​Optical absorptance spectroscopy of polycrystalline CH₃NH₃PbI₃ films usually indicates the presence of a PbI₂ phase, either as a preparation residue or due to film degradation, but gives no insight on how this may affect electrical properties. Here, we apply photocurrent spectroscopy to both perovskite solar cells and coplanar-contacted layers at various stages of degradation. In both cases, we find that the presence of a PbI₂ phase restricts charge-carrier transport, suggesting that PbI₂ encapsulates CH₃NH₃PbI₃ grains. We also find that PbI₂ injects holes into the CH₃NH₃PbI₃grains, increasing the apparent photosensitivity of PbI₂. This phenomenon, known as modulation doping, is absent in the photocurrent spectra of solar cells, where holes and electrons have to be collected in pairs. This interpretation provides insights into the photogeneration and carrier transport in dual-phase perovskites.​