Copper Zinc tin sulfide

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Fabrication and characterization of Molybdenum/Copper Zinc tin sulfide (CZTS)/Aluminum thin film structures. A.El kissani, L. Nkhaili, K. Elassali, A. Outzourhit*.

29th European Photovoltaic Solar Energy Conference and Exhibition

Fabrication and characterization of Molybdenum/Copper Zinc tin sulfide (CZTS)/Aluminum thin film structures

A.El kissani, L. Nkhaili, K. Elassali, A. Outzourhit* Solid State Physics and Thin Films Laboratory, Faculty of Sciences Semlalia, Cadi Ayyad University, PO Box: 2390, Marrakech 40000, Morocco *Corresponding author: [email protected]

ABSTRACT:

Recently, the quaternary semiconducting compound Copper Zinc Tin Sulfide (CZTS) has received increasing attention for applications in thin film solar cells because it offers optical and electronic properties that are comparable to those of the I-II-VI2 materials (CIGS for example) while being composed of abundant and non-toxic elements. In this work CZTS thin films were prepared by the sol-gel process followed by spin coating on Mo-coated glass substrates. The precursor solution consisted of CuCl2 (2M); ZnCl2 (1.4M); SnCl2 (1.3M); thiourea (8M) in a mixture of 70% Ethanol and 30% water. The solution was stirred at ambient temperature for 10 minutes. The CZTS thin films were then deposited from the as-prepared solution at a spinning speed of 4000 rpm for 30 s, followed by heat treatment at temperature of 90°C to 110°C in ambient air. This procedure was repeated three times and each time the film was cooled before the next layer was deposited. The final films were then annealed at 300 to 500°C in an argon atmosphere and in the absence of sulfur. X-ray Diffraction (XRD) studies revealed that the as-deposited films and those annealed in an Ar atmosphere, present the (112), (220), and (312) diffraction peaks that are characteristic of the kësterite phase. The Schottky barrier type structures were completed by depositing Al dots on the annealed CZTS films by thermal evaporation at a vacuum of 4x10-6 mbar. The I-V characteristics these structures showed a rectifying behavior. In addition, both their conductance and capacitance varied with applied voltage and frequency. These measurements are used to analyze and discuss the current transport mechanisms in these structures. Keywords: Cu2ZnSnS4 (CZTS), Sol gel, spin-coating, annealing 1

INTRODUCTION

at temperature 110°C for 5 min. These processes were repeated 3 times. Finally, the films synthesis were annealed for 1h at a temperature between 200°C and 500°C in a argon atmosphere. The structure of the CZTS thin films were characterized by X-ray diffraction (XRD) using copper (Cu) Kα radiation (λ = 0.15418 nm) and X’Pert diffractometer (PANalytical, Almelo, The Netherlands).

The Cu2ZnSnS4 (CZTS) thin films is one of the most promising absorber layer materials for thin film solar cells, having direct band gap between 1.4 eV to 1.5 eV, a large absorption coefficient over 104 cm-1[1, 2], its constituents are nontoxic and earth abundant. For the deposition of Cu2ZnSnS4 (CZTS) thin CZTS, several techniques such as evaporation [3] sputtering, cosputtering [4], spray pyrolysis [5], electro-deposition [6], pulsed laser [7], sol-gel without sulfurization[8], sol-gel with sulfurization [9], etc. have been utilized. However, most of these techniques require a sulfurization step. In this work, we report the synthesis of Cu2ZnSnS4 thin films by the sol-gel process without any further sulfurization and fabrication of Al/p-Cu2ZnSnS4 polycrystalline Schottky diodes. Their current-voltage (IV) as well as capacitance-voltage (C-V) and capacitancefrequency (C-F) characteristics has been measured at room temperature and various junction parameters were calculated.

2.2 Fabrication of Al/p-CZTS Schottky diodes The Al/p-CZTS/Mo Schottky diodes were prepared by depositing Aluminum (Al) circular electrodes on the CZTS films deposited on the Mo-coated substrates. The current-voltage (I-V) characteristics, the capacitance-voltage (C-V) characteristics, Capacitancefrequency (C-ω), and conductance-frequency (G-ω) of Al/p-Cu2ZnSnS4/Mo Schottky diodes were measured at room temperature using a Keithley LCZ3000 meter and Keithley 410 programmable ammeter and voltmeter.

