Temperature dependence of charge localization in high-mobility, solution-crystallized small molecule semiconductors studied by charge modulation spectroscopy

In solution-processable small molecule semiconductors, the extent of charge carrier wavefunction localization induced by dynamic disorder can be probed spectroscopically as a function of temperature using charge modulation spectroscopy (CMS). Here, it is shown based on combined field-effect transistor and CMS measurements as a function of temperature that in certain molecular semiconductors, such as solution-processible pentacene, charge carriers become trapped at low temperatures in environments in which the charges become highly localized on individual molecules, while in some other molecules the charge carrier wavefunction can retain a degree of delocalization similar to what is present at room temperature. The experimental approach sheds new insight into the nature of shallow charge traps in these materials and allows identifying molecular systems in which intrinsic transport properties could, in principle, be observed at low temperatures if other transport bottlenecks associated with grain boundaries or contacts could be removed. Clear spectroscopic evidence has been observed from charge modulation spectroscopy that in certain acene-based organic semiconductors, such as 2,8-difluoro-5,11-triethylsilylethynyl-anthradithiophene (diF-TES-ADT), shallow charge traps within the grains can be prevented. In diF-TES-ADT the spectroscopic properties of charge carriers at low temperatures are very similar to those at room temperature suggesting that charges remain mobile at low temperature within individual domains/grains.