Mechanistic study on the solution-phase n-doping of 1,3-dimethyl-2-aryl-2,3-dihydro-1H-benzoimidazole derivatives

n-Type Doping Effect of Anthracene-Based Cationic Dyes in Organic Electronics

n-Type doping for improving the electrical characteristics and air stability of n-type organic semiconductors (OSCs) is important for realizing advanced future electronics. Herein, we report a selection method for an effective n-type dopant with an optimized structure and thickness based on anthracene cationic dyes with high miscibility induced by a molecular structure similar to that of OSCs. Among the doped OSCs evaluated, rhodamine B (RhoB)-doped OSC exhibits the highest density, a smallest roughness of 2.69 nm, a phase deviation of 0.85° according to atomic force microscopy measurements, and the highest electron mobility (μ), showing its high miscibility. Surface doping of RhoB affords the lowest contact resistance of 2.01 × 105 Ω cm compared to bulk and contact doping, resulting in an effective doping structure. The RhoB-doped OSC retains 81.63% of the original μ value of 6.13 × 10-2 cm2 V-1 s-1 after 15 days, whereas pristine OSC shows a lower μ of 2.33 × 10-2 cm2 V-1 s-1 and maintains only 4.41% of the original value after 15 days. Our findings demonstrate that this methodology is effective for the selection of a high-performance n-type dopant for OSCs toward the development of high-performance and air-stable n-type organic electronics.

On-Demand Catalysed n-Doping of Organic Semiconductors

A new approach to control the n-doping reaction of organic semiconductors is reported using surface-functionalized gold nanoparticles (f-AuNPs) with alkylthiols acting as the catalyst only upon mild thermal activation. To demonstrate the versatility of this methodology, the reaction of the n-type dopant precursor N-DMBI-H with several molecular and polymeric semiconductors at different temperatures with/without f-AuNPs, vis-à-vis the unfunctionalized catalyst AuNPs, was investigated by spectroscopic, morphological, charge transport, and kinetic measurements as well as, computationally, the thermodynamic of catalyst activation. The combined experimental and theoretical data demonstrate that while f-AuNPs is inactive at room temperature both in solution and in the solid state, catalyst activation occurs rapidly at mild temperatures (~70 °C) and the doping reaction completes in few seconds affording large electrical conductivities (~10-140 S cm-1). The implementation of this methodology enables the use of semiconductor+dopant+catalyst solutions and will broaden the use of the corresponding n-doped films in opto-electronic devices such as thin-film transistors, electrochemical transistors, solar cells, and thermoelectrics well as guide the design of new catalysts.

N-DMBI Doping of Carbon Nanotube Yarns for Achieving High n-Type Thermoelectric Power Factor and Figure of Merit

The application of carbon nanotube (CNT) yarns as thermoelectric materials for harvesting energy from low-grade waste heat including that generated by the human body, is attracting considerable attention. However, the lack of efficient n-type CNT yarns hinders their practical implementation in thermoelectric devices. This study reports efficient n-doping of CNT yarns, employing 4-(1, 3-dimethyl-2, 3-dihydro-1H-benzimidazole-2-yl) phenyl) dimethylamine (N-DMBI) in alternative to conventional n-dopants, with o-dichlorobenzene emerging as the optimal solvent. The small molecular size of N-DMBI enables highly efficient doping within a remarkably short duration (10 s) while ensuring prolonged stability in air and at high temperature (150 °C). Furthermore, Joule annealing of the yarns significantly improves the n-doping efficiency. Consequently, thermoelectric power factors (PFs) of 2800, 2390, and 1534 µW m-1 K-2 are achieved at 200, 150, and 30 °C, respectively. The intercalation of N-DMBI molecules significantly suppresses the thermal conductivity, resulting in the high figure of merit (ZT) of 1.69×10-2 at 100 °C. Additionally, a π-type thermoelectric module is successfully demonstrated incorporating both p- and n-doped CNT yarns. This study offers an efficient doping strategy for achieving CNT yarns with high thermoelectric performance, contributing to the realization of lightweight and mechanically flexible CNT-based thermoelectric devices.

Sub-bandgap Photocurrent Spectra of p-i-n Perovskite Solar Cells with n-Doped Fullerene Electron Transport Layers and Bias Illumination

In p-i-n perovskite solar cells optical excitation of defect states at the interface between the perovskite and fullerene electron transport layer (ETL) creates a photocurrent responsible for a distinct sub-bandgap external quantum efficiency (EQE). The precise nature of these signals and their impact on cell performance are largely unknown. Here, the effect of n-doping the fullerene on the EQE spectra is studied. The n-doped fullerene is either deposited from solution or by coevaporation. The latter method is used to create undoped-doped fullerene bilayers and investigate the effect of the proximity of the doped region on the EQE spectra. The intensity of the sub-bandgap EQE increases when the ETL is n-doped and also when the device is biased with green light. Using these results, the sub-bandgap EQE signal is attributed to originate from electron trap states in the perovskite with an energy below the conduction band that are filled by excitation with low-energy photons. The trapped electrons give rise to photocurrent when they are collected at a nearby electrode. The enhanced sub-bandgap EQE observed when the ETL is n-doped or bias light is applied, is related to a higher probability to extract trapped electrons under these conditions.

