Role of doping concentration on the competition between amplified spontaneous emission and nonradiative energy transfer in blends of conjugated polymers

Abstract

We investigated the amplified spontaneous emission (ASE) of a blend of light emitting conjugated polymers. The effects related to the acceptor/donor polymer relative concentration on both cw photoluminescence and ASE were investigated experimentally and interpreted by a quantitative model describing ASE in presence of fast non radiative energy transfer processes. The range of acceptor concentrations where ASE does not occur is experimental determined, whereas outside such interval ASE is observed from the donor or from the acceptor polymer. In particular, the blend with 10% acceptor molecules shows ASE-induced line narrowing for excitation fluences larger than $100\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{J}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, with a maximum optical gain coefficient of $8\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and a loss coefficient as low as $0.3\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. These optical and gain properties make the investigated conjugated polymer blend an appealing material as active layer for laser devices.