Achieving Efficient n-Doping of Conjugated Polymers by Molecular Dopants

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Molecular doping is one of the most central propositions in the field of organic electronics. Unlike classical inorganic semiconductors doped by atomic substitution, organic conjugated materials react with molecular dopants, and then intermolecular charge transfer is involved within. Therefore, the complex noncovalent interactions between two components often cause the molecular dopant to destroy the orderly stacking of the host organic materials and reduce the original properties of the material, such as carrier mobility, which here we call the "doping dilemma." Recently, many studies focus on improving p-doping efficiency and electrical conductivity of doped conjugated polymers; however, the development of n-type molecular doping currently lags far behind that of its p-counterpart. It is well-known that both efficient p-and n-type molecular doping are indispensable in various organic electronic devices, including light-emitting diodes, photovoltaics, field-effect transistors, and thermoelectrics. It is thus an urgent requirement to achieve efficient n-doping in conjugated polymers.In this Account, we give a brief overview of our efforts to improve the n-doping efficiency in conjugated polymers with several strategies from the aspects of the polymer/dopant molecular design and the exploration of the ntype molecular doping mechanism and charge transport mechanism in n-doped organic materials. For the conjugated polymer engineering, we first demonstrate that increasing the electron affinity of the host polymer through halogen substitution can boost the n-doping efficiency. Still, the rigid coplanar backbones of conjugated polymers play a crucial role in the polaron delocalization and final electrical performance. In addition, we emphasize the importance of morphology control in the doped polymers to address the "doping dilemma." For n-dopants designing, we summarize some basic guidelines from molecular sizes and shapes, the interaction between dopants (or dopant cations) and polymers, and the effects of dopants on morphology to design high-efficacy n-type molecular dopants. We propose that the polymers and the dopants need to be treated as a whole system; while enhancing the ionization efficiency, more attention should be paid to the carrierization (free-carrier generation) efficiency of these binary systems.In the end, we adopt the n-type polymer thermoelectric material as an example to discuss the grand challenges encountered in practical applications of n-doped conjugated polymers. The air stability and micrometer-thick thermo-leg processing of n-doped polymers are highlighted for thermoelectric applications. It is our hope that this Account showcases a blueprint for rational approaches and a deep understanding toward the design and development of efficient n-doping in conjugated polymers, bringing ndoped organic materials into the next era.

Disclosing Cyclic(Alkyl)(Amino)Carbenes as One‐Electron Reductants: Synthesis of Acyclic(Amino)(Aryl)Carbene‐Based Kekulé Diradicaloids

“…Variable‐temperature NMR and EPR spectroscopy studies show that the acyclic(amino)(aryl)carbene‐based Chichibabin's hydrocarbon has an appreciable population of the triplet state at room temperature. The newly disclosed properties of CAACs might stimulate respective further exploration in new directions, such as single electron transfer processes [27] and use as n‐type dopant/organic reductant [28] …”

“…The newly disclosed properties of CAACs might stimulate respective further exploration in new directions, such as single electron transfer processes [27] and use as n-type dopant/organic reductant. [28]…”

Toward Efficient Charge Transport of Polymer-Based Organic Field-Effect Transistors: Molecular Design, Processing, and Functional Utilization

“…10 In addition, surface/interface modification and doping of organic semiconductors have been extensively used in OFETs fabrication to obtain efficient charge injection. 4,11,12 For their complex hierarchical structures, the mechanism of charge transport in semiconducting polymers is still not clearly understood, thus leading to the realization of efficient charge transport more complicated. 13 The intrachain charge transport is a prerequisite for total charge transport, and higher carrier mobilities can be observed in polymers with longer conjugated backbones (i.e., higher molecular weight).…”

“…For n-type or ambipolar performance, the lowest unoccupied molecular orbital (LUMO) energy level of the corresponding semiconducting polymer should be lowered or at least deeper than −3.6 eV for air stability . In addition, surface/interface modification and doping of organic semiconductors have been extensively used in OFETs fabrication to obtain efficient charge injection. ,, …”

Characteristic Control of n-Channel Organic Thin-Film Transistors Using a Dimethyl-Substituted Benzimidazole Dopant

“…Especially the creation of versatile electron-donating dopants with easiness to handle is desirable because the stronger donor dopants possess lower ionization energies and are usually comprised by highly air-sensitive substances. In recent years, air-stable n-type dopants have been newly synthesized; for example, organometallic dimers including rhodocene and (pentamethylcyclopentadienyl)­(arene)­ruthenium, , various derivatives of 4-( N , N -dimethylamino)­phenyl-substituted 1,3-dimethyl-2,3-dihydro-1 H -benzimidazole (N-DMBI-H, Figure a), − and triaminomethane derivatives. , These materials are air-stable precursors and become reactive only during the molecular doping process. Actually, these air-stable donor dopants worked effectively not only in OTFTs , but also in OLEDs, , solar cells, − and TEGs. ,− In particular, derivatives of N-DMBI-H are attractive as metal-free, organic dopants in terms of cost-effectiveness and sustainability.…”