We monitor in real time photoinduced charge injection at the interface between a fluorinated copper phthalocyanine layer (CuPcF16) deposited by thermal evaporation on top of a p - doped GaAs (100) wafer. Literature data on the electron affinity of CuPcF16 (5.2 eV respect to vacuum level) combined with photoemission measurements indicates an energy offset of 1.1 eV for the GaAs conduction band respect to the CuPcF16 LUMO level. This suggests that charge transfer at the organic - inorganic interface is feasible. We study bilayers of GaAs and CuPcF16 thin films (25 nm) by pump - probe spectroscopy with 200 fs time resolution. Pump photons at 780 nm excites the CuPcF16 layer whereas probe photons in the visible range, reflected by the GaAs surface, monitor induced changes at the interface. We observe a strong photoinduced absorption band centered around 560 nm which appears during the pulse duration, shows a build-up dynamics and persists beyond 0.2 ns. This band cannot be attributed to single material contribution, as demonstrated by test experiments with single layers. By applying steady state (CW) electromodulated spectroscopy we identify charge state absorption in CuPcF16 in the same spectral region as the photoinduced absorption band. We thus assign our transient dynamics to formation of CuPcF16 ions at the interface, following charge injection. On account of the rapid charge formation we identify this system as a potential candidate for the fabrication of hybrid photodiodes.