Functionally graded materials (FGMs) are widely explored in the context of inorganic thermoelectrics, but not yet in organic thermoelectrics. Here, the impact of doping gradients on the thermoelectric properties of a chemically doped conjugated polymer is studied. The in-plane drift of counterions in moderate electric fields is used to create lateral doping gradients in films composed of a polythiophene with oligoether side chains, doped with 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ). Raman microscopy reveals that a bias voltage of as little as 5 V across a 50 mu m wide channel is sufficient to trigger counterion drift, resulting in doping gradients. The effective electrical conductivity of the graded channel decreases with bias voltage, while an overall increase in Seebeck coefficient is observed, yielding an up to eight-fold enhancement in power factor. Kinetic Monte Carlo simulations of graded films explain the increase in power factor in terms of a roll-off of the Seebeck coefficient at high electrical conductivities in combination with a mobility decay due to increased Coulomb scattering at high dopant concentrations. Therefore, the FGM concept is found to be a way to improve the thermoelectric performance of not yet optimally doped organic semiconductors, which may ease the screening of new materials as well as the fabrication of devices. Electrically programmed doping gradients are found to enhance the thermoelectric power factor of a chemically doped conjugated polymer. The in-plane drift of counterions in moderate electric fields is used to create lateral doping gradients in doped polymer films, resulting in a decrease of the effective electrical conductivity with bias voltage but an overall increase in Seebeck coefficient. image

In order for organic thermoelectrics to successfully establish their own niche as energy-harvesting materials, they must reach several crucial milestones, including high performance, long-term stability, and scalability. Performance and stability are currently being actively studied, whereas demonstrations of large-scale compatibility are far more limited and for carbon nanotubes (CNTs) are still missing. The scalability challenge includes material-related economic considerations as well as the availability of fast deposition methods that produce large-scale films that simultaneously satisfy the thickness constraints required for thermoelectric modules. Here we report on true solutions of CNTs that form gels upon air exposure, which can then be dried into micron-thick films. The CNT ink can be extruded using a slot-shaped nozzle into a continuous film (more than half a meter in the present paper) and patterned into alternating n- and p-type components, which are then folded to obtain the finished thermoelectric module. Starting from a given n-type film, differentiation between the n and p components is achieved by a simple postprocessing step that involves a partial oxidation reaction and neutralization of the dopant. The presented method allows the thermoelectric legs to seamlessly interconnect along the continuous film, thus avoiding the need for metal electrodes, and, most importantly, it is compatible with large-scale printing processes. The resulting thermoelectric legs retain 80% of their power factor after 100 days in air and about 30% after 300 days. Using the proposed methodology, we fabricate two thermoelectric modules of 4 and 10 legs that can produce maximum power outputs of 1 and 2.4 μW, respectively, at a temperature difference ΔT of 46 K.

Soft multi-band compatible electromagnetic wave absorbers have wide applications in 6G communication, radar stealth, detection instruments, and radio astronomy. In this context, a kind of double-layer staggered woodpile absorber is designed and manufactured by the direct ink writing (DIW) technology with a composite ink system composed of reduced graphene oxide (RGO)/spherical carbonyl iron (SCI) and polydimethylsiloxane (PDMS). The as-prepared absorber shows compatible absorption in both microwave and terahertz frequency bands. The effective absorption bandwidth (EAB) and minimum reflection loss (RLmin) reach 8.24 GHz and-52.2 dB at a thickness of 3.2 mm, respectively. In the terahertz band (0.5-3.5 THz), it shows effective absorption (absorption rate greater than 99 %) and achieves RL <-20 dB throughout the entire tested band. In addition, the double-layer staggered woodpile absorber has good flexibility and can be conveniently used for curved substrates. This work enables the potential for rapid manufacturing of flexible and compatible absorbing devices through DIW 3D printing for applications in 5/6G communication, military stealth and flexible wearable intelligent devices.

Conjugated polymers with oligoether side chains make up a promising class of thermoelectric materials. In this work, the impact of the side-chain length on the thermoelectric and mechanical properties of polythiophenes is investigated. Polymers with tri-, tetra-, or hexaethylene glycol side chains are compared, and the shortest length is found to result in thin films with the highest degree of order upon doping with the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ). As a result, a stiff material with an electrical conductivity of up to 830 +/- 15 S cm(-1) is obtained, resulting in a thermoelectric power factor of about 21 mu W m(-1) K-2 in the case of as-cast films. Aging at ambient conditions results in an initial decrease in thermoelectric properties but then yields a highly stable performance for at least 3 months, with values of about 200 S cm(-1) and 5 mu W m(-1) K-2. Evidently, identification of the optimal side-chain length is an important criterion for the design of conjugated polymers for organic thermoelectrics.

