Enhancement of Carrier Collection in Cu 2 ZnSn(SeS) 4 Solar Cell With Hybrid Buffer Layer Homare Hiroi* 1,2 , Jeehwan Kim 3 , Oki Gunawan 3 , Tayfun Gokmen 3 , Masaru Kuwahara 4 , Teodor K Todorov 3 , Dhruv Nair 3 , Yun Seog Lee 3 , Marinus Hopstaken 3 , Yu Zhu 3 , David B. Mitzi 3 and Hiroki Sugimoto 1,2 1 Energy Solution Business Center, Showa Shell Sekiyu K.K. 2 Atsugi Research Center, Solar Frontier K.K., 123-1 Shimokawairi, Atsugi, Kanagawa, 243-0206 Japan 3 IBM TJ Watson Research Center, Yorktown Hts., NY 10598 4 Tokyo Ohka Kogyo Co., Ltd. Nakamaruko Nakahara-ku, Kawasaki Japan *E-Mail: Homare.Hiroi@showa-shell.co.jp ABSTRACT: Several groups have demonstrated high efficiency Cu 2 ZnSnSe 4 (CZTSe), Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSn(SeS) 4 (CZTSeS) solar cells. Especially the highest efficiency has been achieved via the solution-based CZTSeS cells by IBM. However, the power conversion efficiencies are still low as compared with the other solar cells because of the low open circuit voltage (V oc ). Recently, improvement of the V oc in these solar cells by applying a hybrid buffer layer with combination of Cd-based and In-based buffer layers has been reported. The hybrid buffer is considered to drastically boost V oc , and previous reports have focused on V oc and V oc deficit (defined in E g /q-V oc ). In this study we confirm that the hybrid buffer layer can boost not only V oc but also J sc in the CZTSeS solar cells made using a solution process. Specifically improvement of electrostatic potential fluctuation (γ opt ) and bandgap fluctuation (σ g ) which have a strong relation with the V oc was confirmed, which indicates that the hybrid buffer has a potential to improve the band tailing problem in the CZTS-based solar cells. Furthermore, based on external quantum efficiency (EQE) and drive-level capacitance profiling (DLCP) analyses, improvement of EQE in the long wavelength region by enhancement of depletion width (X d ) was revealed. Finally the efficiency (Eff) of 12.7% in-house measurement in IBM (12.3% certificated in Newport) in solution-based CZTSeS cells has been achieved by applying the hybrid buffer layer. Keywords: CZTS-based Solar Cell, Hybrid Buffer Layer, Cd-free Buffer Layer 1 INTRODUCTION The research and development of thin-film solar cells, such as Cu(In,Ga)(Se,S) 2 (CIS-based) and CdTe, has been remarkable in recent years. This is especially the case for the 21.0% efficiency achieved on a CIS-based thin-film single cell by Solibro and 21.0% Eff achieved on CdTe by First Solar [1,2] . In addition, several groups have reported high efficiency Cu 2 ZnSnSe 4 (CZTSe), Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSn(SeS) 4 (CZTSeS) solar cells [3-5] . The highest efficiency was achieved by the solution process CZTSeS solar cells by IBM as a world record of CZTS-based thin film solar cells [6] . However, Eff is still lower than other solar cells. It is caused by the low V oc owing to the poor hetero-junction which means not good interface between CdS buffer layer and CZTS- based bulk should be one of the bottleneck of CZTS- based solar cells development. Thus, we tried various buffer layers for CZTS-based solar cells, for example, Cd-free (In- or Zn-based) and hybrid (CdS/In 2 S 3 ) buffer layers. As results, Cd-free buffer davices showed lower Eff( < 8%), on the other hand, hybrid buffer devices demonstrated higher Eff. Previously, the V oc boost by the hybrid buffer layer on vacuum process CZTS solar cells has been reported at the 39th IEEE PVSC [7] , and that on solution process CZTSeS solar cells has also been reported at the PVSEC-23 and 40th IEEE PVSC [8,9] . Thus, what the hybrid buffer technique improves Voc of CZTS- based solar cells is common sense in the world. On the other hand, nobody argues its effect for current density (Jsc) or fill factor (FF). In this paper, we focus on Jsc improvement on the hybrid buffer CZTSeS devices because we could obsorve not only Voc boosting but also Jsc improvement via hybrid buffer as an interesting result. Additionally, our latest results of Cd-free CZTSeS devices will be presented. 2 EXPERIMENTAL CONDITIONS In this experiment, we prepared a lot of CZTSeS single solar cells with area of approximately 0.45 cm 2 . The baseline structure constituted an In 2 O 3 :Sn (ITO) window layer and an i-ZnO layer by sputtering/ a buffer layer by chemical bath deposition (CBD)/ a CZTSeS absorber by solution process/ an Mo back electrode by DC magnetron sputtering/ a soda-lime glass (SLG) substrate. With regarding to hybrid buffer device, its TEM image and structure are shown in Fig. 1. Current-voltage (J-V) characteristics were measured under a standard test condition with a constant-light solar simulator, then external quantum efficiency (EQE) characteristics of the CZTSeS solar cells were investigated. CZTSeS CdS In 2 S 3 TCO Mo Glass Figure 1: TEM image and the structure of hybrid buffer device.