Carrier Transport Capability Research Articles

Engineering of aggregation-induced emission luminogens by isomeric strategy to achieve high-performance optoelectronic device

Achieving non-doped organic light-emitting diodes (OLEDs) with high electroluminescent (EL) performance is essential for both illumination source and display applications. A source of this difficulty is the aggregation-induce quenching effect of emitters in the solid state, which results in low EL efficiency and severe roll-off. Herein, four isomeric aggregation-induced emission luminogens (AIEgens), namely TPE-3Cz-mPBI, TPE-3Cz-pPBI, TPE-2Cz-mPBI, and TPE-2Cz-pPBI, are constructed by varying the linkage positions of electron-accepter 1,2-diphenyl-1H-benzo[d]imidazole (PBI), electron-donor carbazole, and AIE-active triphenylethene functional groups. A comprehensive investigation is carried out, and the results indicate that position isomerism dramatically affects electronic structures, photophysical properties, carrier transport capabilities, and device performances. Compared with TPE-3Cz-mPBI and TPE-3Cz-pPBI, TPE-2Cz-mPBI, and TPE-2Cz-pPBI exhibit more excellent AIE-active with the fluorescence quantum yield up to 72.5% and 89.7% in the neat films. Moreover, TPE-2Cz-mPBI shows high electron transport property and appropriate energy level, which favor balanced charge carrier injection/transporting in OLEDs. Hence, non-doped device based on TPE-2Cz-mPBI exhibits the best EL performance with external quantum efficiency, current efficiency, and power efficiency of 3.2%, 7.8 cd A−1, and 7.7 lm W−1, respectively. The isomeric strategy presents a promising method to extend the structural diversity of highly efficient AIE emitters, optimize optoelectronic properties, and construct high-performance non-doped devices. Due to intrinsic twisted structure and accessible protonated N-electron in molecular framework, the isomers exhibit impressive mechanochromism and acidochromism.

Multi-functional transparent electrode for reliable flexible perovskite solar cells

Multilayer MoOx/Ag/MoOx (DMD) films are found to be transparent conducting electrodes for use in extremely stable and highly bendable flexible perovskite solar cells (PSCs). The optical transparency and electric properties of DMD and its role as a top electrode of PSCs were studied by changing the thickness of the MoOx layer. Although the MoOx thickness was shown to have a negligible effect on the sheet resistance of DMD, the transmittance of visible light, selective carrier transport capability, and long-term stability of a device considerably depend on this factor. The sandwich structure of a 20-nm-thick MoOx, 7-nm-thick Ag, and 20-nm-thick MoOx exhibits a high transmittance and large photon–electron conversion rate of PSCs. In addition, PSCs using the DMD top electrode maintain 92% of their initial current density after 24 h of continuous operation owing to a UV light cut-off of the top illumination. Moreover, the overall structure of DMD blocks the diffusion of water and oxygen molecules from real environmental conditions. At the same time, the underlying/upper MoOx layer retards the degradation through a chemical reaction between Ag and the halide ions inside the cells, as well as foreign ions from outside the polluted atmosphere. When DMD is applied to flexible PSCs on Ti foil, the PCE reaches 14.5%, and mechanical integrity of the PSCs is maintained at a bending radius of 4 mm.

30% Enhancement of Efficiency in Layered 2D Perovskites Absorbers by Employing Homo‐Tandem Structures

Layered two dimensional (layered 2D) organic–inorganic metal halide perovskites have attracted tremendous interest in photovoltaics due to its acceptable materials stability, especially the moisture resistance, when compared with their three dimensional counterparts. However, the limited carrier transport capability, which originates from the insulativity of bulky organic molecules, has significantly affected the resultant device efficiency. To create a shorter carrier pathway with sufficient optical density, the homo‐tandem device structure by using layered 2D perovskite absorbers is proposed. Following this strategy, the semi‐transparent device and filter bottom cells have been investigated and optimized using the same layered 2D perovskite absorber (BA2MA3Pb4I13). The corresponding four‐terminal tandem device is successfully demonstrated with the champion power conversion efficiency of 14.42%, which is 30% higher than that of single BA2MA3Pb4I13 perovskite devices (11.02%). A stabilized efficiency of 13.57% in the optimized champion tandem device also have been achieved. These results suggest alternatives to develop layered 2D perovskite based solar cells and other optoelectronic devices.

