Organic semiconductors are economical, can be easily processed in large areas, have tunable electronic properties and easily grow into high quality thin films.
Charge transport in organic conductors is fundamental to their operation, yet many aspects of organic charge transport are poorly understood. Techniques such as time-of-flight (TOF) and transport-electroluminescence (TEL) can be useful for comparing properties of thin film device materials prepared under varying conditions. However, it is very difficult to extract intrinsic transport properties from such bulk transport measurements without using large single crystals.
Muon spectroscopy can be used for mobility measurements of conductors with very low concentrations of carriers. This constrasts with more familier methods which are more suited to highly doped materials.
The muon technique is highly sensitive to intrinsic transport properties, even in polycrystalline material. In contrast, macroscopic transport probe measurements are usually dominated by extrinsic properties, such as inter-grain hopping.
A recent ‘proof-of-principle’ study using muon spectroscopy has confirmed the one-dimensional nature of the charge carrier transport in the organic semiconductor Alq3 and given an upper limit to the charge carrier mobility [Drew et al., Physical Review Letters (2008) 100, 116601].
Drew et al. found that the carrier mobility at large electric fields in the most recent TOF and TEL measurements for Alq3 was significantly smaller than the mobility measured in the muon measurements in zero electric field.
But by correcting the macroscopic TOF/TEL measurements for bottlenecks in the transport chain, the macroscopic mobility was found to be very similar to that in the muon measurements.
With the relationships between measurement techniques understood, muon spectroscopy techniques can now be applied to other organic materials.