Influence of Mg doping on electrical properties of Cu(In,Ga)Se 2 bulk materials Mehrdad Monsefi, Dong-Hau Kuo ⇑ Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan article info Article history: Received 8 July 2013 Received in revised form 10 August 2013 Accepted 14 August 2013 Available online 25 August 2013 Keywords: Semiconductor Powder metallurgy Electronic property Scanning electron microscopy, SEM abstract Mg-doped CIGSe bulk materials with the Cu 0.9 [(In 0.7x Mg x )Ga 0.3 ]Se 2 formula at x = 0–0.15 were prepared at 650 °C. A two-step liquid-phase reaction sintering technique has been used in rather low temperature for the densification of Mg-doped CIGSe bulk materials with Sb 2 S 3 and Te sintering aids. Electrical prop- erty was studied by measuring the mobility and concentration of charge carriers. All Mg-doped CIGSe pellets showed a p-type behavior. The favored low hole concentration of 2.86  10 16 cm 3 and mobility of 4.23 cm 2 /V s were achieved for CIGSe materials doped with 10% of magnesium, as compared to 3.25  10 16 cm 3 and 1.16 cm 2 /V s for undoped one. The explanations based upon the Mg-to-Cu antisite defect and the Mg-to-In defect for the changes in electrical property were declared. The study in bulk Mg- doped CIGSe has been based upon defect states and is consistent and supported by the data of lattice parameters. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Cu(In 1x Ga x )Se 2 (CIGSe) solar cell devices with efficiencies above 20% have become a leading material for thin film solar cell applications [1,2]. These quaternary compounds semiconductors have a direct band gap, high efficiency, high absorption coefficient, and long term stability [3]. The most efficient solar cells are ob- tained from absorber layers with Cu/(In + Ga) ratios between 0.8 and 0.92 [1,4]. Investigations in A I B III Se 2 semiconductor materials have been focused on the device performance in order to enhance solar cell efficiency by maximizing the open-circuit voltage, short- circuit current, and fill factor. In the processing of CIGSe thin film deposition, the ratio of Cu:In:Ga was not easily controlled and it influences the electrical properties of the absorber layer and the solar cell performance consequently [5,6]. To present a convincing explanation for the electrical properties of CIGSe and the role of crystal defects, a systematic investigations with the composition of the films formed as a favorable design is inevitable. However, a systematic defect study of CIGSe can be facilitated by using its bulk form. It has been shown that the Cu-deficient CIGSe absorber layers having a lower carrier concentration in the range of 10 16 – 10 17 cm 3 [7,8] implement more efficient solar cells. These mate- rials also have a higher mobility and conductivity as a result. Since the mobility and exciton diffusion length are strongly dependent on the molecular stacking, a high mobility generally could cause a longer exciton diffusion length [9] and a longer exciton diffusion length can contribute to generating more charge carriers [10]. So the current generation as a result of easy trans- port in CIGSe absorber layers with high mobility can improve the PV performance for solar cell applications. An improvement of short circuit current density was observed in CIGSe thin film solar cell with Mg-doped ZnO window layer [11] but there was no report about Mg doping of CIGSe material. The Mg doping in BaTiO 3 –MgO–rare earth oxide-based dielectrics has been com- mercially important for multi-layered ceramic capacitors to X7R specification [12,13]. To have the hole concentration of 10 16 – 10 18 cm 3 , CIGSe absorber needs to become Cu-deficient. Mg 2+ with the ionic size of 0.72 Å is much comparable to Cu + (0.77 Å) instead of In 3+ (0.80 Å) [14]. What is the lattice site for Mg 2+ and what is its effect on the electrical properties and structure are all interesting topics for understanding the property control in CIGSe. If Mg 2+ occupies at the Cu + site, it acts as donor. If Mg 2+ substitutes at the In 3+ site, it becomes acceptor doping. The suitable composition control can have CIGSe with good car- rier concentration, while having the higher Cu content for the consideration of higher carrier mobility. In this work, we fabricated Mg-doped CIGSe bulk materials with different ratios of Mg content to study the electrical property and defect behavior. We executed a liquid phase reaction technique to sinter the Mg-CIGSe bulk materials. The variations of morphol- ogy, structure, and electrical property were investigated in our study and defect mechanism was derived. To support the defect mechanism, the changes in lattice parameters with the Mg doping were also demonstrated. 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.08.101 ⇑ Corresponding author. Tel./fax: +886 2 27303291. E-mail address: dhkuo@mail.ntust.edu.tw (D.-H. Kuo). Journal of Alloys and Compounds 582 (2014) 547–551 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom