<p>InGaP/GaAs dual-junction solar cells are pivotal cell structures for advancing photovoltaic technology in multi-junction solar cell architecture. To evaluate how the top base layer thickness affects cell performance, we investigated the electrical and optical characteristics of two InGaP/GaAs dual-junction solar cell structures with top base layer thicknesses of 0.55 μm and 1 μm. Through electrical simulations and experimental techniques, including power- and temperature-dependent photoluminescence and room-temperature photoreflectance spectroscopies, as contactless techniques, we demonstrate the top base layer thickness’s significant impact on cell performance. The results show that increasing the top base layer thickness from 0.55 to 1 μm increases photoluminescence efficiency, decreases electron–phonon interaction strength, and reduces electron–hole pair entropy. It also reduces the electric field strength in the p-n junction and decreases the trapping time constants. These significant multi-faceted improvements in optical and electrical performance indicate the role of top base layer thickness in influencing optical and electrical properties, which can guide optimization in future solar cell designs. Specifically, the results highlight the importance of the electron–phonon interaction extracted from photoluminescence and the trapping time constants extracted from photoreflectance phase diagrams in characterizing solar cell structures.</p>