  Organic semiconductors have attracted increasing interest as thermoelectric
converters in recent years due to their intrinsically low thermal conductivity
compared to inorganic materials. This boom has led to encouraging practical
results, in which the thermal conductivity has predominantly been treated as an
empirical number. However, in an optimized thermoelectric material, the
electronic component can dominate the thermal conductivity in which case the
figure of merit $ZT$ becomes a function of thermopower and Lorentz factor only.
Hence design of effective organic thermoelectric materials requires
understanding the Lorenz number. Here, analytical modeling and kinetic Monte
Carlo simulations are combined to study the effect of energetic disorder and
length scales on the correlation of electrical and thermal conductivity in
organic semiconductor thermoelectrics. We show that a Lorenz factor up to a
factor $\sim 5$ below the Sommerfeld value can be obtained for weakly
disordered systems, in contrast with what has been observed for materials with
band transport. Although the electronic contribution dominates the thermal
conductivity within the application-relevant parameter space, reaching ZT>1
would require to minimize both the energetic disorder but also the lattice
thermal conductivity to values below $\kappa_\text{lat}<0.2$W/mK.
