DISPLAYS
Incorporating news from O plus E magazine, Tokyo
SENDAI-Scientists at the Institute for Advanced Materials Processing of Tohoku University have discovered a new method of developing high-density, high-luminance phosphor materials. Most next-generation displays, such as plasma and field-emission displays, are self-emitting and use phosphor materials that have increased emission efficiency compared to those used in conventional cathode-ray tube displays.
Current inorganic phosphor materials contain a light-emitting activator integrated into a host-crystal lattice. Increasing the current density makes the phosphor material shine brighter. Beyond a critical density, however, the material reaches a point at which increasing the current density does not improve brightness. This effect occurs because the density of the crucial activators is too low, so-called concentration quenching occurs, and the brightness of the phosphor material actually decreases.
Concentration quenching takes place as a result of the transfer of excitation energy within the crystal. In oxide-type phosphor crystals, concentration quenching is caused by the transfer of excitation energy due to electrostatic resonance transfer between two light-emission centers (activator ions). The excitation energy thus migrates among the different centers within the crystal. There is no problem if the energy eventually reaches the activator ion and is transferred to fluorescent light. But in many cases, the energy is lost to impurities and defects within the crystal, is transferred to heat, and the emission intensity decreases.
Phosphor material takes one of two configurations, depending on the concentration of activator ions. At a low concentration, activator ions form pairs, while at a higher concentration, they form chains. The configurations each emit light efficiently for their respective ion concentrations.
This migration of energy is caused by the forces acting between two activator ions, and is related to the distance between the ions. When the concentration of activator ions is low, the distance between activator ions is large, and energy is less likely to be transferred. Conversely, when the concentration of activator ions is high, it is more likely that energy transfer will occur. This effect, which is related to the dimensionality of the activator ion positions within the crystal, prohibits simply increasing the number of activator ions.
As the dimension of the system goes up from one to two to three, the energy in the crystal migrates across respectively larger volumes per unit time. If the concentration of impurities and defects within a crystal is constant, the probability of encountering one of these defects gets higher as the possible migration volume increases. In other words, by arranging the activator ions in one or two dimensions, the effects of concentration quenching can be reduced.
The research focuses on this point. The phosphor material used in the experiment has a strontium lanthanum gallium oxide host-lattice crystal with europium activators. The critical concentration of the activators is higher than for conventional phosphors. The material also has two emission intensity peaks: one at low concentration, and another at a higher concentration. This is because the positions of the activator ions within the crystal can either be diagonal or linear (see figure). When the position of the ions is diagonal, the nearest neighbor ions form pairs.
When the concentration of activator ions is low, the ions can be seen in both the diagonal and linear configurations, and energy can migrate easily. When the concentration of activator ions is high, any ions in a line move to the diagonal position due to the difference in ionic radii. In this state, energy migrates easily between two ions in a pair, but the energy cannot easily migrate from pair to pair. Thus, the emission peak at high concentration can be seen.
This effect leads to the idea of creating activator ion pairs in which the pairs are far apart from one another. Energy migration goes down, and the transfer of emission energy to non-light-emitting centers (that is, defects) also goes down. A new high-density light-emitting material can be designed because the critical concentration per unit volume increases.
Courtesy O plus E magazine, Tokyo