Tuning Solution-State Aggregation for Shearing-Induced Alignment and High Mobility Transport in Conjugated Polymers.

Alignment of linear polymeric grains for highly stable N-type thin-film transistors

Open AccessCCS ChemistryRESEARCH ARTICLE1 Oct 2021Systematic Investigation of Solution-State Aggregation Effect on Electrical Conductivity in Doped Conjugated Polymers Yang-Yang Zhou, Zi-Yuan Wang, Ze-Fan Yao, Zi-Di Yu, Yang Lu, Xin-Yi Wang, Yi Liu, Qi-Yi Li, Lin Zou, Jie-Yu Wang and Jian Pei Yang-Yang Zhou Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Zi-Yuan Wang Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Ze-Fan Yao Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Zi-Di Yu Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Yang Lu Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Xin-Yi Wang Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Yi Liu College of Chemistry, Shandong Normal University, Jinan 250014 , Qi-Yi Li Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 , Lin Zou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621999 , Jie-Yu Wang Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 and Jian Pei *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 https://doi.org/10.31635/ccschem.021.202101411 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The mesoscopic solution-state aggregation plays a dominant role in the multilevel self-assembly of conjugated polymers. However, its effect on solid-state microstructures and the electrical performance of conjugated polymers is still a puzzle due to the limitation of characterization techniques. Herein, we have developed four isoindigo-based conjugated polymers with varied alkyl chains to reveal the relationship between solution-state aggregation, molecular doping process, and charge transport properties. For the solution-state aggregation, the photophysical and microstructural characterizations were adopted to explore the solvated aggregation structures of these four polymers, where well-differentiated aggregate sizes were observed. IIDDT-C3 showed small solvated aggregate sizes but excellent aggregate connectivity in solutions. The characterization of the solid-state microstructure demonstrated that IIDDT-C3 had ideal crystalline solid-state microstructures with densely fibrillar morphology, which endowed IIDDT-C3 with the highest electrical conductivity up to 531 ± 50 S cm−1 after doping among these four polymers. Our work provides molecular guidance for clarifying the structure–performance relationship between aggregation structures and the electrical properties of the doped conjugated polymers with different alkyl chains. Download figure Download PowerPoint Introduction Solution processing of conjugated polymers is a typical approach for the fabrication of organic optoelectronic devices.1 Conjugated polymers usually form significant pre-aggregates in solutions due to the interchain interactions, including the π–π interaction between the polymer backbones and the dispersion forces among the alkyl side chains.2 Solution-state aggregation structures of conjugated polymers have been demonstrated to be inherited by thin films, and finally, conspicuously reflected on electrical performances.3,4 To improve the optoelectrical performances of conjugated polymers, some methodologies have been developed in the past years through modulating solution aggregation structures,5–9 such as modulating solution preheating temperature,10 tuning solvent quality,1 and changing dissolution temperature.11 In nature, the molecular interactions between side chains of proteins significantly influence their three-dimensional (3D) assembly structures.12 Similar to proteins, the side chains of conjugated polymers affect the solution-state aggregation structures and multilevel assembly. As we know, flexible side chains in conjugated polymers not only ensure their solubility in organic solvents,13–15 but also affect their interchain packing, solid-state microstructures, and their ultimate electrical performance.16–19 In our previous work, moving the branching position of the alkyl chain away from the polymer backbones influenced the interchain π–π stacking distance, and thus, the charge carrier transport.20 Subsequently, diketopyrrolopyrrole (DPP),21 benzodifurandione-based poly(p-phenylene vinylene) (BDPPV),22 naphthalene diimide (NDI)-based polymers,23 and other polymers have also been reported to exhibit a similar effect of branching positions of alkyl chains.24,25 However, the solution-state aggregation behavior of conjugated polymers controlled by the alkyl side chains is not systemically explored. Herein, four isoindigo (IID)-based conjugated polymers with varied alkyl side chains were developed to systematically establish an integrated structure–performance relationship of conjugated polymers from molecular packing structures, mesoscopic solution aggregation, through solid-state microstructures, and finally, to devices with electrical and thermoelectric performances (Figures 1a–1d). Thus, different branching positions in alkyl side chains were designed to control the molecular interaction and the solution-state aggregation. Subsequently, the solution-state morphology of these four polymers was investigated through a series of photophysical and morphological characterizations. To further understand