This study explores the fabrication and performance analysis of ultra-thin III-V solar cells using indium phosphide (InP) epitaxial wafers through two approaches: substrate thinning via lapping and thermocompression bonding. The thinning method reduced the substrate thickness to 160 μm, while the bonding process achieved a final thickness of 0.9 μm. Photoluminescence (PL), internal quantum efficiency (IQE), and external quantum efficiency (EQE) measurements were conducted to evaluate the devices. Results showed that the thinned solar cells exhibited poor photovoltaic characteristics due to suboptimal n-type metal contacts and excessive electron-hole recombination, with no observable light-induced current at zero bias. Backside solar cells fabricated through bonding showed better performance, with improved IQE and EQE due to enhanced light penetration and reduced reflectance from an anti-reflection coating. PL analysis revealed a distinct 1280 nm peak for the backside structure, indicating better light interaction with the active region. Despite these advances, both methods demonstrated low open-circuit current and power efficiency, underscoring the need for further optimization to achieve commercially viable III-V solar cells.