Effects of bursty synthesis in organelle biogenesis.

A fundamental question of cell biology is how cells control the number of organelles. The processes of organelle biogenesis, namely de novo synthesis, fission, fusion, and decay, are inherently stochastic, producing cell-to-cell variability in organelle abundance. In addition, experiments suggest that the synthesis of some organelles can be bursty. We thus ask how bursty synthesis impacts intracellular organelle number distribution. We develop an organelle biogenesis model with bursty de novo synthesis by considering geometrically distributed burst sizes. We analytically solve the model in biologically relevant limits and provide exact expressions for the steady-state organelle number distributions and their means and variances. We also present approximate solutions for the whole model, complementing with exact stochastic simulations. We show that bursts generally increase the noise in organelle numbers, producing distinct signatures in noise profiles depending on different mechanisms of organelle biogenesis. We also find different shapes of organelle number distributions, including bimodal distributions in some parameter regimes. Notably, bursty synthesis broadens the parameter regime of observing bimodality compared to the 'non-bursty' case. Together, our framework utilizes number fluctuations to elucidate the role of bursty synthesis in producing organelle number heterogeneity in cells.

Effects of microRNA-mediated negative feedback on gene expression noise.

MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression post-transcriptionally in eukaryotes by binding with target mRNAs and preventing translation. miRNA-mediated feedback motifs are ubiquitous in various genetic networks that control cellular decision making. A key question is how such a feedback mechanism may affect gene expression noise. To answer this, we have developed a mathematical model to study the effects of a miRNA-dependent negative-feedback loop on mean expression and noise in target mRNAs. Combining analytics and simulations, we show the existence of an expression threshold demarcating repressed and expressed regimes in agreement with earlier studies. The steady-state mRNA distributions are bimodal near the threshold, where copy numbers of mRNAs and miRNAs exhibit enhanced anticorrelated fluctuations. Moreover, variation of negative-feedback strength shifts the threshold locations and modulates the noise profiles. Notably, the miRNA-mRNA binding affinity and feedback strength collectively shape the bimodality. We also compare our model with a direct auto-repression motif, where a gene produces its own repressor. Auto-repression fails to produce bimodal mRNA distributions as found in miRNA-based indirect repression, suggesting the crucial role of miRNAs in creating phenotypic diversity. Together, we demonstrate how miRNA-dependent negative feedback modifies the expression threshold and leads to a broader parameter regime of bimodality compared to the no-feedback case.

An evolutionary divergent thermodynamic brake in ZAP-70 fine-tunes the kinetic proofreading in T cells.

T cell signaling starts with assembling several tyrosine kinases and adapter proteins to the T cell receptor (TCR), following the antigen binding to the TCR. The stability of the TCR-antigen complex and the delay between the recruitment and activation of each kinase determines the T cell response. Integration of such delays constitutes a kinetic proofreading mechanism to regulate T cell response to the antigen binding. However, the mechanism of these delays is not fully understood. Combining biochemical experiments and kinetic modeling, here we report a thermodynamic brake in the regulatory module of the tyrosine kinase ZAP-70, which determines the ligand selectivity, and may delay the ZAP-70 activation upon antigen binding to TCR. The regulatory module of ZAP-70 comprises of a tandem SH2 domain that binds to its ligand, doubly-phosphorylated ITAM peptide (ITAM-Y2P), in two kinetic steps: a fast step and a slow step. We show the initial encounter complex formation between the ITAM-Y2P and tandem SH2 domain follows a fast-kinetic step, whereas the conformational transition to the holo-state follows a slow-kinetic step. We further observed a thermodynamic penalty imposed during the second phosphate-binding event reduces the rate of structural transition to the holo-state. Phylogenetic analysis revealed the evolution of the thermodynamic brake coincides with the divergence of the adaptive immune system to the cell-mediated and humoral responses. In addition, the paralogous kinase Syk expressed in B cells does not possess such a functional thermodynamic brake, which may explain the higher basal activation and lack of ligand selectivity in Syk.

A combined protocol for isolation of T6SS-positive Campylobacter jejuni and assessment of interspecies interaction.

