Transition metal-catalysed molecular n-doping of organic semiconductors

Abstract

Chemical doping is a key process for investigating charge transport in organic semiconductors and improving certain (opto)electronic devices 1-9 . N-(electron)doping is fundamentally more challenging than p-(hole)doping and typically achieves very low doping efficiency (η) <10% 1,10 . An efficient molecular n-dopant should simultaneously exhibit a high reducing power and air stability for broad applicability 1,5,6,9,11 , which is very challenging. Here we show a general concept of catalysed n-doping of organic semiconductors using air-stable precursor-type molecular dopants. Incorporation of a transition metal as vapor-deposited nanoparticles (e.g. Pt, Au, Pd) or solution-processable 2 organometallic complexes (e.g. Pd 2 (dba) 3 ) catalyses the reaction, as assessed by experimental and theoretical evidence, enabling drastically increased η in a much shorter doping time and high electrical conductivities >100 S cm −1 12 . This methodology has technological implications for realizing improved semiconductor devices and offers a broad exploration space of ternary systems comprising catalysts, molecular dopants, and semiconductors, thus opening new opportunities in n-doping research and applications.N-doping of organic semiconductors is important for developing light-emitting diodes 1,6-9 , solar cells 7,8 , thin-film transistors 10 , and thermoelectric devices 12,13 . Although solution-based ndoping is widely investigated, only few air-stable n-dopants have been developed (Fig. S1), with the most prominent being organic hydrides 5,9,14-18 such as benzoimidazole derivatives, dimers of organic radicals 11,19,20 such as nineteen-electron organometallic sandwich compounds, and mono-/multi-valent anions 8,21,22 such as OH − , F − and Ox 2− . These air-stable dopants have a deep ionization potential (IP) in their initial forms, thus, cannot directly transfer electrons to n-dope organic semiconductors with a low electron affinity (EA). For anions, it was shown that dispersion into small anhydrous clusters enables sufficiently high donor levels for n-doping organic semiconductors with EAs up to 2.4 eV 8 . Hydride and dimer dopant precursors (or referred as precursor-type dopants) most undergo a C-H and C-C bond cleavage reaction, respectively, to generate active-doping-species in situ before electron transfer can occur [23][24][25][26] . Thus, their reducing strength and reaction kinetics are strongly affected by the thermodynamics and the activation energies of the doping reaction [23][24][25][26] . If the activation energy to the product is reduced, it is expected that the reaction rate, and extent of doping, will greatly increase (Fig. 1a). 3Transition metal (TM) catalysed C-H and C-C bond cleavage reactions are widely used in organic synthesis, with the most common TMs belonging to group 8-11 elements and the catalysts in the form of nanoparticles (NPs) and organometallic complexes 27,28 . Nanoparticle size, supporting material, and chemical structure of the complex can greatly affect catalytic activities. Thus, an i...

Catalyzing n-doping

“…N-doping induced by transition-metal-nanoparticles-based catalysts provides a very creative solution with more freedom in designing efficient n-precursors/dopants and more ease of handling. 3 However, only limited candidates of n-precursors/dopants are available for practical applications. In terms of fundamental research, it is essential to explore more versatile dopant and catalyst systems and to also quantify the catalytic activity and doping efficiency for screening a good pair of dopant and matrix.…”

“…unraveled an efficient n-doping strategy of organic semiconductors by introducing transition-metal nanoparticles deposited via thermal evaporation at a minimal thickness (e.g., 0.1–1.5 nm), which resulted in a dramatic improvement in the electrical conductivity ( Figure 1 B). 3 Superior to the commonly used organometallic complexes, e.g., Pd 2 (dba) 3 ( E , E -dibenzylidene acetone [dba]), the novel metal-catalyzed n-doping offers faster reactions at mild conditions, e.g., annealing at 120°C for 10 s. As a proof of concept, Au nanoparticles, down to 1.4 ± 0.3 nm in size obtained and assembled after thermal evaporation on glass substrate at a nominal thickness as small as 0.1 nm, can efficiently trigger electron transfer from n-precursor/dopant to n-type organic semiconductors.…”

“…Therefore, electron transfer from n-dopant to matrix is energetically favorable and spontaneous in the presence of Au nanoparticles via a drastic reduction of the activation barriers from precursors to in-situ -generated, active-doping species. 3 …”

“…For example, organic thermoelectric devices, organic thin-film transistors, and laterally conductive organic photodiodes possessing in-plane contacts/channels can be fabricated directly with the catalyzed n-doped layers based on the ternary system, reducing the ohmic loss and thus maximizing device performance. 3 For the vertical stack-based devices, e.g., organic/perovskite solar cells and organic light-emitting devices, unintentional catalysis induced by metal-contact evaporation provides another possible strategy to implement the transition-metal-nanoparticle-based catalyst, although it might depend on the diffusivity to trigger the n-doping.…”

Recent Developments of n‐Type Organic Thermoelectric Materials: Influence of Structure Modification on Molecule Arrangement and Solution Processing

“…Ref. dopants, [60] enabling greatly increased doping efficiency in a much shorter doping time and high electrical conductivity. The exploration of ternary systems comprising catalysts, molecular dopants, and organic semiconductors may open new opportunities for future n-type OTE materials.…”