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

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.