N-type Material Research Articles

Charge-Sharing Based Stabilization of Nano-Porous Organic Electrode for Rechargeable Li-Ion Battery

Organic-based lithium-ion batteries have garnered increasing attention due to their potential for cost-effectiveness, sustainability, and high energy density. Despite the promising theoretical prediction, the actual battery performance of organic-based electrodes has frequently fallen below the expected values. This discrepancy is primarily attributed to a low density of active sites, limited ion diffusivity, and high solubility to the electrolyte. This study introduces 5,10-dihydro-5,10-dimethylphenazine (DMPZ) as an organic active material, demonstrating superior electrochemical performance characterized by high capacity and prolonged cycle performance. To achieve a high capacity, a cryogenic milling is employed to create a porous nanostructure, enhancing the surface-to-volume ratio without altering the molecular structure. Consequently, the initial specific capacity of the porous DMPZ electrode reaches 184 mAh g-1 at 0.6 C, representing a remarkable 180% increase compared to non-milled DMPZ. To improve cycle stability, a charge-sharing reaction among organic active materials is facilitated by forming an organic nanocomposite comprising DMPZ and various n-type organic materials. The organic nanocomposite effectively mitigates the elution of organic active materials, ensuring unprecedented cycling stability with a capacity retention exceeding 90% over 500 cycles with initial specific capacity of 248 mAh g-1. C-rate tests at 1.2, 2, 4, and 8 C demonstrate outstanding rate capability, with an average capacity of 105 mAh g-1 at 8 C. The performance enhancement mechanism of the organic nanocomposite cathode is elucidated through experimental analyses, including ex-situ XPS, PiFM, and FTIR. These analyses collectively contribute to a comprehensive understanding of the mechanisms underlying the observed improvements in battery performance.

Direct Solution Deposition of Large-Area Non-Solvated Fullerene Single-Crystal Films for High-Performance n-Type Field-Effect Transistors.

Fullerene (C60) crystals have attracted considerable attention in the field of optoelectronic devices owing to their excellent performance as n-type semiconductor material. However, a challenge still remains unbeaten as to the continuous crystallization of non-solvated C60 single-crystal films with high coverage and uniform alignment using low-cost solution techniques. Here, a facile bar coating method is used to prepare ribbon-shaped non-solvated C60 crystals with a large area (up to centimeters) and high coverage (>95%) by precisely controlling the crystallization process from specific solvents. Benefiting from the non-solvated crystalline structure, well-distributed thickness, uniform morphological alignment, and crystallographic orientation, organic field-effect transistors fabricated from the C60 single-crystal films exhibit a high average electron mobility of 2.28cm2V-1s-1, along with the coefficient of variance (CV) as small as 13.6%. This efficient manufacturing method will lay a strong foundation for C60 single-crystal films to fit into the future high-performance integrated optoelectronic application.

Enhanced thermoelectric properties of n-type sulfide compound Bi2SeS2 by Cl doping

Sulfides composed of an eco-friendly and low-cost composition have received widespread attention as a substitute for traditional thermoelectric materials. However, n-type sulfides with high performance are rare compared with p-type ones. This work revealed that Bi2SeS2 with n-type conductivity characteristics is a potential thermoelectric material, whose ZT value reaches ∼ 0.70 at 773 K for the Cl doped sample Bi2Se0.98S2Cl0.02. Cl serving as a carrier donor can increase the electrical conductivity by optimizing the carrier concentration. Coupled with a moderate Seebeck coefficient in the range of 100–200 μV/K, a high power factor of ∼6.46 μWcm−1K−2 at 773 K was achieved. In addition, the lattice thermal conductivity reduces because of the formation of precipitates, as well as the induced mass and stress fluctuations due to the doping of heterogeneous Cl element, which effectively suppress the increase in total thermal conductivity. The combination of optimized power factor and low thermal conductivity leads to a final high ZT value, which is relatively higher than or comparable to most other reported n-type sulfide thermoelectric material.