An n-Type Open-Shell Conjugated Polymer with High-Spin Ground-State and High Intrinsic Electrical Conductivity

Open-shell conjugated polymers with a high intrinsic conductivity and high-spin ground state hold considerable promise for applications in organic electronics and spintronics. Herein, two novel acceptor-acceptor (A-A) conjugated polymers based on a highly electron-deficient quinoidal benzodifurandione unit have been developed, namely DPP-BFDO-Th and DPP-BFDO. The incorporation of the quinoidal moiety into the polymers backbones enables deeply aligned lower-lying lowest unoccupied molecular orbital (LUMO) levels of below -4.0 eV. Notably, DPP-BFDO exhibits an exceptionally low LUMO (-4.63 eV) and a high-spin ground state characterized by strong diradical characters. Moreover, a self-doping through intermolecular charge-transfer is observed for DPP-BFDO, as evidenced by X-ray photoelectron spectroscopy (XPS) studies. The high carrier concentration in combination with a planar and linear conjugated backbone yields a remarkable electrical conductivity (σ) of 1.04 S cm-1 in the "undoped" native form, ranking among the highest values reported for n-type radical-based conjugated polymers. When employed as an n-type thermoelectric material, DPP-BFDO achieves a power factor of 12.59 μW m-1 K-2. Furthermore, upon n-doping, the σ could be improved to 65.68 S cm-1. This study underscores the great potential of electron-deficient quinoidal units in constructing dopant-free n-type conductive polymers with a high-spin ground state and exceptional intrinsic conductivity.

Reductive Doping Inhibits the Formation of Isomerization-Derived Structural Defects in N-doped Poly(benzodifurandione) (n-PBDF)

Recently, solution-processable n-doped poly(benzodifurandione) (n-PBDF) has been made through in-situ oxidative polymerization and reductive doping, which exhibited exceptionally high electrical conductivities and optical transparency. The discovery of n-PBDF is considered a breakthrough in the field of organic semiconductors. In the initial report, the possibility of structural defect formation in n-PBDF was proposed, based on the observation of structural isomerization from (E)-2H,2'H-[3,3'-bibenzofuranylidene]-2,2'-dione (isoxindigo) to chromeno[4,3-c]chromene-5,11-dione (dibenzonaphthyrone) in the dimer model reactions. In this study, we present clear evidence that structural isomerization is inhibited during polymerization. We reveal that the dimer (BFD1) and the trimer (BFD2) can be reductively doped by several mechanisms, including hydride transfer, forming charge transfer complexes (CTC) or undergoing an integer charge transfer (ICT) with reactants available during polymerization. Once the hydride transfer adducts, the CTC, or the ICT product forms, structural isomerization can be effectively prevented even at elevated temperatures. Our findings provide a mechanistic understanding of why isomerization-derived structural defects are absent in n-PBDF backbone. It lays a solid foundation for the future development of n-PBDF as a benchmark polymer for organic electronics and beyond.

A Novel N-Type Molecular Dopant With a Closed-Shell Electronic Structure Applicable to the Vacuum-Deposition Process

Rational design, synthesis, and characterization of a new efficient versatile n-type dopant with a closed-shell electronic structure are described. By employing the tetraphenyl-dipyranylidene (DP0) framework with two 7π-electron systems modified with N,N-dimethylamino groups as the strong electron-donating substituent, 2,2',6,6'-tetrakis[4-(dimethylamino)phenyl]-4,4'-dipyranylidene (DP7), a closed-shell molecule with an extremely high-lying energy level of the highest occupied molecular orbital, close to 4.0 eV below the vacuum level, is successfully developed. Thanks to its thermal stability, DP7 is applicable to vacuum deposition, which allows utilization of DP7 in bulk doping for the development of n-type organic thermoelectric materials and contact doping for reducing contact resistance in n-type organic field-effect transistors. As vacuum-deposition processable n-type dopants are very limited, DP7 stands out as a useful n-type dopant, particularly for the latter purpose.

Study of the electron-doping mechanism in single-walled carbon nanotubes using dimethylbenzimidazole

Single-walled carbon nanotubes (SWCNTs) exhibit p-type properties in air, necessitating electron doping using n-dopants (e.g., reducing agents) for the development of SWCNT-based electronic devices. Dimethylbenzimidazole (DMBI-H) derivatives serve as effective electron dopants, not only for SWCNTs, but also for various organic semiconducting materials. However, the doping reaction is still a subject of debate. In this study, the electron-doping reactions of ortho-methoxy-substituted DMBI-H for SWCNTs were analyzed in protic and aprotic solvents in the presence and absence of dioxygen (O2). The presence of O2 was found to cause the reduction of O2 on the SWCNT surface in the protic solvent, resulting in the production of DMBI cations and water through proton-coupled electron transfer (PCET) from the n-doped SWCNT and ethanol. This work elucidates the mechanism behind the air-stability of n-type SWCNTs.

Molecular Doping Modulation and Applications of Structure-Sorted Single-Walled Carbon Nanotubes: A Review

Single-walled carbon nanotubes (SWCNTs) that have a reproducible distribution of chiralities or single chirality are among the most competitive materials for realizing post-silicon electronics. Molecular doping, with its non-destructive and fine-tunable characteristics, is emerging as the primary doping approach for the structure-controlled SWCNTs, enabling their eventual use in various functional devices. This review provides an overview of important advances in the area of molecular doping of structure-controlled SWCNTs and their applications. The first part introduces the underlying physical process of molecular doping, followed by a comprehensive survey of the commonly used dopants for SWCNTs to date. Then, it highlights how the convergence of molecular doping and structure-sorting strategies leads to significantly improved functionality of SWCNT-based field-effect transistor arrays, transparent electrodes in optoelectronics, thermoelectrics, and many emerging devices. At last, several challenges and opportunities in this field are discussed, with the hope of shedding light on promoting the practical application of SWCNTs in future electronics.

