Enhanced Charge Transport Properties Research Articles

Buffer Chain Model for Understanding Crystallization Competition in Conjugated Polymers

AbstractIt remains challenging to comprehensively understand the packing models of conjugated polymers, in which side chains play extremely critical roles. The side chains are typically flexible and non‐conductive and are widely used to improve the polymer solubility in organic solutions. Herein, a buffer chain model is proposed to describe link between conjugated backbone and side chains for understanding the relationship of crystallization competition of conductive conjugated backbones and non‐conductive side chains. A longer buffer chain is beneficial for alleviating such crystallization competition and further promoting the spontaneous packing of conjugated backbones, resulting in enhanced charge transport properties. Our results provide a novel concept for designing conjugated polymers towards ordered organization and enhanced electronic properties and highlight the importance of balancing the competitive interactions between different parts of conjugated polymers.

Buffer Chain Model for Understanding Crystallization Competition in Conjugated Polymers.

It remains challenging to comprehensively understand the packing models of conjugated polymers, in which side chains play extremely critical roles. The side chains are typically flexible and non-conductive and are widely used to improve the polymer solubility in organic solutions. Herein, a buffer chain model is proposed to describe link between conjugated backbone and side chains for understanding the relationship of crystallization competition of conductive conjugated backbones and non-conductive side chains. A longer buffer chain is beneficial for alleviating such crystallization competition and further promoting the spontaneous packing of conjugated backbones, resulting in enhanced charge transport properties. Our results provide a novel concept for designing conjugated polymers towards ordered organization and enhanced electronic properties and highlight the importance of balancing the competitive interactions between different parts of conjugated polymers.

Interfacial Engineering of Selenium‐Based Photodiodes for Extremely Low‐Noise Photodetection and Single‐Pixel Imaging

AbstractSelenium as one of the most historical semiconductors has been used for photovoltaics and photodetection, thanks to its excellent optoelectronic properties, superior stability, facile and low‐cost fabrication. However, commercialized Se‐based devices are mainly based on its amorphous state, which significantly limits the charge transport and requires relatively large bias voltage. The charge carrier dynamics of high‐performance crystalline Se thin films are barely reported, and related photodiodes have not been realized mainly limited by the large dark current and slow response speed. Here, poly‐crystalline Se thin films via vacuum evaporation, in situ thermal annealing, and engineered the interfaces with various metal electrodes are fabricated, which resulted in enhanced charge transport properties and reduced device leakage. The optimized Se photodiodes exhibited excellent performance metrics, including extremely low dark current density, decent specific detectivity, and superior stability. Furthermore, the optimized Se photodetectors are also introduced for single‐pixel imaging, which demonstrates excellent sensitivity and great potential for real applications.

Post-Synthetic Modifications of Ti(IV)-Based Metal-Organic Frameworks for Enhanced Photocatalysis and Conductivity

Metal-Organic Frameworks (MOFs) are coordination networks with organic ligands containing potential voids, ideal for guest-host interactions. These unique characteristics make them interesting materials for several different applications, including catalysis. Unfortunately, most MOFs feature metal oxide nodes that are insulting by nature, which limits their potential in electro- and photochemical catalysts. To combat these issues, post-synthetic modifications (PSM) can be used to tune MOF properties. This research reports on different PSMs incorporated into the Ti(IV)-based MOFs MIL-125(Ti) and MIL-125(Ti)-NH2. PSMs that introduce intentional defect states and dopant metal ions, such as Ce(IV), were explored to determine their influence on the photo-degradation of Rhodamine B in aqueous solution. Post-Synthetic Metal Exchange (PSME) was used to incorporate Ce(IV) at the Ti(IV) sites in MIL-125(Ti) and MIL-125(Ti)-NH2, and intentional defect states were introduced using formic acid during MOF synthesis. In addition, conductive organic polymers were utilized to make novel polymer-MOF hybrid composites that showed enhanced charge transport properties compared to pristine MOF. Ultimately, these results show how PSMs can be used to impart conductivity and enhance photocatalysis in the MOFs MIL-125(Ti) and MIL-125(Ti)-NH2.

