Time Of Flight

Charge transport in organic and hybrid semiconductors is a particularly important issue as it plays a key role in the operation of devices such as light-emitting diodes, solar cells, and field effect transistors. Time of flight (TOF) mobility measurements, in which a sample is photoexcited and the transit time of the charge carriers across the film is measured, provide a powerful way of studying charge transport. Photocurrents induced by short laser pulses can give information not only on charge carrier mobility, but also on free carrier lifetimes, the influence of deep and shallow traps, and on the degree of disorder of the semiconducting material.

The set-up is flexible enough to convert to TOF from CELIV measurements (and vice versa) very rapidly so that it can be used for many different optoelectronic devices with minimal alterations. Figure on the right shows a schematic of the TOF set-up used.

Measurements can be performed under different environmental conditions, such as in air, under inert atmosphere (N₂) or in vacuum, by placing the device inside a customized cryostat in a sample holder which supplied signal to the contacts and gave light access to the device for photo-excitation. The light source is a pulsed LASER tunable over the UV, VIS and NIR spectrum to give the appropriate excitation to a wide range of materials. Finally, measurements can be performed in a temperature range varying from 77 K up to 400 K.