The unique electronic features of highly mismatched alloys such as III-V GaNAs are suitable for the intermediate band solar cell (IBSC) application, in which an intermediate band (IB) acts as a stepping stone to generate additional photocarriers across the host semiconductor bandgap through sequential two-step below-bandgap photon absorption (TSPA). However, the collection of photocarriers in a realistic GaNAs IBSC is much lower and often accompanies S-shape kink features in the current–voltage (J–V) curves under illumination for which a coherent picture is lacking. Based on the solar cell characterization of GaNAs IBSC devices grown with and without barriers, with and without antimony, and with and without indium using molecular beam epitaxy, and also with the photocarrier collection analysis using equivalent circuit models, it was identified that the TSPA and the S-shape J–V of this system depend on two critical factors: (1) high carrier recombination currents (I0CI) across the GaNAs sub-gap between the conduction- and intermediate bands (EgCI) and (2) the counterdiode effect of the AlGaAs IB electron barrier. Dramatic improvements in the S-shape J–V feature of the solar cell characteristics were achieved when lattice-strain was compensated in GaInNAsSb epitaxial layers.