Amplifying planar waveguide structures in Er-doped nanocrystalline II/VI semiconductor layer systems were developed by photolithography and wet chemical etching. 2 μm thick planar waveguides on glass substrates with lateral dimensions down to 5 μm with rectangular cross section were realized. By optical excitation a maximum gain of 82 cm−1 could be determined, which is sufficiently high to allow the design of compact planar amplifiers in this material system. The influence of a thermal sintering step on the gain spectrum and on the fluorescence lifetime has been investigated. By increasing the sintering temperature to 800 °C a consistent increase of gain and fluorescence lifetime was observed.

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traps were involved one would expect it to be more efficient in trapping drifting carriers as the concentration goes up. On the other hand, when the Förster transfer is incomplete, each porphyrin molecule is effectively independent of the others and has its own singlet capture range. Since Dexter transfer is not expected to be of larger spatial extent the same is true for capturing triplets, and hence the ratio of the two contributions is independent of the number (concentration) of porphyrin molecules.To conclude, we have provided several experimental results that, when taken together, prove that it is possible to capture the energy from both singlet and triplet excitons and transform it into light emission. This result suggest that it should be possible to make use of 100 % of electron±hole recombination and it lifts the proposed 25 % limit on EL efficiency. Although it is not trivial to find a material combination that will support Dexter transfer as well as Förster transfer, we believe that careful material design will broaden the scope of this technique. We note that even if the porphyrin acts as a recombination center and not as a triplet scavenger, the 25 % limit can still be broken due to the efficient intersystem crossing on the porphyrin. We also showed that we can separate the contributions arising from either singlets or triplets in the host. In order to comment on the ratio between singlets and triplets as generated in the LED, the above result needs to be supplemented only by a measurement of the fraction of triplets that are actually captured by the guest. Another aspect of this work may be that shortening the triplet lifetime in the polymer blend will remove degradation channels associated with the triplet state. [23] Finally, we would like to comment that although the statistical ratio of 3:1 for the ratio of triplets to singlets formed under EL conditions is widely discussed in the literature, there is considerable speculation that there are spin-dependent electron±hole capture processes that lead to a lower ratio in practice. This discussion is driven by recent reports of unexpectedly high LED efficiencies, e.g., 16 lm/W and above in the green part of the spectrum. [24,25] It is hard to measure this ratio experimentally, unless there is direct measurement of the triplet population, for example by excited state absorption. [5] We note that our measurements of triplets through their transfer to the PtOEP provide another route to achieve this. If we assume that all triplet excitons generated in the PNP are captured at PtOEP sites, the 40 % enhancement in triplet emission for EL in contrast to PL (Fig. 4) sets the triplet:singlet production ratio as low as 0.4:1. This is very much a lower limit, since it is unlikely that all triplets are transferred to the PtOEP. Further work is in progress concerning these points.A more complete description of the work by Forrest et al. [10] has recently been published by Baldo et al., [26] showing evidence for triplet±triplet energy transfer in Alq3 and PtOEP bl...

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