Benzoimidazolium-derived dimeric and hydride n-dopants for organic electron-transport materials: impact of substitution on structures, electrochemistry, and reactivity

1,3-Dimethyl-2,3-dihydrobenzo[d]imidazoles, 1H, and 1,1',3,3'-tetramethyl-2,2',3,3'-tetrahydro-2,2'-bibenzo[d]imidazoles, 12, are of interest as n-dopants for organic electron-transport materials. Salts of 2-(4-(dimethylamino)phenyl)-4,7-dimethoxy-, 2-cyclohexyl-4,7-dimethoxy-, and 2-(5-(dimethylamino)thiophen-2-yl)benzo[d]imidazolium (1g-i+, respectively) have been synthesized and reduced with NaBH4 to 1gH, 1hH, and 1iH, and with Na:Hg to 1g2 and 1h2. Their electrochemistry and reactivity were compared to those derived from 2-(4-(dimethylamino)phenyl)- (1b+) and 2-cyclohexylbenzo[d]imidazolium (1e+) salts. E(1+/1•) values for 2-aryl species are less reducing than for 2-alkyl analogues, i.e., the radicals are stabilized more by aryl groups than the cations, while 4,7-dimethoxy substitution leads to more reducing E(1+/1•) values, as well as cathodic shifts in E(12•+/12) and E(1H•+/1H) values. Both the use of 3,4-dimethoxy and 2-aryl substituents accelerates the reaction of the 1H species with PC61BM. Because 2-aryl groups stabilize radicals, 1b2 and 1g2 exhibit weaker bonds than 1e2 and 1h2 and thus react with 6,13-bis(triisopropylsilylethynyl)pentacene (VII) via a "cleavage-first" pathway, while 1e2 and 1h2 react only via "electron-transfer-first". 1h2 exhibits the most cathodic E(12•+/12) value of the dimers considered here and, therefore, reacts more rapidly than any of the other dimers with VII via "electron-transfer-first". Crystal structures show rather long central C-C bonds for 1b2 (1.5899(11) and 1.6194(8) Å) and 1h2 (1.6299(13) Å).

Optimized Doping of Diffusion Blocking Layers and Their Impact on the Performance of Perovskite Photovoltaics

The roll-to-roll printing production process for hybrid organic-inorganic perovskite solar cells (PSCs) demands thick and high-performance solution-based diffusion blocking layers. Inverted (p-i-n) PSCs usually incorporate solution-processed PC70BM as the electron-transporting layer (ETL), which offers good electron charge extraction and passivation of the perovskite active layer grain boundaries. Thick fullerene diffusion blocking layers could benefit the long-term lifetime performance of inverted PSCs. However, the low conductivity of PC70BM significantly limits the thickness of the PC70BM buffer layer for optimized PSC performance. In this work, we show that by applying just enough N-DMBI doping principle, we can maintain the power conversion efficiency (PCE) of inverted PSCs with a thick (200 nm) PC70BM diffusion blocking layer. To better understand the origin of an optimal doping level, we combined the experimental results with simulations adapted to the PSCs reported here. Importantly, just enough 0.3% wt N-DMBI-doped 200 nm PC70BM diffusion blocking layer-based inverted PCSs retain a high thermal stability at 60 °C of up to 1000 h without sacrificing their PCE photovoltaic parameters.

High-Performance n-Type Organic Thermoelectrics Enabled by Synergistically Achieving High Electron Mobility and Doping Efficiency

n-Doped polymers with high electrical conductivity (σ) are still very scarce in organic thermoelectrics (OTEs), which limits the development of efficient organic thermoelectric generators. A series of fused bithiophene imide dimer-based polymers, PO8, PO12, and PO16, incorporating distinct oligo(ethylene glycol) side-chain to optimize σ is reported here. Three polymers show a monotonic electron mobility decrease as side-chain size increasing due to the gradually lowered film crystallinity and change of backbone orientation. Interestingly, polymer PO12 with a moderate side-chain size delivers a champion σ up to 92.0 S cm-1 and a power factor (PF) as high as 94.3 µW m-1 K-2 in the series when applied in OTE devices. The PF value is among the highest ones for the solution-processing n-doped polymers. In-depth morphology studies unravel that the moderate crystallinity and the formation of 3D conduction channel derived from bimodal orientation synergistically contribute to high doping efficiency and large charge carrier mobility, thus resulting in high performance for the PO12-based OTEs. The results demonstrate the great power of simple tuning of side chain in developing n-type polymers with substantial σ for improving organic thermoelectric performance.

Efficient and air-stable n-type doping in organic semiconductors

Chemical doping of organic semiconductors (OSCs) enables feasible tuning of carrier concentration, charge mobility, and energy levels, which is critical for the applications of OSCs in organic electronic devices. However, in comparison with p-type doping, n-type doping has lagged far behind. The achievement of efficient and air-stable n-type doping in OSCs would help to significantly improve electron transport and device performance, and endow new functionalities, which are, therefore, gaining increasing attention currently. In this review, the issue of doping efficiency and doping air stability in n-type doped OSCs was carefully addressed. We first clarified the main factors that influenced chemical doping efficiency in n-type OSCs and then explain the origin of instability in n-type doped films under ambient conditions. Doping microstructure, charge transfer, and dissociation efficiency were found to determine the overall doping efficiency, which could be precisely tuned by molecular design and post treatments. To further enhance the air stability of n-doped OSCs, design strategies such as tuning the lowest unoccupied molecular orbital (LUMO) energy level, charge delocalization, intermolecular stacking, in situ n-doping, and self-encapsulations are discussed. Moreover, the applications of n-type doping in advanced organic electronics, such as solar cells, light-emitting diodes, field-effect transistors, and thermoelectrics are being introduced. Finally, an outlook is provided on novel doping ways and material systems that are aimed at stable and efficient n-type doped OSCs.

