Controlling and Tracking Dopant Distributions and Morphologies in Molecularly-Doped n-Type Conjugated Polymers via Pendant Group Modification

The structural diversity of n-type conjugated polymers is somewhat lacking compared to their p-type equivalents, which have benefited considerably from nearly three decades of research in polymer-fullerene photovoltaic cells. However, n-type materials are finding applications in their doped form, which presents different synthetic design challenges than the aforementioned p-type semiconductor paradigm. In this contribution I will present some of our recent work on the versatile naphthalenediimide-bithiophene (PNDI2) framework and its performance as a thermoelectric material.<br/>Charged dopants will phase-separate from conjugated polymers with non-polar pendant groups, hence the now-common use of polar (typically ether-containing) pendant groups. Using the PNDI2 backbone doped with n-DMBI, we investigated three different pendant groups: 1) branched alkyl (non-polar); glycol ether (polar); and alkyl-glycol-ether (non-polar and polar). We varied the regiochemistry of these chains such that the naphthalenediimide and bithiophene units had (mis)matching pairs of the various combinations of polar, non-polar and mixed pendant groups.<br/>Counter-intuitively, the PNDI2 derivative with all polar pendant groups did not give the best performance. Rather, two polymers stood out. The highest doping efficiency (40 fold compared to the benchmark) correlated to both the regiochemistry and identity of the groups, while the highest power factor (5 fold compared to the benchmark) correlated to pendant groups with alkyl spacers between the backbone and the glycol ether functionality.<br/>We ascribe these observations to the ability of the pendant groups to direct the charged dopant molecules, which is supported by spatially resolved absorbance spectroscopy. A central problem with doped, n-type polymers is that the polarons tend to localize more easily than in their p-type counterparts. This localization is exacerbated by coulomb interactions with the ionized dopant. Thus, pendant groups that place a polar phase far from the backbone and, importantly, farther from the acceptor units than the donor units in the backbone achieve morphologies that mitigate phase separation while achieving high charge-carrier mobilities by avoiding the localization of carriers via nearby dopants. The use of spatially resolved absorbance spectroscopy allowed us to correlate topographical features from AFM to the intensity of absorption bands associated with doped polymer, linking morphological and electronic features to figures of merit.<br/><br/><i>Macromolecules</i> <b>2021</b>, <i>54</i>, 3886-3896<br/><i>Adv. Mater.</i> <b>2021</b>, <i>33</i>, 2006694