Improved performance of perovskite solar cells through using (FA)x(MA)1-xPbI3 optical absorber layer

In this work, the perovskite solar cells (PSCs) were fabricated with the bandgap-tunable (FA)x(MA)1-xPbI3 absorber layers through a facile two-stage deposition route. The doping was realized by adding the formamidinium iodide (FAI) into a precursor MAI solution. Both the surface morphology and electrochemical impedance spectra (EIS) were conducted to evaluate the absorber layers or solar cells. After the optimization, the best PSC performance of 14.73% was achieved at a nominal FAI content of 12.5 at.%. The performance enhancement was attributed to both the enhancement of visible light harvesting and carrier transport capability. Besides, the stability of a PSC device based on the single MAPbI3 absorber layer was also investigated, and a power conversion efficiency (PCE) of 11.27% remained even after laying in vacuum for 10 days.

A new carbon phase with direct bandgap and high carrier mobility as electron transport material for perovskite solar cells

Rapid development of perovskite solar cells is challenged by the fact that current semiconductors hardly act as efficient electron transport materials that can feature both high electron mobility and a well-matched energy level to that of the perovskite. Here we show that T-carbon, a newly emerging carbon allotrope, could be an ideal candidate to meet this challenge. By using first-principles calculations and deformation potential theory, it is found that T-carbon is a semiconductor with a direct bandgap of 2.273 eV, and the energy level in the conduction band is lower than that of perovskite by 0.5 eV, showing a larger force of electron injection. Moreover, the calculated electron mobility can reach up to 2.36 × 103 cm2 s–1 V–1, superior to conventional electron transport materials such as TiO2, ZnO and SnO2, which will facilitate more efficient electron separation and more rapid diffusion away from their locus of generation within the perovskite absorbers. Furthermore, the bandgap of T-carbon is highly sensitive to strain, thus providing a convenient method to tune the carrier transport capability. Overall, T-carbon satisfies the requirements for a potential efficient electron transport material and could therefore be capable of accelerating the development of perovskite solar cells.

How does the Zn-precursor nature impact carrier transfer in ZnO/Zn-TiO2 nanostructures? organic vs. inorganic anions

The carrier transport capability of ZnO/Zn-TiO2 nanostructures is affected by Zn-precursor anions, generating donor, acceptor and interfacial energy states.

Construction of vesicle CdSe nano-semiconductors photocatalysts with improved photocatalytic activity: Enhanced photo induced carriers separation efficiency and mechanism insight

Visible-light-driven photocatalysis as a green technology has attracted a lot of attention due to its potential applications in environmental remediation. Vesicle CdSe nano-semiconductor photocatalyst are successfully prepared by a gas template method and characterized by a variety of methods. The vesicle CdSe nano-semiconductors display enhanced photocatalytic performance for the degradation of tetracycline hydrochloride, the photodegradation rate of 78.824% was achieved by vesicle CdSe, which exhibited an increase of 31.779% compared to granular CdSe. Such an exceptional photocatalytic capability can be attributed to the unique structure of the vesicle CdSe nano-semiconductor with enhanced light absorption ability and excellent carrier transport capability. Meanwhile, the large surface area of the vesicle CdSe nano-semiconductor can increase the contact probability between catalyst and target and provide more surface-active centers. The photocatalytic mechanisms are analyzed by active species quenching. It indicates that h+ and O2− are the main active species which play a major role in catalyzing environmental toxic pollutants. Simultaneously, the vesicle CdSe nano-semiconductor had high efficiency and stability.

