Progress in Emission Efficiency of Organic Light-Emitting Diodes: Basic Understanding and Its Technical Application
The technical history of when and how the basic understanding of the emission efficiency of organic light-emitting diodes (OLEDs) was established over the last 50 years is described. At first, our understanding of emission efficiency in single-crystal and thin-film electroluminescence (EL) devices i...
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| Published in | Japanese Journal of Applied Physics Vol. 52; no. 11; pp. 110001 - 110001-10 |
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| Main Authors | , |
| Format | Journal Article |
| Language | English |
| Published |
The Japan Society of Applied Physics
01.11.2013
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| Subjects | |
| Online Access | Get full text |
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| Summary: | The technical history of when and how the basic understanding of the emission efficiency of organic light-emitting diodes (OLEDs) was established over the last 50 years is described. At first, our understanding of emission efficiency in single-crystal and thin-film electroluminescence (EL) devices in the early stages before the Eastman-Kodak breakthrough, that is, the introduction of the concept of multilayer structures, is examined. Then our contemplation travels from the Eastman-Kodak breakthrough towards the presently widely accepted concept of emission efficiency. The essential issues concerning the emission efficiency of OLEDs are summarized to help readers to obtain a common understanding of OLED efficiency problems, and detailed discussions on the primary factors that determine emission efficiency are given. Finally, some comments on remaining issues are presented. |
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| Bibliography: | (Color online) Explanation of the meaning of recombination current $J_{\text{r}}$. The numbers of holes and electrons injected from an anode and a cathode to an organic layer are represented by $J_{\text{h}}$ and $J_{\text{e}}$, respectively. The number of holes transfered from an organic layer to a cathode and that of electrons transferred from an organic layer to an anode are given by $J_{\text{h}}'$ and $J_{\text{e}}'$, respectively. The external current $J$ and the electron--hole recombination current $J_{\text{r}}$ are expressed using $J_{\text{h}}$, $J_{\text{e}}$, $J_{\text{h}}'$, and $J_{\text{e}}'$. (Color online) Diagram for explaining the meaning of the carrier balance factor introduced by Scott et al. The left and right corners of the triangle represent hole-only and electron-only devices, respectively, and the apex of the triangle represents the case when an equal numbers of holes and electrons are injected from electrodes, and they all recombine within EML. Schematic representation of the processes from carrier injection to external photon emission. Once the definition of the carrier balance factor is given, Eq. ( ) can be derived straightforwardly. (Color online) Three basic multilayer device structures: two double-layer devices and a triple-layer device. (Color online) Out-coupling of light from a simplified device structure. Dependence of power conversion efficiency on drive voltage. External quantum efficiency is assumed to take a constant value of 0.2, and average photon energy of emitted light is assumed to be 2.4 eV. Two major unsolved problems related to device efficiency. (a) Roll-off in external quantum efficiency under high-current-density drive conditions. (b) Long-term decay in external quantum efficiency in constant-current-density mode. Multiple lines in both figures indicate that roll-off or efficiency decay originates from multiple unknown parameters. ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
| ISSN: | 0021-4922 1347-4065 |
| DOI: | 10.7567/JJAP.52.110001 |