Over the past few decades, semiconducting polymers have been investigated to materialize efficient molecular photonic and electric devices such as organic solar cells, printing electric circuit, organic light-emitting diodes, and so on. Among those, considerable efforts have been devoted to organic solar cells as a substitute energy source to overcome an energy crisis. Bulk heterojunction (BHJ) organic photovoltaic (OPV) cells have various advantages, low cost, ease fabrication, and small environmental impact relative to silicon or heavy metal based solar cells. The solar energy conversion efficiency of OPV cells is improving and especially polymer-based OPV cells are promising for commercialization.
We have investigated two representative OPV polymers, poly(thienothiophenebenzodithiophene) (PTB7) and poly-3-hexylthiophene (P3HT), which are alternating copolymer and homopolymer, respectively (Scheme 1). This work revealed that the initial excited states of PTB7 and P3HT after photoexcitation are emissive and that their origins can be assigned as optically allowed exciton states. Interestingly, the initial excitons with excess energy in PTB7 coherently diffuse before vibrational relaxation and then the cold excitons sequentially show incoherent exciton diffusion processes to neighboring local state. On the other hand, although P3HT show efficient deactivation processes, these processes are not involved in exciton diffusion. This contrary feature results from that the initial excitons are experimentally generated on long conjugated local sites with low electronic energies and thus they are energetically isolated by neighboring short local sites and that there is no conformational motions within a few picoseconds.