In this paper, we use transient absorption spectroscopy to examine the charge photogeneration yields of a series of low band gap polythiophenes and polyselenophenes in blend films with 6,6-phenyl C61-butyric acid methyl ester (PCBM). The polymers are selected to have approximately matched ionization potentials, allowing us to focus upon the importance of the polymer lowest unoccupied molecular orbital (LUMO) level in determining photogeneration efficiency. Data are collected as a function of PCBM composition. A correlation is observed between the yield of dissociated polarons, as measured by the amplitude of the transient absorption signal, and the polymer LUMO level. Lower band gap polyselenophenes produce lower polaron yields, in quantitative agreement with a previously proposed model in which the excess thermal energy of initially generated bound radical pairs determines their dissociation efficiency. Increasing the PCBM concentration from 5 to 50 wt % results in an increase in charge photogeneration. Photoluminescence data demonstrate that this dependence is not primarily associated with an increase in exciton quenching; instead, this increase is assigned to the additional influence of PCBM domain size and/or electron mobility on the dissociation efficiency of the bound radical pairs. These observations are then discussed in terms of their implications for the development of polymer semiconductor materials for organic photovoltaics, and in particular the development of guidelines for the design of polymers for efficiency charge photogeneration in such devices.