Molecular Doping Research Articles

Polaron interfacial entropy as a route to high thermoelectric performance in DAE-doped PEDOT:PSS films.

Enhancing the thermoelectric transport properties of conductive polymer materials has been a long-term challenge, in spite of the success seen with molecular doping strategies. However, the strong coupling between the thermopower and the electrical conductivity limits thermoelectric performance. Here, we use polaron interfacial occupied entropy engineering to break through this intercoupling for a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate)) thin film by using photochromic diarylethene (DAE) dopants coupled with UV-light modulation. With a 10-fold enhancement of the thermopower from 13.5 μV K-1 to 135.4 μV K-1 and almost unchanged electrical conductivity, the DAE-doped PEDOT:PSS thin film achieved an extremely high power factor of 521.28 μW m-1 K-2 from an original value of 6.78 μW m-1 K-2. The thermopower was positively correlated with the UV-light intensity but decreased with increasing temperature, indicating resonant coupling between the planar closed DAE molecule and PEDOT. Both the experiments and theoretical calculations consistently confirmed the formation of an interface state due to this resonant coupling. Interfacial entropy engineering of polarons could play a critical role in enhancing the thermoelectric performance of the organic film.

Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance.

High-performance semiconductor materials and devices are needed to supply the growing energy and computing demand. Organic semiconductors (OSCs) are attractive options for opto-electronic devices, due to their low cost, extensive tunability, easy fabrication, and flexibility. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been extensively studied due to their high carrier mobility, stability and opto-electronic tunability. Although molecular charge transfer doping affords widely tunable carrier density and conductivity in s-SWCNTs (and OSCs in general), a pervasive challenge for such systems is reliable measurement of charge carrier density and mobility. In this work we demonstrate a direct quantification of charge carrier density, and by extension carrier mobility, in chemically doped s-SWCNTs by a nuclear magnetic resonance approach. The experimental results are verified by a phase-space filling doping model, and we suggest this approach should be broadly applicable for OSCs. Our results show that hole mobility in doped s-SWCNT networks increases with increasing charge carrier density, a finding that is contrary to that expected for mobility limited by ionized impurity scattering. We discuss the implications of this important finding for additional tunability and applicability of s-SWCNT and OSC devices.

Activating a high-spin iron(II) complex to thermal spin-crossover with an inert non-isomorphous molecular dopant.

[Fe(bpp)2][ClO4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C2/c) is high-spin between 5-300 K, and crystallises with a highly distorted molecular geometry that lies along the octahedral-trigonal prismatic distortion pathway. In contrast, [Ni(bpp)2][ClO4]2 (monoclinic, P21) adopts a more regular, near-octahedral coordination geometry. Gas phase DFT minimisations (ω-B97X-D/6-311G**) of [M(bpp)2]2+ complexes show the energy penalty associated with that coordination geometry distortion runs as M2+ = Fe2+ (HS) ≈ Mn2+ (HS) < Zn2+ ≈ Co2+ (HS) ≲ Cu2+ ≪ Ni2+ ≪ Ru2+ (LS; HS = high-spin, LS = low-spin). Slowly crystallised solid solutions [FexNi1-x(bpp)2][ClO4]2 with x = 0.53 (1a) and 0.74 (2a) adopt the P21 lattice, while x = 0.87 (3a) and 0.94 (4a) are mixed-phase materials with the high-spin C2/c phase as the major component. These materials exhibit thermal spin-transitions at T½ = 250 ± 1 K which occurs gradually in 1a, and abruptly and with narrow thermal hysteresis in 2a-4a. The transition proceeds to 100% completeness in 1a and 2a; that is, the 26% Ni doping in 2a is enough to convert high-spin [Fe(bpp)2][ClO4]2 into a cooperative, fully SCO-active material. These results were confirmed crystallographically for 1a and 2a, which revealed similarities and differences between these materials and the previously published [FexNi1-x(bpp)2][BF4]2 series. Rapidly precipitated powders with the same compositions (1b-4b) mostly resemble 1a-4a, except that 2b is a mixed-phase material; 2b-4b also contain a fraction of amorphous solid in addition to the two crystal phases. The largest iron fraction that can be accommodated by the P21 phase in this system is 0.7 ± 0.1.

Topotactic N-doped carbon for efficient oxygen reduction reaction

A topotactic molecular doping strategy is developed via in situ coupling of dechlorinated carbon with ringed heterocyclic molecules under mild conditions.

