Novel n-Doping and Characterization of Pbttt-C14 for Applications in Organic Diodes

The use of organic semiconductors (OSCs), i.e., conductive polymers, has risen greatly over the past few decades, finding applications in organic light-emitting diodes (OLEDs), solar cells, and wearable technologies. Developing OSCs requires effective n-type and p-type doping to enhance their electrical conductivity, but it is challenging to develop n-doped polymers that reach the same conductivity of p-doped OSCs. Given that many electronic devices such as thermoelectrics and OLEDs require both n- and p-doped components, this stark difference raises the need for further research into better-performing n-doped polymers to allow for further expansion of OSC use.

This work contributes to the pool of available n-doped polymers by demonstrating the feasibility of a novel combination of polymer and n-type dopant, poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno[3,2-b]thiophene] (PBTTT-C14) and (pentamethyl cyclopentadienyl) (1,3,5-trimethylbenzene) ruthenium dimer (RuCp*Mes)2, respectively. PBTTT-C14 is commonly p-doped due to its high morphological order, which is advantageous for electron mobility in OSCs, and (RuCp*Mes)2 is a powerful n-dopant that has previously been used in the Kahn Lab. We hypothesized that PBTTT-C14’s relatively high electron affinity would allow for this n-doping reaction.

First, UV-Vis spectroscopy was used to identify the formation of polarons, which are characteristic of electron localizations, in the n-doped PBTTT-C14 to confirm the doping reaction. Next, this work characterized how the conductivity of this new material changes as a function of different preparation and measurement conditions. Ultra-high vacuum current-voltage measurements revealed positive correlations between conductivity and dopant concentration, temperature, and UV light exposure, reaching a maximum value of 1.05 x 10-4 S/m. In general, the conductivity can be precisely tuned and heightened through all of these conditions.

At the same time, this work revealed that conventional spin coating techniques prove ineffective for n-doped PBTTT-C14, and alternative film preparation methods create a more disordered polymer morphology. Additionally, evaporation doping, which previous literature showed to be effective for raising conductivity in OSCs, did not show any improvement over mixed solution doping. Despite these concerns, n-doped PBTTT-C14 still reaches within an order of magnitude to the conductivity of p-doped PBTTT-C14 prepared in a comparable manner in literature. Therefore, this comparison suggests that, if developed, synthesis methods which produce more ordered morphology can yield highly-conductive n-doped PBTTT-C14. Paired with current p-doped OSCs in literature and industry, this new material could be deployed in a wide variety of fully organic diode devices.