Crystals Structural Impact on Transport

In the second part of this section we will discuss the effects of structural ordering on the transport for the case of diindenoperylene (DIP). As we have demonstrated in the former section, DIP has proven to be less susceptible to chemical degradation and, therefore, represents a prototypical organic semiconductor to study the relations between structure and electronic transport. 

As indicated by the measurements performed in specular 0-20 geometry, the structural phase transition is completed at around 400 K. The [001] lattice spacing of 1.68 nm (20 = 5.31°) in the high temperature phase coincides with that of DIP thin films grown on weakly interacting substrates such as Si02 at room temperature, (see colour plates p. XCIII) 

To analyse the effeets of those statie and dynamie struetural modifieations on the eleetronie properties at the phase transition, the mobility measured by TOF, SCLC and FET was ehosen as an indieator of high sensitivity. By this eoneerted experimental approaeh the earrier transport ean be studied independently for eleetrons and holes and with speeial foeus on the strueture-transport eorrelation along different erystallographie direetions. 

At first, by injeetion-free TOF speetroseopy transients for eleetrons and holes were observed between 300 K and 420 K, i.e. above the struetural phase transition. The two eharge earrier types obey a thermally aetivated transport behaviour with mobility maxima of 0.03 emWs for holes and 0.2 em A s for electrons both at 400 K. The detection of well-defined electron transients in a sublimation purified material aheady indicates its aforementioned chemical stability. For many polyaromatic materials the formation of the respective quinones, e.g. upon (photo-)oxidation, shifts the molecular levels to lower energies because of the higher electronegativity and causes stronger binding of electrons on the molecular skeleton [2]. The LUMO (lowest unoccupied molecular orbital) of those oxidation-products is often positioned below the 

LUCO of the host and acts as an effective electron trap [49]. In combination with the comparable mobility of electrons and holes, the chemical inertness evidences the potential of diindenoperylene for application in e.g. photovol-taics where a balanced charge transport is required. 

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