Efficient and stabilized molecular doping of hole-transporting materials driven by a cyclic-anion strategy for perovskite solar cells

A cyclic-anion dopant (Li-CYCLIC) with high doping performance and intrinsic stability was used for efficient and stable perovskite solar cells.

Enhancing the photoelectrochemical hydrogen evolution activity of polyaniline through small molecular acid doping to introduce polariton-enhanced active sites

Development of organic polymer photoelectrodes with high surface activity is the key to efficient water splitting for hydrogen production. Polyaniline (PANI) prepared under alkaline conditions has semiconducting properties, but high reaction rates are difficult to achieve due to its poor surface activity. In this study, electrochemical doping was used to introduce small molecular acids (2,3-hydroxybutanedioic acid (TA), 2-hydroxybutanedioic acid (MA), and succinic acid (SA)) containing different numbers of hydroxyl groups into the PANI backbone to form polarizers, dipolarizers and active sites. Density Functional Theory (DFT) shows that small molecular acids interact with the imine (=N-) structure of the PANI backbone via electron cloud stacking to form complex active sites. Meanwhile, the transition from polarons to dipolarons under light conditions increases the polarity of the PANI molecular chain, which further improves the photoelectrocatalytic performance. This study will bring new ideas for efficient photoelectrocatalysis based on organic polymers.

Formula omitted]-[formula omitted] junction from localized doping of metal-WSe2 contact with N2H4, NH4 and BF4 molecules: A first-principles investigation

Using the density-functional theory, we investigate the possibility of obtaining a p–n junction in transition metal–semiconductor junctions. The systems consist in metal surfaces, M = Ti(0001), Co(0001), Pd(111) or Pt(111) in proximity to the semiconductor WSe2 monolayer. The latter is doped with hydrazine (N2H4), ammonium (NH4), or tetrafuoroborate (BF4). We determine for the M-WSe2 systems the binding energy, work function of metal surfaces, the Schottky barrier energy with and without molecular doping and the spin polarization. Results show a strong binding between the metal and WSe2 which implies a stable M-WSe2 contacts. The Spin-polarization in Ti-WSe2 (35%), Co-WSe2 (37,5%), Pd-WSe2 (13%) and Pt-WSe2 (-0.2%) suggest a significant metal-to-WSe2 spin injection in M-WSe2. Localization of hydrazine or ammonium on the WSe2 in M-WSe2 (M = Pd, Pt) changes its conductivity from p to n-type. The strong binding, the finite spin-injection, the change in conductivity and the reduction in Shottky barrier, raise the potential for a molecule-based p–n junction in metal-WSe2 field-effect transistor.

Multifunctional Regulation of SnO2 Nanocrystals by Snail Mucus for Preparation of Rigid or Flexible Perovskite Solar Cells in Air.

Tin oxide (SnO2) is widely used as an inorganic electron transport layer (ETL) for rigid and flexible perovskite solar cells (PSCs). In this work, an extract of snail shell, the sodium salt of polyaspartic acid (S-PASP), a water-soluble polypeptide polymer, has been used to multifunctionally regulate SnO2 nanograins. S-PASP has a strong chelating and dispersing effect; thus, chemically adsorbed SnO2 can inhibit agglomeration. The S-PASP:SnO2 ETL also improved the extraction and transferability of carriers, reducing body defects and interfacial charge. Moreover, the S-PASP:SnO2 ETL promotes the vertical growth of the perovskite crystals due to its bottom-up morphology, wettability, and strain release, which is conducive to improving the photoelectric performance of the device. The optimized rigid device prepared under open-air conditions obtained a PCE of 20.92%. In addition, due to the stress compensation of the S-PASP long chain, which prevented the cracking and displacement of the ETL, the optimal PCE of the flexible device was 17.96%, and the initial efficiency was maintained at 82.8% after 100 bends. This work introduces a molecular doping mechanism for organic-inorganic hybrid electronics.

