Distance-dependent excitonic couplings in P3HT stacks of 30-mer long chains (orange circles) versus 6-mer long chains (blue triangles). To ease the comparison, the excitonic couplings have been renormalized (divided) by the nearest-neighbor values ( meV in 30-mer stacks and meV in 6-mer stacks). The lines show fits of the data using the following double exponential functions: ( ) ( ) for 30-mer stack and ( ) ( ) for 6-mer stack, is the separation between two polymer chains in the stack. Note that the falloff parameters are smaller in 6-mer versus 30-mer stacks.