Atomic Layer Deposited MoS2 Counter Electrodes for the Bifacial Dye-Sensitized Solar Cells

Abstract

Of two-dimensional (2D) transition-metal dichalcogenides (TMDCs), molybdenum disulfide (MoS2) is a promising electrocatalyst for application in dye-sensitized solar cells (DSSCs) as a Pt-free counter electrode (CE). The DSSCs have attracted much attention as a promising next-generation solar cell, because of their low-cost, high efficiency and aesthetically favorable properties. In particular, the semitransparent property distinguishes the DSSCs for applications in building-integrated photovoltaics (BIPVs), however, the previously reported MoS2 CEs prepared by conventional chemical reaction have the opaque property. Therefore, for the fabrication of bifacial DSSC, and for application in BIPVs, it is required to prepare semitransparent MoS2 CEs. In this study, we prepared semitransparent MoS2 films with precisely controlled thickness by using atomic layer deposition (ALD). These semitransparent MoS2 films were applied in the bifacial DSSCs as the CEs. By controlling the ALD cycle, the thickness of deposited MoS2 film was varied from 3 to 16 nm. The electrocatalytic activity of MoS2 CEs depending on the thickness was investigated by cyclic voltammetry (CV), Tafel plot, and impedance analysis. As the thickness of MoS2 layer was increased, the electrocatalytic activity and the surface area for electrochemical reaction were gradually enhanced. As a result, 16 nm-thick-MoS2 CE exhibited the highest conversion efficiency of 6.86%. In addition, because of the sufficient transmittance in the whole visible light wavelength region, the DSSCs employing MoS2 CEs were operated under back-illumination. Unlike the case of front-illumination, the conversion efficiency of DSSCs was gradually decreased as the thickness of MoS2 layer was increased, due to the reduced optical transmittance. As a result, 3 nm-thick-MoS2 CE showed the highest conversion efficiency of 4.24% under back-illumination, on the other hand, 16 nm-thick-MoS2 CE exhibited the lowest conversion efficiency of 3.32%.