  Exciton polaritons, arising from the interaction of electronic transitions
with confined electromagnetic fields, have emerged as a powerful tool to
manipulate the properties of organic materials. However, standard experimental
and theoretical approaches overlook the significant energetic disorder present
in most materials now studied. Using the conjugated polymer P3HT as a model
platform, we systematically tune the degree of energetic disorder and observe a
corresponding redistribution of photonic character within the polariton
manifold. Based on these subtle spectral features, we develop a more
generalized approach to describe strong light-matter coupling in disordered
systems that captures the key spectroscopic observables and provides a
description of the rich manifold of states intermediate between bright and
dark. Applied to a wide range of organic systems, our method challenges
prevailing notions about ultrastrong coupling and whether it can be achieved
with broad, disordered absorbers.
