We report recent advances in the development of conjugated polymer device structures. We have fabricated vertical microcavity structures in which the optical path distance is tunable through control of the orientation of a liquid crystal layer, and demonstrate cavity tuning over 59 nm. We show that this structure allows convenient matching of cavity modes to the freespace emission spectrum from poly(r henylene vinylene), PPV. We have investigated the operation of electrically-excited polymer LEDs with short pulse operation, and have achieved current densities above 103 A/cm2 and peak brightnesses of up to 5 × 106 cd/m 2. We have measured the optical absorption due to the injected carriers under these drive conditions, and find an absorption band near 1.6 eV which we attribute to the presence of polarons. This allows measurement of the injected charge density, from which we deduce that carrier mobilities are strongly enhanced at high fields, reaching values of 4 x 10−2 cm2/Vsec for PPV diodes.