A common way of determining the majority charge carriers of pristine and doped semiconducting polymers is to measure the sign of the Seebeck coefficient. However, a polarity change of the Seebeck coefficient has recently been observed to occur in highly doped polymers. Here, it is shown that the Seebeck coefficient inversion is the result of the density of states filling and opening of a hard Coulomb gap around the Fermi energy at high doping levels. Electrochemical n‐doping is used to induce high carrier density (>1 charge/monomer) in the model system poly(benzimidazobenzophenanthroline) (BBL). By combining conductivity and Seebeck coefficient measurements with in situ electron paramagnetic resonance, UV–vis–NIR, Raman spectroelectrochemistry, density functional theory calculations, and kinetic Monte Carlo simulations, the formation of multiply charged species and the opening of a hard Coulomb gap in the density of states, which is responsible for the Seebeck coefficient inversion and drop in electrical conductivity, are uncovered. The findings provide a simple picture that clarifies the roles of energetic disorder and Coulomb interactions in highly doped polymers and have implications for the molecular design of next‐generation conjugated polymers.

A combinatorial study of the effect of in‐mixing of various guests on the thermoelectric properties of the host workhorse polymer poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (PBTTT) is presented. Specifically, the composition and thickness for doped films of PBTTT blended with different polymers are varied. Some blends at guest weight fractions around 10–15% exhibit up to a fivefold increase in power factor compared to the reference material, leading to zT values around 0.1. Spectroscopic analysis of the charge‐transfer species, structural characterization using grazing‐incidence wide‐angle X‐ray scattering, differential scanning calorimetry, Raman, and atomic force microscopy, and Monte Carlo simulations are employed to determine that the key to improved performance is for the guest to promote long‐range electrical connectivity and low disorder, together with similar highest occupied molecular orbital levels for both materials in order to ensure electronic connectivity are combined.

Conducting polymers, such as the p -doped poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), have enabled the development of an array of opto- and bio-electronics devices. However, to make these technologies truly pervasive, stable and easily processable, n -doped conducting polymers are also needed. Despite major efforts, no n -type equivalents to the benchmark PEDOT:PSS exist to date. Here, we report on the development of poly(benzimidazobenzophenanthroline):poly(ethyleneimine) (BBL:PEI) as an ethanol-based n -type conductive ink. BBL:PEI thin films yield an n -type electrical conductivity reaching 8 S cm −1 , along with excellent thermal, ambient, and solvent stability. This printable n -type mixed ion-electron conductor has several technological implications for realizing high-performance organic electronic devices, as demonstrated for organic thermoelectric generators with record high power output and n -type organic electrochemical transistors with a unique depletion mode of operation. BBL:PEI inks hold promise for the development of next-generation bioelectronics and wearable devices, in particular targeting novel functionality, efficiency, and power performance.

By spin-coating silver nanowires (AgNWs) and polymethyl methacrylate (PMMA), applying pressure imprint and plasma treatment, we obtained flat AgNW thin film with a sheet resistance of 20 Ω/sq and a transmittance of 78% at 550 nm with low surface roughness. No significant change in sheet resistance was observed after cyclic bending (bending radius is 5 mm) test and tape test. After 1 000 bending tests, the change rate of sheet resistance was only 8.3%. The organic light-emitting devices (OLEDs) were prepared by using such AgNW electrodes and a maximum brightness of 5 090 cd/m2 was obtained. Compared with the AgNWs electrode without any treatment, the present AgNW electrodes have lower sheet resistance and better hole injection. Our results show spin-coated with flat layers, embossed and plasma-treated AgNW electrodes are suitable for manufacturing flexible organic optoelectronic devices.

In this paper, co-doping method is used to improve the current efficiency of solution-processed organic light-emitting diodes (OLEDs). By changing the ratio of two thermally activated delayed fluorescent (TADF) emitters, we studied the performance of device and its mechanism. A solution processed OLED with a structure of indium tin oxide (ITO, 150 nm)/PEDOT:PSS (30 nm)/CBP:4CzIPN-x%:4CzPN-y% (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) was fabricated. The current efficiencies of 26.6 cd/A and 26.4 cd/A were achieved by the devices with dopant ratio of 6% 4CzIPN:2% 4CzPN and 2% 4CzIPN:6% 4CzPN in emitting material layer (EML), respectively. By investigating the tendency of current density change in devices with different doping ratio, we suggested that the enhancement of the current efficiency should be due to the charge transport balance improvement induced by assist dopant in EML.