the influence of solution-state aggregation structures on the film's electrical performances of these polymers, characterizations of electrical conductivity, Seebeck coefficient and power factor were performed in detail. These conjugated polymers were doped through immersion the annealed films in 10 mM FeCl3 solution in orthogonal nitromethane. These FeCl3 doped conjugated polymers displayed different electrical conductivities and thermoelectric performance. All results demonstrated that further alkyl chain branching position would cause smaller aggregate sizes and more excellent connectivity among aggregates in solution, and the corresponding polymer showed better charge transport performance and thermoelectric performance. As a result, IIDDT-C3 exhibited superior conductivity as high as 531 ± 50 S cm−1 and a notable power factor with 84 μW m−1 K−2 after doping with FeCl3. Figure 1 | The structure–performance relationship of these four IID-based conjugated polymers with varied alkyl chain branching positions. (a) Molecular packing of IID-based conjugated polymers. (b) Aggregate structures in solutions. (c) Microstructures in films. (d) The different electrical and thermoelectric performance of conjugated polymers with varied alkyl chain branching positions. Download figure Download PowerPoint Experimental Section Materials and characterization IIDDT-C1, IIDDT-C2, IIDDT-C3, and IIDDT-C4 were synthesized, as described in the routes shown in the Supporting Information. Anhydrous FeCl3 and solvents [anhydrous nitromethane and 1,2-dichlorobenzene (oDCB)] were obtained from Sigma Aldrich (Shanghai, China). All the chemicals were used as received. Molecular weights were determined by gel permeation chromatography (GPC) performed on Polymer Laboratories PL-GPC220 (Palo Alto, CA) at 150 °C using 1,2,4-trichlorobenzene as eluent. Thin-film fabrication and immersion doping All devices were fabricated through spin-coating polymer solutions on substrates. The substrates were subjected to cleaning using ultrasonication in acetone, deionized water (twice), and isopropanol. Thin films of these four polymers were deposited on the substrates by spin-coating 5 g L−1 polymer solutions at 1500 rpm for 60 s and 3000 rpm for 3 s, followed by annealing at 180 °C for 10 min. The preparation of the samples for grazing incidence wide-angle X-ray scattering (GIWAXS), atomic force microscopy (AFM), and scanning electron microscopy (SEM) analysis uniformly used Si substrates. All doping experiments were carried out in the air atmosphere. A 10 mM FeCl3 solution was prepared by dissolving 32 mg of FeCl3 in 20 mL anhydrous nitromethane in a sample vial and then divided into four equal solutions. Doping was done by dipping the annealed polymer film in the dopant solution for 10 s then the residual solution was blown away by N2. Solubility experiments The solubility experiments were performed by comparing the absorption spectra of several dilute solutions with known concentrations and the solutions diluted from saturated solutions to specific multiple. The Lambert–Beer law was adopted for data fitting, and the absorption maximum λmax of the diluted saturation solutions was interpolated to obtain the saturation concentration and polymer solubility. Solution characterization Small-angle neutron scattering (SANS) measurements were conducted on a Suanni Instrument (Mianyang, Sichuan, China) at the 20 MW China Mianyang Research Reactor (CMRR) at the China Academy of Engineering Physics. (The detailed description of SANS is demonstrated in the Supporting Information.) Thin-film characterization Absorption spectra were recorded on PerkinElmer Lambda 750 UV–vis spectrometer. SEM experiments were performed with Hitachi S-4800 field emission SEM (Tokyo, Japan) operated at an accelerating voltage of 2 kV. GIWAXS was carried out at beamline BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 1.2398 Å. Scattering data were collected with an incident beam energy of 10 keV, an incidence angle of 0.14 degrees, and a sample-to-detector distance of about 425 mm. The setup geometry was corrected with a LaB6 standard sample. AFM studies of thin films were performed with Cypher S microscope (Asylum Research, Oxford Instruments, Abingdon, Oxfordshire, England) at tapping mode under ambient conditions, using a silicon cantilever (AC240TS-R3) with a resonant frequency around 70 kHz. All the pristine films were annealed at 180 °C for 10 min before running GIWAXS measurements. Thermoelectric property and conductivity measurements Four-point conductivity measurements were conducted in the air with Keithley 4200SCS. Seebeck coefficient measurements were performed in air using homemade equipment. (The detailed description of Seebeck coefficient measurement is in the Supporting Information.) Electrical conductivity was measured with a co-linear four-point-probe bar geometry. Results and Discussion Solution-state aggregation Here, the Stille-coupling polymerization between 6,6′-dibromo-N,N′-(2-decyltetradecyl)-isoindigo, 6,6′-dibromo-N,N′-(3-decyltetradecyl)-isoindigo, 6,6′-dibromo-N,N′-(4-decyltetradecyl)-isoindigo, or 6,6′-dibromo-N,N′-(5-decyltetradecyl)-isoindigo and 5,5′-bis(trimethylstannyl)-2,2′-bithiophene was employed to give four IID-based conjugated polymers IIDDT-C1, IIDDT-C2, IIDDT-C3, and IIDDT-C4. Figure 2a