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RESULLTS AND DISCUSSIONS

EXPERIMENTAL 3.1 Structure analysis Fig. 3 shows the XRD patterns of the as-deposited thin films and annealed at different temperatures (300°C, 400°C, 500°C), in an atmosphere of Argon (Ar). The major diffraction peaks of the as-deposited thin films on soda lime glass (SLG) and of those annealed at different temperatures corresponding to (112), (220) and (312) planes of the kesterite phase CZTS (JCPDS: 00026-0575) can be clearly seen. The intensity of (112) diffraction peak became relatively more intense for CZTS thin films annealed at 400°C.

2.1 Synthesis of the Cu2ZnSnS4 (CZTS) thin films By using spin coating technique the Cu2ZnSnS4 (CZTS) thin films were synthesized on soda lime glass (SLG), and Molybdenum/SLG substrates. The sol gel precursor solution containing copper (II) chloride (1.4M), zinc (II) chloride (1.3M), tin (II) chloride (1M), and thiourea (8M), respectively dissolved in mixture solution of 70% ethanol and 30% deionized water. The solution was stirred at room temperature and then spinning coated on the soda lime glass (SLG) and Mo/SLG at a speed of 3000 rpm. The films were baked for drying on a hot plate

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29th European Photovoltaic Solar Energy Conference and Exhibition

Fig1. XRD patterns of the Cu2ZnSnS4 (CZTS) thin films as-deposited and annealed at 300°C, 400°C and 500°C.

Fig.5 Capacitance voltage characteristics of Al/pCZTS/Mo

3.2 Current-voltage (I-V) characteristics Fig2. Shows the current voltage (I-V) characteristics of Al/Cu2ZnSnS4 (annealed at 400°C)/Mo. The I-V curve is nonlinear and asymmetric. The turn-on voltage in the forward-bias region is on the order of 0.5 V. The I-V characteristics were analyzed using the modified Shockley model. The ideality factor is found to be n= 4, the series resistance is on the order of 1 KΩ, and Schottky barrier height ∅ = 0.71 eV The high value of series resistance may be attributed to the presence of an interfacial layer as well as the reactive nature of the Al top contact.

varied from 40Hz to 100 kHz. This variation may be attributed to deep sates within the depletion layer or to the large series resistance..

Fig. 6 Capacitance frequency characteristics of Al/pCZTS/Mo

Fig4. I-V characteristics Of Al/p-Cu2ZnSnS4 (annealted at 400°C) Schottky diode

3.3 Capacitance-volatge (C-V) characteristics Fig.5 shows the capacitance voltage (C-V) characteristics of Al/p-CZTS diodes Schottky. The (C-V) characteristics showed a typical trend observed in Schottky barrier-type devices. The capciatnce deccreases as the reverse bias is increased especially in the case of annealed films.These results were used to determine the doping contration in the two cases. The doping density Na was found to increase from 3.1013 for the as-deposited films to 5.1018 for the films anealed at 400°C.

Fig. 7 Condutance frequency characteristics of Al/pCZTS/Mo

3.4 Capacitance and conductance-frequency characteristics Figures 6 and 7 show the capacitance-frequency and the conductance frequency characteristics of the investigated Schottky diodes. The capacitance decreases while the conductance increases as the frequency is

The kesterite Cu2ZnSnS4 (CZTS) thin films were deposited by a simple and low cost spin coating technique without any further sulfurization, and a Al/pCu2nSnS4/Mo structure was formed by deposition of Cu2ZnSnS4 (CZTS) on top of a Mo-coated glass substrate and by annealing at 400°C. The effect of annealing at 300°C, 400°C and 500°C on the structural of the kesterite

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Conclusion

29th European Photovoltaic Solar Energy Conference and Exhibition

Cu2ZnSnS4 (CZTS) thin films was investigated. The Schottky diode fabricated shows a rectifying behavior.

5 References [1] M. P. Suryawanshi, and Al, status review. Mater. Technol. 28, (2013), 98-109. [2] K. Ito and T. Nakazawa, J. Appl. Phys. 27, (1988), 2094- 2097 [3] A. Ennaoui and al. Thin Solid Films 517 (2009) 2511–2514 [4] H. Katagiri, K. Jimbo,and Al., Appl. Phys. Express1, (2008), 041201. [5] T. Prabhakar and Al., Solar Energy Materials & Solar Cells 95 (2011) 1001–1004 [6] X. Fontané and al. Applied Physics Letters 98, (2011), 1819058-1 _ 181905-3 [7] L. Sun, J. He, H. Kong, F. Yue, P. Yang, J. Chu; Sol. Energy. Mater. Sol. Cells (2011), 95, 2907–2913 [8] A. El kissani and al. Spectroscopy Letters, 47:387– 391, 2014 [9] S. Kahraman and al.; Ceramics International; (2013); 1-8

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