High n-type and p-type conductivities and power factors achieved in a single conjugated polymer

The charge transport properties of conjugated polymers are commonly limited by the energetic disorder. Recently, several amorphous conjugated polymers with planar backbone conformations and low energetic disorder have been investigated for applications in field-effect transistors and thermoelectrics. However, there is a lack of strategy to finely tune the interchain π-π contacts of these polymers that severely restricts the energetic disorder of interchain charge transport. Here, we demonstrate that it is feasible to achieve excellent conductivity and thermoelectric performance in polymers based on thiophene-fused benzodifurandione oligo(p-phenylenevinylene) through reducing the crystallization rate of side chains and, in this way, carefully controlling the degree of interchain π-π contacts. N-type (p-type) conductivities of more than 100 S cm^-1 (400 S cm^-1) and power factors of more than 200 μW m^-1 K^-2 (100 μW m^-1 K^-2) were achieved within a single polymer doped by different dopants. It further demonstrated the state-of-the-art power output of the first flexible single-polymer thermoelectric generator.

Crystalline Anions Based on Classical N-Heterocyclic Carbenes

Herein, the first stable anions K[SIPr^Bp ] (4 a-K) and K[IPr^Bp ] (4 b-K) (SIPr^Bp =BpC{N(Dipp)CH₂ }₂ , IPr^Bp =BpC{N(Dipp)CH}₂ ; Bp=4-PhC₆ H₄ ; Dipp=2,6-iPr₂ C₆ H₃ ) derived from classical N-heterocyclic carbenes (NHCs) (i.e. SIPr and IPr) have been isolated as violet crystalline solids. 4 a-K and 4 b-K are prepared by KC₈ reduction of the neutral radicals [SIPr^Bp ] (3 a) and [IPr^Bp ] (3 b), respectively. The radicals 3 a and 3 b as well as [Me-IPr^Bp ] 3 c (Me-IPr^Bp =BpC{N(Dipp)CMe}₂ ) are accessible as crystalline solids on treatment of the respective 1,3-imidazoli(ni)um bromides (SIPr^Bp )Br (2 a), (IPr^Bp )Br (2 b), and (Me-IPr^Bp )Br (2 c) with KC₈ . The cyclic voltammograms of 2 a-2 c exhibit two one-electron reversible redox processes in -0.5 to -2.5 V region that correspond to the radicals 3 a-3 c and the anions (4 a-4 c)^- . Computational calculations suggest a closed-shell singlet ground state for (4 a-4 c)^- with the singlet-triplet energy gap of 17-24 kcal mol^-1 .

Enhancement Effect of the C60 Derivative on the Thermoelectric Properties of n-Type Single-Walled Carbon Nanotube-Based Films

Obtaining air-stable and high-performance flexible n-type single-walled carbon nanotube (SWCNT)-based thermoelectric films used in wearable electronic devices is a challenge. In this work, the microstructure and thermoelectric properties of n-type SWCNT-based films have been optimized via doping C₆₀ and its derivative into polyethylenimine/single-walled carbon nanotube (PEI/SWCNT) films. The result demonstrated that the dispersity of triethylene glycol-modified C₆₀ (TEG-C₆₀) was better in PEI/SWCNT films than that of pure C₆₀. Among the prepared composite films, TEG-C₆₀-doped PEI/SWCNT (TEG-C₆₀/PEI/SWCNT) films exhibited the highest TE performance, achieving a peak electrical conductivity of 923 S cm^-1 with a Seebeck coefficient of -42 μV K^-1 at a TEG-C₆₀/SWCNT mass ratio of 1:100. Compared to that of PEI/SWCNT, the power factor was increased significantly from 40 to 162 μW m^-1 K^-2 after the addition of TEG-C₆₀, which was higher than that of films after the addition of C₆₀. In addition, the n-type doped SWCNT films had good air stability at high temperatures, and the Seebeck coefficients of C₆₀/PEI/SWCNT and TEG-C₆₀/PEI/SWCNT at 120 °C were still negative and remained at 92% and 85%, respectively, after 20 days. Furthermore, a flexible TE device consisting of five pairs of p-n junctions was assembled using the optimum hybrid film, which generated a maximum output power of 3.6 μW at a temperature gradient of 50.2 K. Therefore, this study provides a facile way to enhance the thermoelectric properties of n-type carbon nanotube-based materials, which have potential application in flexible power generators.

Thermal Insolubilization of Electrically n-Doped Films Achieved Using 7-Alkoxy-Benzocyclobutene-Substituted Fullerene and Dopant Molecules

Insoluble electrically n-doped fullerene-containing films have been obtained by thermal annealing of a fullerene compound and a 1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazole n-dopant moiety, both of which are functionalized with a 7-butoxybenzocyclobutene group. The covalent tethering and electrical doping reactions are studied by mass spectrometry as well as electron paramagnetic resonance. Optical absorption spectra on BBCB-N-DMBI-H-doped BBCBP indicate films heated at 150 °C for 10 min are unaffected by immersion for 10 min in ortho-dichlorobenzene. Although films containing a 10 mol % loading of the dopant showed electrical conductivity values of 1.1 × 10^-5 ± 3.4 × 10^-7 S cm^-1 prior to heating, the thermal insolubilization process led to values around two orders-of-magnitude lower. However, the thermal insolubilization also leads to immobilization of the dopant molecule and the corresponding cation, reducing their ability to diffuse into an adjacent layer of a stronger electron acceptor.