Insights into enhanced thermoelectric performance of the n-type Mg3Sb2-based materials by amphoteric Al doping

Doping has the potential to alter the levels of anharmonicity in compounds by attenuating bonding strength. In this study, we explore the efficacy of amphoteric Al doping for stimulating anharmonicity in n-type Mg3.25AlxSb1.48Bi0.48Te0.04 to attain enhanced phonon scattering and thermoelectric performance. First-principles calculations and experimental data reveal the occupation of both Sb and Mg2 sites by amphoteric Al atoms in the anionic framework of Mg3.25AlxSb1.48Bi0.48Te0.04. A marginal variation in both carrier concentration and mobility sustains the high power factor without affecting the Seebeck coefficient, implying amphoteric doping induced charge compensation. While phonon velocity, Grüneisen parameter, and crystal orbital Hamilton population calculations results indicate that phonon softening and bond weakening are realized via Al doping, leading to an enhanced lattice anharmonicity and a reduced lattice thermal conductivity. A remarkable enhancement ∼16% in the peak figure of merit ZTpeak and the average ZTavg, was attained for the x = 0.015 sample, when compared with the un-doped sample. Hence, the amphoteric doping can serve as an effective means to optimize ZT values by decoupling the intertwined thermoelectric transport properties.

N-type bismuth telluride-based thermoelectric materials, devices, and applications

n-type bismuth telluride-based thermoelectric materials, devices, and applications

Enhanced thermoelectric properties of 3D printed Bi2Te2.7Se0.3 by atomic interface engineering

The performance of n-type Bi2Te3 materials prepared by selective laser melting (SLM) 3D printing technology tends to be lower than p-type materials. Herein, a bottom-up interface engineering strategy based on Atomic Layer Deposition (ALD) is presented to improve the properties of n-type Bi2Te2.7Se0.3 (BTS) materials prepared by SLM. To validate this strategy, a ZnO ultrathin layer was applied to the grain boundaries of BTS, synergistically optimizing carrier/phonon transport. In terms of electrical transport, the energy filtering effect triggered by the constructed ZnO/BTS interface effectively enhances the Seebeck coefficient of the Bi2Te3 material printed by SLM, substantially increasing the power factor. Additionally, the high thermal stability of the ALD-deposited ZnO ultrathin layer effectively suppresses the volatilization of Te in BTS, regulating the carrier concentration. Regarding thermal transport properties, the multiscale defects introduced by the non-equilibrium solidification process during SLM printing cause effective phonon scattering across a wide frequency range, which in turn reduces the thermal conductivity of the material. Atomic-level interface modification in SLM-printed BTS enhanced the ZT value to 1.25. Exploiting this superior thermoelectric performance, a thermoelectric module was constructed. It demonstrated a conversion efficiency of 5.7 % with a temperature difference of 200 K, comparable to the advanced Bi2Te3-based thermoelectric modules. The integration of an ALD interface modification strategy with SLM technology has not only demonstrated potential applications in Bi2Te3-based thermoelectric material development, but also bears significant importance for the accelerated advancement of high-performance thermoelectric materials in other systems.

Recombination lifetimes and mechanisms of In0.75Ga0.25As and In0.53Ga0.47As as a function of doping density

The minority carrier lifetimes in InGaAs ternary alloys are investigated both experimentally and theoretically. Two groups of InGaAs/InP photodetector samples are designed for experiments, one with In0.75Ga0.25As and the other with In0.53Ga0.47As as the absorption layers. The lifetimes are obtained to be about 100–200 ns in In0.75Ga0.25As and 600 ns-1 µs in In0.53Ga0.47As by microwave photoconductivity decay technique. The calculated lifetimes are also found to be consistent with the experimental ones. Besides, for both InGaAs materials, the Shockley-Read-Hall (SRH), radiative and Auger recombination mechanisms are identified in calculations to be the dominant mechanisms at low, intermediate and high doping densities at room temperature, respectively. Furthermore, the effects of density, capture cross-sections and energy levels (Ets) of recombination centers, and bandgaps (Egs) on the SRH lifetimes are determined and compared for the two InGaAs materials. For an n-type material, when Et is near the mid-bandgap (Eg/2), the SRH lifetime is only inversely proportional to the product of the density and the capture cross-section of the recombination centers. When Et is not near the mid-bandgap, the magnitude of Eg also plays an important role in the SRH lifetime. By increasing the In-content from 53 % in In0.53Ga0.47As to 75 % in In0.75Ga0.25As, where the bandgap narrows from 0.75 to 0.52 eV, the SRH lifetime in In0.75Ga0.25As is shortened by a factor of about 1–42 for Et < Eg/2 and 1–65 for Et > Eg/2 within the bandgap, respectively.

