ZnS buffer layer for Cu 2 ZnSn(SSe) 4 monograin layer solar cell Mai Nguyen a,⇑ , Kaia Ernits b,⇑ , Kong Fai Tai a,c , Chin Fan Ng c , Stevin Snellius Pramana d,e , Wardhana A. Sasangka e , Sudip K. Batabyal a , Timo Holopainen b , Dieter Meissner b , Axel Neisser b , Lydia H.Wong a,e,⇑ a Energy Research Institute @ Nanyang Technological University, Singapore b crystalsol OU, Akadeemia tee 15a, Tallinn, Estonia c School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore d Facility of Analysis, Characterization, Testing and Simulation (FACTS), Nanyang Technological University, Singapore e School of Materials Science and Engineering, Nanyang Technological University, Singapore Received 2 September 2014; received in revised form 4 November 2014; accepted 5 November 2014 Available online 26 November 2014 Communicated by: Associate Editor Nicola Romeo Abstract Copper zinc tin sulfo-selenide Cu 2 ZnSn(SSe) 4 (CZTSSe) is a low-cost alternative semiconductor material that can be used as an absor- ber in solar cells. CdS deposited by chemical bath deposition (CBD) is the most efficient buffer layer for Cu 2 ZnSn(SSe) 4 and Cu(InGa)(SSe) 2 (CIGS) solar cells. However, there is a strong demand for the development of a Cd-free buffer layer due to the toxicity of Cd and its associated concerns with respect to the disposal and long term safety of Cd containing solar modules. In this work, we report for the first time, the successful use of a ZnS buffer layer for CZTSSe monograin solar cell that shows similar functionality level as a CdS buffer layer. ZnS buffer layer was deposited onto CZTSSe absorber layer by employing a scalable non-vacuum CBD method. The effect of morphology, thickness, as well as chemical composition of the ZnS buffer layer on the efficiency of the CZTSSe solar cell was investigated, and the best CZTSSe monograin solar cell had an efficiency of 4.50 (±0.16)%. External quantum efficiency (EQE) showed higher transmission in the blue light region for the ZnS buffer compared to CdS. Increased number of ZnS layers decreased the EQE signal in 400–800 nm regions, resulting in decreased J–V parameters, suggesting a single layer of 10–25 nm ZnS as a most effi- cient alternative buffer for CZTSSe. Ó 2014 Elsevier Ltd. All rights reserved. Keywords: ZnS buffer layer; CBD; Cd free; Kesterite CZTSSe; Monograin layer solar cell 1. Introduction Thin film solar cells based on copper chalcogenide absorber materials, such as CuInSe 2 (CIS), Cu(InGa)(SSe) 2 (CIGS), and Cu 2 ZnSn(SSe) 4 (CZTSSe), are promising can- didates for future solar cell technologies. Copper–zinc– tin–chalcogenide (denoted as CZTS), composed of only earth-abundant elements, has attracted increasing atten- tion as low-cost light absorbing material for photovoltaic applications (Abermann, 2013). The best CIGS solar cell has traditionally been deposited using costly vacuum based deposition techniques, such as thermal evaporation and sputtering, where the devices showed a maximum power conversion efficiency (PCE) of 20.8% (Jackson et al., 2014). Recently, a CZTSSe based solar cell with PCE of http://dx.doi.org/10.1016/j.solener.2014.11.006 0038-092X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors at: Energy Research Institute @ Nanyang Technological University, Singapore. E-mail addresses: nguyen.mai@ntu.edu.sg (M. Nguyen), kaia.ernits@ crystalsol.com (K. Ernits), LydiaWong@ntu.edu.sg (L. H.Wong). www.elsevier.com/locate/solener Available online at www.sciencedirect.com ScienceDirect Solar Energy 111 (2015) 344–349