Self-powered solution-processed photodetectors utilizing FAPbBr3:C60 bulk heterojunction

Hybrid halide perovskites have emerged as pivotal optoelectronic materials due to their easy and cost-effective solution-based manufacturing processes. In this study, we have combined formamidinium lead bromide (FAPbBr3) with buckminsterfullerene (C60) to create the bulk heterojunction (BHJ) photoactive layer for solution-processed photodetectors. Remarkably, the FAPbBr3:C60 BHJ-based photodetectors exhibited a substantial enhancement in optoelectronic performance, with the responsivity increasing from 0.87 A/W to 1.47 A/W compared to the pristine FAPbBr3. This improvement can be attributed to the superior surface morphology and crystallinity of the FAPbBr3:C60 BHJ thin film, which facilitated enhanced charge transport properties. Furthermore, the fabricated photodetectors demonstrated remarkable detectivity even at 0V, enabling self-powered operation. By employing the space charge limited conduction (SCLC) model, we were able to elucidate the dominant charge transport mechanism. At lower voltages, trap-free SCLC was identified as the primary mechanism, while traps governed charge transport at higher voltages. Additionally, we conducted impedance analysis to investigate the underlying factors contributing to the distinctive optoelectronic performance observed in the photodetectors.

Epitaxial Growth of Lead‐Free Double Perovskite Shell for CsPbX3/Cs2SnX6 (X = Cl, Br, and I) Core/Shell Perovskite Nanocrystals with Enhanced Photoelectric Properties and Stability

AbstractAll‐inorganic lead halide perovskite nanocrystals (NCs) have great optoelectronic properties with promising applications in light‐emitting diodes (LEDs), lasers, photodetectors, solar cells, and photocatalysis. However, the intrinsic toxicity of Pb and instability of the NCs impede their broad applications. Shell‐coating is an effective method for enhanced environmental stability while reducing toxicity by choosing non‐toxic shell materials such as metal oxides, polymers, silica, etc. However, multiple perovskite NCs can be encapsulated within the shell material and a uniform epitaxial‐type shell growth of well‐isolated NCs is still challenging. In this work, lead‐free vacancy‐ordered double perovskite Cs2SnX6 (X = Cl, Br, and I) shells are epitaxially grown on the surface of CsPbX3 NCs by a hot‐injection method. The effectiveness of the non‐toxic double perovskite shell protection is demonstrated by the enhanced environmental and phase stability against UV illumination and water. In addition, the photoluminescence quantum yields (PL QYs) increase for the CsPbCl3 and CsPbBr3 NCs after shelling because of the type I band alignment of the core/shell materials, while enhanced charge transport properties obtained from CsPbI3/Cs2SnI6 core/shell NCs are due to the efficient charge separation in the type II core/shell band alignment.

High performance platinum contacts on high-flux CdZnTe detectors

The need for direct X-ray detection under high photon flux with moderate or high energies (30–100 keV range) has strongly increased with the rise of the 4th Generation Synchrotron Light Sources, characterised by extremely brilliant beamlines, and of other applications such as spectral computed tomography in medicine and non-destructive tests for industry. The novel Cadmium Zinc Telluride (CZT) developed by Redlen Technologies can be considered the reference material for high-flux applications (HF-CZT). The enhanced charge transport properties of the holes allow the mitigation of the effects of radiation induced polarization phenomena, typically observed in standard CZT materials (LF-CZT) under high photon flux. However, standard LF-CZT electrical contacts led to inacceptable high dark leakage currents on HF-CZT devices. In this work, a detailed study on the characteristics of new optimized sputtered platinum electrical contacts on HF-CZT detectors is reported. The results from electrical and spectroscopic investigations, showed the best performances on HF-CZT detectors with platinum anode, coupled with both platinum or gold cathode. The morphology, structure, and composition of Pt/CZT contact have been analysed by means of Transmission Electron Microscopy (TEM) on microscopic lamellas obtained by Focused Ion Beam (FIB), highlighting the presence of CdTeO3 oxide at the metal semiconductor interface.

Single Crystalline α-Fe2O3 Nanosheets with Improved PEC Performance for Water Splitting.

