We consider directed polymers in a random environment.
Under some mild assumptions on the environment, we prove equivalence between the decay rate of the partition function
and some natural localization properties of the path; some quantitative estimates of the decay
of the partition function in one or two dimensions, or at sufficiently low temperature; and the existence of quenched free energy. In particular, we generalize to general environments the results recently obtained by Carmona and Hu for a Gaussian environment. Our approach is based on martingale decomposition and martingale analysis. It leads to a natural, asymptotic relation between the partition function, and the probability that two polymers in the same environment, but otherwise independent, end up at the same point.