Improving the Device Performance of Tin Monosulfide Planar Solar Cells via Modifying the Electronic Properties at SnS/CdS Interface

Thi-Thong Ho^1,2*, Cheng-Ying Chen³, Shaham Quadir^1,2, Kuei-Hsien Chen^2,3, Li-Chyong Chen³

¹Physics, National Central University, Taoyuan, Taiwan
²Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
³Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan

* Presenter:Thi-Thong Ho, email:thongho1990@gmail.com

Among earth-abundant absorber layers, orthorhombic tin monosulfide (SnS) is one of the potential candidates due to its earth-abundant elements, suitable bandgap (around 1.3 eV), and high absorption coefficient (104 cm-1) in the visible region. To improve the device performance, grain boundaries (GBs) in SnS thin films have been investigated in this study. Owing to more defects and impurities, GBs are considered as recombination sites that caused the degradation in power conversion efficiency. However, it has been figured out that a charge accumulation at GBs can create a lateral electric field, eventually assisted the charge separation process. To generate this local electric field at GBs, a wide bandgap material – MgF2 with different thicknesses (0 nm, 5 nm, and 10 nm) is introduced at the interface between SnS and buffer layer (CdS). From Scanning Kelvin Probe Microscopy (SKPM) measurement and the device characterization tools, the effect of MgF2 is explored. Contact potential difference (CPD) becomes more distinguishable between Grain Interior (GIs) and GBs when MgF2 is introduced; CPD variation across GBs illustrates the downward behavior, i.e. the upward band bending. The band bending becomes deeper with an increase of MgF2 thickness. This typical band bending repels electrons away GBs tend to promote the charge transport and reduce recombination at GBs. Consistently, the presence of MgF2 improved series resistance (Rs), and short-circuit current (Jsc) boosts the efficiency from 1% to nearly 1.5%. Furthermore, external quantum efficiency measurement supports the impact of MgF2 via the rise of charge collection in the region of 500 nm to 800 nm.

Keywords: Tin monosulfide , SnS-based solar cells, SnS’s grain boundaries