We report the photoelectrochemical (PEC) performance of a densely grown single crystalline hematite (α-Fe2O3) nanosheet photoanode for water splitting. Unlike expensive ITO/FTO substrates, the sheets were grown on a piece of pure Fe through controlled thermal oxidation, which is a facile low cost and one-step synthesis route. The sheets grow with a widest surface parallel to basal plane (0001). Iron oxide formed on Fe consisting of layer structure α-Fe2O3-Fe3O4-Fe is elucidated from GIXRD and correlated to spectral features observed in Raman and UV-vis spectroscopy. The top α-Fe2O3 nanosheet layer serves as a photoanode, whereas the conducting Fe3O4 layer serves to transport photogenerated electrons to the counter electrode through its back contact. Time-resolved photoluminescence (TRPL) measurements revealed significantly prolonged carrier lifetime compared to that of bulk. Compared to the thin film of α-Fe2O3 grown on the FTO substrate, ∼3 times higher photocurrent density (0.33 mA cm-2 at 1.23 VRHE) was achieved in the nanosheet sample under solar simulated AM 1.5 G illumination. The sample shows a bandgap of 2.1 eV and n-type conductivity with carrier density 9.59 × 1017 cm-3. Electrochemical impedance spectroscopy (EIS) measurements reveal enhanced charge transport properties. The results suggest that nanosheets synthesized by the simple method yield far better PEC performance than the thin film on the FTO substrate. The anodic shifts of flat band potential, delayed electron-hole recombination, and growth direction parallel to the highly conducting basal plane (0001) being some of the contributing factors to the higher photocurrent observed in the NS photoanode are discussed. Characterizations carried out before and after the PEC reaction show excellent stability of the nanosheets in an alkaline electrochemical environment.

Enhanced charge transport properties of an LFP/C/graphite composite as a cathode material for aqueous rechargeable lithium batteries.

Electrodes that offer quick ion transport, a large surface area, and excellent electrical conductivity support high performance aqueous rechargeable lithium batteries. LiFePO4 (LFP) nanoparticles have been successfully coated with carbon by a chemical sol-gel route, and assembled on graphite by an ultrasonication method to acquire LFP/C/graphite. This LFP/C/graphite composite exhibits exceptional electrochemical performance at various current densities (1C to 20C). LFP/C/graphite delivers better capacity that is higher than that of LFP/C particles and high stability after 60 cycles at a current density of 1C for aqueous rechargeable lithium batteries as a cathode material. The graphite serves as a good volume buffer in improving the lithium performance of LFP/C/graphite during the charge/discharge process. The LFP/C/graphite composite shows high rate capability at 20C that returned to the initial capacity at 1C after 25 cycles with coulombic efficiency of 97%. Therefore, this effort presents a super low-cost route to fabricate high performance cathode materials in aqueous rechargeable lithium batteries and other energy storage appliances.

Rational design of W-doped BiVO4 photoanode coupled with FeOOH for highly efficient photoelectrochemical catalyzing water oxidation

Developments of promising photocatalyst for PEC water oxidation gain significant interest in the research field of PEC water splitting. The BiVO4 has been envisioned as suitable photocatalyst material for the PEC water oxidation due to suitable bandgap with favorable band edge positions. Nevertheless, the poor electron-hole separation and low charge transfer efficiency of BiVO4 yield sluggish surface catalysis reaction. Herein, facile electrodeposition and annealing techniques are proposed to fabricate W-doped BiVO4 photoanode coupled with FeOOH (W–BiVO4/FeOOH) for efficient photocatalytic water oxidation. This synthesis is simple, cost-effective and less time consuming. The doping concentration of W and deposition time of FeOOH are optimized to improve photocatalytic ability of BiVO4. At 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination, the W–BiVO4/FeOOH photoanode exhibits a high photocurrent density of 2.2 mA/cm2, which is seven folds higher than that of the pristine BiVO4 photoanode (0.31 mA/cm2 1.23 V vs. RHE). The enhanced photocatalytic ability of W–BiVO4/FeOOH photoanode is due to the enhanced charge transport properties and synergistic effects of W doping and FeOOH deposition. The excellent long-term stability with the photocurrent density retention of 90% after continuous light illumination for 1000 s is also achieved for the W–BiVO4/FeOOH photoanode.

Intramolecular Noncovalent Interaction-Enabled Dopant-Free Hole-Transporting Materials for High-Performance Inverted Perovskite Solar Cells.