illustrates the chemical structures of these four polymers with varied alkyl chain branching positions. High-temperature GPC with 1,2,4-trichlorobenzene as the eluent gave the molecular weights of these four polymers ( Supporting Information Table S1 and Figure S1). To clarify our story more clearly, we also explained in detail and schematic diagrams of some academic terms in Supporting Information Figure S2. The aggregation behavior of these four polymers in solution was a crucial bridge to connect the molecular conformation and the solid-state microstructure ( Supporting Information Figure S2). First, room-temperature solubility of these three IID-based polymers was measured in oDCB except for IIDDT-C2.26 The solubility of IIDDT-C2 was characterized at 110 °C due to its gel state in oDCB at room temperature. These four conjugated polymers showed distinct solubility in oDCB. With the branching positions moving further from the backbones, IIDDT-C3 and IIDDT-C4 showed higher solubility in oDCB. On the contrary, IIDDT-C1 and IIDDT-C2 showed lower solubility ( Supporting Information Table S3 and Figure S3). To gain an insight into their solution-state aggregation, temperature-dependent absorption spectra of 0.01 g L−1 solutions of these four polymers in oDCB were performed (Figures 2c–2f). In terms of IID-based conjugated polymers, the maximum absorption (A0) and its shoulder absorption (A1) at low-energy characteristic absorption bands were related to the aggregation and well-dissolved states, respectively.27 Thus, the A0/A1 ratios at room temperature were used to show a magnitude of aggregation. The A0/A1 ratios of IIDDT-C1 and IIDDT-C2 showed higher values than those of IIDDT-C3 and IIDDT-C4 at room temperature, demonstrating that strong aggregation of IIDDT-C1 and IIDDT-C2 occurred in oDCB ( Supporting Information Table S2). By monitoring the relative intensity of the A0 transition peak as a function of temperature, the disaggregation rates among these four polymers were calculated ( Supporting Information Figure S2).28 With increasing temperature, IIDDT-C3 and IIDDT-C4 displayed slower disaggregation rates than IIDDT-C1 and IIDDT-C2 (Figure 2b), indicating that it was more difficult to break the interactions between neighboring chains of IIDDT-C3 and IIDDT-C4 than IIDDT-C1 and IIDDT-C2 in oDCB. Besides, we measured the temperature-dependent spectra of IIDDT-C3 with different molecular weight distributions ( Supporting Information Figure S4). The similar evolution of "relative aggregation strength" estimated from the temperature-dependent spectra of IIDDT-C3 indicated that the molecular weight and distribution did not influence the aggregation of IIDDT-C3 appreciably. Figure 2 | Solution-state aggregation behavior of these four IID-based conjugated polymers. (a) Chemical structures of IIDDT-C1, IIDDT-C2, IIDDT-C3, and IIDDT-C4. (b) Plots of "relative aggregation strength" versus solution temperature. The intensity of the varying A0 peak at different temperatures relative to the intensity of the peak absorbance at room temperature (30 °C) was described as the "relative aggregation strength." (c–f) Temperature-dependent absorption spectra of 0.01 g L−1 polymers in oDCB. Download figure Download PowerPoint SANS was adopted to gain a better understanding of the solution-aggregation structures of these four polymers. Due to the difference in scattering length density between the polymer and the deuterated solvent, SANS can effectively probe the mesoscale structures of conjugated polymers in solutions. Solutions of these four polymers in 1,2-dichlorobenzene-d4 (oDCB-d4) at the concentration of 5 g L−1 were used to perform the SANS experiments. Figure 3a plots the scattering intensity I(q) as the function of scattering vector q of these four polymers. The magnitude of scattering intensity at the low range of the scattering vector (q) decreased in the order of IIDDT-C2, IIDDT-C1, IIDDT-C4, and IIDDT-C3, indicating statistical reduced solvated aggregate sizes in the order of IIDDT-C2, IIDDT-C1, IIDDT-C4, and IIDDT-C3. The scattering data fitted well in the formula of I(q) ≈ q−α, where α is the Porod exponent ( Supporting Information Figure S5). The Porod exponent shows the interactions of the solvated aggregates.29 These four IID-based polymers showed different Porod exponents, indicating their existence as different solution-aggregation states: The Porod exponent of IIDDT-C2 was about 2.53, suggesting that it might exhibit strong interaction between polymer chains in oDCB. The Porod exponent of IIDDT-C1 was around 1.68, which might indicate flexible chains with self-avoiding walk conformations in a good solvent ( Supporting Information Figure S2).30 However, IIDDT-C3 and IIDDT-C4 showed similar Porod exponents of 0.99 and 1.09, respectively. The Porod exponent close to 1 corresponded to a rod-like solution-aggregation structure.31 Figure 3 | Solution-state aggregate structures of these four polymers. (a) SANS intensity of these four IID-based polymers in oDCB. (b) Size distributions of solvated aggregates in IIDDT-C1, IIDDT-C2, IIDDT-C3, and IIDDT-C4 in oDCB. (c–f) AFM height images and (g–j) SEM images of these four polymers freeze-dried from 5 g L−1oDCB solutions. Supporting Information Figure S6b shows the corresponding height profiles of the white lines. The green lines