Elsevier

Journal of Alloys and Compounds

Volume 727, 15 December 2017, Pages 1089-1094
Journal of Alloys and Compounds

Structural and optical properties of (AgxCu1-x)2ZnSnS4 thin films synthesised via solution route

https://doi.org/10.1016/j.jallcom.2017.08.222Get rights and content

Highlights

  • A simple solution route for the fabrication of (AgxCu1-x)2ZnSnS4 thin films.

  • Grain size, optical band gap dramatically increase with the increased Ag amount.

  • Lattice constant a (and b) increases with the increase Ag amount.

  • Tuning band gap, charge-carrier density, mobility of CZTS thin film.

Abstract

Silver (Ag) has been introduced in the quaternary compound Cu2ZnSnS4 synthesised via solution chemistry. Thin films of the resulting pentanary alloys (AgxCu1-x)2ZnSnS4 (0 ≤ x ≤ 1) show remarkable change in their microstructure and electronic properties with the increasing Ag content. Going from Cu2ZnSnS4 (x = 0) to Ag2ZnSnS4 (x = 1), the grain size increased from 0.13 to 2 μm which could be attributed to the liquid assisted grain growth mechanism. The optical band gap increased from 1.5 to 2.0 eV due to the influence of d-orbitals of Ag atoms on the valence band edge. The charge carrier density decreased by two orders of magnitude with only 4 atomic % Ag (x = 0.04) incorporated in the films, which can be attributed to the reduction in the density of CuZn and VCu point defects with Ag atoms occupying the Cu-sublattice sites. Also, with increasing Ag content the lattice parameters a and b increased from 5.42 to 5.82 Å, whereas there was negligible change in the lattice parameter c.

Introduction

Active research and development efforts for the pentanary compound Cu(Inx,Ga1-x)(SeyS1-y)2 (CIGSSe) has allowed the CIGSSe thin film based solar cells to achieve a photoconversion efficiency (PCE) of more than 20% [1]. However, the high commercial demand in other applications [2], [3], [4], and the relatively limited availability of constituent elements gallium (Ga) and indium (In) are going to limit the potential of CIGSSe based photovoltaics for the terawatt scale implementation. The pentanary compound [Cu2ZnSn(SeyS1-y)4] (CZTSSe) is a promising alternative because of the relatively abundant elements zinc and tin in it. However, the highest PCE reported for the corresponding PV device is 12.6% [5] which is far behind that of CIGSSe (20%) [1]. To increase the PCE of Cu2ZnSnS4 based PV devices the corresponding open circuit voltage (Voc) needs to be increased beyond its current best value of 0.67 V [6] and take it closer to the theoretically expected value of 1.25 V [7]. The low Voc has been attributed to the high density of CuZn point defects that causes Fermi level pinning in the middle of the band gap [8], [9]. These point defects also form defect-complexes viz. CuZn + ZnCu, VCu + ZnCu and 2CuZn + SnZn that can cause band gap fluctuation as well as act as recombination centers thereby further deteriorating Voc [9], [10], [11], [12], [13]

Theoretical studies by Z. K. Yuan et al. [8] and E. Chagarov et al. [14] have suggested that alloying of Ag in Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) could lead to reduced density of defect-complexes and also allow the band gap tuning. C. J. Hages et al. [15] have alloyed Ag in CZTSe and found an increased PCE of 7.1% as compared to 6.6% for the case of no Ag alloying. Similarly, W. Li et al. [16] have also obtained increased PCE of 4.42% from (Ag0.12Cu0.88)2ZnSnS4 based PV devices as compared to 2.31% from CZTS without Ag alloying.

The compound Ag2ZnSnS4 (AZTS) itself has been investigated as a photoelectrode in photoelectrochemical cells [17] and photocatalyst for hydrogen generation [18]. Its synthesis has been carried out by hot injection [15], solvo thermal [18], sputtering [16], [19], and chemical bath deposition [17].

In the present study, Ag has been introduced in varying amounts in the precursor solution meant for CZTS synthesis and the thin films of thus obtained pentanary compounds [(AgxCu1-x)2ZnSnS4, (0 ≤ x ≤ 1)] (ACZTS) have been investigated for their microstructural and electrical characteristics.

Section snippets

Materials

Copper chloride dihydrate (CuCl2.2H2O), copper acetate monohydrate (Cu(Ace)2.H2O), zinc acetate dihydrate (Zn(Ace)2.2H2O), and tin chloride dihydrate (SnCl2.2H2O) from Alfa Aesar, thiourea (CS(NH2)2) from Sigma-Aldrich, silver nitrate (AgNO3) from Rankem, and 2-methoxyethanol (2ME) from LOBA Chemicals (India) were used as received.

Methods

Soda lime glass substrates were cleaned in acetone, isopropyl alcohol, and piranha (98% H2SO4/30% H2O2 = 3:1, v/v) in that order for 10 min each. Precursor solutions

Surface morphology and composition characterization

Fig. 1 shows FESEM micrographs for the ACZTS thin films having varying Ag content. The corresponding targeted (XTarget) and experimentally obtained Ag/(Ag + Cu) ratios (XExp) have been shown in Table 1. Except for XTarget = 0.02 and 0.05 the value of XExp remained within 20% of the XTarget. The Zn/Sn and (Cu + Ag)/(Zn + Sn) ratios corresponding to these films have been shown in S2.

From the FESEM images (Fig. 1) and the accompanying grain size vs. XExp plot in Fig. 2 it is evident that with the

Conclusion

In summary, this study reports the synthesis of thin films of pentanary compounds (AgxCu1-x)2ZnSnS4 (0 ≤ x ≤ 1) via solution route. The Ag alloying in CZTS dramatically affects the grains size, charge carrier density, and mobility. The increase in the band gap obtained with increasing Ag alloying could possibly be used for band gap tuning along the CZTS thin film thickness. Also, the larger grains and higher mobility with Ag alloying in CZTS are very promising for the possible improvement in

Acknowledgment

The research was supported by Indo-German Science & Technology Centre (IGSTC/MET/2014269). Quartz ampoule making and sample sealing were done at Central Glass Blowing section at IIT Kanpur. Scanning electron microscopy was conducted in Materials Science and Engineering Department of IIT Kanpur. X-ray diffraction study was performed at Advance Center for Materials Science at IIT Kanpur. The authors also thank Profs. Ashish Garg and Rajdip Mukherjee of Materials Science and Engineering, IIT

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