Photoelectrochemical Microscopy of Ultrathin Liquid Junction Solar Cells

Abstract

Solid state photovoltaic devices made of monolayer two-dimensional (2D) transition metal dichalcogenide (TMDs) monolayers such as WSe2, MoS2, and MoS2/WSe2have achieved 1-3% power conversion efficiency. However, photo-generated charge carriers in the middle of the device typically traverse tens of micrometers toward end-on electrical contacts at the device edge. The long transport distances parallel to the TMD layers give rise to transport-limited device physics that limit overall device performance. Here we demonstrate ultrathin liquid junction photovoltaic systems that use an iodide-based hole-selective electrolyte contact that facilitates a charge transport pathway perpendicular to the 3-atom thick TMD materials. These proof-of-concept indium doped tin oxide (ITO)|monolayer TMD|I–/I2|Pt cells exhibit peak internal quantum efficiencies (IQE) of 7.1%, 34.5%, and 8.2% and open circuit voltages (V OC) of 0.46 V, 0.26 V, and 0.28 V for WSe2, MoS2, and MoS2/WSe2 heterojunctions. The photocurrent efficiency values are competitive with solid state devices. Surprisingly, we observe a non-linear scaling relation between IQE and layer thickness for homojunction and heterojunction TMDs that can be attributed to either thickness-dependent charge carrier dynamics or a buffer layer screening effect at the ITO/TMD interface. We discuss opportunities to improve the fill factor and V OCby interface engineering and tuning the energy level offset between the TMD composition and redox electrolyte potential.