Synergistic Improvement of BiI3 and In on Thermoelectric Properties of Zone-Melted n-Type Bi2Te2.7Se0.3.

Bismuth telluride (Bi2Te3) is the only commercial thermoelectric material so far, and it is also the best thermoelectric material with the best performance at room temperature. However, up to now, the zT value of n-type materials used on a large scale is only about 1.0; this makes the thermoelectric conversion efficiency of thermoelectric devices and thermoelectric applications stagnant. Therefore, under the synergistic action of BiI3 and In, the properties of n-type Bi2Te2.7Se0.3 material are improved. The experiments show that BiI3, which is nontoxic and non-absorbent, can effectively improve the power factor of the material and inhibit the bipolar effect and is an effective dopant. After the inclusion of In, due to the low bond energy of the In-Te bond, it is easy to form the InTe phase in the matrix material and then introduce the second phase, and the presence of the second phase in the material will scatter phonons and reduce the lattice thermal conductivity so that it can reach 0.31 W m-1 K-1 at 350 K. Ultimately, a high maximum zT of 1.20 at 325 K and a remarkable average zT of 1.04 (300-500 K) are attained in the In0.005Bi1.995Te2.7Se0.3 + 0.13 wt % BiI3 sample.

Metallic Degenerately Doped Free-Electron-Confined Plasmonic Nanocrystal and Infrared Extinction Response

In this paper, synthetically scaled-up degenerately n-type doped indium tin oxide (Sn:In2O3) nanocrystals are described as highly transparent conductive materials possessing both optoelectronic and crystalline properties. With tin dopants serving as n-type semiconductor materials, they can generate free-electron carriers. These free electrons, vibrating in resonance with infrared radiation, induce strong localized surface plasmon resonance (LSPR), resulting in efficient infrared absorption. To commercialize products featuring Sn:In2O3 with localized surface plasmon resonance, a scaled-up synthetic process is essential. To reduce the cost of raw materials during synthesis, we aim to proceed with synthesis in a large reactor using industrial raw materials. Sn:In2O3 can be formulated into ink dispersed in solvents. Infrared-absorbing ink formulations can capitalize on their infrared absorption properties to render opaque in the infrared spectrum while remaining transparent in the visible light spectrum. The ink can serve as a security ink material visible only through infrared cameras and as a paint absorbing infrared light. We verified the transparency and infrared absorption properties of the ink produced in this study, demonstrating consistent characteristics in scaled-up synthesis. Due to potential applications requiring infrared absorption properties, it holds significant promise as a robust platform material in various fields.

Enhanced Thermoelectric Performance of N-Type PbSe Through Semi-Coherent Nanostructure by AgCuTe Alloying.

N-type PbSe thermoelectric materials encounter challenges in improving the power factor due to the single-band structure near the Fermi level, which obstructs typical band convergence. The primary strategy for enhancing the thermoelectric figureof merit (ZT) for n-type PbSe involves reducing lattice thermal conductivity (κlat) by introducing various defect structures. However, lattice mismatches resulting from internal defects within the matrix can diminish carrier mobility, thereby affecting electrical transport properties. In this study, n-type AgCuTe-alloyed PbSe systems achieve a peak ZT value of ≈1.5 at 773 K. Transmission electron microscopy reveals nanoprecipitates of Ag2Te, the room temperature second phase of AgCuTe, within the PbSe matrix. Meanwhile, a unique semi-coherent phase boundary is observed between the PbSe matrix and the Ag2Te nanoprecipitates. This semi-coherent phase interface effectively scatters low-frequency phonons while minimizing damage to carrier mobility. Additionally, the dynamic doping effect of Cu atoms from the decomposition of AgCuTe within the matrix further optimize the high-temperature thermoelectric performance. Overall, these factors significantly enhance the ZT across the whole temperature range. The ZT value of ≈1.5 indicates high competitiveness compared to the latest reported n-type PbSe materials, suggesting that these findings hold promise for advancing the development of efficient thermoelectricsystems.

Tailoring the Density of State of n-Type Conjugated Polymers through Solvent Engineering for Organic Electrochemical Transistors.