CsPbBr3 all-inorganic perovskite light-emitting diodes (PeLEDs) have received increasing attention in recent years due to their unique luminescent property and superior thermal stability. However, the low solubility of bromide precursors and the fast crystallization of perovskites usually cause the formation of discontinuous CsPbBr3 films with rough grains and large pinholes. This would increase the defects and nonradiative recombination and severely degrade the device electroluminescence (EL) performance. To tackle this issue, herein, the 1,3-bis(9H-carbazol-9-yl)benzene (mCP) small molecule was elaborately selected as an effective additive to enhance the film-forming ability of CsPbBr3 emissive layers. With the addition of mCP, the submicrometer-sized CsPbBr3 grains were reduced to nanometer range, and the resulting perovskite films became uniform and continuous. It was found that the electron pairs of N2- in mCP would donate to metal Pb2+ in CsPbBr3. Thus, the so-called Lewis acid-base reaction occurred, which could retard the fast crystallization process and contribute to the connection of perovskite grains. As such, the film roughness was decreased from 8.28 to 1.62 nm, and the carrier lifetime was increased from 2.49 to 68.39 ns. The CsPbBr3 :mCP device with an optimized mass ratio of 1:0.10 exhibited a maximum luminance (L) of 21008 cd/m(2), a maximum current efficiency (CE) of 3.74 cd/A, and the corresponding maximum external quantum efficiency (EQE) of 1.21%, far surpassing the EL performance of the pristine CsPbBr3 PeLEDs.

Doping of organic semiconductors is crucial for the operation of organic (opto)electronic and electrochemical devices. Typically, this is achieved by adding heterogeneous dopant molecules to the polymer bulk, often resulting in poor stability and performance due to dopant sublimation or aggregation. In small-molecule donor-acceptor systems, charge transfer can yield high and stable electrical conductivities, an approach not yet explored in all-conjugated polymer systems. Here, we report ground-state electron transfer in all-polymer donor-acceptor heterojunctions. Combining low-ionization-energy polymers with high-electron-affinity counterparts yields conducting interfaces with resistivity values five to six orders of magnitude lower than the separate single-layer polymers. The large decrease in resistivity originates from two parallel quasi-two-dimensional electron and hole distributions reaching a concentration of ∼1013 cm-2. Furthermore, we transfer the concept to three-dimensional bulk heterojunctions, displaying exceptional thermal stability due to the absence of molecular dopants. Our findings hold promise for electro-active composites of potential use in, for example, thermoelectrics and wearable electronics.

N-type semiconducting polymers have been recently utilized in thermoelectric devices, however they have typically exhibited low electrical conductivities and poor device stability, in contrast to p-type semiconductors, which have been much higher performing.

A novel strategy of doping organic small molecule mCP in light-emitting layers (EMLs) is first proposed to improve the film-forming ability of CdSe/ZnS quantum dots (QDs), which tremendously enhances the device luminescence performance of quantum-dot light-emitting diodes (QLEDs). Accordingly, with the addition of appropriate amount of mCP, the surface roughness of the emitting layer is reduced from 3.31 nm to 1.59 nm, the maximum brightness of the device increases from 5340.3 cd/m(2) to 10651 cd/m(2), and the corresponding maximum current efficiency (CE) of 18.0 cd/A and external quantum efficiency (EQE) of 3.30% are both achieved. Therefore, our method successfully overcomes the QDs aggregation blocking issue and paves the way towards high-performance QLEDs.

Quantum dot light-emitting diode (QLED) displays are considered a next-generation technology, but previously reported quantum dots (QDs) consisting of heavy metals are toxic and harmful. This work examined earth-abundant, metal-free, graphitic C3N4 (g-C3N4) with exceptional optical and electronic properties, excellent chemical and thermal stability, an appropriate band gap, and non-toxicity for QLED applications. The dependence of the luminescence performance on the reaction atmosphere and temperature; the transformation of the crystal and electronic structures during the reaction, including crystal defects and surface functional groups; and the luminescence mechanisms of g-C3N4 were uncovered. The highest quantum yield of 49.8% was achieved by the sample possessing the highest graphitic-to-triazine carbon ratio synthesized at 500 degrees C under N-2 atmosphere. The disappearance of the charge-transfer band, crystal defects (traps), and non-radiative transition (due to fast relaxation) from the absorption spectra demonstrates the enhanced quantum efficiency of the g-C3N4 QDs over that of the bulk powders. A QLED prototype device employing g-C3N4 QDs as the blue-emitting layer was demonstrated.

