Deposition of Cu2ZnSnS4 Thin Film by Pulsed Laser Deposition and Assembly of Thin Film Solar Cell with the Novel Structure of FTO/CdS/

Cu2ZnSnS4/Mo

Min Yao1,2,a, Chengwu Shi*1,2,b Yanru Zhang1,2,c, Xiaoyan Dai1,2,d 1 School of Chemical Engineering, Hefei University of Technology, Hefei 230009, China

2 Key Lab of Novel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, China

ayaomin100@163.com, bshicw506@gmail.com, czhangyanru137@126.com, ddaixiaoyanhfut1991@126.com

Keywords: Cu2ZnSnS4; Thin film; Pulsed laser deposition; solar cell

Abstract. Cu2ZnSnS4 (CZTS) thin film was successfully prepared by pulsed laser deposition using

CZTS nanocrystal as a source and the thin film solar cell with the novel structure of

FTO/CdS/CZTS/Mo was assembled. At the laser incident energy of 6 J·cm-2

, the chemical

composition of the CZTS thin film was Cu1.74Zn0.80Sn1.00S4.04, near to stoichiometric ratio. The thin

film solar cell with the novel structure of FTO/CdS/CZTS/Mo gave short circuit photocurrent density

of 1.01 mA·cm-2

, open circuit voltage of 0.39 V, and fill factor of 0.485, corresponding to the

photoelectric conversion efficiency of 0.19% at the illumination (Air Mass 1.5, 100 mW·cm-2

).

Introduction

Earth abundant and relatively low toxicity copper-zinc-tin-chalcogenide compounds such as

Cu2ZnSnS4(CZTS) [1-3], which have large absorption coefficient (>104

cm-1

) and direct band gap

(Eg=1.4-1.5 eV) [4,5], was the promising absorber materials for the low-cost thin film solar cells. The

preparation of CZTS thin films was widely investigated, such as thermal evaporation [6,7], spray

pyrolysis [8,9], electrodeposition [10,11] and pulsed laser deposition (PLD) [12-13], and the typical

structure of CZTS thin film solar cells was SLG/Mo/CZTS/CdS/Al:ZnO/Al. Up to now, the

preparation of CZTS thin films by pulsed laser deposition (PLD) directly using CZTS nanocrystal as

a source and the assembly of thin film solar cell with the novel structure of FTO/CdS/CZTS/Mo was

rarely reported.

In this paper, CZTS thin films were prepared by PLD using the target from the CZTS nanocrystal

at the substrate temperature of 400 oC. The chemical composition, crystal structure, surface

morphology and band gap of CZTS thin film were systemically investigated by energy dispersive

X-ray spectroscopy (EDS), X-ray diffraction (XRD), scanning electron microscope (SEM) and

ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra, respectively. And the CZTS thin

film solar cell with the novel and simple structure of FTO/CdS/CZTS/Mo was assembled.

Experimental

All the analytical chemicals were purchased from the commercial market and used without further

purification. The CZTS nanocrystal was prepared using the hydrothermal method by the mixture of

newly synthesized CuS, ZnS, and SnS precursors [14]. The CZTS target was shaped and pressed into

a pellet using the CZTS nanocrystal.

CZTS thin film was deposited by PLD with a KrF excimer laser (Lambda Physik, COMPEXPro

102, λ=248 nm, 20 ns pulse width). The distance between the CZTS target and the substrate was 50

mm. The CZTS target was fixed on a rotating holder with rotation at 10 rpm and the substrate was

fixed on a rotating holder with rotation at 5 rpm. Then the substrate was heated to 400 oC during

laser ablation. The deposition chamber was evacuated to 2.0×10-4

Pa using a turbo molecular pump.

The laser incident energy was 6 J·cm-2

and a repetition rate was 2 Hz. The deposition time was 150

min.

Advanced Materials Research Vol. 716 (2013) pp 328-331Online available since 2013/Jul/15 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.716.328

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The CZTS thin film solar cell with the novel and simple structure of FTO/CdS/CZTS/Mo was

fabricated. Firstly, CdS thin film was deposited on SnO2:F transparent conductive glass sheets (FTO,

TEC-8, LOF) using the ultrasonic agitation chemical bath deposition method, followed by

CdCl2-treatment and annealing at 450 °C for 50 min under high purity N2 atmosphere [15].

Secondly, the CZTS thin film was successively deposited on the CdS thin film by PLD using the

CZTS target. Lastly, the Mo back electrode was deposited by DC magnetron sputtering. The active

area of the CZTS thin film solar cell was 0.25 cm2

.

