Spectrally separated optical gain and triplet absorption: Towards continuous wave lasing in organic thin film lasers

We studied laser dynamics of poly(9,9'-dioctylfluorene) (PFO) based organic thin films lasers. The favorably used host/guest system PFO, containing 6,6'-(2,2'-octyloxy-1,1'-bianphthalene) binaphthyl spacer groups (BN-PFO), doped with 1,4-bis(2-(4-(N,N-di(p-tolyl)amino)phenyl) vinylbenzene (DPAVB) shows a dramatic decrease of laser emission for excitation pulses longer than a few nanoseconds, whereas the undoped material offers a stable emission. By measuring the optical gain and the triplet absorption spectra we show that the spectral overlap of the two spectra leads to a dramatic increase of the laser threshold in the doped system due to a pile-up of long lived triplet states. On the contrary for BN-PFO, optical gain and triplet absorption are found to be spectrally separated. As a result, accumulated triplet excitons hardly affect the laser emission. (C) 2011 Elsevier B.V. All rights reserved.