This paper describes the influence of an irradiance-dependent photo-induced leakage current through a semi-insulating GaAs substrate on the performance of photovoltaic monolithically series-interconnected multisegment laser power converters. A reciprocal relation between the resistivity of a semi-insulating GaAs substrate and irradiance of monochromatic light is experimentally observed. A reduced resistivity of the substrate with an increasing irradiation results in a substantial increase of a leakage current through the semi-insulating GaAs substrate between adjacent segments. For a multisegment laser power converter, this photo-induced leakage current is identified as a major shunting mechanism between adjacent segments that arises under high irradiances. Open-circuit voltage $V_{{\rm{oc}}}$ , fill factor (FF), and consequently conversion efficiency of a multisegment laser power converters are highly affected by the shunting mechanism. Based on a shading experiment, we observed that $V_{{\rm{oc}}}$ drops up to 21.5 mV per segment at a short-circuit current density $J_{{\rm{sc}}}= 47.3{\rm{\,A/ cm}}^{2}$ for the studied six-segment MIM specimen. For the same device, FF drops by 4.1% absolute at $J_{{\rm{sc}}}= 40.5{\rm{\,A/ cm}}^{2}$ . For the two-segment specimen, 5.8 mV drop of $V_{{\rm{oc}}}$ per segment and 1.5% absolute drop in FF is reported at $J_{{\rm{sc}}}= 47.3$ and $43.7{\rm{\,A/ cm}}^{2}$ , respectively.