  In crystalline semiconductors, absorption onset sharpness is characterized by
temperature dependent Urbach energies. These energies quantify the static,
structural disorder causing localized exponential-tail states, and dynamic
disorder from electron-phonon scattering. Applicability of this
exponential-tail model to disordered solids has been long debated. Nonetheless,
exponential fittings are routinely applied to sub-gap absorption analysis of
organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes
of organic semiconductors and their blends by temperature-dependent quantum
efficiency measurements. We find that sub-gap absorption due to singlet
excitons is universally dominated by thermal broadening at low photon energies
and the associated Urbach energy equals the thermal energy, regardless of
static disorder. This is consistent with absorptions obtained from a
convolution of Gaussian density of excitonic states weighted by Boltzmann-like
thermally activated optical transitions. A simple model is presented that
explains absorption line-shapes of disordered systems, and we also provide a
strategy to determine the excitonic disorder energy. Our findings elaborate the
meaning of the Urbach energy in molecular solids and relate the photo-physics
to static disorder, crucial for optimizing organic solar cells for which we
present a new radiative open-circuit voltage limit.