in AFM height images and orange lines in SEM images show some aggregate tracks. Download figure Download PowerPoint To analyze the aggregate size distribution of these four polymers quantitatively, further filtration experiments were adopted. IIDDT-C2 showed almost single aggregate size distribution with 98% large aggregates (Figure 3b and Supporting Information Figure S7). IIDDT-C1 showed 51% large aggregates (diameter > 450 nm). However, IIDDT-C3 in solution showed a significant increase in medium-size aggregates (450 nm > diameter > 220 nm), and IIDDT-C4 showed many small aggregates of ∼37%. Therefore, IIDDT-C3 and IIDDT-C4 displayed smaller statistical solvated aggregate sizes. The difference in the aggregate size distribution of these four conjugated polymers was consistent with the SANS results. To observe the characteristics of the solution aggregates, including the aggregates size distribution and connectivity with neighboring aggregates, we performed freeze-drying experiments with 5 g L−1 polymer solutions in oDCB and characterized their morphology using AFM and SEM. In these experiments, the polymer solutions were quickly frozen to solid-state, and the solvents were subsequently removed using a vacuum pump; thus, their aggregate structures in solution could be reserved for further characterization ( Supporting Information Figure S2). IIDDT-C2 possessed the largest fiber-like aggregates among these four polymers (Figures 3c and 3g); these aggregates were distributed separately in oDCB like many isolated islands and barely exhibited effective connectivity between each other. However, the fiber-like aggregates in IIDDT-C1 solution showed significant diversity in sizes with an average diameter of 56.98 ± 20.69 nm in AFM height images ( Supporting Information Figure S6). Also, these aggregates were intricately entangled with each other and displayed cluttered connectivity (Figures 3d and 3h). In comparison, IIDDT-C3 possessed relatively smaller aggregate sizes, and the dense aggregate networks grew further into many fiber-like morphologies, showing superior connectivity (Figures 3e and 3i). The average diameter of the IIDDT-C3 aggregates was 33.16 ± 11.73 nm. Although IIDDT-C4 showed a similar aggregate network as IIDDT-C3, its network density was significantly sparse than IIDDT-C3 (Figures 3f and 3j). Hence, the different electrical performances of IIDDT-C3 and IIDDT-C4 might be partly attributed to the distinct connectivity between adjacent aggregates. Moreover, the characterization results of SEM provided further strong evidence for AFM height images data. On the scale of several nanometers, the contrast of partial organic thin-film would have been too low to make a clear distinction of the whole film in SEM images, so trivial differences were observed between the SEM and AFM height images of IIDDT-C3. Thin-film microstructure To gain further insight into the solid-state microstructures influenced by solution-state aggregation, GIWAXS and AFM were performed on pristine spin-coated polymer films, which were annealed at 180 °C for 10 min. The observed higher diffraction orders out-of-plane diffraction peaks (h00), which indicated stronger crystallinity of IIDDT-C3 and IIDDT-C4 than IIDDT-C1 and IIDDT-C2 (Figures 4a–4d). The arc-like diffraction pattern of the IIDDT-C2 film showed a broad crystallite orientation distribution, containing both edge-on and face-on packing modes. However, the out-of-plane (h00) diffractions and in-plane (0k0) diffraction intensities indicated that IIDDT-C1, IIDDT-C3, and IIDDT-C4 films displayed edge-on packing mode. The gradually shorter π–π stacking and longer lamellar distances reflected the influence of varied alkyl chain branching positions on molecular packing structures in these four pristine polymer films ( Supporting Information Table S4). The AFM height images showed different surface morphologies of these four conjugated polymers (Figures 4e–4h). The surface root-mean-square (RMS) of IIDDT-C2 was ∼5.21 nm, representing a relatively rough surface with rod-like aggregates. However, IIDDT-C1 showed long fibrillar aggregates with RMS ∼4.64 nm. Both IIDDT-C3 and IIDDT-C4 displayed short and densely aligning fibrillar aggregates on their surfaces with RMS of ∼0.41 and 2.12 nm, respectively. Nevertheless, some trenches were distributed on the surface, making some regions disconnect in the IIDDT-C4 film. The GIWAXS and AFM results demonstrated that the diversiform solution-state aggregation behavior was inherited into the films of these four IID-based polymers, which affected their solid-state microstructures dramatically. Figure 4 | Solid-state microstructures of these four polymer films. (a–d) GIWAXS patterns and (e–h) AFM height images of IIDDT-C1, IIDDT-C2, IIDDT-C3, and IIDDT-C4. Download figure Download PowerPoint Electrical and thermoelectric performance of doped films Electrical conductivity is a critical parameter to evaluate the charge transport performance of conjugated polymers. However, a few reports have described how solution-state aggregation affects doped binary films due to the complexity of doped conjugated polymers and limited characterization techniques. To investigate the association between the film microstructure and electrical performance, sequential doping was performed by immersing the annealed polymer