These materials offer the prospect of flat, and even flexible, displays that operate at low voltage and emit light, giving excellent contrast and viewing angle. When a voltage is applied to the contacts, charges are injected into the device. Figure 1 A fourfold increase in the efficiency of an organic light-emitting device. Light emission in organic materials commonly occurs when the material absorbs energy and becomes excited, then rapidly returns to its ground state by emitting fluorescence.
Light emission can also occur by phosphorescence, which gives longer-lived emission.
These processes share the same general mechanism but involve different excited states with different electronic configurations. An electron has spin, which can be either up or down, and when two electrons have opposite spins they are said to be spin-paired or in a 'singlet' state. The ground electronic state of most molecules is a singlet state and fluorescence usually involves an excited singlet state relaxing back to the singlet ground state. Transitions of this type are forbidden by the quantum-mechanical selection rules, which in practice means they have low probability and occur more slowly, allowing other processes such as quenching to occur. This generally means that phosphorescent materials emit light with low efficiency.