Microstructure and high-temperature deformation performance of liquid-dispersed evaporated Nano-ZrTiO4 doping molybdenum alloy

To enhance the low-temperature plasticity and high-temperature strength of molybdenum alloys, we introduce a method of uniformly dispersing nano-ZrTiO4 particles within and between molybdenum grains. This approach effectively eliminates the formation of hard agglomerations between particles and leads to a further refinement of molybdenum grains. The research employs molecular level solid-liquid doping and subsequent liquid dispersion evaporation techniques to successfully synthesize Mo-ZrTiO4 nano-composite powder following H2 reduction. Subsequently, a novel fine-grained Mo-ZrTiO4 alloy sintered billet is successfully fabricated through cold isostatic pressing and high-temperature sintering. To assess the efficacy of the process, three sets of comparative experiments are designed, each employing different methods. The comparative studies have revealed compelling advantages of the liquid dispersed evaporated fine-grained samples of Mo-ZrTiO4. The key advantages identified are as follows:the samples demonstrate a refined grain size, ranging from 2 μm to 4 μm. Within the samples, the nano ZrTiO4 particles have an average size of approximately 200 nm. The density and Vickers hardness of the samples is measured at 10.21 g/cm3 and 268.5 ± 17HV0.2, respectively. The samples exhibit impressive toughness even at room temperature, notably, the high-temperature compressive strength of these samples surpasses that of the deformed samples reported in previous literature concerning this system. The mechanism of liquid dispersion evaporation method in grain nucleation, growth, and powder drying, reduction, and sintering processes was systematically elaborated. Simultaneously, a detailed analysis was conducted on two high-temperature strengthening mechanisms that impact the mechanical properties. The results indicate that this excellent performance can be attributed to the synergistic effect of both fine grain strengthening and uniform dispersion strengthening of second-phase nanoparticles.

Improving Performance of Organic Photodetectors by Using TCNQ Doped Copper Thiocyanate as the Anode Interfacial Layer

AbstractThe responsivity and specific detectivity are critical figures of merits for organic photodetectors; however, it remains challenging to simultaneously enhance two such parameters, since enhancing external quantum efficiency corresponding to responsivities will inevitably increase leakage current and noises, resulting in decreased specific detectivity. To address such a trade‐off, an effective anode interfacial layer of 7,7,8,8‐tetracyanoquinodimethane (TCNQ) doped copper thiocyanate (CuSCN) is developed herein, which can enhance shunt resistance, increase depletion width, enhance hole extraction efficiency, as well as reduce trap density of states of organic photodetectors. The organic photodetectors based on doped CuSCN present a very low dark current density of 4.4 × 10−11 A cm−2 and an impressively high specific detectivity of 1.1 × 1014 Jones. As evidenced by X‐ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy measurements, the energy levels of the doped CuSCN:TCNQ film are deepened, associated with the occurrence of dopant bonding and charge transfer. In addition, it is found that increasing the delay time can suppress the hysteresis effects of current density–voltage characteristics of organic photodetectors and make the minimum dark current density obtainable at 0 V. These findings demonstrate the great potential of using molecular doping to adjust the anode interfacial layer for developing high‐performance organic photodetectors.

Optimization Strategy of Molecular Modified Graphene Films for High-Performance Aqueous Zinc-Ion Hybrid Supercapacitors

Molecular modified graphene films (M-GFs) have the advantages of simple preparation and high electrochemical activity, which has a promising future in the field of electrochemical energy storage. However, the influence of molecular doping technology is rarely studied for electrochemical performance of the M-GFs, hindering the development of high-performance M-GFs. Herein, we have demonstrated that a selective adsorption self-assembly strategy can prepare M-GFs with higher performance than physical mixing method. The prepared M-GF exhibits a three-dimensional porous structure with active organic molecules support, and its interlayer spacing is evenly widened. The resulting M-GFs as cathode material of aqueous Zinc-ion hybrid supercapacitors (AZISCs) exhibits a high specific capacitance of 228F g-1, and a good capacitance retention of 42% (@10 A g-1), which are 25% and 13% higher than those of the physical mixing method, respectively. Besides, the energy and power density of the devices can reach 82 Wh kg-1 and 20.3 kW kg-1, respectively. After 5000 cycles of charge and discharge testing, the capacitance retention rate of the device can still reach 94%. The impressive results indicate that the M-GFs by selective adsorption self-assembly strategy could be a potential high-performance electrode material for the AZISCs..