films into 10 mM FeCl3 solution in orthogonal nitromethane (Figure 5a). A study has shown that sequential doping constantly improves electrical performances, while preserving the pristine film microstructures to a great extent.32 As expected, doping did not dramatically change the crystallinity and morphology of the polymer films, and the ordering of these four polymers was sustained. The coherence lengths (Lc, reflects crystalline grain sizes) and paracrystallinity disorder (g factor, reflects an accumulation of intrinsic and extrinsic defects that yields a statistical static disorder) of the doped films were close to the pristine films ( Supporting Information Figure S13), indicating that a large proportion of charge transport pathways might have been reserved during the immersion process. An increase of lamellar distances in polymer films after doping indicated that the dopant molecules preferred to reside in the alkyl side chain regions ( Supporting Information Figure S11 and Table S4).33,34 Also, the tighter π–π stacking distances after doping were more beneficial to interchain charge transport ( Supporting Information Figure S12 and Table S4).35 Some clusters with a diameter of ∼100 nm were distributed on the film surfaces of IIDDT-C3 and IIDDT-C4 ( Supporting Information Figure S10), which might be the residual dopant molecules. All the doped films almost kept consistent morphology with their pristine films, except that some residual dopant molecules were present. Therefore, the doping process had little influence on the morphology and crystallinity of these four polymers but decreased the π–π stacking distances and increased the lamellar distances. Figure 5 | Electrical performance of these doped polymers. (a) Diagrams of doping immersion and color change in pristine and doped films. (b) Electrical conductivities were obtained from FeCl3-doped polymers immersed for 1, 10, 30, and 300 s, respectively. (c) Seebeck coefficients and power factors of these four polymers doped by 10 mM FeCl3 for 1, 2, 5, 10, and 30 s, respectively. (d) Electrical conductivities of IIDDT-C2 and IIDDT-C3 spin-coated at 30 °C and 120 °C, respectively. (e and f) AFM height images of IIDDT-C2 and IIDDT-C3 doped films spin-coated at 120 °C. Download figure Download PowerPoint Given the influence of solution-aggregation structures on film microstructures, these four polymers showed different electrical performances after doping. An apparent color change from light blue to mauve was observed in these four polymer films after a very short doping time (Figure 5a), demonstrating that an efficient doping reaction occurred between the polymers and FeCl3 during the immersing period ( Supporting Information Figure S9). Compared with the pristine films, new, high-intensity absorption bands appeared at 800–2000 nm after doping, which were ascribed to the polarons of the polymers ( Supporting Information Figure S8). Through four-point electrical conductivity measurements, it was observed that these four conjugated polymers displayed distinct electrical IIDDT-C3 exhibited the highest conductivity of 531 ± 50 S followed by ± S cm−1 for IIDDT-C4, ± S cm−1 for IIDDT-C1, and ± S cm−1 for IIDDT-C2 (Figure The electrical conductivity properties of conjugated polymers of from charge carrier density and charge transport to the conductivity formula field effect measurements were adopted to explore the from charge carrier concentration and charge transport ( Supporting Information Figure The of IIDDT-C2 was due to the film and the charge carrier density by the effect was too high to the conjugated polymer characteristic so the of IIDDT-C2 was from the With longer doping the decreased and increased charge carrier concentrations of IIDDT-C1, IIDDT-C3, and IIDDT-C4 reflected the high doping among these IID-based polymers. The charge carrier density of IIDDT-C3 was to three that of IIDDT-C1 and IIDDT-C4 after doping, indicating that IIDDT-C3 more The magnitude differences of conductivities were with differences in values of carrier concentrations among these three polymers. The of conductivities also showed a similar with in the carrier Thus, the results of the effect measurement demonstrated that the different charge carrier concentrations affect the electrical which might be ascribed to the differences in solid-state The highest conductivity and

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Efficient charge transport in organic semiconductors is essential for construction of high performance optoelectronic devices. Herein, for the first time, we demonstrate that poly(amic acid) (PAA), a facilely deposited and annealing-free dielectric layer, can tailor the growth of organic semiconductor films with large area and high crystallinity toward efficient charge transport and high mobility in their thin film transistors. Pentacene is used as a model system to demonstrate the concept with mobility up to 30.6 cm2 V-1 s-1, comparable to its high quality single crystal devices. The structure of PAA has corrugations with OH groups pointing out of the surface, and the presence of an amide bond further allows adjacent polymer strands to interact via hydrogen bonding, leading to a self-rippled surface perpendicular to the corrugation. On the other hand, the strong polar groups (-COOH/-CONH) of PAA could provide repulsive