Recombination singlet excitons

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

W Wohlgenannt, XM Jiang, C Yang, OJ Korovyanko, and ZV Vardeny, Spin-dependent polaron pair recombination in TT-conjugated polymers enhanced singlet exciton densities, Synth. Met., 139 921-924, 2003. 

Exciton decay When an exciton decays radiatively a photon is emitted. When the excitons form in fluorescent materials radiative decay is limited to singlet excitons and emission occurs close to the recombination region [7] of the OLED due to the relatively short lifetime of the excited state (of the order of 10 ns). For phosphorescent materials, emission can occur from triplet excitons. Due to the longer excited state lifetime (of the order of hundreds of nanoseconds), triplet excitons can diffuse further before decaying. 

For a critical concentration of excitons 17 = 7/. the critical radius (, below which bimolecular annihilation process predominates over singlet exciton recombination can be expressed as [5],... 

The study of the radiative recombination of excitons makes it possible to investigate the influence of the radiation-stimulated destruction of C60 fullerenes (partially of C70) on changes of the singlet states within the energy gap. It is known that the emission of excitons in this case is the result of the presence of own dimeric traps [11] and X-centers, caused by the chemically bound with fullerenes and intercalated impurities [8], and also of taking into account the corresponding phonon states. 

Therefore, at least in some CPs, the lowest singlet exciton is a g state. If recombination in such a CP is by the exciton process, no EL, or only a very weak one, is expected. The case of direct (nonexciton) recombination has not been considered yet, but one may expect that if a low-lying g state exists, it will be formed by recombination just as well, and no EL will be emitted. 

To summarize, the microscopic recombination processes in CPs are still debated. If recombination proceeds via the formation of a singlet exciton, a good CP for LED applications should have at least three properties It should not form bipolarons, or only slowly the u exciton should be the lowest singlet state and that state should, of course, have minimal non-radiative decay processes, in particular to the triplet state [129]. But, in addition, the diode geometry and the electrodes should be chosen so as to maximize carrier recombination and minimize carrier extraction through the electrodes. 

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