Impact of benzimidazole functional groups on the n-doping properties of benzimidazole derivatives

n-Dopants play a crucial role in improving organic electronic devices through controlled doping of organic semiconductors. Benzimidazoline-based dopants have been reported as one of the best solution-processed n-type dopant precursors. In this study, two benzimidazoline-based dopants (BIBDTO and BBIBDTO) were prepared using benzo[1,2-b:4,5-b′]dithiophene as the 2-Ar unit, and their n-doping properties on the fullerene derivative PTEG-2 as the host material were carried out. For BIBDTO and BBIBDTO, respectively, the temperature at which 5% weight loss was achieved was 229 and 265°C. By comparing the ultraviolet-visible absorption spectroscopy, cyclic voltammetry, and density functional theory calculated data, it is found that BBIBDTO has a higher energy level, which is more favorable for charge transfer. Additionally, both the oxidative titration experiments and conductivity characterization of the dopants showed that BBIBDTO was more advantageous at low doping concentrations, and the BBIBDTO-doped PTEG-2 films obtained a conductivity of 0.15 S cm−1 at 10 mol% doping concentration. However, at high dopant concentrations, the dopant volume increases, potentially disrupting the microstructure. The highest conductivity of 0.29 S cm–1 was obtained at a BIBDTO doping concentration of 15 mol%. This study delves into the effect of benzimidazole functional groups on the doping performance of benzimidazoline-based dopant molecules, providing insight into designing novel efficient n-type dopant molecules and further selecting the type of dopant for various doping systems.

Use of a Multiple Hydride Donor To Achieve an n-Doped Polymer with High Solvent Resistance

The ability to insolubilize doped semiconducting polymer layers can help enable the fabrication of efficient multilayer solution-processed electronic and optoelectronic devices. Here, we present a promising approach to simultaneously n-dope and largely insolubilize conjugated polymer films using tetrakis[{4-(1,3-dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)phenoxy}methyl]methane (tetrakis-O-DMBI-H), which consists of four 2,3-dihydro-1H-benzoimidazole (DMBI-H) n-dopant moieties covalently linked to one another. Doping a thiophene-fused benzodifurandione-based oligo(p-phenylenevinylene)-co-thiophene polymer (TBDOPV-T) with tetrakis-O-DMBI-H results in a highly n-doped film with bulk conductivity of 15 S cm^-1. Optical absorption spectra provide evidence for film retention of ∼93% after immersion in o-dichlorobenzene for 5 min. The optical absorption signature of the charge carriers in the n-doped polymer decreases only slightly more than that of the neutral polymer under these conditions, indicating that the exposure to solvent also results in negligible dedoping of the film. Moreover, thermal treatment studies on a tetrakis-O-DMBI-H-doped TBDOPV-T film in contact with another undoped polymer film indicate immobilization of the molecular dopant in TBDOPV-T. This is attributed to the multiple electrostatic interactions between each dopant tetracation and up to four nearby anionic doped polymer segments.

Cobalt sandwich-stabilized rhodium nanocatalysts for ammonia borane and tetrahydroxydiboron hydrolysis

A bulky organocobalt sandwich-supported Rh nanoparticle is an efficient, stable and recyclable nanocatalyst for hydrolysis of both ammonia borane and tetrahydroxydiboron to H2.

3,4,5-Trimethoxy Substitution on an N-DMBI Dopant with New N-Type Polymers: Polymer-Dopant Matching for Improved Conductivity-Seebeck Coefficient Relationship

Achieving high electrical conductivity and thermoelectric power factor simultaneously for n-type organic thermoelectrics is still challenging. By constructing two new acceptor-acceptor n-type conjugated polymers with different backbones and introducing the 3,4,5-trimethoxyphenyl group to form the new n-type dopant 1,3-dimethyl-2-(3,4,5-trimethoxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (TP-DMBI), high electrical conductivity of 11 S cm^-1 and power factor of 32 μW m^-1  K^-2 are achieved. Calculations using Density Functional Theory show that TP-DMBI presents a higher singly occupied molecular orbital (SOMO) energy level of -1.94 eV than that of the common dopant 4-(1, 3-dimethyl-2, 3-dihydro-1H-benzoimidazol-2-yl) phenyl) dimethylamine (N-DMBI) (-2.36 eV), which can result in a larger offset between the SOMO of dopant and lowest unoccupied molecular orbital (LUMO) of n-type polymers, though that effect may not be dominant in the present work. The doped polymer films exhibit higher Seebeck coefficient and power factor than films using N-DMBI at the same doping levels or similar electrical conductivity levels. Moreover, TP-DMBI doped polymer films offer much higher electron mobility of up to 0.53 cm²  V^-1  s^-1 than films with N-DMBI doping, demonstrating the potential of TP-DMBI, and 3,4,5-trialkoxy DMBIs more broadly, for high performance n-type organic thermoelectrics.

