EXCITONS IN CRYSTALLINE THIN-FILMS OF 3,4,9,10-PERYLENETETRACARBOXYLIC DIANHYDRIDE STUDIED BY PHOTOCURRENT RESPONSE

We study excitons in crystalline thin films of the archetype molecular compound: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) by measuring the photocurrent response under small electric fields (<10(4) V/cm). A previously uncharacterized exciton at a wavelength of lambda = 590 nm, or at 2.10 eV above the PTCDA highest occupied molecular orbital, is identified. Lower in energy than either of the previously observed charge transfer excitons at lambda = 475 and 555 nm in PTCDA, the lambda = 590 nm exciton is speculated to be Frenkel-like. The diffusion length of this exciton is 225 +/- 15 nm, compared to 88 +/- 6 nm for excitons generated between 475 and 555 nm.