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