Thin-film Foerster-transfer-based conjugated polymer blend lasers and light emitting diode

Since the discovery of stimulated emission in solid films of conjugated polymers and subsequent demonstration of optically pumped lasing, the focus has shifted towards making diode lasers. Low mobilities and difficult charge carrier injection limit the current densities in light emitting diodes made using polymers. The current densities corresponding to the optically pumped lasing thresholds are difficult to achieve in electrically pumped devices. In order to make diode lasers, the thresholds for lasing must be reduced.;Self-absorption losses limit the thresholds for lasing in conjugated polymers. We have reduced these losses by spectrally shifting the emission away from the absorption edge. This is achieved by making blends of conjugated polymers, in which the excitations are transferred non-radiatively (Forster transfer) from a higher band-gap polymer (donor) to a lower band-gap polymer (acceptor).;Forster transfer has been characterized by measuring the absorption and emission spectra of various blends. Photoluminescence excitation spectroscopy and time resolved pump-probe spectroscopy confirm the presence of Forster transfer. The rate of Forster transfer is dependent on the spectral overlap of the donor emission with acceptor absorption and on the donor-acceptor spacing. For optimized blends, the transfer takes place within 10 ps of excitation. This is faster than the excitation lifetimes in the donor (600 ps) and the efficiency of transfer can be as high as 99.8%.;Propagation losses in the films were reduced from ∼90 cm-1 to 3 cm-1 by using blends. Thresholds as low as 100 W/cm2 were achieved in distributed feedback lasers made using these blends, an order of magnitude lower than the thresholds reported earlier. Unfortunately, the light emitting diodes made using the blends of conjugated polymers have high turn-on voltages, a result of charge carrier trapping in the acceptor molecules. Thus, the prospects of making direct injection diode lasers using the blends are not promising.;The output of the GaN LED's made at UCSB is around 80 W/cm2, close to the thresholds of the DFB lasers made using conjugated polymer blends. The emission from GaN LED's and lasers can be used to optically pump the polymers to make hybrid lasers.