Excited state interaction in the dopant system in ZnS: Mn electroluminescence devices

Luminescence Saturation via Mn2+–Exciton Cross Relaxation in Colloidal Doped Semiconductor Nanocrystals

“…The data in Figure C and those reported previously in ref for related CdS/Mn 2+ :ZnS nanocrystals demonstrate that the Mn 2+ PL decay time is unaltered in the saturation regime. This behavior contrasts with the reduced Mn 2+ PL lifetimes observed with increased excitation densities in related bulk materials, which suggested Mn* energy migration followed by Mn*–Mn* cross relaxation. − Such processes occur on the 10 μs time scale, , and they become slower at lower Mn 2+ concentrations . Mn*–Mn* cross relaxation is therefore far too slow to be responsible for the PL saturation in Figures –.…”

“…The difference in PL saturation between bulk and nanocrystalline Mn 2+ -doped semiconductors is also an interesting issue. Given the numerous reports of PL saturation in related phosphors, − saturation of the slow Mn 2+ PL is not unexpected, but the low excitation rates and low Mn 2+ concentrations at which it occurs in these nanocrystals appear to differentiate this saturation from those described in closely related bulk materials. − The same Mn 2+ -exciton cross relaxation is undoubtedly active in bulk Mn 2+ -doped semiconductors, but it is apparently not the dominant cause of saturation. In the nanocrystals investigated here, the Mn 2+ concentrations are too small for energy migration of the type observed in bulk, and time-resolved measurements reveal no slow contributions to the Mn 2+ excited-state decay dynamics.…”

“…Mn 2+ is an effective luminescence activator in many semiconductor matrices, and consequently Mn 2+ -doped semiconductors have been employed as phosphors in photoluminescent and electroluminescent technologies for many years. − In photoluminescence (PL), photoexcitation of the semiconductor is followed by rapid (picosecond) nonradiative energy transfer to excite Mn 2+ to its 4 T 1 state (Mn*), which then relaxes back to the 6 A 1 ground state radiatively with approximately millisecond lifetimes, depending on the lattice and temperature. − The long Mn 2+ 4 T 1 excited-state lifetime is a bottleneck at large excitation rates, however, causing luminescence saturation. − In bulk Mn 2+ -doped semiconductors, a combination of Mn 2+ concentration-dependence and PL time-dependence measurements has led to the conclusion that saturation is not only associated with depletion of ground-state Mn 2+ but also depends on Mn* energy migration and nonradiative Mn*–Mn* cross relaxation. − In electroluminescent devices, Mn*–electron Auger-type de-excitation has also been identified as an important deactivation mechanism. ,− …”

Photoluminescence Behavior of Manganese‐Doped Zinc Silicate Phosphors

“…Moreover, there should be more complications by considering the fact that cross relaxation also occurs. 13,14 On the other hand, trap-involved processes, which can retard the overall decay process, occur to a certain extent when excited in the UV range above E T . [15][16][17] Even if the trap-involved charge transfer between the host and the Mn ions were a controlling step, the shortening of the decay time with increasing manganese content could be interpreted in terms of a trapinvolved model, 9 where it is assumed that the rate-controlling step for manganese emission could be recombination between the trapped electrons and the Mn ions that have been ionized through either thermal emission or tunneling.…”

“…Such a difference might be related to the complexity of interaction behavior between Mn centers that include cross relaxation. 13,14 The lifetime in this range can be expected to decrease, in comparison to ranges II or III, by considering the fact that no trap-involved process occurs. However, in actuality, the difference is not great between ranges I and II, as far as the near-resonant ( 4 T 1 level) excitation is concerned, even if range I shows a slightly shorter lifetime than the others.…”