Conjugated polymers with ethylene glycol-type side chains are commonly used as channel materials in organic electrochemical transistors (OECTs). To improve the performance of these materials, new chemical structures are often created through synthetic routines. Herein, we demonstrate that the OECT performance of these polymers can also be improved by changing their density-of-state (DOS) profile through solvent engineering. Depending on the solvent polarity, it solvates the backbone and side chains of the conjugated polymer differently, leading to differences in molecule orientation, π-stacking paracrystallinity, and film defects, such as grain boundaries and pinholes. This then results in a change in the DOS profile of the polymer. A more intense and narrow-width DOS distribution is usually observed in organic films with an "edge on" orientation and fewer film defects, while films with a "face on" orientation and apparent defects show a broadened DOS profile. The OECT devices that use the polymer film with a more intense and narrow-width DOS profile exhibit a better-normalized transconductance and figure-of-merit μC* than those with a broadened DOS profile (0.74 to 4.29 S cm-1 and 3.5 to 14.3 F cm-1 V-1 s-1). This study provides useful insights into how the DOS profile affects the mixed ionic-electronic conduction performance and presents a new avenue for improving n-type OECT materials.

Synthesis and Characterization of Octacyano-Cu-Phthalocyanine.

Octacyano-metal-substituted phthalocyanine MPc(CN)8 is a promising n-type stable organic semiconductor material with eight cyano groups, including a strong electron-withdrawing group at its molecular terminals. However, most MPc(CN)8 have not been thoroughly investigated. Therefore, CuPc(CN)8 was synthesized in this study and its crystal structure, chemical and electronic states, thermal stability, and electrical properties were investigated. This article discusses the various properties of CuPc(CN)8, as compared to those of CuPc and FePc(CN)8. The previously reported FePc(CN)8 is an organic semiconductor molecule with a molecular structure similar to that of CuPc(CN)8. X-ray diffraction (XRD) measurements revealed that CuPc(CN)8 has a crystalline structure in the P1̅ space group. The crystal structure forms an in-plane network parallel to the molecular plane through multiple hydrogen bonds by the cyano groups at the molecular terminals. Interestingly, the crystal structure, especially the molecular stacking, of CuPc(CN)8 differs from that of FePc(CN)8. The absorption edge observed in the ultraviolet-visible spectrum of CuPc(CN)8 shifted to a longer wavelength than that of CuPc, which was attributed to the energy gap of CuPc(CN)8 being smaller than that of CuPc owing to the influence of the cyano groups at the molecular terminals, according to the molecular orbital calculation results using density functional theory. Ultraviolet photoelectron spectroscopy measurements confirmed that CuPc(CN)8 had a stronger n-type character than CuPc because of the orbital energy stabilization by the cyano groups. Thermogravimetry/differential thermal analysis measurements revealed that the thermal stability of CuPc(CN)8 was significantly higher than that of FePc(CN)8. CuPc(CN)8 exhibited photoconduction upon visible-light irradiation, and its electrical conductivity was higher than that of CuPc, which was attributed to a reduction in the electron injection barrier at the electrode interfaces.

High Mechanical Property and Texture Degree of Hot-Extruded Bi0.905Sb0.095

Bi1−xSbx crystal is one of the best n-type thermoelectric materials below 200 K, but its weak mechanical strength hinders practical applications for deep refrigeration. Herein, we adopted the mechanical enhancement method of hot extrusion to investigate the comprehensive mechanical and thermoelectric properties of Bi0.905Sb0.095. It revealed that reducing the grain size of the matrix and increasing the extrusion ratio can improve the gain size uniformity and mechanical properties. Meanwhile, the thermoelectric performance depends on the texture, grain size, and local composition. The extruded sample prepared by ingot with the high extrusion ratio of 9:1 generated uniform small grains, which resulted in the high bending strength of Bi1−xSbx~130 Mpa and a high power factor of ~68 μW·cm−1·K−2@173 K, as well as the relatively high figure of merit of 0.25@173K. This work highlights the importance of the uniform distribution of the grain size and the compositions for Bi1−xSbx, as well as the required universal key parameter for the hot extrusion method.

