Molecular and Electronic Structure in Thin Films of Semiconducting Polymers as Studied in the Infrared and UV/Vis Spectral Range

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Semiconducting polymers combine advantages of neat polymers, such as mechanical robustness and light weight, with semiconducting electronic characteristics and the possibility of low-cost large-area deposition, which provides considerable technological benefits. Despite the intensive research in the field of organic electronics and the commercial success of organic light emitting diodes (OLEDs), many very fundamental questions remain unanswered. For example, in well-ordered organic semiconductor films charges and excitons are thought to behave as intermediate between highly ordered inorganic semiconductors, with band-like transport, and very disordered systems with hopping transport. However, we do not yet understand how the structure can be controlled to improve charge or exciton coherence length. This lack of understanding is in part due to the lack of suitable microstructural probes that are sensitive to disorder on the nanometer scale. In this project, it is propose to build up upon recent work at Leipzig University using infrared transition moment orientational analysis (IR-TMOA) to probe the moiety-specific orientation and order of sub-molecular structures in organic semiconductors. This technique –so far unique and only available in Leipzig– enables to build up a comprehensive picture of the structure and the degree of disorder at the length scale relevant for energy and charge transfer. Using time-resolved spectroscopy measurements at the University of Sheffield, the microstructural findings from IR-TMOA will be correlated with measurements of the exciton and charge transfer coherence length. Measurements are planned to be examined at the model polymer P3HT and their novel bottlebrush derivatives. The degree of crystallinity and the microstructural environment will be altered by the molecular weight and chemical structure. Furthermore, the model co-polymer PNDIT2 (or P(NDI2OD-T2)) and its thionated derivative 2S-trans-PNDIT2 allows for studying the influence of chemical substitution and hence altered molecular orbital energy levels on the molecular orientation and order as well as charge transfer and exciton coherence length. So far this combination of experimental techniques has not been published previously and promises new insights into the interplay of molecular structure and electronic properties. In addition to the use of time-resolved spectroscopy and steady-state IR-TMOA as independent methods, it is planned to combine the measurement principle of TMOA with time-resolved infrared spectroscopy. This will lead for the first time to a measurement technique which allows for studying the dynamics of orientation and order of molecular units.