Intramolecular noncovalent interactions (INIs) have served as a powerful strategy for accessing organic semiconductors with enhanced charge transport properties. Herein, we apply the INI strategy for developing dopant-free hole-transporting materials (HTMs) by constructing two small-molecular HTMs featuring an INI-integrated backbone for high-performance perovskite solar cells (PVSCs). Upon incorporating noncovalent S⋅⋅⋅O interaction into their simple-structured backbones, the resulting HTMs, BTORA and BTORCNA, showed self-planarized backbones, tuned energy levels, enhanced thermal properties, appropriate film morphology, and effective defect passivation. More importantly, the high film crystallinity enables the materials with substantial hole mobilities, thus rendering them as promising dopant-free HTMs. Consequently, the BTORCNA-based inverted PVSCs delivered a power conversion efficiency of 21.10 % with encouraging long-term device stability, outperforming the devices based on BTRA without S⋅⋅⋅O interaction (18.40 %). This work offers a practical approach to designing charge transporting layers with high intrinsic mobilities for high-performance PVSCs.

Intramolecular Noncovalent Interaction‐Enabled Dopant‐Free Hole‐Transporting Materials for High‐Performance Inverted Perovskite Solar Cells

AbstractIntramolecular noncovalent interactions (INIs) have served as a powerful strategy for accessing organic semiconductors with enhanced charge transport properties. Herein, we apply the INI strategy for developing dopant‐free hole‐transporting materials (HTMs) by constructing two small‐molecular HTMs featuring an INI‐integrated backbone for high‐performance perovskite solar cells (PVSCs). Upon incorporating noncovalent S⋅⋅⋅O interaction into their simple‐structured backbones, the resulting HTMs, BTORA and BTORCNA, showed self‐planarized backbones, tuned energy levels, enhanced thermal properties, appropriate film morphology, and effective defect passivation. More importantly, the high film crystallinity enables the materials with substantial hole mobilities, thus rendering them as promising dopant‐free HTMs. Consequently, the BTORCNA‐based inverted PVSCs delivered a power conversion efficiency of 21.10 % with encouraging long‐term device stability, outperforming the devices based on BTRA without S⋅⋅⋅O interaction (18.40 %). This work offers a practical approach to designing charge transporting layers with high intrinsic mobilities for high‐performance PVSCs.

Strong Suppression of Thermal Conductivity in the Presence of Long Terminal Alkyl Chains in Low-Disorder Molecular Semiconductors.

While the charge transport properties of organic semiconductors have been extensively studied over the recent years, the field of organics-based thermoelectrics is still limited by a lack of experimental data on thermal transport and of understanding of the associated structure-property relationships. To fill this gap, a comprehensive experimental and theoretical investigation of the lattice thermal conductivity in polycrystalline thin films of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (Cn-DNTT-Cn with n= 0, 8) semiconductors is reported. Strikingly, thermal conductivity appears to be much more isotropic than charge transport, which is confined to the 2D molecular layers. A direct comparison between experimental measurements (3ω-Völklein method) and theoretical estimations (approach-to-equilibrium molecular dynamics (AEMD) method) indicates that the in-plane thermal conductivity is strongly reduced in the presence of the long terminal alkyl chains. This evolution can be rationalized by the strong localization of the intermolecular vibrational modes in C8-DNTT-C8 in comparison to unsubstituted DNTT cores, as evidenced by a vibrational mode analysis. Combined with the enhanced charge transport properties of alkylated DNTT systems, this opens the possibility to decouple electron and phonon transport in these materials, which provides great potential for enhancing the thermoelectric figure of merit ZT.

Enhanced Photocatalytic Performance of Poly(3,4-ethylenedioxythiophene)- Coated TiO2 Nanotube Electrodes

This study presents the design of a mixed organic–inorganic system to be utilized in solar-light chemical energy conversion applications, where the visible-light absorption properties of the conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), are combined with the enhanced charge transport properties of TiO2 nanotubes, thus creating an efficient photoelectrode. A dispersion of PEDOT doped with p-toluenesulfonate (tosylate) ions was synthesized using a simple solution-casting polymerization process, which was then spin-coated onto an ordered array of anodically fabricated TiO2 nanotubes to be used for solar energy conversion. Electron microscopy observation confirmed the presence of PEDOT in the hybrid (PEDOT–TiO2) and Fourier transform infrared spectroscopy verified the vibrational modes of the polymerized PEDOT. The ultraviolet-visible diffuse reflectance data indicated a change in the absorption edge of the pure TiO2 nanotubes to longer wavelengths after PEDOT coating. The developed photo-electrodes exhibited a significant increase in photocurrent response and lower charge transfer resistance with respect to the pure TiO2 nanotubes, as verified by electrochemical impedance spectroscopy.

