The effect of active-layer thickness on PM6:Y6 organic solar cells' performance was studied using several characterization techniques, such as current density vs. voltage (J-V), capacitance vs. voltage (C-V), charge vs. voltage (Q-V), charge extraction by linearly increasing voltage (CELIV), and sun-dependent. Results show that increasing thickness increased light absorption and therefore short-circuit current density (J_SC), but excessive thickness caused transport limitations and higher recombination, resulting in reduced fill factor (FF). On the other hand, open-circuit voltage (V_OC) remained weakly dependent on thickness variation. As a result, power conversion efficiency (PCE) has initially increased and then declined beyond the optimum thickness. C-V and Q-V results indicated reduced charge extraction efficiency and increased series resistance in thick films. Recombination and mobility analyses showed higher loss rates and lower effective mobilities with increasing thickness, consistent with longer transport paths. CELIV confirmed more dispersive, trap-limited transport in thicker layers. Light-intensity-dependent J_SC followed a near-linear dependence (α ≈ 1), and Suns-V_OC measurements demonstrated thickness-insensitive voltage governed mainly by recombination kinetics.