Transparent 3 nm-thick MoS₂ counter electrodes for bifacial dye-sensitized solar cells

Abstract

Molybdenum disulfide (MoS2) counter electrode (CE) is considered one of the most viable alternatives to Pt CE in dye-sensitized solar cells (DSSCs) owing to its abundance, low cost, and superior electrocatalytic activity. However, mostly, MoS2 CEs for DSSCs are prepared by conventional chemical reactions and annealing at a high temperature. By these conventional processes, deposition of sufficiently thin and transparent MoS2 layers is challenging; therefore, bifacial DSSCs employing transparent MoS2 CEs have not been studied. Here, we report transparent few-nanometer-thick MoS2 CEs prepared by atomic layer deposition at a relatively low temperature (98 degrees C) for bifacial DSSC applications. MoS2 nanofilms with precisely controlled thicknesses of 3-16 nm are conformally coated on transparent conducting oxide glass substrates. With increase in the MoS2 nanofilm thickness, the MoS2 CE electrocatalytic activity for the iodide/triiodide redox couple enhances, but its transparency decreases. Notably, the application of a thinner MoS2 nanofilm in a bifacial DSSC leads to lower conversion efficiency under front-illumination, but higher conversion efficiency under back-illumination. In particular, only the 3 nm-thick MoS2 nanofilm shows reasonable photovoltaic performances under both front- and back-illumination conditions.