Over 12% Efficiency CU2ZnSn(SeS)4 Solar Cell Via Hybrid Buffer Layer

Homare Hiroi*I,2, Jeehwan Kim3, Masaru Kuwahara4, Teodor K Todorov3, Dhruv Nair3

Marinus Hopstaken3, Yu Zhu3, Oki Gunawan3, David B. Mitzi3 and Hiroki Sugimotol,2

IEnergy Solution Business Center, Showa Shell Sekiyu K.K. 2Atsugi Research Center, Solar Frontier K.K., 123-1 Shimokawairi, Atsugi, Kanagawa, 243-0206 Japan

3IBM TJ Watson Research Center, Yorktown Hts., NY 10598 4Tokyo Ohka Kogyo Co., Ltd. Nakamaruko Nakahara-ku, Kawasaki Japan

*E-Mail: Homare.Hiroi@showa-shell.co.jp

Abstract Efficiency of 12.3% on solution-based CU2ZnSn(SeS)4 (CZTSeS) solar cells was achieved by applying hybrid buffer layer with combination of In-based buffer and Cd­based buffer layers. Previously, the Voc boost by the hybrid buffer layer on CU2ZnSnS4 (CZTS) and CZTSeS solar cells have been reportedll-

21. In this paper, the latest results and some analyses of the hybrid buffer for solution-based CZTSeS cells will be presented.

Index Terms - CZTS, buffer layer, CdS, In2S3

I. INTRODUCTION

Recently, several groups have reported high efficiency CU2ZnSnSe4 (CZTSe), CZTS and CZTSeS solar cells[3-61. The highest efficiency was achieved by the solution process CZTSeS solar cells by IBM as a world record of CZTS-based thin film solar cells[31. However, the low Voc is still the biggest problem as compared with the other solar cells. Previously, the Voc boost by the hybrid buffer layer on vacuum process CZTS solar cells has been reported at the 39th IEEE PVSC[ll, and that on solution process CZTSeS solar cells has also been reported at the PVSEC-23[21. In this work, the hybrid buffer was optimized for the high-quality solution process CZTSeS absorbers, and finally the efficiency of 12.3% on the solution­based CZTSeS solar cells was achieved, as shown in Fig. l.

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

II. RESULTS AND DISCUSSION

In this experiment, we prepared many CZTSeS solar cells with an area of approximately 0.45 cm2. The baseline structure constituted an In203:Sn (ITO) window layer and an i-ZnO layer by sputtering/ a buffer layer by chemical bath deposition (CBD)/ a CZTSeS absorber by solution process/ an Mo back electrode by DC magnetron sputtering! a soda-lime glass (SLG) substrate. With regard to each buffer layer, a comparative process flow chart is described in Fig. 2. The In­based buffer layer between CdS and i-ZnO layers on the hybrid buffer device was confirmed by STEM-ED X images, as shown in Fig. 3. Current-voltage (J-V) characteristics were measured under a standard test condition with a constant-light solar simulator, then external quantum efficiency (EQE) characteristics of the CZTSeS cells were investigated.

CdS ClTSeS absorber (Solution process)

CdS buffer (CeO)

I i-lnO/ITO (Sputtering)

ClTSeS atliorber (Solution process)

CdS buffer (CeO)

Fig. 2. Comparison of fabrication process and device structure.

0030

(a) CZTSeS/CdSIi-ZnO/ITO r-------....,

(b) CZTSeS/CdS/ln2S31i-ZnO/ITO

'� ",,:> , ' f :.'\ '. ': ' I ( . \, J 'I. ,: . t�·� · 1 • I�

" '\ . . ' . ' - .' .

Fig. 3. STEM-ED X analyses of (a) CdS and (b) the hybrid buffer

layers. Both cells have an identical C ZTSeS absorbers.

Figure 4 shows the comparison of Voc between the hybrid and the only CdS buffer devices, and the Voc boost in the hybrid buffer devices was clearly observed. Figure 5 summarizes a comparison between Voc and Eg on our CZTSeS cells with the CdS and hybrid buffer layers. Larger Voc deficit (Eglq-Voc=delta V) was observed at the higher Eg. The Voc on the CZTSeS cells with and without hybrid buffer layer was plotted as red circles and blue triangles in Fig. 5, respectively. The Voc improvement was confirmed by applying the hybrid buffer. This result points to the possibility that the interface between buffer and absorber layers could be one of the reasons for the low Voc> and it is very effective for Voc improvement to apply the hybrid buffer into the CZTSeS cells.

600

550

500 >" .§. 450 >'5

400 � •

350

300 0 100

•

• •

)_\!_ V �,& � l ��l � .,'P i. . It

. . . .. .. i • • •

�. •

200 300 400 500 Number of sumples

600

Fig. 4. Comparison of Voc between CdS and hybrid buffer devices.

978-1-4799-4398-2/141$31.00 ©2014 IEEE

600

575

550

525 >" .§. 500 >g 475

450

425

400 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45

Eg [eV] Fig. 5. Correlation between Voc and Eg on our C ZTSeS cells with

CdS and with hybrid buffer layers.

Comparison of J-V characteristics between the CdS and hybrid buffer layers on the identical absorbers is shown in Fig. 6. The hybrid buffer device showed higher Voc than the CdS buffer device, however FF was degraded due to higher series resistance (Rs). The origin of the high Rs is assumed to be an inter-diffusion between absorber and buffer layer based on STEM-EDX analysis of the hybrid buffer device, as shown in Fig. 7. Then, the fabrication process of the hybrid buffer was optimized for the CZTSeS absorber, which resulted in significant improvement of R" as shown in Fig. 8. The FF of the hybrid buffer device was also improved as good as that of just CdS buffer device, while the Voc remained higher than that of the CdS buffer devices. Finally, higher Eff was achieved by the hybrid buffer and 12.3% the best Eff was certified by Newport, as shown in Fig. 9 and Fig. 1.

We are now intensively developing the hybrid buffer for the CZTSeS solar cells, and further improvement will be reported at the conference.

35 30 -Hybrid

-CdS � 25 Hybrid

� 20 Voc: 480[mV] ct FF: 51.2r/o] .§.. 15 Eg: 1.17eV ..., CdS

10 Voc: 454[mV] 5 FF: 64.9[%]

Eg: 1.17eV 0

0 50 100 150 200 250 300 350 400 450 500 V (mV)

Fig. 6. Comparison of J-V characteristics between CdS and hybrid

buffer layers before optimization.

0031

i-ZnO

CdS

CZTSeS

Fig. 7.

35 30 25

E 20 (J

;;;( 15 .s ...., 10

5 o

o

Cu BII In

An enlarged STEM-EDX image of the hybrid buffer layers.

Hybrid Voc: 466[mV] FF: 68.4[%] Eg: 1.0geV CdS Voc: 436[mV] FF: 68.9[%] Eg: 1.0geV

50 100 150 200 250 300 350 400 450 500 V (mV)

Fig. 8. Comparison of J-V characteristics between CdS and hybrid

buffer layers after optimization.

11.2 11.0 10.8

- 10.6 � 0 10.4 -

.. . Hybrid u

.. . CdS J l

.... .... 10.2 W

10.0 9.8 -r 9.6 9.4

I (Without anti-reflection coating)

Fig. 9. Comparison of Eff between CdS and hybrid buffer devices.

(w/o anti reflection coating).

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

ACKNOWLEDGEMENT

This work was conducted as part of a joint development project between IBM Corporation, Tokyo Ohka Kogyo Co., Ltd., Showa Shell Sekiyu K.K. and Solar Frontier K.K.

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