Multi-interface novel devices (MIND) exhibit a dramatically low UV- and blue-spectrum photovoltaic (PV) performance. A paradox could even be observed, the better the electronic passivation the poorer the PV performance. The paradox appears under relatively low excitations in comparison with intense laser fluxes usually at its origin. The effect can be explained by solar light induced opacity, which reduces considerably or even totally the photon penetration into deeper layers, from which exclusively the photocarrier collection is possible. This opacity results from a feedback occasioned by the free-carrier absorption: better surface passivation, higher free-carrier density, stronger surface dead zone absorptance. The total energy of the incident short wavelength beam can be absorbed before a carrier collection limit buried in the emitter. This limit acts simultaneously on the electronic performance, blocking free-carriers, and on the optical performance, being at the origin of an enhancement of the surface absorptance. As a consequence, a thin surface zone dominates the optical functions of MIND cells through the free-carrier gas confined inside it. In this work we report specific effects concerning the solar-light induced opacity in MIND cells. The investigation allows modification of the free-carrier confinement using different device architectures. The main characterization methods were reflectivity and spectral response with a varying incident beam. The results prove the domination of the free-carrier optical functions on the MIND PV conversion.