Azahomofullerenes as New n-Type Acceptor Materials for Efficient and Stable Inverted Planar Perovskite Solar Cells.

Fullerene derivatives with a strong electron-accepting ability play a crucial role in enhancing both the performance and stability of perovskite solar cells (PSCs). However, most of the used fullerene molecules are based on [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which limits the device performance due to difficulties in preparing high-quality and uniform thin films. Herein, solution-processable azahomofullerene (AHF) derivatives (abbreviated as AHF-1 and AHF-2) are reported as novel and effective electron-transport layers (ETLs) in p-i-n planar PSCs. Compared to the control PCBM ETL-based PSCs, the devices based on AHFs exhibit higher photovoltaic performances, which is attributed to the enhanced charge-transport properties and improved layer morphology leading to a maximum power conversion efficiency (PCE) of 20.21% in the case of the device based on AHF-2 ETL. Besides, due to the preferable energy band alignment with the perovskite layer, reduced trap states, and suppressed charge recombination, the device with AHF-2 ETL exhibits significantly suppressed hysteresis and improved stability under both ambient and thermal conditions.

Water-Surface Drag Coating: A New Route Toward High-Quality Conjugated Small-Molecule Thin Films with Enhanced Charge Transport Properties.

Electronic properties of organic semiconductor (OSC) thin films are largely determined by their morphologies and crystallinities. However, solution-processed conjugated small-molecule OSC thin films usually exhibit abundant grain boundaries and impure grain orientations because of complex fluid dynamics during solution coating. Here, a novel methodology, water-surface drag coating, is demonstrated to fabricate high-quality OSC thin films with greatly enhanced charge transport properties. This method utilizes the water surface to alter the evaporation dynamics of solution to enlarge the grain size, and a unique drag-coating process to achieve the unidirectional growth of organic crystals. Using 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (Dif-TES-ADT) as an example, thin films with millimeter-sized single-crystal domains and pure crystallographic orientations are achieved, revealing a significant enhancement (4.7 times) of carrier mobility. More importantly, the resulting film can be directly transferred onto any desired flexible substrates, and flexible transistors based on the Dif-TES-ADT thin films show a mobility as high as 16.1 cm2 V-1 s-1 , which represents the highest mobility value for the flexible transistors reported thus far. The method is general for the growth of various high-quality OSC thin films, thus opening up opportunities for high-performance organic flexible electronics.

Cocrystallization-Promoted Charge Mobility in All-Conjugated Diblock Copolymers for High-Performance Field-Effect Transistors.

The cocrystallization method that combines various constituents into cocrystals yields the newly formed materials with significantly enhanced charge transport properties. However, this strategy has not been greatly utilized in all-conjugated block copolymers (BCPs). Herein, we scrutinize the relationship between cocrystals and charge mobilities in all-conjugated BCPs (i.e., poly(3-butylthiophene)-block-poly(3-hexylthiophene); denoted P3BT-b-P3HT) by tuning their molecular weights and thermal annealing process. All the rod-rod BCPs form cocrystals with high charge mobilities than P3BT and P3HT homopolymers and P3BT/P3HT blend, imparting the cocrystal-facilitated charge transport because of the synergy of two conjugated components. Upon 150 °C treatment, their crystallinities increase and their charge mobilities at 15k, 18k, and 28k increase slightly. In contrast, P3BT-b-P3HT-12k shows decreased charge mobilities. It is due to the preferential increase of crystal size and order through the π-π stacking direction in the former while through the alkyl stacking direction in the latter. Intriguingly, when these P3BT-b-P3HT cocrystals experience two-step thermal treatment, P3BT-b-P3HT-12k retains its cocrystalline structure, while microphase separation of P3BT and P3HT occurs in P3BT-b-P3HT-15k, 18k, and 28k with different degrees. All P3BT-b-P3HT BCPs exhibit decreased charge mobilities. This study demonstrates the cocrystallization-promoted charge mobility in all-conjugated BCPs, which may facilitate their application in a wide range of optoelectronic devices.

