One of the most effective ways to tune the electronic properties of conjugated polymers is to dope them with small-molecule oxidizing agents, creating holes on the polymer and molecular anions. Undesirably, strong electrostatic attraction from the anions of most dopants localize the holes created on the polymer, reducing their mobility. Here, we employ a new strategy utilizing a substituted boron cluster as a molecular dopant for conjugated polymers. By designing the cluster to have a high redox potential and steric protection of the corelocalized electron density, we obtain highly delocalized polarons with mobilities equivalent to films doped with no anions present. AC Hall effect measurements show that P3HT films doped with our boron clusters have conductivities and polaron mobilities roughly an order of magnitude higher than films doped with F 4 TCNQ, even though the boron-cluster-doped films have poor crystallinity. Moreover, the number of free carriers approximately matches the number of boron clusters, yielding a doping efficiency of ∼100%. These results suggest that shielding the polaron from the anion is a critically important aspect for producing high carrier mobility, and that the high polymer crystallinity required with dopants such as F 4 TCNQ is primarily to keep the counterions far from the polymer backbone.
Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)-based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox-potential-driven. Remarkably, X-ray scattering shows that despite their large 2-nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.
The polymer chain orientation and degree of crystallinity within a polymer:fullerene bulk heterojunction (BHJ) photovoltaic can greatly impact device performance. In general, a face-on chain orientation is preferred for charge conduction through sandwich-structure photovoltaic devices, but for many conjugated polymers, an edge-on conformation is energetically favored. In this work, we examine the effects of different processing techniques on photovoltaics based on the poly [4,8-bis(2ethylhexyloxy)]-phenyl-C 71 -butyric-acid-methylester (PC 71 BM) materials combination. We examine the extent of polymer crystallinity and crystalline domain orientation using both traditional blend-casting (BC), where the polymer and fullerene are cast from a single, codissolved solution, as well as sequential processing (SqP), where the polymer film is deposited first, and then the fullerene is infiltrated into the polymer film in a second solution processing step. We show using two-dimensional grazing-incidence wide-angle X-ray scattering (GIWAXS) that BC leads to a disordered, isotropic polymer network in the resulting BHJ film with a correspondingly poor device efficiency. By contrast, SqP preserves the preferred face-on chain orientation seen in pure polymer films, yielding higher short-circuit currents that are consistent with the increased hole mobility of face-on oriented polymer chains. We also study the effects of the widely used processing additive 1,8-diiodooctane (DIO) on polymer chain orientation and crystallinity in photovoltaic devices made by both processing techniques. We show that DIO results in increased polymer crystallinity, and in devices made by BC, DIO also causes a partial recovery of the face-on PBDTTT-C domain orientation, improving device performance. The face-on chain orientation in SqP devices produces efficiencies similar to those of optimized BC devices made with DIO but without the need for solvent additives or other postprocessing steps.
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