To date optoelectronic devices have been dominated by inorganic semiconductors such as silicon and gallium nitride. However organic semiconductors have many potential advantages and have shown significant research progress in both organic light-emitting diodes and organic photovoltaics.

Tightly-bound excitons form intermediate states in the operation of both organic light-emitting diodes and organic photovoltaics. Many properties of an exciton are determined by its spin state, which can either be a spin-zero singlet or a spin-one triplet. In both organic light-emitting diodes and organic photovoltaics triplet excitons can be a source of loss due to poor radiative coupling to the ground state and energy loss on formation from the singlet state. In this thesis, methods of avoiding loss pathways through triplet states are investigated in both organic light-emitting diodes and organic photovoltaics.

Carbene-metal-amides are a new class of emitter material for organic light-emitting diodes. High-performance devices are reported with maximum external quantum efficiency of 27%, which indicates effective triplet utilisation. Emission colour can be selected through both molecular design and host environment allowing the demonstration of blue-emitting devices with 19% external quantum efficiency. Host-free devices have been fabricated with record external quantum efficiency of 23%, enabled by a resistance to concentration-dependent luminescence quenching.

Electron spin resonance has been used to investigate the mechanism of intersystem crossing between singlets and triplets in the important organic light-emitting diode material, 4CzIPN. It is demonstrated that the spin-vibronic model of intersystem crossing breaks down in device-relevant solid films. Evidence is instead found for back transfer from a spin-correlated radical pair state being the dominant mode of intersystem crossing. With increased film concentration the signal becomes dominated by free polarons, suggesting that charge separation may be a mechanism of luminescence quenching at high concentration.

In many organic photovoltaic material systems triplets are low-lying states from which charge separation is not energetically favoured. In this thesis, attempts are made to negate this triplet loss pathway by the use of low singlet-triplet energy gap materials to ensure that the charge-transfer state can be endothermically accessed from all molecular exciton states.