We performed advanced opto-electrical simulations on thin-film BaSi₂ solar cells. First, absorption spectra of BaSi₂-pn homojunction solar cells on Si substrate were calculated based on flat and/or pyramidally-textured surfaces, wherein 20-nm-thick n⁺-BaSi₂ was the topmost electron transport layer. By changing the front surface structure from flat to texture, the reflectance decreased in the wavelength (λ) range 700–1200 nm and the photocurrent density (J_ph) delivered by the photogenerated carriers in the 500-nm thick p-BaSi₂ layer increased by 1.2 mA/cm². Simulations revealed that the key factor inhibiting light absorption in the p-BaSi₂ layer was parasitic absorption in the n⁺-BaSi₂ and in the c-Si substrate. To solve these optical issues, we propose a new device structure, Al-doped n⁺-ZnO (AZO, 50 nm)/i-ZnO (20 nm)/p-BaSi₂ (500 nm) heterojunction solar cell (HJSC). In this device structure, the parasitic absorption reduced drastically, and J_ph reached 30.23 mA/cm². Furthermore, by replacing the Si substrate with a glass substrate, the light trapping worked more effectively, and the absorber layer thickness required for J_ph to saturate was reduced to 1 μm, yielding 32.06 mA/cm². Based on these simulation results, we manufactured n⁺-AZO/p-BaSi₂ HJSC. The internal quantum efficiency exceeded 30% at λ = 600 nm, meaning that we demonstrated the operation of n⁺-AZO/p-BaSi₂ HJSC for the first time. We investigated origins of small efficiencies compared to those simulated, and found that the passivation of defects in the p-BaSi₂ layer and the reduction of carrier recombination at the i-ZnO/p-BaSi₂ interface would significantly improve the solar cell performance.