Enhanced N‐Type Doping of a Naphthalene Diimide Based Copolymer by Modification of the Donor Unit

Abstract

Doped conjugated organic semiconductors are suitable materials to be used as building blocks of flexible and cost‐effective thermoelectric generators. While several efficient solution‐processable p‐type organic thermoelectric materials are reported, n‐doped materials are fewer because of lack of good electron‐transporting materials and stable n‐dopants. Here, n‐doping process is investigated on a relevant n‐type class, namely naphthalene‐diimide‐based copolymers. Among these, copolymer incorporating bithiophene (T2) donor units, largely studied poly{N,N″‐bis(2‐octyl‐dodecyl)‐1,4,5,8‐naphthalenedicarboximide‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (PNDI‐T2), is one of first polymers reported to achieve good solution‐based n‐doping. By substituting T2 with thienylenevinylene‐thienylene (TVT), resulting PNDI‐TVT copolymer exhibits improvements on both structural and transport properties, offering a suitable basis to improve thermoelectric properties upon doping. When doped with 1H‐benzimidazoles, PNDI‐TVT achieves maximum in‐plane electrical conductivity at room temperature of 2.4 × 10−2 S cm−1, being the highest value for PNDI‐T2 solution‐doped derivatives excluding those with oligoethylene‐glycol chains. Electron paramagnetic resonance and variable temperature electrical conductivity measurements relate this enhancement to more efficient charge‐transfer between n‐dopant molecules and host polymeric matrix, and easier charge carrier transport within the system. This electrical conductivity large enhancement also improves in‐plane power factor of almost three times with respect to similar doped PNDI‐T2 films.

Doping Approaches for Organic Semiconductors

“…Naturally, polymer backbone design is primarily associated with changes to the electronic properties of a material which fundamentally influence its ability to be doped. For example, the use of comonomers, which widen the gap between donor and host frontier molecular orbitals, tends to result in higher doping efficiency. − Another important consideration when aiming to improve conductivity of the doped species is backbone planarization. This can be achieved by opting for comonomers that result in a more planar configuration or through the use of backbone linkers with low steric hindrance, e.g., through the coupling of NDI with bithiazole (Tz2) instead of bithiophene (T2). − Although generally attributed to inherent properties of the materials, such as morphology, some studies also highlight the important role of polaron delocalization in the doped species toward high conductivity. ,, Interestingly, many of these materials display higher conductivities in their doped states despite lower mobilities of the pristine materials. ,, …”

Donor Functionalization Tuning the N‐Type Performance of Donor–Acceptor Copolymers for Aqueous‐Based Electrochemical Devices

“…[36][37][38] Furthermore, by fine-tuning the donor units, the polymers' packing orientation, electron mobility and film crystallinity can be optimized to give high performance of electrochemical devices. [39][40][41] However, little attention has been paid to the effect of electron-donating strength of donor units on the n-type D-A copolymers in aqueous media.…”

Strategic Insights into Semiconducting Polymer Thermoelectrics by Leveraging Molecular Structures and Chemical Doping

“…p-Type: D−π, − ,,,,,,,,− ,− A−π, − and D–A − ,,− structures. n-Type: A–A, ,,,,,,, A−π, − ,,,,− ,,− and D–A ,,,,,,,, structures.…”