We fabricated phosphorescent organic light-emitting diodes (PhOLEDs) using thermally activated delayed fluorescence (TADF) material 10,10′-(4,4′-sulfonylbis(4,1-phenylene)) bis(9,9-dimethyl-9,10-dihydroacridine) (DMAC-DPS) with low concentration, which showed better performance compared with 1,3-bis(carbazole-9-yl) benzene (mCP) based devices. When the concentration of DMAC-DPS was 1wt%, the driving voltage of the device was only 3.3 V at 1 000 cd/m2, and the efficiency and lifetime of the device were effectively improved compared with those of mCP based devices. The result indicated that DMAC-DPS could effectively improve the performance of phosphorescent devices. We believe that the better device performance can be attributed to the optimization of the energy transfer process in the emitter layer and lifetime of triplet excitons by DMAC-DPS. The study may provide a simple and effective strategy to achieve high-performance OLEDs.

In this study, we simulate the transient electroluminescence (EL) in organic light-emitting diodes. The forming mechanism of transient EL spikes is discussed in detail. After applying a voltage pulse, the remaining mobile charges drift to the opposite trapped charges and lead to an increase in the exciton recombination rate, which corresponds to the EL spike phenomenon. We observe an EL spike in a solution-processed 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenz-ene(4CzIPN)-doped emitting device, in which 4CzIPN acts as an electron trapping center and confines the electrons within the emitting layer. To further study the effect of the mobility on the EL spike, we simulate it at different electron/hole mobilities. The results show that increasing the hole mobility increases the EL spike. Moreover, when the charge mobility is temperature- and electric field-dependent, the simulation results suggest that if the charge mobility is more susceptible to the electric field, then a lower EL spike is observed, whereas with temperature decrease, a longer tailing is noticeable at the falling edge.

In this paper, p-chlorophenylacetic acid and p-fluorophenylacetic acid were applied to modify the indium tin oxide (ITO) electrodes. The surface work functions of unmodified ITO, p-chlorophenylacetic acid modified ITO (Cl-ITO) and p-fluorophenylacetic acid modified ITO (F-ITO) are 5.0 eV, 5.26 eV and 5.14 eV, respectively, and the water contact angles are 7.3°, 59.1° and 46.5°, respectively. The increase of the work function makes the hole injection ability of the devices improved, which is proved by the hole transport devices. The self-assembly (SAM) layers transfer hydrophilic ITO to hydrophobic ITO, which makes ITO more compatible with the hydrophobic organic layers, making the organic film more stable during the operation. After modification, the organic light emitting diodes (OLEDs), SAM-modified ITO/NPB/Alq3/LiF/Al, with better performance and stability were fabricated. Especially, the OLED with Cl-ITO (Cl-OLED) has a maximum luminance of 22 428 cd/m2 (improved by 32.9%) and a half-lifetime of 46 h. Our results suggest that employing organic acids to modify ITO surface can enhance the stability and the luminescent properties of OLED devices.

To achieve uniform distribution of silver nano clusters（SNCs） on substrate and reveal its effect on the performance of organic light-emitting diode（OLED）, the SNCs incorporated OLED was fabricated and SNCs were coated by multi-step spin coating. Compared with the device without SNCs film, the brightness and current efficiency of the OLED devices with SNCs film were highly raised. The enhancement is attributed to SNCs induced local surface plasmon（LSP） oscillation, which can increase the radiative rate of excitons on Alq 3 molecules.

In this data article, the normalized emission and excitation spectra, the ultraviolet-visible (UV-vis) absorption spectra, and the X-ray photoelectron spectroscopy (XPS) of bulk-powders and nano-structured graphitic C3N4 (g-C3N4) were presented, which are helpful to get insight into the crystal and electronic structures of g-C3N4, especially on determining the energy levels and the mechanisms of luminescence originating from electron transitions. This data article is related to our recent publication (He et al., in press) [1]. The absorption, excitation and emission spectra are vital to illustrate the optoelectronic performances in terms of photoluminescence, photocatalysis, electroluminescence, etc., from the viewpoint of electron transitions intrinsically.

We have developed an assistant dopant system in phosphorescent OLEDs. The system is consisted of a conventional host material 4,4'-Bis(carbazol-9-yl)biphenyl (CBP), a thermally activated delayed fluorescence (TADF) 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) and a red phosphor Bis[2-(4-n-hexylphenyl)quinoline](acetylacetonate)iridium(III) (Hex-Ir(phq)(2)(acac)), it has achieved maximum current efficiency and external quantum efficiency of 21.52 cd/A and 9.22%, respectively. The intrinsic mechanism and energy transfer process in the system have been discussed in detail. Moreover, a satisfactory critical current density Jo of 106.1 mA/cm(2) has been obtained when the assistant dopant concentration ascends to a suitable level, which indicated low efficiency roll-off of the device. By investigating the exciton generation mechanism and the transient electroluminescence (EL), the main cause of the improvement has been revealed. (C) 2017 Elsevier B.V. All rights reserved.