forces between PAA and pentacene, which results in the vertical orientation of pentacene on the dielectric surface. Indeed, in comparison with its imidized counterpart polyimide (PI), PAA dielectric significantly enhances the film crystallinity, drastically increases the domain size, and decreases the interface trap density, giving rise to superior device performance with high mobility. This concept can be extended to more organic semiconducting systems, e.g., 2,6-diphenylanthracene (DPA), tetracene, copper phthalocyanine (CuPc), and copper hexadecafluorophthalocyanine (F16CuPc), demonstrating the general applicability. The results show the importance of combining surface nanogrooves with the strong polarity in orienting the molecular arrangement for high crystallinity toward efficient charge transport in organic semiconductors.

An all-perovskite tandem cell based on narrow-bandgap mixed tin-lead (Sn-Pb) alloyed perovskites is a potential photovoltaic device whose power conversion efficiency can exceed the Shockley-Queisser limit of a single-junction solar cell, 33%. However, comprehensive descriptions of the charge-carrier mobilities and transport mechanisms in the mixed Sn-Pb perovskite system remain elusive. Herein, we integrate density functional theory (DFT) calculations with charge transport models to provide more insight into the electronic structures and transport behaviors of these materials. We specifically employ a model using large polaron transport based on LO phonon scattering and ionized impurity scattering due to the oxidation of Sn2+ to Sn4+. DFT simulations revealed that there is a crossover in the electronic properties of pure-Sn and pure-Pb perovskites at Pb contents of more than 0.5, which causes increasing reduced effective mass as the Pb content increases. Theoretical calculations for charge-carrier mobility were in agreement with the experimental data between 23 and 89 cm2 V-1 s-1 at room temperature. In mixed Sn-Pb perovskites, LO phonon scattering predominates. However, in pure-Sn perovskites, transport scatterings based on LO phonon and ionized impurity scatterings are important. This discovery advances our knowledge of the transport mechanisms underlying the system of mixed Sn-Pb compounds.

ConspectusTwo-dimensional (2D) molecular materials, in which the major interactions are confined in 2D planes with contrasted force fields acting in between the planes, have been key electronic functional materials since the past decade. Even without referring to the functionals of graphene-based systems, 2D electronic conjugated systems are expected to show extrawide dynamic ranges in electronic density of states (DOS) tuning, effective electron mass, electron mobility, and conductivity. A major advantage of 2D electronic systems is their compatibility with the ubiquitous electronic devices designed using planar structures, such as transistors and memories, which is associated with the utility of 2D active materials. The mobility of electrons in 2D systems is the key to their utility, and various conjugated molecular and 2D materials have been designed to optimize the mobility. This Account begins with an introduction for mobility assessment: using noncontact time-resolved microwave conductivity (TRMC) measurements as a technique to probe differential conductivity upon transient charge carrier injection into the materials. Electronic transport over 2D electronic materials such as graphenes, covalent organic frameworks (COFs), and metal-organic frameworks (MOFs) is discussed with a special emphasis on molecular building blocks, fine-tuning conducting species and linkages, topology of the framework, and controlling molecular doping. The superiority of β-ketoenamine-linked COF over imine-linked COF films in charge transport and dominant in-plane charge carrier mobility over out-of-plane mobility is also illustrated. Systematic molecular engineering of the building blocks of β-ketoenamine-linked COFs with varying degrees of donor-acceptor (D-A) conjugation, torsional angles, and reaction conditions resulted in the modulation of the efficiency of charge carrier generation/transport as well as exciton migration. The advantages of 2D systems are finally discussed in terms of the mobility interplaying with spatial arrangements of molecules as well as the substantial role of intermolecular interactions in stabilizing their condensed phases. The strong correlation between the dispersion of mobility and hierarchical intermolecular interactions sheds light on the way to overcome structural fluctuation on the optimization of charge transport in molecular electronic materials. The point of singularity in the dispersion at an intermolecular distance of d ∼ 0.3 nm is deduced from the overall mobility assessment in condensed phases of conjugated molecules, suggesting key roles of intermolecular electronic coupling: the new concept of electronic conjugation. Exceptional electronic coupling with relatively high charge carrier mobility was also observed, particularly in 2D spatial arrangements of chiral molecules in contrast to 3D analogues, where the reduction of gravitational density of the molecular condensates was impacting DOS: the Wallach's rule. 2D electronic systems are strong candidates for the violation of the long-lasting Wallach's rule in terms of DOS.

