  Although organic heterojunctions can separate charges with near-unity
efficiency and on a sub-picosecond timescale, the full details of the
charge-separation process remain unclear. In typical models, the Coulomb
binding between the electron and the hole can exceed the thermal energy
$k_\mathrm{B}T$ by an order of magnitude, suggesting that it is impossible for
the charges to separate before recombining. Here, we consider the entropic
contribution to charge separation in the presence of disorder and find that
even modest amounts of disorder have a decisive effect, reducing the
charge-separation barrier to about $k_\mathrm{B}T$ or eliminating it
altogether. Therefore, the charges are usually not thermodynamically bound at
all and could separate spontaneously if the kinetics otherwise allowed it. Our
conclusion holds despite the worst-case assumption of localised, thermalised
carriers, and is only strengthened if mechanisms like delocalisation or `hot'
states are also present.