Enhanced carrier mobility and direct tunneling probability of biaxially strained Ge1−xSnx alloys for field-effect transistors applications

The carrier transport and tunneling capabilities of biaxially strained Ge1−xSnx alloys with (001), (110), and (111) orientations were comprehensively investigated and compared. The electron band structures of biaxially strained Ge1−xSnx alloys were calculated by the nonlocal empirical pseudopotential method and the modified virtual crystal approximation was adopted in the calculation. The electron and hole effective masses at the band edges were extracted using a parabolic line fit. It is shown that the applied biaxial strain and the high Sn composition are both helpful for the reduction of carrier effective masses, which leads to the enhanced carrier mobility and the boosted direct band-to-band-tunneling probability. Furthermore, the strain induced valance band splitting reduces the hole interband scattering, and the splitting also results in the significantly enhanced direct tunneling rate along the out-of-plane direction compared with that along the in-plane direction. The biaxially strained (111) Ge1−xSnx alloys exhibit the smallest band gaps compared with (001) and (110) orientations, leading to the highest in-plane and out-of-plane direct tunneling probabilities. The small effective masses on (110) and (111) planes in some strained conditions also contribute to the enhanced carrier mobility and tunneling probability. Therefore, the biaxially strained (110) and (111) Ge1−xSnx alloys have the potential to outperform the corresponding (001) Ge1−xSnx devices. It is important to optimize the applied biaxial strain, the Sn composition, and the substrate orientation for the design of high performance Ge1−xSnx field-effect transistors.

Thermoelectric performance of poly(3-hexylthiophene) films doped by iodine vapor with promising high seebeck coefficient

Poly(3-hexylthiophene) (P3HT) films doped with iodine vapor have been prepared by casting a P3HT solution on glass substrates and their thermoelectric (TE) performances has been investigated. The maximum Seebeck coefficient and electrical conductivity of iodine-doped P3HT films were 386 µV·K−1 (at room temperature) and 4.7 × 10−1 S·cm−1, which is about five orders of magnitude higher than that of pristine P3HT films. The power factor of these iodine-doped P3HT films was estimated to be 7.0 µW·m−1·K−2 at room temperature, which is a relative high value for organic TE materials. The UV-vis spectra of iodine-doped P3HT films showed a slight red shift of the iodine-doped P3HT compared to those of pristine P3HT films. Atomic force microscopy images indicated the conformational changes in P3HT chains after treatment with iodine vapor. During this treatment, the P3HT chains self-organized into a more ordered structure, this organization improved the charge carrier transport capability and the TE performance of P3HT the films.

Spray-on Thin Film PV Solar Cells: Advances, Potentials and Challenges

The capability to fabricate photovoltaic (PV) solar cells on a large scale and at a competitive price is a milestone waiting to be achieved. Currently, such a fabrication method is lacking because the effective methods are either difficult to scale up or expensive due to the necessity for fabrication in a vacuum environment. Nevertheless, for a class of thin film solar cells, in which the solar cell materials can be processed in a solution, up scalable and vacuum-free fabrication techniques can be envisioned. In this context, all or some layers of polymer, dye-sensitized, quantum dot, and copper indium gallium selenide thin film solar cells illustrate some examples that may be processed in solution. The solution-processed materials may be transferred to the substrate by atomizing the solution and carrying the spray droplets to the substrate, a process that will form a thin film after evaporation of the solvent. Spray coating is performed at atmospheric pressure using low cost equipment with a roll-to-roll process capability, making it an attractive fabrication technique, provided that fairly uniform layers with high charge carrier separation and transport capability can be made. In this paper, the feasibility, the recent advances and challenges of fabricating spray-on thin film solar cells, the dynamics of spray and droplet impaction on the substrate, the photo-induced electron transfer in spray-on solar cells, the challenges on characterization and simulation, and the commercialization status of spray-on solar cells are discussed.

