Experimental and theoretical studies of the electronic structure of bulk heterojunction (BHJ) organic solar cells are reported. The photoconductivity spectral response of the solar cells has a weak absorption band extending from the band-gap energy down to 1 eV due to charge-transfer optical excitation at the interface between the polymer and the fullerene. The low-energy absorption indicates an exponential band tail of localized states and an absorption model based on the one-electron joint density of electronic states accounts for the data. Transient photoconductivity measurements of the carrier mobility exhibit a temperature-dependent carrier dispersion. Data analysis for the particular case of transport in the BHJ structure is developed. A multiple trapping model of the dispersive transport is consistent with localized band tail states having a comparable density-of-states distribution to those observed by optical absorption. Theoretical calculations of the density of states including disorder in the \ensuremath{\pi}-\ensuremath{\pi} spacing of the polymer chains also shows exponential band tailing. A density-of-states model is developed from the data and is discussed.