An electron mobility enhancement is the very important phenomenon of an electron in the electronic device, where the high electronic device performance has the good electron mobility, which is obtained by the overall electron drift velocity in the electronic material driven potential difference. The increase in electron mobility by the injected high group velocity pulse is proposed in this article. By using light pulse input into the nonlinear microring resonator, light pulse group velocity can be tuned and increased, from which the required output group velocity can be obtained, which can be used to drive electron within the plasmonic waveguide, where eventually, the relative electron mobility can be obtained, the increasing in the electron mobility after adding up by the driven optical fields can be connected to the external electronic devices and circuits, which can be useful for many applications.

Recently, 2D materials of indium selenide (InSe) layers have attracted much attention from the scientific community due to their high mobility transport and fascinating physical properties. To date, reports on the synthesis of high-quality and scalable InSe atomic films are limited. Here, a synthesis of InSe atomic layers by vapor phase selenization of In2 O3 in a chemical vapor deposition (CVD) system, resulting in large-area monolayer flakes or thin films, is reported. The atomic films are continuous and uniform over a large area of 1 × 1 cm2 , comprising of primarily InSe monolayers. Spectroscopic and microscopic measurements reveal the highly crystalline nature of the synthesized InSe monolayers. The ion-gel-gated field-effect transistors based on CVD InSe monolayers exhibit n-type channel behaviors, where the field effect electron mobility values can be up to ≈30 cm2 V-1 s-1 along with an on/off current ratio, of >104 at room temperature. In addition, the graphene can serve as a protection layer to prevent the oxidation between InSe and the ambient environment. Meanwhile, the synthesized InSe films can be transferred to arbitrary substrates, enabling the possibility of reassembly of various 2D materials into vertically stacked heterostructures, prompting research efforts to probe its characteristics and applications.

We designed a series of 5,6,7-trithiapentacene-13-one (TTPO) derivatives (from 1 to 20) by attaching cyano groups (–CN) and by replacing S atoms with Se. The incoherent charge-hopping model combined with the Marcus–Hush electron transfer theory were used to evaluate the charge mobility of the title compounds as potential n-type organic field-effect transistors (OFETs) candidates. Based on the optimized face-to-face dimers, the crystal structures were predicted. The replacement of S with Se can decrease the electron reorganization energy (λ –). The introduction of −CN can decrease the energy levels of both HOMOs and LUMOs, which increases the stability. According to the electronic affinities (EA) and structures of dimers predicted by the DFT method coupled with dispersion corrections (−D), we selected compounds 2, 14, 15, and 19 as initial well-performance OFETs materials and predicted their crystal structures. Based on the optimized crystal structures, the charge (both electron and hole) transport mobilities are estimated. Electron mobilities of 14 and 15 are as high as 2.32 and 2.48 cm2 V−1 s−1, respectively, indicating that 14 and 15 are the promising n-type OFETs materials. What’s more, the predicted crystals show remarkable anisotropic charge mobilities. The maximum electron mobility appears along the direction of face-to-face π-stacking.

Ambipolar thin‐film transistors based on a series of air‐stable, solution‐processed blends of an n‐type polymer poly(benzobisimidazobenzophenanthroline) (BBL) and a p‐type small molecule, copper phthalocyanine (CuPc) are demonstrated, where all fabrication and measurements are performed under ambient conditions. The hole mobilities are in the range of 6.0 × 10–6 to 2.0 × 10–4 cm2 V–1 s–1 and electron mobilities are in the range of 2.0 × 10–6 to 3.0 × 10–5 cm2 V–1 s–1, depending on the blend composition. UV‐vis spectroscopy and electron diffraction show crystallization of CuPc in the metastable α‐crystal form within the semicrystalline BBL matrix. These CuPc domains develop into elongated ribbon‐like crystalline nanostructures when the blend films are processed in methanol, but not when they are processed in water. On methylene chloride vapor annealing of the blend films, a phase transformation of CuPc from the α‐form to the β‐form is observed, as shown by optical absorption spectroscopy and electron diffraction. Ambipolar charge transport is only observed in the blend films where CuPc crystallized in the elongated ribbon‐like nanostructures (α‐form). Ambipolar behavior is not observed with CuPc in the β‐polymorph. Unipolar hole mobilities as high as 2.0 × 10–3 cm2 V–1 s–1 are observed in these solution‐processed blend field‐effect transistors (FETs) on prolonged treatment in methanol, comparable to previously reported hole mobilities in thermally evaporated CuPc FETs. These results show that ambipolar charge transport and carrier mobilities in multicomponent organic semiconductors are intricately related to the phase‐separated nanoscale and crystalline morphology.

