  Transition metal dichalcogenide (TMD) materials have emerged as promising
candidates for thin film solar cells due to their wide bandgap range across the
visible wavelengths, high absorption coefficient and ease of integration with
both arbitrary substrates as well as conventional semiconductor technologies.
However, reported TMD-based solar cells suffer from relatively low external
quantum efficiencies (EQE) and low open circuit voltage due to unoptimized
design and device fabrication. This paper studies $Pt/WSe_2$ vertical Schottky
junction solar cells with various $WSe_2$ thicknesses in order to find the
optimum absorber thickness.Also, we show that the photovoltaic performance can
be improved via $Al_2O_3$ passivation which increases the EQE by up to 29.5% at
410 nm wavelength incident light. The overall resulting short circuit current
improves through antireflection coating, surface doping, and surface trap
passivation effects. Thanks to the ${Al_2O_3}$ coating, this work demonstrates
a device with open circuit voltage ($V_{OC}$) of 380 mV and short circuit
current density ($J_{SC}$) of 10.7 $mA/cm^2$. Finally, the impact of Schottky
barrier height inhomogeneity at the $Pt/WSe_2$ contact is investigated as a
source of open circuit voltage lowering in these devices