The bacterial Type VI Secretion System (T6SS) functions as a nanomachine used by many gut pathogens. In the present protocol, we outlined how such molecular activities during interspecies interaction can be demonstrated at a population level. To this end, we first present a comprehensive protocol for isolation, identification, and functional characterization of T6SS-positive Campylobacter jejuni. Further, we developed straightforward techniques for unraveling how the T6SS targets prey populations and host cells when growing with or without environmental stressors. For complete details on the use and execution of this protocol, please refer to Gupta et al. (2021).

Effects of length-dependent positive feedback on length distributions of microtubules undergoing hydrolysis.

Despite a rapid turnover of subunits, how cells control the lengths of cytoskeletal filaments (such as microtubules) is a fundamental question in cell biology. Here, we theoretically investigate how microscopic processes affect the length distributions of multiple microtubules growing stochastically in a shared subunit pool. In particular, we consider length-dependent positive feedback on filament growth and the chemical conversion from GTP-tubulin to GDP-tubulin (hydrolysis) inside a filament. We found different dynamical regimes for a single filament by simulating a model of microtubule kinetics, where both bimodal and unimodal (bell-shaped) length distributions emerge in the steady state. More significantly, the length distributions of multiple filaments were not unimodal, predicting a collective effect for more than one filament. Interestingly, when length distributions were bimodal, we also observed bistable toggling of individual lengths. Therefore, regulation of biophysical parameters (e.g., hydrolysis rate and feedback strength) can lead to length diversity in an ensemble of multiple microtubules.

Effects of random hydrolysis on biofilament length distributions in a shared subunit pool.

The sizes of filamentous structures in a cell are often regulated for many physiological processes. A key question in cell biology is how such size control is achieved. Here, we theoretically study the length distributions of multiple filaments, growing by stochastic assembly and disassembly of subunits from a limiting subunit pool. Importantly, we consider a chemical switching of subunits (hydrolysis) prevalent in many biofilaments like microtubules (MTs). We show by simulations of different models that hydrolysis leads to a skewed unimodal length distribution for a single MT. In contrast, hydrolysis can lead to bimodal distributions of individual lengths for two MTs, where individual filaments toggle stochastically between bigger and smaller sizes. For more than two MTs, length distributions are also bimodal, although the bimodality becomes less prominent. We further show that this collective phenomenon is connected with the nonequilibrium nature of hydrolysis, and the bimodality disappears for reversible dynamics. Consistent with earlier theoretical studies, a homogeneous subunit pool, without hydrolysis, cannot control filament lengths. We thus elucidate the role of hydrolysis as a control mechanism on MT length diversity.

The cost of bacterial predation via type VI secretion system leads to predator extinction under environmental stress.

As a common gut pathogen, Campylobacter jejuni (C. jejuni) harbors the Type VI Secretion System (T6SS) that injects toxic effectors into neighboring cells, modulating microbial competitions in the harsh gut environment. Using bile salt as a natural stressor and T6SS-positive C. jejuni as a predator, we show that T6SS activity could entail a cost during bacterial predation under environmental stress. Our data suggest bile salt influx and subsequent DNA damage due to the prey-driven activation of the T6SS. We further combined experiments and mathematical modeling to explore how the stress-induced "predation cost" determines ecological outcomes. Consistent with a population-dynamics model, we found predator extinction above a critical bile salt concentration and prey-predator coexistence below this level. Moreover, we utilized the predation cost as an effective strategy facilitating host defense against C. jejuni infection. Together, we elucidate how predator dominance versus extinction emerges from the interplay between environmental stress and the T6SS machinery.

Fibronectin is a smart adhesive that both influences and responds to the mechanics of early spinal column development.

An extracellular matrix of Fibronectin adheres the neural tube to the two flanking columns of paraxial mesoderm and is required for normal vertebrate development. Here, we find that the bilaterally symmetric interfaces between the zebrafish neural tube and paraxial mesoderm function as optimally engineered adhesive lap joints with rounded edges, graded Fibronectin 'adhesive' and an arced adhesive spew filet. Fibronectin is a 'smart adhesive' that remodels to the lateral edges of the neural tube-paraxial mesoderm interfaces where shear stress is highest. Fibronectin remodeling is mechanically responsive to contralateral variation morphogenesis, and Fibronectin-mediated inter-tissue adhesion is required for bilaterally symmetric morphogenesis of the paraxial mesoderm. Strikingly, however, perturbation of the Fibronectin matrix rescues the neural tube convergence defect of cadherin 2 mutants. Therefore, Fibronectin-mediated inter-tissue adhesion dynamically coordinates bilaterally symmetric morphogenesis of the vertebrate trunk but predisposes the neural tube to convergence defects that lead to spina bifida.