Proteins as Solid-State Electronic Conductors

Protein structures can facilitate long-range electron transfer in solution. But a fundamental question remains: can these structures also serve as solid-state electronic conductors? Answering this question requires methods for studying conductivity of the "dry" protein (which only contains tightly bound structured water molecules) sandwiched between two electronic conductors in a solid-state type configuration. If successful, such systems could serve as the basis for future, bioinspired electronic device technology. In this Account, we survey, analyze, and compare macroscopic and nanoscopic (scanning probe) solid-state conductivities of proteins, noting the inherent constraints of each of these, and provide the first status report on this research area. This analysis shows convincing evidence that "dry" proteins pass orders of magnitude higher currents than saturated molecules with comparable thickness and that proteins with known electrical activity show electronic conductivity, nearly comparable to that of conjugated molecules ("wires"). These findings suggest that the structural features of proteins must have elements that facilitate electronic conductivity, even if they do not have a known electron transfer function. As a result, proteins could serve not only as sensing, polar,or photoactive elements in devices (such as field-effect transistor configurations) but also as electronic conductors. Current knowledge of peptide synthesis and protein modification paves the way toward a greater understanding of how changes in a protein's structure affect its conductivity. Such an approach could minimize the need for biochemical cascades in systems such as enzyme-based circuits, which transduce the protein's response to electronic current. In addition, as precision and sensitivity of solid-state measurements increase, and as knowledge of the structure and function of "dry" proteins grows, electronic conductivity may become an additional approach to study electron transfer in proteins and solvent effects without the introduction of donor or acceptor moieties. We are particularly interested in whether evolution might have prompted the electronic carrier transport capabilities of proteins for which no electrically active function is known in their native biological environment and anticipate that further research may help address this fascinating question.

Organic Photovoltaic Cells with Improved Performance Using Bathophenanthroline as a Buffer Layer

The role of bathophenanthroline (Bphen) as a buffer layer inserted between fullerene (C60) and Ag cathode in organic photovoltaic (OPV) cell was discussed. By introducing Bphen as a buffer layer with thicknes from 0 to 2.5 nm, the power conversion efficiency of the OPV cell based on copper phthalocyanine (CuPc) and C60 was increased from 0.87% to 2.25% under AM 1.5 solar illumination at an intensity of 100 mW/cm2, which was higher than that of bathocuproine used as a buffer layer. The photocurrent-voltage characteristics showed that Bphen effectively improves electron transport through C60 layer into Ag electrode and leads to balance charge carrier transport capability. The influence of Bphen thickness on OPV cells was also investigated. Furthermore, the absorption spectrum shows that an additional Bphen layer enhances the light harvest capability of CuPc/C60.

Carrier Transport and Luminescence Properties of Nanocomposites of Poly[2-methoxy-5-(2-ethyl hexyloxy)-p-phenylene vinylene] and Dehydrated Nanotubes Titanic Acid

The carrier transport capability and luminescence efficiency of poly(2-methoxy-5-(2-ethyl hexyloxy)-p-phenylene vinylene) (MEH-PPV) films are enhanced by doping with dehydrated nanotubed titanic acid (DNTA). MEH-PPV molecules, either wrapped on the outer surface of or encapsulated into DNTA pores, have a more open, straighter conformation than undoped molecules, which induces a longer conjugated backbone and stronger interchain interactions, thereby, enhancing carrier mobility. MEH-PPV molecules within DNTA pores have higher exciton recombination efficiency owing to quantum confinement and the antenna effect.

Effect of fabrication process on characteristics of phosphorescence organic light emitting diodes with methoxy-substituted starburst low-molecule as a host

The effect of dry process and wet process on the characteristics of phosphorescence organic light-emitting devices (OLEDs) employing a phosphorescent dye fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy) 3) doped into a methoxy-substituted starburst low-molecule material methoxy-substituted 1,3,5-tris[4-(diphenylamino) phenyl]benzene (TDAPB) are investigated. The FT-IR and absorption spectra of TDAPB films fabricated by a dry process, and a wet process are almost same, and the PL spectra of those films are different. The carrier transport capability of TDAPB by a dry process is lower than that by a wet process. The photoluminescence intensity of Ir(ppy) 3 doped in TDAPB fabricated by a wet process is higher than that by a dry process. A maximum external current efficiency of more than 20 cd/A and luminance of more than 10,000 cd/m 2 were obtained. Maximum luminance of devices monotonously decreases with increasing the thickness of a dry-processed emitting layer. The main emission zone of the OLED was located in almost at the center of the emitting layer. The improvement of device performance in the OLED fabricated by a wet process was achieved due to the high efficient energy transfer from TDAPB to Ir(ppy) 3, high carrier transporting capability and the formation of homogeneous film, compared with that fabricated by a dry process.

