Title : The Flexible Solar Cells Using Earth Abundant Materials
Authors: Kang Jin-Kyu , Kim Dae-Hwan , Sung Shi-Joon , Yang Kee-Jeong , Hwang Dae-Kue , Kim Seung-Hyun , Jo Hyo-Jeong , Maeng Sun-Jeong , Yun Ju-Young 
Published: December 2024

The development of flexible solar cells utilizing non-toxic, earth-abundant materials is of paramount importance in driving market competitiveness and fostering the adoption of innovative business models such as Building Integrated Photovoltaics. While CZTS-based (Cu2ZnSnS4, Cu2ZnSnSe4, Cu2ZnSn(S,Se)4) solar cells offer promising cost advantages due to their low-cost available absorber materials, their progress has been impeded by challenges like narrow phase stability and secondary phase defects.

To address these challenges, researchers have been actively exploring the potential of metal-doping effects, employing elements such as Ag, Li, Na, K, and Rb to enhance the performance and stability of CZTS-based solar cells. Understanding the reaction mechanisms involved in dopant incorporation is crucial for optimizing fabrication processes and ultimately improving solar cell efficiency.

In this study, CZTSSe solar cell devices were designed, comprising an Mo back contact, CZTSSe absorber, CdS buffer layer, ZnO layer, Al-doped ZnO layer, and an Al collection grid. The doping effects of Ag (applied on a rigid SLG substrate with Na impurity) and Na (applied on a flexible substrate without Na impurity) at different dopant layer positions of the Sn/Cu/Zn precursor structure were investigated.

Without a dopant layer, the precursors revealed the formation of Cu-Zn and Cu-Sn alloys despite the deposition sequence of Zn, Cu, Sn at room temperature. Upon pre-annealing at 300 °C, the formation of the ZnSSe layer was observed from the Cu-Zn alloy, followed by the formation of Cu2Se and SnSe at temperatures exceeding 400 °C [1]. In the Sn/Cu/dopant layer/Zn precursor structure, the reaction pathways of CZTSSe were found to be influenced by the presence of dopant layers, which acted as barriers, blocking Zn and Cu/Sn diffusion, and altering the initial reaction mechanism during the annealing process.

It was observed that the application of Sn/Cu/Zn/dopant layer or Sn/dopant layer/Cu/Zn precursor significantly enhanced the characteristics of the solar cell devices. By mitigating defect levels and reducing the Voc deficit, dopants led to an improvement in power conversion efficiency [2]. This enhancement can be attributed to the associated decrease in defects, which are considered recombination centers within the CZTSSe absorber layer and can be effectively passivated by dopants.

Overall, the CZTSSe solar cells exhibited promising efficiencies of 12.6% and 12.3% on rigid and flexible substrates, respectively.

[1] Kim, S.-Y., Son, D. H., Kim, Y. I., Kim, S. H., Kim, S. M., Ahn, K. S., Sung, S. J., Hwang, D. K., Yang, K. J., Kang, J. -K., Kim, D.-H., Nano Energy 59 (2019) 399-411

[2] K.-J.Yang, S.Kim, S.-Y.Kim, D.-H.Son, J.Lee, Y.-I.Kim, S.-J.Sung, D.H.Kim, T.Enkhbat, J.H.Kim, J.Kim, W.Jo, J.-K. Kang, Advanced Functional Materials 31 (2021) 2102238

Acknowledgment: This work was supported in part by the program of Phased development of carbon-neutral technologies (2022M3J1A1085371) through NRF (National Research Foundation of Korea) and in part by DGSIT R&D Program of the Ministry of Science and ICT of Korea (Grant numbers 23-ET-08 and 23-CoE-ET-01).