In embryos, the spinal cord starts out as a flat sheet of cells that curls up to form a closed cylinder called the neural tube. The folding tube is attached to the surrounding tissues through an extracellular matrix of proteins and sugars. Overlapping strands of a protein from the extracellular matrix called Fibronectin connect the neural tube to adjacent tissues, like a kind of biological glue. However, it remained unclear what effect this attachment had on the embryonic development of the spinal cord. Connecting two overlapping objects with glue to form what is known as an 'adhesive lap joint' is common in fields such as woodworking and aeronautical engineering. The glue in these joints comes under shearing stress whenever the two objects it connects try to pull apart. But, thanks to work in engineering, it is possible to predict how different joints will perform under tension. Now, Guillon et al. have deployed these engineering principles to shed light on neural tube development. Using zebrafish embryos and computational models, Guillon et al. investigated what happens when the strength of the adhesive lap joints in the developing spine changes. This revealed that Fibronectin works like a smart adhesive: rather than staying in one place like a conventional glue, it moves around. As the neural tube closes, cells remodel the Fibronectin, concentrating it on the areas under the highest stress. This seemed to both help and hinder neural tube development. On the one hand, by anchoring the tube equally to the left and right sides of the embryo, the Fibronectin glue helped the spine to develop symmetrically. On the other hand, the strength of the adhesive lap joints made it harder for the neural tube to curl up and close. If the neural tube fails to close properly, it can lead to birth defects like spina bifida. One of the best-known causes of these birth defects in humans is a lack of a vitamin known as folic acid. Cell culture experiments suggest that this might have something to do with the mechanics of the cells during development. It may be that faulty neural tubes could close more easily if they were able to unglue themselves from the surrounding tissues. Further use of engineering principles could shed more light on this idea in the future.

Probing Mechanisms of Transcription Elongation Through Cell-to-Cell Variability of RNA Polymerase.

The process of transcription initiation and elongation are primary points of control in the regulation of gene expression. Although biochemical studies have uncovered the mechanisms involved in controlling transcription at each step, how these mechanisms manifest in vivo at the level of individual genes is still unclear. Recent experimental advances have enabled single-cell measurements of RNA polymerase (RNAP) molecules engaged in the process of transcribing a gene of interest. In this article, we use Gillespie simulations to show that measurements of cell-to-cell variability of RNAP numbers and interpolymerase distances can reveal the prevailing mode of regulation of a given gene. Mechanisms of regulation at each step, from initiation to elongation dynamics, produce qualitatively distinct signatures, which can further be used to discern between them. Most intriguingly, depending on the initiation kinetics, stochastic elongation can either enhance or suppress cell-to-cell variability at the RNAP level. To demonstrate the value of this framework, we analyze RNAP number distribution data for ribosomal genes in Saccharomyces cerevisiae from three previously published studies and show that this approach provides crucial mechanistic insights into the transcriptional regulation of these genes.

Broad spectral responsivity in highly photoconductive InZnO/MoS2 heterojunction phototransistor with ultrathin transparent metal electrode.

An amorphous InZnO/MoS2 heterojunction-based phototransistor with excellent photoconductive gain and responsivity over the entire visible range has been demonstrated. The photogenerated current of the InZnO phototransistor at long light wavelength (>600 nm) was significantly improved by utilizing narrow bandgap MoS2 as the capping layer (1.3 eV). At lower wavelength, photocarriers are generated due to the optical absorption of both InZnO and MoS2 layers, whereas the latter ensures significant photocarrier generation even at the higher wavelength region of the visible spectrum. The photogenerated carriers subsequently transfer to the underlying InZnO layer of superior carrier mobility that has a high channel conduction of additional electrons from the optically-induced doubly positively charged oxygen vacancies (Vo++) where the gate field is screening, thereby leading to the higher photoconductive gain of the InZnO/MoS2 phototransistors. The dynamic photosensitivity behaviour of the aforesaid phototransistor reveals the presence of persistent photoconductivity (PPC) due to the oxygen vacancy associated with InZnO which can be removed by applying a reset gate pulse from -15 to +5 V. The optical properties of these phototransistors were further enhanced by replacing the opaque Ti/Au electrode by an ultrathin transparent Ti/Au electrode. Utilization of the transparent electrode results in enhanced electron injection from source to channel due to a reduced barrier height under illumination giving rise to a ten-fold improvement in the photocurrent and responsivity of the phototransistors. A position-dependent study of the photocurrent w.r.t beam position also reveals that the enhancement in photocurrent is strongly dependent on the position and is at its maximum when the beam is placed near the source region.