First-principles calculations to investigate effects of strains on structural, electronic, thermodynamic, optical and thermoelectric properties of NaAs3P5 alloy

Herein, the structural, optical, electronic and thermoelectric properties of the NaAs3P5 were investigated by Ab-initio calculations. The electronic results of NaAs3P5 showed that this compound has a metallic characteristic. The optical response of this compound has been studied using optical parameters such as complex dielectric function, refractive index, optical conductivity, electron energy loss, and absorption coefficient. This compound is an n-type material, as determined by thermoelectric experiments because it has a negative Seebeck coefficient when the strain is at 20 %. The highest value of the figure of merit (ZT) is about 0.021. Besides, the obtained value of PF is 7595.5 W−1.K−2.s−1 for 20 % of the strain. As the ZT trend grows, the alloy is being promoted as a potential contender for thermoelectric power production. Applying dilatation strain can improve the mechanical and thermodynamic stability of NaAs3P5 because many solid-state physical characteristics, such as heat capacity, thermal expansion coefficient, and entropy, are related to the Debye temperature.

N-type small molecule electrolyte cathode interface layer with thickness insensitivity for organic solar cells

Interface engineering has a critical impact on the performances of organic solar cells (OSCs). And cathode interface layer (CIL) with thickness insensitivity is urgently pursued to improve the possibility of industrialization of OSCs. N-type self-doping has been proven effective in increasing electron mobility. Here, four novel n-type small molecule electrolytes (SMEs) with diverse counter anions (CAs), PDIN-BF4, PDIN-BPh4, PDIN-BPhF4, and PDIN-BIm4 were synthesized and employed as cathode interface layers (CILs). Among them, PDIN-BIm4-based OSCs with PM6:Y6 active layer achieved the most glorious electron mobility and thickness insensitivity with a power conversion efficiency (PCE) of 16.98 % due to outstanding self-doping effect and interfacial regulation ability. However, the multi-F atoms on PDIN-BPhF4 may prevent self-doping progress and impede electron transport, thus leading to a low PCE of 11.53 %. Meanwhile, the PDIN-BIm4-based device can maintain over 80 % of the optimal PCE with a thickness of 43 nm or storing in a glove box for 600 h. In addition, PM6: BTP-eC9-based device with PDIN-BIm4 CIL acquired a PCE of 17.82 %, highlighting the broad applicability of PDIN-BIm4. Our work demonstrates that the introduction of CAs into n-type organic materials helps promote the progress of efficient and stable OSCs.

Enhanced n-Type Thermoelectric Properties and Structure Evolution of Carbonized Metal-Coordination Polydopamine.

Carbonized polydopamine (cPDA) exhibits a nitrogenous graphite-like structure with n-type semiconductor property. However, the low electrical conductivity and Seebeck coefficient of cPDA cannot meet the needs of flexible thermoelectric devices. In this study, a series of metal ions were coordinated with cPDA to enhance n-type thermoelectric properties. At 300 K, all metal-coordination cPDA (metal-cPDA) samples obtain lower thermal conductivity compared to cPDA. Mn-cPDA exhibits the greatest Seebeck coefficient of -25.94 μV K-1, which is almost six times higher than cPDA. Fe-cPDA shows the best electrical conductivity of 2.45 × 105 S m-1. An optimal power factor (PF) value of 11.93 μW m-1 K-2 is achieved in Ca-cPDA with the enhanced electrical conductivity of 9.5 × 104 S m-1 and Seebeck coefficient of -11.24 μV K-1. Using Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), we revealed the structural characterization of metal-cPDA. Our results indictate that the different metal ions (Mn2+, Zn2+, Mg2+, Al3+, Ca2+, and Fe3+) exert varying influences on the growth of graphite-like structure within metal-cPDA, which lead to the evolution of electrical conductivity. We observe that the carrier density and carrier mobility depend on both the degree of graphitization and the metal-ion coordination, which work together on electrical conductivity and Seebeck coefficient. These findings and understanding of the thermoelectric properties of PDA-based materials will help to realize high-performance n-type thermoelectric materials for flexible electronic device applications.

