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We report a study of the triplet exciton dynamics and their effect on the performance of fluorescent organic light-emitting diodes. These polymer light-emitting diodes comprise metal oxide, injection electrodes, and poly(9,9'-dioctylfluorene-co-benzothiadiazole) as the emissive material and exhibit external quantum efficiencies up to 6.5%. Transient optical absorption measurements following a short (0.5 to 50 µs) electrical drive pulse were used to monitor triplet dynamics during device operation. Triplet generation and decay processes were modeled, and we find that triplet-triplet annihilation is the dominant triplet decay mechanism. Singlet states, generated from triplet-triplet annihilation were monitored as delayed electroluminescence after the end of the drive pulse. From the delayed electroluminescence dynamics, we determine monomolecular as well as bimolecular triplet decay rates and estimate the triplet-charge annihilation rate. Singlet states generated from bimolecular triplet-triplet annihilation contribute up to 33% of the total amount of singlets generated in these fluorescent devices. To model these results, we require that triplet states can undergo bimolecular annihilation several times. With this model, we show that singlets can reach a maximum fraction of 40% of all excitons generated by charge recombination, without violating spin statistics. Singlet states generated from triplet-triplet annihilation are one important explanation for high external quantum efficiencies found in these fluorescent devices. [ABSTRACT FROM AUTHOR]

We present a means to optically tune the emission of an organic laser reversibly over a range of 14 nm. This is enabled by incorporating a photochromic spiropyran (10,30-dihydro-10,30,30-trimethyl-6-nitrospiro[2H-1-benzopyran-2,20-(2H)-indole]) into the gain medium formed by the laser dye Rhodamine 700 in poly(methyl methacrylate). This is structured by nanoimprinting to form a distributed feedback resonator. Exposure of the device to ultraviolet and visible light leads to a reversible refractive index change of the photochromic additive and the gain medium, which enables the continuous shift of the laser wavelength. This is possible without compromising the optical gain properties of the device. [ABSTRACT FROM AUTHOR]

A new zwitterionic conjugated polyelectrolyte without free counterions has been used as an electron injection material in polymer light-emitting diodes. Both the efficiency and maximum brightness were considerably improved in comparison with standard Ca cathode devices. The devices showed very fast response times, indicating that the improved performance is, in addition to hole blocking, due to dipoles at the cathode interface, which facilitate electron injection. [ABSTRACT FROM AUTHOR]