Boosting the Optoelectronic Properties of Molybdenum Diselenide by Combining Phase Transition Engineering with Organic Cationic Dye Doping

Two-dimensional layered transition metal dichalcogenides (TMDs) have been investigated intensively as next-generation semiconducting materials. However, conventional TMD-based devices exhibit large contact resistance at the interface between the TMD and the metal electrode because of Fermi level pinning and the Schottky barrier, which results in poor charge injection. Here, we present enhanced charge transport characteristics in molybdenum diselenide (MoSe₂) by means of a sequential engineering process called PESOD-2H/1T (i.e., phase transition engineering combined with surface transfer organic cationic dye doping; 2H and 1T represent the trigonal prismatic and octahedral phases, respectively). Substantial improvements are observed in PESOD-processed MoSe₂ phototransistors, specifically, an approximately 40 000-fold increase in effective carrier mobility and a 100 000-fold increase in photoresponsivity, compared with the mobility and photoresponsivity of intact MoSe₂ phototransistors. Moreover, the PESOD-processed MoSe₂ phototransistor on a flexible substrate maintains its optoelectronic properties under tensile stress, with a bending radius of 5 mm.

Doping Approaches for Organic Semiconductors

Electronic doping in organic materials has remained an elusive concept for several decades. It drew considerable attention in the early days in the quest for organic materials with high electrical conductivity, paving the way for the pioneering work on pristine organic semiconductors (OSCs) and their eventual use in a plethora of applications. Despite this early trend, however, recent strides in the field of organic electronics have been made hand in hand with the development and use of dopants to the point that are now ubiquitous. Here, we give an overview of all important advances in the area of doping of organic semiconductors and their applications. We first review the relevant literature with particular focus on the physical processes involved, discussing established mechanisms but also newly proposed theories. We then continue with a comprehensive summary of the most widely studied dopants to date, placing particular emphasis on the chemical strategies toward the synthesis of molecules with improved functionality. The processing routes toward doped organic films and the important doping-processing-nanostructure relationships, are also discussed. We conclude the review by highlighting how doping can enhance the operating characteristics of various organic devices.

Transition metal-catalysed molecular n-doping of organic semiconductors

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 a very low doping efficiency (η) of less than 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 (for example, Pt, Au, Pd) as vapour-deposited nanoparticles or solution-processable organometallic complexes (for example, Pd₂(dba)₃) catalyses the reaction, as assessed by experimental and theoretical evidence, enabling greatly increased η in a much shorter doping time and high electrical conductivities (above 100 S cm^-1; ref. 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^{12, 13}.

One-way rotation of a chemically anchored single molecule-rotor

We present the chemical anchoring of a DMBI-P molecule-rotor to the Au(111) surface after a dissociation reaction. At the temperature of 5 K, the anchored rotor shows a sequential unidirectional rotational motion through six defined stations induced by tunneling electrons. A typical voltage pulse of 400 mV applied on a specific location of the molecule causes a unidirectional rotation of 60° with a probability higher than 95%. When the temperature of the substrate increases above 20 K, the anchoring is maintained and the rotation stops being unidirectional and randomly explores the same six stations. Density functional theory simulations confirm the anchoring reaction. Experimentally, the rotation shows a clear threshold at the onset of the C-H stretch manifold, showing that the molecule is first vibrationally excited and later it decays into the rotational degrees of freedom.

Approaching disorder-tolerant semiconducting polymers

Doping has been widely used to control the charge carrier concentration in organic semiconductors. However, in conjugated polymers, n-doping is often limited by the tradeoff between doping efficiency and charge carrier mobilities, since dopants often randomly distribute within polymers, leading to significant structural and energetic disorder. Here, we screen a large number of polymer building block combinations and explore the possibility of designing n-type conjugated polymers with good tolerance to dopant-induced disorder. We show that a carefully designed conjugated polymer with a single dominant planar backbone conformation, high torsional barrier at each dihedral angle, and zigzag backbone curvature is highly dopable and can tolerate dopant-induced disorder. With these features, the designed diketopyrrolopyrrole (DPP)-based polymer can be efficiently n-doped and exhibit high n-type electrical conductivities over 120 S cm⁻¹, much higher than the reference polymers with similar chemical structures. This work provides a polymer design concept for highly dopable and highly conductive polymeric semiconductors.

In conjugated polymers, n-doping is often limited by the tradeoff between doping efficiency and charge carrier mobilities, since dopants often randomly distribute within polymers, leading to significant structural and energetic disorder. Here, the authors screen a large number of polymer building block combinations and explore the possibility of designing n-type conjugated polymers with good tolerance to dopant-induced disorder.

B←N-Incorporated Dibenzo-azaacenes as n-Type Thermoelectric Materials

Organic thermoelectric materials play a vital role in flexible power generating applications, such as wearable electronics and sensor networks. While there is a wealth of research on p-type organic thermoelectric materials, developments on n-type counterparts as complementary are comparatively limited. Herein, we report a new kind of n-type small-molecule thermoelectric materials based on B←N-incorporated dibenzo-azaacenes 1,2-DBNA-2 and 1,2-DBNA-5. Because of the low-lying lowest unoccupied molecular orbital (LUMO) energy levels, 1,2-DBNA-2 and 1,2-DBNA-5 could be efficiently n-doped, and the rigid and almost planar skeleton could ensure good carrier transfer. When doped with a typical n-dopant (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), 1,2-DBNA-5 exhibits a moderate conductivity of 0.01 S cm^-1 and a power factor of 0.06 μW m^-1 K^-2 with a Seebeck coefficient of -244.4 μV K^-1 in thermoelectric devices. These results not only demonstrate that B←N-incorporated dibenzo-azaacenes are a novel class of n-type thermoelectric materials but also highlight a new strategy to develop n-type organic thermoelectric materials.

