A promising sputtering route for dense Cu 2 ZnSnS 4 absorber ﬁlms and their photovoltaic performance Bao-Tang Jheng a , Po-Tsun Liu b,n , Meng-Chyi Wu a a Department of Electrical Engineering, National Tsing Hua University, Hsinchu City, Taiwan b Department of Photonics & Display Institute, National Chiao Tung University, Hsinchu City, Taiwan article info Article history: Received 25 January 2014 Received in revised form 13 April 2014 Accepted 13 May 2014 Available online 6 June 2014 Keywords: Solar cell Antireﬂective coating Photoelectric devices Zinc oxide abstract Copper zinc tin sulﬁde (Cu 2 ZnSnS 4 , CZTS) is highly abundant in nature. It is an important absorber material for the development of low-cost and sustainable next-generation I 2 –II–IV–VI 4 thin-ﬁlm solar cells because it has a the tunable direct band gap energy, inexpensive constituent elements, and a large absorption coefﬁcient in the visible wavelength region. This work develops an efﬁcient one-step vacuum-based approach to depositing CZTS ﬁlms without the need to supply excess sulfur during/after deposition or to perform any post-sulfurization treatment. This one-step RF sputtering process produces CZTS ﬁlms that are crystalline, phase-pure, dense, smooth, and continuous. Air Mass 1.5 G power conversion efﬁciencies of as high as 6% have been achieved with an antireﬂection coating, demonstrating that this new approach has great potential as a low-cost alternative for high-efﬁciency CZTS solar cell production. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Copper ternary chalcogenide compounds and alloys are among the most promising absorber materials for use in photovoltaic (PV) devices, because their energy band gaps can be directly tuned by adding additional elements such as group III elements; they have high optical absorption coefﬁcients in the visible to near infrared spectral range, and they have moderate surface recombination velocities and radiation hardness. The best chalcopyrite CuInSe 2 (CISe)-type thin ﬁlm solar cells with Ga-containing absorber layers of CuInGaSe 2 have already demonstrated a conversion efﬁciency of up to 20.3% [1] and excellent longevity. However, challenges include environmental issues and the scarcity, expense and rarity of the constituent elements. The constituent elements of chalcopyrite- type compound CuInGaSe 2 are expensive (In and Ga) and toxic (Se), inhibiting cost-effective large scale production. Therefore, a high- quality semiconductor that comprises abundant elements with low toxicity is a favored alternative for large-scale commercial applications. The alternative kesterite-type compound Cu 2 ZnSnS 4 (CZTS) is a quaternary semiconductor that is derived from CIS by replacing In (III) with Zn (II) and Sn (IV) in the ratio 50:50. It contains elements that are abundant in the earth's crust and has an ideal direct band gap in the range 1.0–1.5 eV and a large absorption coefﬁcient (  10 4 cm 1 ) [2,3]. Besides, the widely accepted Shockley–Queisser limit conversion efﬁciency of CZTS solar cells reaches 28% [4]. All of these advantages make it one of the most promising materials for thin ﬁlm solar cells. Many deposition methods have been used to grow CZTS ﬁlms, including sputtering, evaporation, electrodepos- ited, sol–gel, liquid-processing, pulsed laser deposition, etc. [5–15]. The record efﬁciency of 11.1% for CZTS thin ﬁlm solar cells was achieved by Todorov et al. through non-vacuum methods [16], and the highest efﬁciency of CZTS thin ﬁlm solar cells fabricated through vacuum based methods reaches 9.3% so far [17] . Currently, the production of Cu 2 ZnSnS 4 thin-ﬁlm solar cell devices as described in the literature cited above faces some problems. These include (i) the high price of precursor material, the toxicity of the required organic solvents, and the quick agglomeration of particles at high tempera- ture; (ii) the need for high-temperature sulfurization, which involves the addition of toxic (N 2 þ H 2 S) gas and sulfur vapor, in the subsequent stage of deposition of precursor layer; and (iii) the formation of the precursor by the stacking of layers, such as of Cu, CuS, ZnS and ZnSn, which make the process complex and the composition stoichiometry difﬁcult to control. The ultimate aim of this study was the development of a relatively simple and stable growth technique for the production of device quality CZTS thin ﬁlms. Unlike the methods that were developed in the works cited above, the approach herein is a single-stage sputtering process that involves no sulfurization; has a low production cost, and does not use a solvent that pollutes the environment. Therefore it is highly suited the commercial mass production of solar with a large area. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2014.05.033 0927-0248/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel: 886-3-5712121 ext. 52994. E-mail address: ptliu@mail.nctu.edu.tw (P.-T. Liu). Solar Energy Materials & Solar Cells 128 (2014) 275–282