The field, temperature and concentration dependence of the dark injection of holes and electrons from metal electrodes into molecular dispersions of respectively 5′-[4-[bis(4-ethylphenyl)amino]phenyl]-N,N,N',N'-tetrakis(4-ethylphenyl)-[1,1′:3′,1′'- terphenyl]-4,4′'-diamine and 1,3-bisdicyanomethylene-2-methyl-allyl-indane in polymers has been investigated. All results point to a two step process: in a first step charge carriers are injected to the bottom of the potential well created by an image potential between the charge transport material and the electrode. The competition between escape of the charge carriers from this well and recombination with empty levels in the electrode governs the final rate of charge carrier injection. While in a qualitative way this approach resembles an extension to doped polymers of the Willig-Gerischer model, developed for organic single crystals, a quantitative description of this system requires us to consider also the diagonal and non-diagonal disorder of the hole transport material.