  The photophysics of $\pi$-conjugated polymers has been of considerable
interest over the last three decades because of their organic semiconductor
properties. Primary photoexcitations, Frenkel excitons, can be probed optically
by means of numerous linear spectroscopies, providing a wealth of information
on the the strength of excitonic coupling, the exciton coherence length, and on
the nature of the disordered energy landscape. Nonetheless, there are intrinsic
limitations in the information that can be obtained with linear spectroscopy
compared to non-linear coherent techniques. Examples of this are the separation
of homogeneous and inhomogeneous broadening contributions to the total exciton
spectral lineshape, the detailed spectral structure of exciton-vibrational
coupling, and correlations between optical excitations, including
exciton-charge and exciton-exciton correlations. In the present work, we
discuss the role of two-dimensional coherent excitation spectroscopy and review
its applications towards unravelling the basic photophysics of $\pi$-conjugated
polymers as well as polymer:fullerene blends, and we argue that these
techniques are now valuable mainstream materials optical probes.
