ABSTRACT
References .....................................................................................................................................390
A review of the early work on electroluminescence (EL) in organic single crystals in the 1960s is very useful for understanding the more recent works on high-performance organic multilayered thin-film EL. In particular, Helfrich and Schneider studied the mechanism of EL in anthracene single crystals in the 1960s. They used anthracene cation-and anion-containing electrolyte solutions as an anode and cathode, respectively, and double charge injection of electrons and holes was only possible using solution contacts for both electrodes, leading to the production of bright EL.1,2 Figure 32 shows the light intensity versus current density characteristics replotted from one of their original figures. It is seen that the emission intensity scales linearly with injected current density over more than three decades, providing clear evidence of charge injection, transport, and recombination processes which determine quantum efficiencies in terms of photons emitted per charge injected. Helfrich and Schneider also reported that the emission arose predominantly from a region near the positive electrode, indicating that electrons are injected and transported through the crystal to recombine with the holes injected from the positive electrode. This clearly shows that bulk-controlled charge transport and recombination control the process of chargeinjection EL.
