5x1016 1.2xl0-16

5.7x1017 5.5xl0-18

1.1x1016 1.8xl0-17

1.4x1016 9.9xl0-18

5.1x1017 2.5xl0-17

IV. CONCLUSIONS

The activation energy, defect density and capture cross

section of defects in CZTSSe grown with different CU/Sn ratio

were obtained by AS measurements. We found two kinds of

defect levels in CZTSSe. With increasing Cu/Sn ratio in

CZTSSe, the activation energy of defects decreased. The

defect density increased with increasing Cu/Sn ratio. The

capture cross section of the two kinds of defect was estimated

to be in the range from 10-18 cm2

to 10-16 cm2

depending on

Cu/Sn ratio. From the value of the cross section, the defects

detected by AS possibly have no effective impact on the

CZTSSe solar cell performance.

REFERENCES [I] H. katagiri, "Cu2ZnSnS4 thin film solar cells," Thin Solid Films,

vol. 426, pp. 480-481, 200S. [2] H. katagiri, K. Jimbo, W. Maw, K. Oishi, M. Yamazaki, H.

Araki, A. Takeuchi, "Development of CZTS-based thin film solar cells," Thin Solid Films, vol.Sl7, pp.24SS-2460, 200S.

[3] H. Duan, W. Yang, B. Bob, C. Hsu, B. Lei, Y. Yang, "The role of sulfur in solution-processed CU2ZnSn(S,Se)4 and its effect on defect properties," Advanced Functional Materials, vol. 23, pp. 1466-1471, 2013.

[4] H. Sugimoto, C. Liao, H. Hiroi, N. Sakai, T. Kato, "Lifetime improvement for high efficiency Cu2ZnSnS4 submodules," in proceedings of the 391h IEEE photovoltaic specialist conference, 2013 . .

[S] W. Wang, M. T . Winkler, O. Gunawan, T. Gokmen, T . K. Todorov, Y. Zhu, and D. B.Mitzi, "Device Characteristics of CZTSSe Thin-Film Solar Cells with 12. 6% Efficiency," Advanced Energy Materials, doi:10. 1002/aenm.20130146S, 2013.

[6] T. Walter, R. Herberholz, C. Muller, H. Schock, "Determination of defect distributions from admittance measurements and

2368

application to Cu(ln,Ga)Se2 based heterojuncions," Journal of Applied Physics, vol. 80, pp. 4411-4420, 1996.

[7] D. Losee, "Admittance spectroscopy of impurity levels in schottky barriers," Journal of Applied Physics, vol. 46, pp. 2204-2214, 1975.

[8] M. Islam, N. Miyashita, N. Ahsan, Y. Okada, "Defect study of molecular beam epitaxy grown undoped GalnNAsSb thin film using junction-capacitance spectroscopy," Applied Physics Letters, vol. 102, pp. 74104-(1-5), 2013.

[9] E. Kask, M. Grossberg, R. Josepson, P. Salu, K. Timmo, 1. Krustok, "Defect studies in CU2ZnSnSe4 and CU2ZnSn(Seo7sSo2s)4by admittance and photoluminescence spectroscopy" Materials Science in Semiconductor Processing, vol. 16, pp. 992-996, 2013.

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

[10] T. Sakurai, N. Ishida, S. Ishizuka, K. Matsubara, K. Sakurai, A. Yamada, G.K. Paul, K. Akimoto, S. Niki, "Investigation of relation between Ga concentration and defect levels of AI/Cu(ln,Ga)Se2 Schottky junctions using admittance spectroscopy" Thin Solids, vol. 515, pp. 6208-6211, 2007.

[11] T. Sakurai, M. Islam, H. Uehigashi, S. Ishizuka, A. Yamada, K. Matsubara, S. Niki, K. Akimoto, "Dependence of Se beam pressure on defect states in CIGS-based solar cells" Solar Energy Materials and Solar Cells, vol. 95, pp. 227-230, 2011.

[12] H. Liu, S. Chen, Y Zhai, H. Xiang, X. Gong, S Wei, "First­principles study on the effective masses of zinc-blend-derived Cu2Zn-IV-VI4 (IV=Sn, Ge, Si and VI=S, Se)" Journal of Applied Physics, vol. 112, pp. 93717-(1-6), 2012.

2369