Molecular doping of organic semiconductors and beyond: resolving fundamental processes and increasing doping efficiency

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Doping inorganic semiconductors such as crystalline silicon by introducing impurities is the basis of all transistor technology, which enables the functionality of today's electronic devices. In the past decades, organic semiconductors emerged as fascinating alternative to inorganics promising large-area processability on flexible-substrates and enormous versatility, as their opto-electronic properties, their bandgap in particular, can be tuned by their chemical structure. The concept of doping has only recently been adapted for organics by employing molecular dopants, where all novel display technology based on organic light emitting diodes (OLEDs) today uses doped conjugated materials. Despite enormous success in applications, however, the fundamental processes of doping are still poorly understood for organic semiconductors and material design remains largely empirical. In previous work, we identified the chemical interaction between semiconductor and dopant as key factor for their low doping efficiency, as, rather than ion pairs, charge transfer complexes tend to form and take over the role of the dopants, however, of significantly reduced strength.In the present multidisciplinary research program, we aim to, first, exploit this novel understanding for regaining dopant strength by modifying the mechanism of intermolecular coupling. We will employ semiconductors capable of interacting with more than one dopant, which will make effective acceptor levels energetically more accessible. Alternatively, as charge-transfer complex formation is due to the coupling, we will inhibit the coupling by employing species of sterically shielded functional cores. Second, as we found that complex formation only occurs for molecular semiconductors, while conjugated polymers indeed form ion pairs upon doping, we aim to derive a unifying picture valid for both, from which their different doping behavior emerges naturally. To this end, we will increase the number of repeat units from the oligo- to the polymer limit, thereby assess the role of interaction locality, and observe the transition between the seemingly incongruous doping phenomenologies. Third, we aim to augment molecular doping by chemical selectivity through using Lewis acids as p-dopants, where charge transfer is restricted to species capable of acting as Lewis base. Finally, we will go beyond traditional doping to gain spatial control over the charge density in buried layers. Instead of dopant admixture, tailored energy-level alignment will render layers p-/n-doped via energy level pinning with the substrate. Overall, the proposed program will establish a complete picture of doping in organic semiconductors. It will enable knowledge-based chemical design for controlling charge density just as precisely as we customarily do today with inorganic semiconductors, for ultimately approaching similar functionality.