Ambipolar Carrier Transport in Amorphous Bis(triazinyl)arenes and Their Application in OLED

Bis(4,6-diaryl-1,3,5-triazin-2-yl)arenes were prepared for new semiconductive materials, and their thermal and morphological natures were characterized. The glass formation properties of the materials were discussed with respect to the effect of the substituents and the molecular conformations. The carrier mobility measurement revealed that the bis-triazines showed ambipolar carrier transport capabilities in the films. We also demonstrated to fabricate an organic light emitting diode using the his-triazine as an electron transport layer.

Enhancement of bipolar carrier transport in oligofluorene films through alignment in the liquid-crystalline phase

We investigate the formation of aligned liquid-crystal glasses of an oligofluorene and perform comparative studies of carrier-transport properties of the oligofluorene in both amorphous films and glassy liquid-crystal films. With mesophase-mediated molecular alignment, the bipolar carrier-transport capability of oligofluorene solid films is enhanced by more than an order of magnitude. High bipolar carrier mobilities, up to 2.0×10−2cm2∕Vs for holes and up to 2.3×10−2cm2∕Vs for electrons, are observed in the aligned glassy liquid-crystal films.

Integration of Electrically Isolated Porous Silicon Leds for Applications in CMOS Technology

AbstractPreviously we reported on integrated porous silicon (PSi) based LEDs with local bipolar drivers that would have possible applications in an active-matrix configuration for display or optical signal transmission [1]. We now report further progress in device engineering and integration that has enabled the fabrication of improved light-emitting silicon devices based on oxide-passivated nanocrystalline silicon (OPNSi) that can be integrated with standard CMOS technology. Design of experiments methodologv was used to direct device engineering experiments, providing a better understanding of how process parameters influence the resulting device performance. It was discovered that the formation of stacking fault defects in the LED region prior to anodization has a predominant effect on both carrier transport and electroluminescence (EL) capabilities of the devices. Process development work has resulted in the fabrication of OPNSi LEDs that offer many of the required attributes of a useful silicon light-emitter. The devices exhibit EL at a bias < 5V, diode-like rectifying I-V characteristics, good stability under DC bias, and uniform emission over the LED contact area. These LEDs have been combined with a new fabrication technique which enables the formation of electrically isolated integrated LEDs in the single-crystal substrate. Each diode structure is formed in its own individual well, enabling junction-isolated devices without a common substrate electrode. Such a process is required to realize integrated LEDs which can be individually addressed in an X-Y array configuration using full-rail voltage modulation. Details of the LED characteristics and device integration will be discussed.

Light Emitting thin Film Devices Based on Self-assembled Multilayer Heterostructures of PPV

ABSTRACTPPV based light emitting thin film devices were fabricated using a layer-by-layer deposition technique involving the alternate spontaneous adsorption of a PPV precursor polymer and either poly(styrene-4-sulfonate) (SPS) or poly(methacrylic acid) (PMA). It was demonstrated that the polyanion used to self-assemble the PPV precursor strongly influences the characteristics and performance of the resulting LEDs. Devices fabricated with PPV created in the presence of SPS exhibited symmetric I–V curves, low luminance levels and very high current densities while PPV/PMA devices exhibited luminance levels in the range of 10–60 cd/m2 and classical rectifying behavior. These dramatic differences are primarily due to a low level of p-type doping activated during the thermal conversion of PPV and/or during device operation that confers excellent hole carrier transport capabilities to the PPV/SPS combination. Fabrication of a multi-slab type heterostructure device comprised of a PPV/SPS block (hole transporting block) and a PPV/PMA block (emitting block) resulted in improved performance with luminance levels significantly higher than previously obtained for a single slab PPV/PMA device (typically > 100 cd/m2). It was also demonstrated that the presence of very thin (about 20–30 Å thick) insulating layers at the Al/polymer interface improves device efficiency by a factor of 2–4.