Achieving Efficient n-Doping of Conjugated Polymers by Molecular Dopants

ConspectusMolecular 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 n-type 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 n-doped organic materials into the next era.

Molecular Doping Directed by a Neutral Radical

Molecular doping makes possible tunable electronic properties of organic semiconductors, yet a lack of control of the doping process narrows its scope for advancing organic electronics. Here, we demonstrate that the molecular doping process can be improved by introducing a neutral radical molecule, namely nitroxyl radical (2,2,6,6-teramethylpiperidin-i-yl) oxyl (TEMPO). Fullerene derivatives are used as the host and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazoles (DMBI-H) as the n-type dopant. TEMPO can abstract a hydrogen atom from DMBI-H and transform the latter into a much stronger reducing agent DMBI•, which efficiently dopes the fullerene derivative to yield an electrical conductivity of 4.4 S cm-1. However, without TEMPO, the fullerene derivative is only weakly doped likely by a hydride transfer following by an inefficient electron transfer. This work unambiguously identifies the doping pathway in fullerene derivative/DMBI-H systems in the presence of TEMPO as the transfer of a hydrogen atom accompanied by electron transfer. In the absence of TEMPO, the doping process inevitably leads to the formation of less symmetrical hydrogenated fullerene derivative anions or radicals, which adversely affect the molecular packing. By adding TEMPO we can exclude the formation of such species and, thus, improve charge transport. In addition, a lower temperature is sufficient to meet an efficient doping process in the presence of TEMPO. Thereby, we provide an extra control of the doping process, enabling enhanced thermoelectric performance at a low processing temperature.

Recent Progress on Addressing the Key Challenges in Organic Thermoelectrics

Compared with inorganic thermoelectric materials, organic thermoelectric (OTE) materials have attracted increasing attention due to their advantages of low toxicity, high mechanical flexibility, and large-scale solution processability. In the past few years, OTE materials have made remarkable progress in terms of their design, synthesis, and device performance. However, some challenges remain, including the low doping efficiency in n-type materials, poor doping stability with molecular dopants, and the largely reduced Seebeck coefficient after heavily doping, etc. All these factors hinder the further development of OTEs for commercial applications. In this Minireview, we highlight several key challenges during the development of OTEs and summarize recent understandings and efforts to address these challenges.

Oxygen-Rich Polymer Polyethylene Glycol-Functionalized Single-Walled Carbon Nanotubes Toward Air-Stable n-Type Thermoelectric Materials

It is crucial for thermoelectric (TE) devices to obtain both p-type and n-type materials and control charge carrier density. However, n-type thermoelectric materials are quite deficient and have lower thermoelectric properties. We report one oxygen-rich polymer named polyethylene glycol (PEG) for converting p-type single-walled carbon nanotubes (SWCNTs) to air-stable n-type thermoelectric materials. When pristine SWCNTs were doped with 2 mg·mL^-1 PEG in an ethanol solution, the optimal Seebeck coefficient of PEG/SWCNT composites reached -50.8 μV·K^-1. The result of ultraviolet photoelectron spectroscopy demonstrated that the lone pair of oxygen atoms in the PEG chain has electron transferability to SWCNTs. According to the hard and soft acid and base theory, sodium hydroxide (NaOH) was further introduced to improve air stability and thermoelectric performance of doped SWCNTs. As a result, PEG/NaOH/SWCNT composites achieved the highest power factor of 173.8 μW·m^-1·K^-2 at 300 K. Meanwhile, their final changes in electrical conductivity and the Seebeck coefficient are less than 8% in the investigation of air stability over two months. Inspired by this finding, we fabricated the TE generator composed of the pristine p-type SWCNTs and n-type PEG/NaOH/SWCNT composites. The maximum output power of this robust TE device reached 5.3 μW at a temperature gradient of 76 K, which is superior to many reported TE devices. Moreover, the experimental procedure is attractive as a sustainable process for materials preparation. Our study has indicated that the oxygen-rich polymer-functionalized SWCNTs have huge potential for developing air-stable n-type carbon-based thermoelectric materials.

Efficient n-Doping of Polymeric Semiconductors through Controlling the Dynamics of Solution-State Polymer Aggregates

Doping of polymeric semiconductors limits the miscibility between polymers and dopants. Although significant efforts have been devoted to enhancing miscibility through chemical modification, the electrical conductivities of n-doped polymeric semiconductors are usually below 10 S cm^-1 . We report a different approach to overcome the miscibility issue by modulating the solution-state aggregates of conjugated polymers. We found that the solution-state aggregates of conjugated polymers not only changed with solvent and temperature but also changed with solution aging time. Modulating the solution-state polymer aggregates can directly influence their solid-state microstructures and miscibility with dopants. As a result, both high doping efficiency and high charge-carrier mobility were simultaneously obtained. The n-doped electrical conductivity of P(PzDPP-CT2) can be tuned up to 32.1 S cm^-1 . This method can also be used to improve the doping efficiency of other polymer systems (e.g. N2200) with different aggregation tendencies and behaviors.

