Unraveling the inhomogeneously broadened absorption spectrum of conjugated polymers by single-molecule light-harvesting action spectroscopy.

The distribution of chromophores in single polymer chains is revealed by photoluminescence excitation spectroscopy under excitation of the backbone and detection of emission from an end cap. Spectral broadening in excitation exceeds that in emission. An increase in vibronic coupling for shorter (higher energy) chromophores is resolved, leading to intrinsic spectral broadening and making higher energy units more effective donors. The results suggest routes to increasing absorption breadth while minimizing disorder as required for efficient photovoltaics.