High open-circuit voltage Mos2 homojunction - effect of Schottky barriers at the contacts

Abstract

Van der Waals structures made of layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have been proposed for the development of ultra-thin photovoltaic devices. The main limitation of these solar cells up to now has been their low open-circuit voltage (Voc), which is typically below 0.55 V even for high illumination levels. Recently, we have presented a p-n Mos2 homojunction that exhibits a Voc of 1.02 V under broadband illumination equivalent to 40 suns. The use of substitutionally-doped $p$ and $n$ Mos2 material instead of a heterojunction is crucial to produce a band alignment that enables high Voc. Another important aspect for the realization of large photovoltages in TMDC solar cells is the optimization of metallic contacts. We demonstrate using a simple circuital model that the presence of Schottky barriers at the semiconductor/metal interfaces does not only introduce a non-ohmic series resistance, but also reduces the Voc because the Schottky diodes are photoactive. We characterize the Schottky barrier produced by different metals in combination with $p$ and $n$ Mos2. When p-flakes are deposited directly onto a SiO2/Si substrate, we find that they are depleted from carriers by a surface doping effect. This depletion contributes to aggravate the effect of the p-MoS2/metal Schottky. We show that inserting a flake of hexagonal boron nitride (h-BN) between the p-material and the SiO2 surface eliminates this effect. Given the already demonstrated strong light absorption of TMDC ultra-thin devices, the achievement of high Voc is a turning point in the path towards high-efficiency TMDC solar cells.