Atomically thin 2D layered semiconductor materials such as Transition Metal Di-Chalcogenides (TMDCs) have great potential for use as flexible, ultra-thin photovoltaic materials in solar cells due to their favorable photon absorption and electronic transport properties. In this dissertation, the electronic properties, such as band structure and bandgap, and optical absorption properties of a TMDC known as Tungsten Disulfide (WS2) were obtained from Density Functional Theory (DFT) calculations to design conventional and unconventional solar cells. Using these properties, a 1 μm thick heterojunction solar cell based on monolayer and bulk WS2 together with amorphous silicon (a-Si) was modeled using numerical calculations and simulations. The maximum efficiency of this cell is 23.3% with Voc = 0.84 V and Jsc = 33.5 mA/cm2 under the AM1.5G terrestrial solar spectrum. Next, a similar but even thinner solar cell with a thickness of 200 nm, together with a light trapping structure and an anti-reflection coating layer, was modeled under the AM0 space solar spectrum; similar device performance efficiencies around 21-23% were obtained. The performance of these solar cell models is comparable to many commercial cells in both terrestrial and space photovoltaics. As conventional photovoltaics approach the Shockley-Queisser limit, the need for unconventional materials and approaches has become more apparent. Hybrid alloys of TMDCs exhibit tunable direct bandgaps and significant dipole moments. Dark state protection induced by dipole-dipole interactions forms new bright and dark states in the conduction band that reduce radiative recombination and enhance photon-to-electron conversion, leading to significantly higher photocurrents. In our work, current enhancement of up to 35% has been demonstrated by modeling dark state protection in a solar cell composed of Tungsten Diselenide (WSe2) and Tungsten Sulfo-Selenide (WSeS), with the potential to exceed the Shockley-Queisser limit under ideal conditions.