The chemical composition and crystal structure of CZTS thin film were measured by EDS using

Japan EDX spectrometer (JSM6490/LV, Japan) and XRD, respectively. XRD patterns were

measured using CuKα radiation, λ=0.154056 nm, 40 kV and 50 mA (Philips X' Pert PRO SUPER,

Netherlands). A scanning rate of 0.017°⋅s-1

was applied to record the patterns in the 2θ range of

10-80°. The surface morphology of the CZTS thin film was observed by a scanning electron

microscope (SEM, Sirion 200, USA). The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption

spectra were measured with a double beam UV-Vis-NIR (DUV-3700, Shimadzu, Japan) in the

wavelength range of 400-1400 nm at the resolution of 1 nm. The photovoltaic performance of the

CZTS thin film solar cell was measured with a solar simulator (Class AAA, Oriel, Newport, USA,

Air Mass 1.5, 100 mW·cm-2

) and a Keithley 2420 source meter controlled by Testpoint software.

The irradiation intensity was calibrated with standard crystalline silicon solar cell (Oriel, Newport,

USA).

Results and Discussion

Chemical composition of CZTS thin films. The chemical composition of the CZTS thin film was

determined by the EDS analysis as shown in Table 1. The chemical composition of the CZTS thin

film was Cu1.74Zn0.80Sn1.00S4.04, near to stoichiometric except slightly Cu-poor and Zn-poor. Zn was

volatile elements and the loss of Zn in CZTS thin film was possibly due to the fact that the

re-evaporation of Zn element during thin film growth at the heating substrate [13].

Table 1 Chemical composition of the CZTS thin film

Sample Chemical composition Atomic ratio

Cu:Zn:Sn:S Cu/(Zn+Sn) Zn/Sn S/metal

CZTS thin film 1.74 : 0.80 : 1.00 : 4.04 0.97 0.80 1.14

Crystal structure of CZTS thin films. Figure 1 showed X-ray diffraction pattern of the CZTS

thin film. The peaks at 2θ = 15.8°, 28.1°, 47.1° corresponding to (002), (112) and (220) planes of

CZTS and a preferred orientation along (220) plane (JCPDS NO. 26-0575) appeared.

Surface morphology of CZTS thin films. Figure 2 showed the SEM image of the CZTS thin

film, which was densely packed. Some particles on the surface of CZTS thin film can be observed.

This may be because the plasma plume had nucleation process during the pulsed laser deposition.

Advanced Materials Research Vol. 716 329

Figure 1 XRD pattern of the CZTS thin film Figure 2 SEM image of the CZTS thin film

The optical band gap of CZTS thin films. Figure 3 displayed the UV-Vis-NIR absorption

spectrum of the CZTS thin film. The absorption edge of CZTS thin film was about 1000 nm. The

direct band gap of the CZTS thin film was estimated to be 1.21 eV by extrapolating the linear

region of a plot of (αhν)2 versus (hν) as shown in the inset of Fig. 3.

Figure 3 UV-Vis-NIR absorption of CZTS thin film

and inset of panel showed the (αhν)2 vs hν for the

CZTS thin film

Figure 4 Photocurrent-voltage curve of the CZTS

thin film solar cell with the novel and simple

structure of FTO/CdS/CZTS/Mo

Photovoltaic performance of the thin film solar cell. Figure 4 showed the

photocurrent-voltage curve of the CZTS thin film solar cell with the novel and simple structure of

FTO/CdS/CZTS/Mo. The CZTS thin film solar cell gave the short circuit current density of 1.01

mA·cm-2

, open circuit voltage of 0.39 V, and fill factor of 0.485, corresponding to the photoelectric

conversion efficiency of 0.19%.

Summary

CZTS thin film was successfully prepared by pulsed laser deposition using CZTS nanocrystal as a

source and the chemical composition of the CZTS thin film was nearly stoichiometric. The CZTS thin

film solar cell with the novel and simple structure of FTO/CdS/CZTS/Mo gave the short circuit

current density of 1.01 mA·cm-2

, open circuit voltage of 0.39 V, and fill factor of 0.485,

corresponding to the photoelectric conversion efficiency of 0.19%. Further optimization of the CZTS

thin film deposition and the solar cell fabrication should lead to the increased photoelectric

conversion efficiency.

330 Materials Science and Technology II

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (51072043,

51272061), National Basic Research Program of China (2011CBA00700), Anhui Province Science

and Technology Plan Project of China (2010AKND0794).

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Materials Science and Technology II 10.4028/www.scientific.net/AMR.716 Deposition of Cu2ZnSnS4 Thin Film by Pulsed Laser Deposition and Assembly of Thin Film Solar Cell

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