  p-Conjugated materials, including small molecules and polymers, are
attracting substantial attention as novel gain media in semiconductor lasers;
they offer many potential advantages not achievable with conventional inorganic
semiconductors: simple processing, low cost, easy tuneability of the spectrum,
and large-area integration on flexible substrates. Optically pumped lasing
action in various small molecular and polymeric p-conjugated materials has been
demonstrated using several resonator configurations. However, electrically
pumped organic semiconductor lasers, i.e., organic injection or diode lasers,
remain elusive, presumably due to various loss mechanisms, e.g., charge
(polaron)-induced absorption and metal electrode absorption. Here we report on
evidence for amplified spontaneous emission (ASE), also known as mirrorless
lasing (i.e., wherein some of the spontaneously emitted photons are amplified
by stimulated emission during their propagation) in DC-driven small molecular
organic light-emitting diodes (SMOLEDs). The evidence includes a dramatic
spectral line narrowing, with a full width at half maximum (FWHM) of only 5 -
10 nm, and optical gain, of the edge-emission from SMOLEDs at room temperature.
However, there is no clear indication of threshold behavior associated with
this spectral narrowing. Nevertheless, this discovery should pave the way
towards the realization of an organic diode laser.
