Influence of conjugation length on ultrafast electronic tunneling in organic semiconductor thin films

Electron delocalization in conjugated organic molecules is a rate-limiting step in maximizing the charge generation efficiency of next generation photovoltaics and molecular electronics. In particular, ultrafast (<50 fs) delocalization is an important aspect that has been beyond the scope of traditional optical experiments. In this work, we use resonant photoemission spectroscopy to probe electron delocalization timescales as a function of conjugation length by examining an oligothiophene chemical series containing 4-, 5- and 6-mers. We find that above a certain photon energy threshold, the 5-mer, quinquenthiophene, displays the largest ultrafast tunneling rates, roughly three times faster than the 6-mer, sexithiophene. We argue that differences in thin-film molecular packing cannot satisfactorily explain our results, and we speculate that the differences in ultrafast electron dynamics may be a manifestation of the odd/even effect.