Sequential Doping of Carbon Nanotube Wrapped by Conjugated Polymer for Highly Conductive Platform and Thermoelectric Application

Doping of conjugated polymers (CPs) is a promising strategy to obtain solution‐processable and highly conductive films; however, the improvement in electrical conductivity is limited owing to the relatively poor carrier mobility of CPs. Herein, a CP with excellent molecular doping ability, i.e., poly[2‐([2,2'‐bithiophen]‐5‐yl)‐3,8‐difluoro‐5,10‐bis(5‐octylpentadecyl)‐5,10‐dihydroindolo[3,2‐b]indole] (PIDF‐BT) is wrapped onto the surface of single‐walled carbon nanotubes (SWCNTs). The resulting PIDF‐BT@SWCNT simultaneously achieves excellent solution dispersibility and a high electrical conductivity of over 5000 S cm−1 through AuCl3 doping. The doping mechanism is systematically studied using spectroscopic analysis, and the four‐probe field‐effect transistor based on the doped PIDF‐BT@SWCNT confirms a carrier mobility up to 138 cm2 V−1 s−1. The carrier‐transfer barrier energy is related to the Schottky barrier between the SWCNT and PIDF‐BT, which can be controlled by doping. Finally, when the doped PIDF‐BT@SWCNT is applied to a thermoelectric device, a power factor exceeding 210 μW m−1 K−2 is achieved because of its high electrical conductivity, even if the increased carrier density reduces the Seebeck coefficient.

Improving the Performance of Layer-by-Layer Organic Solar Cells by n-Doping of the Acceptor Layer.

Molecular dopants can effectively improve the performance of organic solar cells (OSCs). Here, PM6/BTP-eC9-4Cl-based OSCs are fabricated by a layer-by-layer (LbL) deposition method, and the electron acceptor BTP-eC9-4Cl layer is properly doped by n-type dopant benzyl viologen (BV) or [4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl]dimethyl-amine (N-DMBI-H). The power conversion efficiency (PCE) of OSCs increases from 16.80 to 17.61 or 17.84% when the acceptor layer is doped by BV (0.01 wt %) or N-DMBI-H (0.01 wt %), respectively. At the optimal doping concentration, the device exhibits more balanced charge transport, fewer bimolecular recombinations, faster charge separation and transfer, and better stability. This doping strategy has good universality; when the acceptor layer L8-BO of LbL OSCs is doped by 0.01 wt % BV or 0.01 wt % N-DMBI-H, the PCE increases from 17.49 to 18.35 or 18.25%, respectively. All in all, our studies have demonstrated that the doping strategy is effective in enhancing the performance of OSCs.

Coordination of Tetracyanoquinodimethane-Derivatives with Tris(pentafluorophenyl)borane Provides Stronger p-Dopants with Enhanced Stability.

Strong molecular dopants for organic semiconductors that are stable against diffusion are in demand, enhancing the performance of organic optoelectronic devices. The conventionally used p-dopants based on 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its derivatives "FxTCN(N)Q", such as 2,3,4,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and 1,3,4,5,7,8-hexafluorotetracyano-naphthoquinodimethane (F6TCNNQ), feature limited oxidation strength, especially for modern polymer semiconductors with high ionization energy (IE). These small molecular dopants also exhibit pronounced diffusion in the polymer hosts. Here, we demonstrate a facile approach to increase the oxidation strength of FxTCN(N)Q by coordination with four tris(pentafluorophenyl)borane (BCF) molecules using a single-step solution mixing process, resulting in bulky dopant complexes "FxTCN(N)Q-4(BCF)". Using a series of polymer semiconductors with IE up to 5.9 eV, we show by optical absorption spectroscopy of solutions and thin films that the efficiency of doping using FxTCN(N)Q-4(BCF) is significantly higher compared to that using FxTCN(N)Q or BCF alone. Electrical transport measurements with the prototypical poly(3-hexylthiophene-2,5-diyl) (P3HT) confirm the higher doping efficiency of F4TCNQ-4(BCF) compared to F4TCNQ. Additionally, the bulkier structure of F4TCNQ-4(BCF) is shown to result in higher stability against drift in P3HT under an applied electric field as compared to F4TCNQ. The simple approach of solution-mixing of readily accessible molecules thus offers access to enhanced molecular p-dopants for the community.

Long-term stability of molecular doped epigraphene quantum Hall standards: single elements and large arrays (R K/236 ≈ 109 Ω)