Competitive Desulfonylative Reduction and Oxidation of α-Sulfonylketones Promoted by Photoinduced Electron Transfer with 2-Hydroxyaryl-1,3-dimethylbenzimidazolines under Air

Desulfonylation reactions of α-sulfonylketones promoted by photoinduced electron transfer with 2-hydroxyarylbenzimidazolines (BIH-ArOH) were investigated. Under aerobic conditions, photoexcited 2-hydroxynaphthylbenzimidazoline (BIH-NapOH) promotes competitive reduction (forming alkylketones) and oxidation (producing α-hydroxyketones) of sulfonylketones through pathways involving the intermediacy of α-ketoalkyl radicals. The results of an examination of the effects of solvents, radical trapping reagents, substituents of sulfonylketones, and a variety of hydroxyaryl- and aryl-benzimidazolines (BIH-ArOH and BIH-Ar) suggest that the oxidation products are produced by dissociation of α-ketoalkyl radicals from the initially formed solvent-caged radical ion pairs followed by reaction with molecular oxygen. In addition, the observations indicate that the reduction products are generated by proton or hydrogen atom transfer in solvent-caged radical ion pairs derived from benzimidazolines and sulfonylketones. The results also suggest that arylsulfinate anions arising by carbon-sulfur bond cleavage of sulfonylketone radical anions act as reductants in the oxidation pathway to convert initially formed α-hydroperoxyketones to α-hydroxyketones. Finally, density functional theory calculations were performed to explore the structures and properties of radical ions of sulfonylketones as well as BIH-NapOH.

Cyano-Functionalized Bithiophene Imide-Based n-Type Polymer Semiconductors: Synthesis, Structure-Property Correlations, and Thermoelectric Performance

n-Type polymers with deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels are essential for enabling n-type organic thin-film transistors (OTFTs) with high stability and n-type organic thermoelectrics (OTEs) with high doping efficiency and promising thermoelectric performance. Bithiophene imide (BTI) and its derivatives have been demonstrated as promising acceptor units for constructing high-performance n-type polymers. However, the electron-rich thiophene moiety in BTI leads to elevated LUMOs for the resultant polymers and hence limits their n-type performance and intrinsic stability. Herein, we addressed this issue by introducing strong electron-withdrawing cyano functionality on BTI and its derivatives. We have successfully overcome the synthetic challenges and developed a series of novel acceptor building blocks, CNI, CNTI, and CNDTI, which show substantially higher electron deficiencies than does BTI. On the basis of these novel building blocks, acceptor-acceptor type homopolymers and copolymers were successfully synthesized and featured greatly suppressed LUMOs (-3.64 to -4.11 eV) versus that (-3.48 eV) of the control polymer PBTI. Their deep-positioned LUMOs resulted in improved stability in OTFTs and more efficient n-doping in OTEs for the corresponding polymers with a highest electrical conductivity of 23.3 S cm^-1 and a power factor of ∼10 μW m^-1 K^-2. The conductivity and power factor are among the highest values reported for solution-processed molecularly n-doped polymers. The new CNI, CNTI, and CNDTI offer a remarkable platform for constructing n-type polymers, and this study demonstrates that cyano-functionalization of BTI is a very effective strategy for developing polymers with deep-lying LUMOs for high-performance n-type organic electronic devices.

N-type organic thermoelectrics: demonstration of ZT > 0.3

The ‘phonon-glass electron-crystal’ concept has triggered most of the progress that has been achieved in inorganic thermoelectrics in the past two decades. Organic thermoelectric materials, unlike their inorganic counterparts, exhibit molecular diversity, flexible mechanical properties and easy fabrication, and are mostly ‘phonon glasses’. However, the thermoelectric performances of these organic materials are largely limited by low molecular order and they are therefore far from being ‘electron crystals’. Here, we report a molecularly n-doped fullerene derivative with meticulous design of the side chain that approaches an organic ‘PGEC’ thermoelectric material. This thermoelectric material exhibits an excellent electrical conductivity of >10 S cm⁻¹ and an ultralow thermal conductivity of <0.1 Wm⁻¹K⁻¹, leading to the best figure of merit ZT = 0.34 (at 120 °C) among all reported single-host n-type organic thermoelectric materials. The key factor to achieving the record performance is to use ‘arm-shaped’ double-triethylene-glycol-type side chains, which not only offer excellent doping efficiency (~60%) but also induce a disorder-to-order transition upon thermal annealing. This study illustrates the vast potential of organic semiconductors as thermoelectric materials.

Achieved high thermoelectric figure of merit (ZT) in organic thermoelectric materials remains a challenge due to their low packing order and poor host/dopant miscibility. Here, the authors report side chain-engineered n-doped fullerene derivatives with record ZT >0.3 for organic thermoelectrics.

Towards commercialization: the operational stability of perovskite solar cells

Recently, perovskite solar cells (PSCs) have attracted much attention owing to their high power conversion efficiency (25.2%) and low fabrication cost. However, the short lifetime under operation is the major obstacle for their commercialization. With efforts from the entire PSC research community, significant advances have been witnessed to improve the device operational stability, and a timely summary on the progress is urgently needed. In this review, we first clarify the definition of operational stability and its significance in the context of practical use. By analyzing the mechanisms in established approaches for operational stability improvement, we summarize several effective strategies to extend device lifetime in a layer-by-layer sequence across the entire PSC. These mechanisms are discussed in the contexts of chemical reactions, photo-physical management, technological modification, etc., which may inspire future R&D for stable PSCs. Finally, emerging operational stability standards with respect to testing and reporting device operational stability are summarized and discussed, which may help reliable device stability data circulate in the research community. The main target of this review is gaining insight into the operational stability of PSCs, as well as providing useful guidance to further improve their operational lifetime by rational materials processing and device fabrication, which would finally promote the commercialization of perovskite solar cells.