電界発光セルの発光強度

抄録

Brightness of electroluminescent (EL) cell consisting of a thin layer film of dielectric material in which phosphor powder is dispersed is analyzed. The brightness depends on optical properties of the cell material and electric field in the phosphor particle.
By macroscopical optical analysis on the layer, relations of brightness of the cell to the amount of light given out by each powder particle, transmittance and reflectance of the layer and reflectance of back surface of the layer are clarified.
By electrical analysis, relation of electric field in the particle to mean field strength impressed on EL layer is first derived. Generally in a system composed of two components, mean field strength E₁ in component 1 can be expressed as
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where ε₁ and ε₂ are the dielectric constants of the two components, ε the mean dielectric constant of the system, δ₁ the volume ratio, and E₀ the mean field strength in the system. By introducing Bruggeman's relation
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into the above, one obtains the expression for the inner field strength of the particle. This expression was proved valid for the practical EL cell.
According to the above reasoning, the field strength, that is the brightness, of the cell becomes higher when a matrix of higher dielectric constant is used. Dielectric constant of a matrix can be increased by mixing powder of high dielectric constant with plastic. Use of a dielectric material of low optical absorption-barium titanate powder of reflectance 0.92-increased the mean dielectric constant, hence the brightness, of the cell as expected from the foregoing reasoning.