Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener A modeling study on utilizing SnS 2 as the buffer layer of CZT(S, Se) solar cells Maryam Haghighi a , Mehran Minbashi c , Nima Taghavinia a,b, ⁎ , Dae-Hwan Kim d , Seyed Mohammad Mahdavi a,b , Amirhossein Ahmadkhan Kordbacheh c a Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588-89694, Iran b Department of Physics, Sharif University of Technology, Tehran 11155-9161, Iran c Department of Physics, Iran University of Science and Technology, Tehran 16846-13114, Iran d Convergence Research Center for Solar Energy, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea ARTICLE INFO Keywords: CZT(S, Se) solar cell Buffer layer SnS 2 SCAPS software Pulsed laser deposition (PLD) Electron transport layer (ETL) ABSTRACT CdS is conventionally used as the n-type buffer layer in chalcopyrite (CIG(S, Se)) and Kesterite (CZT(S, Se)) solar cells. CdS is toxic and there are wide attempts to find substitutes for it. Here, we suggest SnS 2 as a possible alternative. SnS 2 films were deposited by pulsed laser deposition (PLD), characterized to estimate carrier con- centration and electron affinity values, and the obtained values were used to model a CZT(S, Se) solar cell. The experimental values of a benchmark CZT(S, Se) cell with efficiency of 12.3% were employed to obtain the density and energy position of defects in CZT(S, Se) and validating the model. We observed that SnS 2 results in almost identical performance as CdS, showing slightly better current density, due to smaller conduction band offset of 0.21 eV compared to 0.28 eV for CdS. 1. Introduction Photovoltaic (PV) conversion of solar energy has gained increasing importance due to both energy economy and environmental reasons. In recent years, new semiconductor materials and device structures are being explored, in order to develop solar cells with lower cost, higher efficiency and better manufacturability. Solar cells based on Cu 2 ZnSn(S, Se) 4 (CZT(S, Se)) absorbers are made of earth-abundant low-cost elements and are being extensively studied for improved efficiencies. While CZT(S, Se) is an ideal PV material with bandgap of 1.0–1.5 eV and absorption coefficient of about 10 4 cm -1 (Yang et al., 2016b), the maximum efficiency of CZT(S, Se) cells is about 12.6% (National Renewable Energy Laboratory. Best Research Cell Efficiencies, 2017; Wang et al., 2014), which is almost half the efficiency of CIG(S, Se) cells. The relatively low efficiency of CZT(S, Se) solar cells indicates that more optimizations and control are required for device fabrication. One of the known problems of CZT(S, Se) solar cells is the variety of defect types existing in the grown layers, including vacancies (V Cu ,V Zn ,V Sn and V S(e) ), interstitials (Cu i , Zn i , Sn i and S(e) i ) and anti-sites (Cu Zn , Zn Cu , Zn Sn , Sn Zn , …), as well as zero charge defect clusters (e.g. V Cu + Zn Cu , 2Cu Sn + Zn Sn )(Li et al., 2017; Shin et al., 2017; Yang et al., 2017). Some of these defects are also known in CIGS (Wang et al., 2017). In addition to compositional defects, other factors including good crystallinity of the absorber layer, avoiding secondary phases (such as Zn(S, Se), Cu 2 Sn(S, Se) 3 )(Kumar et al., 2015; Yang et al., 2017) and control of bandgap also are important in improving the CZT(S, Se) solar cells efficiencies. (Bourdais et al., 2016; Yang et al., 2016b) The buffer layer, which is the n-type electron transport layer (ETL) also, plays an important role in open circuit voltage (V OC ) and short circuit current density (J SC ) of the devices. The band alignment at the interface of absorber/buffer highly affects the V OC . The conduction band offset (CBO) determines whether the band bending at the interface is spike, flat or cliff. A spike at the interface, inhibits recombination at the interface, while a cliff type band bending results in higher interface recombination and lower V OC (Redinger et al., 2013; Yan et al., 2014). Nevertheless, the height of the spike should be sufficiently low, or it forms a barrier against the electron transfer from the absorber layer to the buffer layer, reducing the current density and fill factor (FF) (Yan et al., 2014). The structural configuration of CZT(S, Se) solar cells is very similar to CIG(S, Se) cells, where CdS is conventionally considered as the buffer layer (Liu et al., 2012; Wang et al., 2010). CdS shows very good performance in CIG(S, Se) solar cells (Burton et al., 2013; Voznyi et al., 2016). CdS con- tains Cd which is not environment friendly and there are serious https://doi.org/10.1016/j.solener.2018.04.010 Received 23 January 2018; Received in revised form 21 March 2018; Accepted 4 April 2018 ⁎ Corresponding author at: Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588-89694, Iran. Department of Physics, Sharif University of Technology, Tehran 11155-9161, Iran. E-mail address: taghavinia@sharif.edu (N. Taghavinia). Solar Energy 167 (2018) 165–171 0038-092X/ © 2018 Elsevier Ltd. All rights reserved. T