Simple hole-transporting materials containing twin-carbazole moiety and unconjugated flexible linker for efficient and stable perovskite solar cells

Three twin-carbazole-based hole-transporting materials (HTMs) bearing an unconjugated flexible linker and four peripheral diphenylamine groups with different methoxy positions (pp-, pm-, and po-) are first synthesized (GJ-pp, GJ-pm, and GJ-po) and successfully employed in perovskite solar cells (PSCs). The HTMs were obtained from simple synthetic steps and facile purification techniques. The thermal stability, optical and electrochemical properties, density functional theory calculations, hole-transporting properties, X-ray diffraction, hole transfer dynamics, hydrophobic properties, surface morphology, and photovoltaic performances were measured and discussed. Device performance depends on the methoxy group positions. GJ-pp shows the best power conversion efficiency (17.23%) among the three twin-carbazole-based HTMs, and slightly lower than that of the spiro-OMeTAD-based devices (17.96%). However, the GJ-pp-based devices show outstanding thermal stability compared with the spiro-OMeTAD-based ones. Laboratory synthetic cost of GJ-pp is merely 29.0% of the purchasable spiro-OMeTAD price, and the concentration of GJ-pp is lower than that of spiro-OMeTAD (50.0 vs. 72.3 mg mL−1). The results demonstrate that the twin-carbazole moiety can endow enhanced charge transport properties due to its potential intermolecular interactions, and the conjugated core is unnecessary for promising HTMs.

Enhanced charge transport properties in heteroatomic (NH, O, Se) analogs of benzotrithiophene (BTT) isomers: a DFT insight

ABSTRACTA comprehensive comparative study of the geometry, optoelectronic and charge transport properties in benzotrithiophene (BTT), benzotripyrrole (BTP), benzotrifuran (BTF) and benzotriselenophene (BTSe) and their possible isomers are reported by employing density functional theory (DFT). The motive of this work is to examine the role of different heteroatoms (NH, O and Se) on optical and charge transport properties. The optimised molecules display planar geometry, which is ideal for organic semiconductor applications. The absorption properties of all the molecules are calculated using time-dependent density functional theory (TDDFT). Nucleus independent chemical shift (NICS) calculations are performed to see the stability and aromatic character in these molecules. The calculated hole and electron reorganisation energies (λh and λe) for majority of the isomers in each series are smaller than that of typical hole and electron transport material (TPD and Alq3). BTP series isomers are with exceptionally small λe values (10–60 meV), which means these isomers are better for electron transport materials. The quantum chemical results show that HOMO, LUMO, HOMO–LUMO gap, and reorganisation energies along with ionisation potentials and electron affinities are significantly affected by changing the heteroatoms. The smaller hole and electron reorganisation energies of these molecules suggest possible use in OLEDs.

Enhancement of Charge Transport in Polythiophene Semiconducting Polymer by Blending with Graphene Nanoparticles.

This paper describes a study on the charge transport in a composite of liquid-exfoliated graphene nanoparticles (GNPs) and a polythiophene semiconducting polymer. While the former component is highly conducting, although it consists of isolated nanostructures, the latter offers an efficient charge transport path between the individual GNPs within the film, overall yielding enhanced charge transport properties of the resulting bi-component system. The electrical characteristics of the composite layers were investigated by means of measurements of time-of-flight photoconductivity and transconductance in field-effect transistors. In order to analyze both phenomena separately, charge density and charge mobility contributions to the conductivity were singled out. With the increasing GNP concentration, the charge mobility was found to increase, thereby reducing the time spent by the carriers on the polymer chains. In addition, for GNP loading above 0.2 % (wt.), an increase of free charge density was observed that highlights an additional key role played by doping. Variable-range hopping model of a mixed two- and three-dimensional transport is explained using temperature dependence of mobility and free charge density. The temperature variation of free charge density was related to the electron transfer from polythiophene to GNP, with an energy barrier of 24 meV.