Cell-to-cell variability in organelle abundance reveals mechanisms of organelle biogenesis.

How cells regulate the number of organelles is a fundamental question in cell biology. While decades of experimental work have uncovered four fundamental processes that regulate organelle biogenesis, namely, de novo synthesis, fission, fusion, and decay, a comprehensive understanding of how these processes together control organelle abundance remains elusive. Recent fluorescence microscopy experiments allow for the counting of organelles at the single-cell level. These measurements provide information about the cell-to-cell variability in organelle abundance in addition to the mean level. Motivated by such measurements, we build upon a recent study and analyze a general stochastic model of organelle biogenesis. We compute the exact analytical expressions for the probability distribution of organelle numbers, their mean, and variance across a population of single cells. It is shown that different mechanisms of organelle biogenesis lead to distinct signatures in the distribution of organelle numbers which allow us to discriminate between these various mechanisms. By comparing our theory against published data for peroxisome abundance measurements in yeast, we show that a widely believed model of peroxisome biogenesis that involves de novo synthesis, fission, and decay is inadequate in explaining the data. Also, our theory predicts bimodality in certain limits of the model. Overall, the framework developed here can be harnessed to gain mechanistic insights into the process of organelle biogenesis.

Improved optical performance of multi-layer MoS2 phototransistor with see-through metal electrode.

In recent years, MoS2 has emerged as a prime material for photodetector as well as phototransistor applications. Usually, the higher density of state and relatively narrow bandgap of multi-layer MoS2 give it an edge over monolayer MoS2 for phototransistor applications. However, MoS2 demonstrates thickness-dependent energy bandgap properties, with multi-layer MoS2 having indirect bandgap characteristics and therefore possess inferior optical properties. Herein, we investigate the electrical as well as optical properties of single-layer and multi-layer MoS2-based phototransistors and demonstrate improved optical properties of multi-layer MoS2 phototransistor through the use of see-through metal electrode instead of the traditional global bottom gate or patterned local bottom gate structures. The see-through metal electrode utilized in this study shows transmittance of more than 70% under 532 nm visible light, thereby allowing the incident light to reach the entire active area below the source and drain electrodes. The effect of contact electrodes on the MoS2 phototransistors was investigated further by comparing the proposed electrode with conventional opaque electrodes and transparent IZO electrodes. A position-dependent photocurrent measurement was also carried out by locally illuminating the MoS2 channel at different positions in order to gain better insight into the behavior of the photocurrent mechanism of the multi-layer MoS2 phototransistor with the transparent metal. It was observed that more electrons are injected from the source when the beam is placed on the source side due to the reduced barrier height, giving rise to a significant enhancement of the photocurrent.

White light emitting diode based on purely organic fluorescent to modern thermally activated delayed fluorescence (TADF) and perovskite materials.

White organic/polymer light emitting diode (WOLED/WPLED) processed from solution has attracted significant research interest in recent years due to their low device production cost, device flexibility, easy fabrication over large area including roll to roll and ability to print in various designs and shapes providing enormous design possibilities. Although WOLEDs fabricated using solution process lack their thermally evaporated counterparts in terms of device efficiency, remarkable progress has been made in this regard in recent years by utilizing new materials and device structures. In the present review, we have summarized and extrapolated an excellent association of old and modern concept of cost-effective materials and device structure for realization of white light. In particular, this article demonstrated and focused on design, and development of novel synthesis strategy, mechanistic insights and device engineering for solution process low cost WOLEDs device. Herein, an overview of the prevailing routes towards white light emitting devices (WLEDs) and corresponding materials used, including polymer based WLED, small molecules emitters based thermally activated delayed fluorescence (TADF), perovskite light-emitting diodes (PeLEDs) and hybrid materials based LEDs, color down-converting coatings with corresponding best efficiencies ever realized. We presume that this exhaustive review on WLEDs will offer a broad overview of the latest developments on white SSL and stonework the approach en route for innovations in the immediate future.