Tailoring interfacial states for improved n-type bismuth telluride thermoelectrics

Efforts to enhance the thermoelectric (TE) efficiency of n-type bismuth telluride (BTS) have focused on leveraging 2D nanostructures, as they have been proven effective in inhibiting phonon transport and simultaneously improving electrical properties. Herein, we introduce g-C3N4 as a suitable candidate for disrupting phonon transport, with the potential to finely tune charge carrier mobility. Crucially, our investigation provides insights into the potential of externally introduced conducting states as a feasible strategy for facilitating carrier transport at heterogeneous interfaces. In contrast, the carrier scattering and localization events play an inverse role. By carefully modulating the competition, we effectively increase carrier mobility, boosting electrical conductivity and optimizing the power factor. This orchestrated strategy leads to a high figure of merit (ZT) of 1.29 at 400 K and an average ZT (ZTave) of 1.20 within 300–500 K. At temperature difference ∆T = 180 K, a maximum power output Pmax = 0.91 W and a 6.2 % conversion efficiency (η) can be achieved over the fabricated TE module. Our findings underscore the potential of 2D nanomaterials and interfacial engineering (IE) as a promising avenue for unlocking the full potential of n-type thermoelectric materials, advancing sustainable and efficient TE power generation.

SYNTHESIS OF STRUCTURED ZINC SELENIDE FOR OPTOELECTRONIC APPLICATIONS

In this paper, synthesis of Zinc Selenide (ZnSe) based photovoltaic materials deposited by electrode position method. The material lms have been characterized for their optical properties using UV-VIS spectrophotometer with wavelength range of 300 nm – 900 nm and electrical conductivity using photoelectrochemical (PEC) cell measurement. The conductivity type of the electrodeposited materials was investigated together with the tuning of the optical phenomena. The materials deposited were obtained by varying deposition time from 5 minutes to 15 minutes for ZnSe. It was observed that the optical properties of the materials increased with an increase in the time of deposition. The optical band gap of ZnSe was found to be in the range between 2.03 and 2.46 eV. The PEC signal results revealed ZnSe as an n-type material.

Titanium-coated high-density Fe2O3 single crystal nanowire array for solar water splitting

Fe2O3 holds promising n-type semiconductor material in the field of solar water splitting due to its excellent photocatalytic properties. However, the photoelectrochemical performance of Fe2O3 is limited by its inherent properties such as poor conductivity, and charge separation efficiency owing to its recombination rate. Therefore, researchers are more focused on nanostructuring, doping, and surface coating to overcome these issues of Fe2O3. In this study, we have investigated a low-cost way to fabricate a Ti coating layer on a high-density Fe2O3 single-crystal nanowire array for solar water splitting. Firstly, we have prepared a high-density single-crystal Fe2O3 nanowire array at lower temperatures by a new approach stress-induced atomic diffusion method. Thereafter, the prepared nanowire array was coated by Ti film using RF sputtering. The optimal film thickness of 13 nm titanium coatings layer into Fe2O3 single crystal nanowire array exhibited a high photocurrent density of 1.36 mA/cm2 at 1.23 V versus RHE and solar to hydrogen conversion efficiency (STH) of 1.67%, which could be resulting from adjusted optoelectronic properties of the nanowires.

A Highly Conductive n-Type Conjugated Polymer Synthesized in Water.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a benchmark hole-transporting (p-type) polymer that finds applications in diverse electronic devices. Most of its success is due to its facile synthesis in water, exceptional processability from aqueous solutions, and outstanding electrical performance in ambient. Applications in fields like (opto-)electronics, bioelectronics, and energy harvesting/storage devices often necessitate the complementary use of both p-type and n-type (electron-transporting) materials. However, the availability of n-type materials amenable to water-based polymerization and processing remains limited. Herein, we present a novel synthesis method enabling direct polymerization in water, yielding a highly conductive, water-processable n-type conjugated polymer, namely, poly[(2,2'-(2,5-dihydroxy-1,4-phenylene)diacetic acid)-stat-3,7-dihydrobenzo[1,2-b:4,5-b']difuran-2,6-dione] (PDADF), with remarkable electrical conductivity as high as 66 S cm-1, ranking among the highest for n-type polymers processed using green solvents. The new n-type polymer PDADF also exhibits outstanding stability, maintaining 90% of its initial conductivity after 146 days of storage in air. Our synthetic approach, along with the novel polymer it yields, promises significant advancements for the sustainable development of organic electronic materials and devices.