Designing Conjugated Polymers for Molecular Doping: The Roles of Crystallinity, Swelling, and Conductivity in Sequentially-Doped Selenophene-Based Copolymers

Abstract

Although chemical doping is widely used to tune the optical and electrical properties of semiconducting polymers, it is not clear how the degree of doping and the electrical properties of the doped materials vary with the bandgap, valence band level, and crystallinity of the polymer. We addressed these questions utilizing a series of statistical copolymers of poly­(3-hexylthiophene) (P3HT) and poly­(3-heptylselenophene) (P37S) with controlled gradients in bandgap, valence band position, and variable crystallinity. We doped the copolymers in our series with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) using solution sequential processing. We then examined the structures of the films using grazing incidence wide-angle X-ray scattering, differential scanning calorimetry, and ellipsometric porosimetry, and the electrical properties of the films via the AC Hall effect. We found that the ability of a particular copolymer to be doped is largely determined by the offset of the polymer’s valence band energy level relative to the LUMO of F4TCNQ. The ability of the carriers created by doping to be highly mobile and thus contribute to the electrical conductivity, however, is controlled by how well the polymer can incorporate the dopant into its crystalline structure, which is in turn influenced by how well it can be swelled by the solvent used for dopant incorporation. The interplay of these effects varies in a nonmonotonic way across our thiophene:selenophene copolymer series. The position and shape of the polaron absorption spectrum correlate well with the polymer crystallinity and carrier mobility, but the polaron absorption amplitude does not reflect the number of mobile carriers, precluding the use of optical spectroscopy to accurately estimate the mobile carrier concentration. Overall, we found that the degree of crystallinity of the doped films is what best correlates with conductivity, suggesting that only carriers in crystalline regions of the film, where the dopant counterions and polarons are forced apart by molecular packing constraints, produce highly mobile carriers. With this understanding, we are able to achieve conductivities in this class of materials exceeding 20 S/cm.

Synergistically Improved Molecular Doping and Carrier Mobility by Copolymerization of Donor–Acceptor and Donor–Donor Building Blocks for Thermoelectric Application

“…[ 17 ] Subsequently, a Hall effect measurement was employed to further elucidate the fundamental charge transport in doped polymers. [ 33,41 ] The mobilities and carrier concentrations of PDPP‐TT, PDPP‐g 3 2T 0.3 , and Pg 3 2T‐TT with varied dopant concentration are shown in Figure . For PDPP‐TT film (Figure 5a), its mobility decreases with the increase of carrier concentration, which is probably due to the dopant‐induced scattering [ 23 ] or the disruption of the nanostructure at high dopant concentration (the resistance is high over the instrument limit for dopant concentration below 10 × 10 −3 m ).…”

“…Random copolymers are conventionally treated as irregular polymers with structural disorder along the backbone. Nevertheless, the large space for the adjustment of energy levels [ 29,30 ] and electrical properties [ 31–33 ] endows them with potentially enhanced TE behaviors. Herein, we introduce planar diketopyrrolopyrrole (DPP) as acceptor unit, thienothiophene and oligo ethylene glycol functionalized bithiophene as donor units, to synthesize a series of new random copolymers (Figure 1, bottom layer) consisting of DPP‐TT (green shadow, abbreviated to DPP‐TT in following text) and g 3 2T‐TT building blocks (red shadow, abbreviated to g 3 2T‐TT).…”

Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers

“…In this paper, we focus on the nature of the charge carriers and their optical properties in a push–pull conjugated polymer oxidized with the commonly used molecular dopants FeCl 3 and F 4 TCNQ (2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane), [ 8,20–25 ] whose chemical structure is shown at the lower right of Figure 1. The push–pull copolymers we have chosen to study include (poly[(4‐(2‐hexyldecyl)‐4H‐dithieno[3,2‐b:2′,3′‐d′]pyrrole)‐2,6‐diyl‐ alt ‐(2,5‐bis(3‐dodecylthiophen‐2‐yl)benzo[1,2‐d;4,5‐[4]d′]bisthiazole)]), PBTDTP, whose molecular structure is also shown in Figure a, and poly({4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl}{3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl})), PTB7, whose molecular structure is given in the Supporting Information.…”

Impact of Morphology on Charge Carrier Transport and Thermoelectric Properties of N‐Type FBDOPV‐Based Polymers

“…[ 27 ] Nevertheless, in the case of amorphous p‐doped polymers, high values of doping efficiency (up to 97%) are not always associated to a high electrical conductivity, [ 28 ] meaning that the formation of electron‐hole pairs does not necessarily result in mobile charges. [ 28,29 ] Controlling the organization of polymer chains is, therefore, paramount to find the trade‐off between doping efficiency and charge carrier transport to ultimately achieve high thermoelectric performances.…”

“…We show that doping of P(FBDOPV‐F) with N‐DMBI is highly efficient due to its amorphous nature but lead to a very low electrical conductivity (10 –6 –10 −7 S cm –1 range). On the contrary, despite a lower doping efficiency, N‐DMBI‐doped P(FBDOPV‐2T‐C 12 ) films demonstrated a much higher σ of 4.2 × 10 −2 S cm −1 , superior to P(NDI2OD‐T2), [ 27–32 ] a reference material in the field. A decent PF of 0.30 µW m −1 K −2 was attained for this polymer produced from a simplified synthesis route compared to the “champion” BDOPV‐based polymers.…”