In this work we investigate the long-term stability of epitaxial graphene (epigraphene) quantum Hall resistance standards, including single devices and an array device composed of 236 elements providing R K/236 ≈ 109 Ω, with R K the von Klitzing constant. All devices utilize the established technique of chemical doping via molecular dopants to achieve homogenous doping and control over carrier density. However, optimal storage conditions and the long-term stability of molecular dopants for metrological applications have not been widely studied. In this work we aim to identify simple storage techniques that use readily available and cost-effective materials which provide long-term stability for devices without the need for advanced laboratory equipment. The devices are stored in glass bottles with four different environments: ambient, oxygen absorber, silica gel desiccant, and oxygen absorber/desiccant mixture. We have tracked the carrier densities, mobilities, and quantization accuracies of eight different epigraphene quantum Hall chips for over two years. We observe the highest stability (i.e. lowest change in carrier density) for samples stored in oxygen absorber/desiccant mixture, with a relative change in carrier density below 0.01% per day and no discernable degradation of quantization accuracy at the part-per-billion level. This storage technique yields a comparable stability to the currently established best storage method of inert nitrogen atmosphere, but it is much easier to realize in practice. It is possible to further optimize the mixture of oxygen absorber/desiccant for even greater stability performance in the future. We foresee that this technique can allow for simple and stable long-term storage of polymer-encapsulated molecular doped epigraphene quantum Hall standards, removing another barrier for their wide-spread use in practical metrology.

A Highly Conductive n-Type Polythiophene Derivative: Effect of Molecular Weight on n-Doping Behavior and Thermoelectric Performance.

Here, we examine the impact of the molecular weight of an n-type conjugated polymer (n-PT2) on molecular doping and thermoelectric parameters. Two common dopants TDAE and N-DMBI with different doping mechanisms are used for molecular doping of n-PT2. It turns out that n-PT2 with a higher molecular weight is more miscible with the dopant, leading to more charge carriers. Moreover, the crystal structures and morphology of n-PT2 with a higher molecular weight are more tolerant against the intrusion of dopant molecules and charging. Finally, these factors work in synergy to endow the doped n-PT2 with the best conductivity and power factor (144 S cm-1/75.0 μW m-1 K-2 and 75.4 S cm-1/98.5 μW m-1 K-2 after doping by TDAE and N-DMBI, respectively). This study indicates that regulating the molecular weight allows for synergistic regulation of conductivity and Seebeck coefficient and is a feasible means to improve the performance for a given n-type organic thermoelectric material.

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.

Contrasting Transport Performance of Electron- and Hole-Doped Epitaxial Graphene for Quantum Resistance Metrology

Epitaxial graphene grown on silicon carbide (SiC/graphene) is a promising solution for achieving a high-precision quantum Hall resistance standard. Previous research mainly focused on the quantum resistance metrology of n-type SiC/graphene, while a comprehensive understanding of the quantum resistance metrology behavior of graphene with different doping types is lacking. Here, we fabricated both n- and p-type SiC/graphene devices via polymer-assisted molecular adsorption and conducted systematic magneto-transport measurements in a wide parameter space of carrier density and temperature. It is demonstrated that n-type devices show greater potential for development of quantum resistance metrology compared with p-type devices, as evidenced by their higher carrier mobility, lower critical magnetic field for entering quantized Hall plateaus, and higher robustness of the quantum Hall effect against thermal degeneration. These discrepancies can be reasonably attributed to the weaker scattering from molecular dopants for n-type devices, which is further supported by the analyses on the quantum interference effect in multiple devices. These results enrich our understanding of the charged impurity on electronic transport performance of graphene and, more importantly, provide a useful reference for future development of graphene-based quantum resistance metrology.

Tuning of electrical properties of CVD grown graphene by surface doping with organic molecules

Tailoring the charge carriers of two-dimensional (2D) materials is essential for high performance optoelectronic devices. The surface transfer doping by adsorption of molecules on 2D crystals is an attractive technique to tune the properties. Here, we study the change in the electronic transport properties of monolayer graphene (MLG) by surface doping with two different types of molecules. An effect of methyl isobutyl ketone (MIBK) and chlorobenzene molecular doping on the carrier concentration and electrical conductivity of chemical vapor deposition(CVD)-grown MLG was carried out by Raman spectroscopy and electrical transport measurement. The shifting of Raman peaks toward higher wave number and shifting of Dirac points toward positive gate voltage confirmed that the surface doping of graphene with MIBK and chlorobenzene molecules induced holes doping effect. The molecular doping approach significantly improved the carrier concentration of CVD grown MLG, which is a promising result. Our study will be useful for understanding as well as improvement of graphene based electronic device research.

Bulk Incorporation of Molecular Dopants into Ruddlesden–Popper Organic Metal–Halide Perovskites for Charge Transfer Doping (Adv. Funct. Mater. 38/2023)

Bulk Incorporation of Molecular Dopants into Ruddlesden–Popper Organic Metal–Halide Perovskites for Charge Transfer Doping (Adv. Funct. Mater. 38/2023)