Using pulsed laser deposition, the synthesis of epitaxial La-doped BaSnO₃ (LBSO) thin films was optimized by controlling plasma plume dynamics with parameters such as pressure, laser energy, and target-substrate distance. This allowed precise tuning of the Ba/Sn ratio, ultimately leading to LBSO films with high electron mobility and carrier density. During post-growth hydrogen annealing, precise control of oxygen partial pressure and temperature simultaneously increased electron density and mobility, achieving a room-temperature mobility of 130 cm2V⁻1s⁻1. While hydrogen annealing could potentially reduce mobility due to charged oxygen vacancies, careful regulation of oxygen partial pressure increased the electron mobility significantly. This improvement was primarily attributed to oxygen-vacancy-assisted recovery in the ionic lattice, which induces reduced threading dislocation density and crystal mosaicity. Furthermore, this research provided a comprehensive guideline for defect engineering through thermal treatment. While reducing conditions improved crystallinity and mobility, they also induced Sn vacancy formation, which strongly trapped free electrons. Therefore, slightly Sn-excess LBSO films were found to be optimal for maximizing room-temperature mobility after hydrogen annealing. This study utilized LBSO thin films as a model system to explore defect control during synthesis and post-treatment, investigating the correlation between defect engineering and electronic properties. The findings highlight that thermodynamic defect control strategies during film deposition and annealing can offer new pathways to enhance room-temperature electron mobility in epitaxial oxide thin films.

Ambipolar transport characteristics of thin film transistors fabricated from nontoxic white light emitting quantum dot complexes (QDCs) are herein reported to exhibit efficient carrier mobilities. The QDCs are synthesized by forming bluish green emitting zinc quinolate complex on the surface of orange emitting Mn2+‐doped ZnS quantum dots (Qdots) using 8‐hydroxyquinoline 5‐sulfonic acid as the chelating ligand. The device exhibits efficient ambipolar transport characteristics with high ION/IOFF ratio of 104 and electron mobility and hole mobility of 2.95 × 10−02 and 1.06 × 10−02 cm2 V−1 s−1, respectively. The subthreshold slope of Qdot complex–integrated thin film transistor increases from that of Mn2+‐doped ZnS Qdot–integrated thin film transistor from 0.35 to 0.79 V dec−1 in p‐field effect transistor (FET) and from 0.59 to 0.97 V dec−1 in n‐FET operations, which annotates an increase in trap state density due to surface complexation of the Qdot. These results suggest that white light emitting QDC can be used as an efficient transport as well as an emissive material, which open up new paradigm for advanced optoelectronic applications.

The presence of hotspots with extreme heat fluxes and temperatures in high-performance electronics has led to severe thermal management challenges in the semiconductor industry. Our work experimentally investigates the potential of capillary-fed thin-film evaporation as a thermal management solution for devices where hotspots are superposed with mild background heating. The front side of our test device incorporates well-defined silicon micropillar arrays for passive fluidic transport via capillary wicking, and the backside is patterned with thin-film resistive heaters to emulate $640 \times 620~\mu \text{m}^{2}$ hotspots and $1 \times 1$ cm2 uniform background heating. Our micropillar wick design facilitates efficient fluidic and thermal transport and dissipated ${\approx} 6$ kW/cm2 from a single hotspot without background heating beyond which viscous losses exceeded the capillary pressure and dryout occurred due to insufficient liquid supply. While the capillary-limited hotspot dryout heat flux decreased by creating concurrent hotspots as well as by superposing a hotspot with mild uniform background heating, it was unaltered (within the measurement error) when the location of the hotspot was varied within the $1 \times 1$ cm2 microstructured area. The ultrahigh heat fluxes in our experiments were achieved by suppressing boiling by judiciously designing the fluidic transport where the microstructured surface maintains a thin liquid film and syphons only the required amount of liquid to sustain evaporation. Our experimental results show that thin-film evaporation is a viable thermal management solution for the next-generation high-performance power electronics, among others, where hotspots pose a significant challenge.