Abstract text (incl. figure legends and references)
Introduction
The pivotal property of organic semiconductors (OSCs) to achieve high performance devices is a high charge carrier mobility. However, the mobility is commonly measured from a model device, most often a field-effect transistor, making the mobility value material- and device-dependent. Both, the intrinsic factors - such as energy levels of all materials involved, molecular packing of the OSC, its crystalline domain size and orientation - and the extrinsic factors - likewise device structure, morphological defect, contact defect - will influence the final result.
On the other hand, computational methods can provide intrinsic mobilities, which deviate frequently over several orders of magnitudes from the experimental data. Thus, material scientists need a reliable method to (re-)evaluate the existing and newly developed materials in order to gain a rational structure-function-relationship.
Objectives
Herein, we present an electron-spectroscopic method to evaluate the charge carrier mobility directly from thin-films or microcrystals. Such samples can easily be obtained for most materials. With the proposed method the effects of the extrinsic impact factors are reduced to only one - the contact resistance. This factor remains, as it is necessary to ground the sample for electron spectroscopic data collection. We show that the relative lateral mobilities between different OCSs can thus be directly determined.
Materials & methods
Polycrystalline thin-films of small molecules were prepared via thermal evaporation under vacuum. Microcrystals of small molecules and polymers were obtained by wet techniques. We use a demonstrator of an electron-spectroscopic SEM, i.e. Zeiss Delta-SEM, for recording the secondary electron (SE) emission spectrum. The dynamic charging map, generated from the SE spectra on each pixel, is used for the mobility evaluation (see Figure 1).
Results
First, the beam-material interaction was investigated and the electrically neutral energy points were determined under various charging conditions. Depending on n-type or p-type OSCs, suitable charging conditions that results in negative or positive charging was selected.1 With step-wise increase of charging intensity, the OSCs turned from electrically neutral to severely charged. By comparing the stage currents among different OSCs, which exhibit the same/similar charging maps, one can evaluate OSCs in terms of lateral charge carrier mobility (example see Figure 2). The horizontal mobility is also accessible by analyzing the charging maps near and on the metal contacts. In addition, space charge, impacts from crystal orientation and molecular packing on mobility can be observed.
Conclusion
The lateral mobilities estimated with the presented method agree very well with the theoretical calculation (eg. transfer integral)2. Contact between OSC and electrode can be individually tested and thus a suitable device structure can be proposed according to the intrinsic lateral mobility.
Ref. [1] Adv. Electron. Mater. 2021, 2100400. [2] Chem. Eur. J. 2015, 21, 17691-17700.