  The density of states (DOS) is fundamentally important for understanding
physical processes in organic disordered semiconductors, yet hard to determine
experimentally. We evaluated the DOS by considering recombination via tail
states and using the temperature and open-circuit voltage ($V_\mathrm{oc}$)
dependence of the ideality factor in organic solar cells. By performing
Suns-$V_\mathrm{oc}$ measurements, we find that gaussian and exponential
distributions describe the DOS only at a given quasi-Fermi level splitting. The
DOS width increases linearly with the DOS depth, revealing the power-law DOS in
these materials.
