The lifetime of optically injected carriers is determined in polySi/SiO2/Si structures grown by LPCVD at 625°C. These samples are as-grown, have undergone H diffusion or have been implanted by phosphorous ions, followed by various annealing schedules. The lifetime measurement is done in an all-optical, contactless fashion, using the tools of picosecond time-resolved spectroscopy. We find that the lifetime due to recombination at the grain boundaries (trapping time) increases after implantation only if subsequent annealing increases the grain size. The trapping time also increases after hydrogen diffusion. After trapping in relatively shallow grain boundary states, thermalization into deeper lying states is slow on a subnanosecond time scale.