  By comparing photoemission spectroscopy with a non-perturbative dynamical
mean field theory extension to many-body ab initio calculations, we show in the
prominent case of pentacene crystals that an excellent agreement with
experiment for the bandwidth, dispersion and lifetime of the hole carrier bands
can be achieved in organic semiconductors provided that one properly accounts
for the coupling to molecular vibrational modes and the presence of disorder.
Our findings rationalize the growing experimental evidence that even the best
band structure theories based on a many-body treatment of electronic
interactions cannot reproduce the experimental photoemission data in this
important class of materials.
