Electroluminescence from Phosphor-doped nanocrystals
Alternating current thin-film electroluminescent (ACTFEL) devices already occupy a segment of the largearea, high-resolution, flat-panel-display market. The AC-TFEL displays, which consist of a phosphor layer, such as manganese doped-zinc sulfide (ZnS:Mn), vertically sandwiched between two insulators that are contacted by electrodes, are robust, possess long lifetimes, and offer high luminance with relatively low power consumption [1], [2]. While fabrication of AC-TFEL devices has been the subject of considerable study over the past three decades, significant challenges remain. Development of multicolor displays with balanced red, green, and blue (RGB) emission has proven difficult as the most efficient red, green, and blue phosphors comprise different materials systems that require different deposition and annealing steps. Transparent AC-TFEL displays have recently been demonstrated by Sharp, Inc.; however, the processing of the phosphor to achieve transparency is difficult and has not yet been developed for phosphors other than ZnS:Mn [3].
ELECTRICAL CHARACTERIZATION OF THIN-FILM ELECTROLUMINESCENT DEVICES
▪ Abstract Electrical characterization methods for the analysis of alternating current thin-film electroluminescent (ACTFEL) devices are reviewed. Particular emphasis is devoted to electrical characterization techniques because ACTFEL devices are electro-optic display devices whose performance is to a large extent determined by their electrical properties. A systematic procedure for ACTFEL electrical assessment is described. The utility of transient charge, voltage, current, and phosphor field analysis is explained. Steady-state electrical characterization methods discussed in this review include charge-voltage (Q-V), capacitance-voltage (C-V), internal charge-phosphor field (Q-Fp), and maximum charge-maximum applied voltage (Qmax-Vmax) analysis. These electrical characterization methods are illustrated by reviewing relevant results obtained from the analysis of evaporated ZnS:Mn and atomic layer epitaxy (ALE) SrS:Ce ACTFEL devices.
Charge transfer in ZnS‐type electroluminescence
In this paper, a simple model for the charge transfer in thin‐film, ac‐driven electroluminescent structures of doped ZnS type is proposed. We first review the minimal assumptions needed to account for the carrier emission under high‐field conditions and its subsequent feedback on the field strength; it opens the way to a quantitative description of the field variation with time and the conduction current under arbitrary low‐frequency drive conditions and without using any adjustable parameter. The charge‐voltage relationships are also examined in detail. Conduction in the phosphor layer is assumed to originate from deep levels (traps) in the phosphor forbidden band gap, located at the insulator‐phosphor interface. These levels act for charge storage, too. When a discrete trap level is considered, field clamping in the active layer is obtained; when a smooth interface‐state energy density is assumed, deviations from the field clamping are possible and simply related to the interface parameters. Most of the...