Organization of Embryonic Morphogenesis via Mechanical Information.

Embryonic organizers establish gradients of diffusible signaling molecules to pattern the surrounding cells. Here, we elucidate an additional mechanism of embryonic organizers that is a secondary consequence of morphogen signaling. Using pharmacological and localized transgenic perturbations, 4D imaging of the zebrafish embryo, systematic analysis of cell motion, and computational modeling, we find that the vertebrate tail organizer orchestrates morphogenesis over distances beyond the range of morphogen signaling. The organizer regulates the rate and coherence of cell motion in the elongating embryo using mechanical information that is transmitted via relay between neighboring cells. This mechanism is similar to a pressure front in granular media and other jammed systems, but in the embryo the mechanical information emerges from self-propelled cell movement and not force transfer between cells. The propagation likely relies upon local biochemical signaling that affects cell contractility, cell adhesion, and/or cell polarity but is independent of transcription and translation.

Patterned Disordered Cell Motion Ensures Vertebral Column Symmetry.

The biomechanics of posterior embryonic growth must be dynamically regulated to ensure bilateral symmetry of the spinal column. Throughout vertebrate trunk elongation, motile mesodermal progenitors undergo an order-to-disorder transition via an epithelial-to-mesenchymal transition and sort symmetrically into the left and right paraxial mesoderm. We combine theoretical modeling of cell migration in a tail-bud-like geometry with experimental data analysis to assess the importance of ordered and disordered cell motion. We find that increasing order in cell motion causes a phase transition from symmetric to asymmetric body elongation. In silico and in vivo, overly ordered cell motion converts normal anisotropic fluxes into stable vortices near the posterior tail bud, contributing to asymmetric cell sorting. Thus, disorder is a physical mechanism that ensures the bilateral symmetry of the spinal column. These physical properties of the tissue connect across scales such that patterned disorder at the cellular level leads to the emergence of organism-level order.

Effect of transcription factor resource sharing on gene expression noise.

Gene expression is intrinsically a stochastic (noisy) process with important implications for cellular functions. Deciphering the underlying mechanisms of gene expression noise remains one of the key challenges of regulatory biology. Theoretical models of transcription often incorporate the kinetics of how transcription factors (TFs) interact with a single promoter to impact gene expression noise. However, inside single cells multiple identical gene copies as well as additional binding sites can compete for a limiting pool of TFs. Here we develop a simple kinetic model of transcription, which explicitly incorporates this interplay between TF copy number and its binding sites. We show that TF sharing enhances noise in mRNA distribution across an isogenic population of cells. Moreover, when a single gene copy shares it's TFs with multiple competitor sites, the mRNA variance as a function of the mean remains unaltered by their presence. Hence, all the data for variance as a function of mean expression collapse onto a single master curve independent of the strength and number of competitor sites. However, this result does not hold true when the competition stems from multiple copies of the same gene. Therefore, although previous studies showed that the mean expression follows a universal master curve, our findings suggest that different scenarios of competition bear distinct signatures at the level of variance. Intriguingly, the introduction of competitor sites can transform a unimodal mRNA distribution into a multimodal distribution. These results demonstrate the impact of limited availability of TF resource on the regulation of noise in gene expression.

Sufficient conditions for the additivity of stall forces generated by multiple filaments or motors.

Molecular motors and cytoskeletal filaments work collectively most of the time under opposing forces. This opposing force may be due to cargo carried by motors or resistance coming from the cell membrane pressing against the cytoskeletal filaments. Some recent studies have shown that the collective maximum force (stall force) generated by multiple cytoskeletal filaments or molecular motors may not always be just a simple sum of the stall forces of the individual filaments or motors. To understand this excess or deficit in the collective force, we study a broad class of models of both cytoskeletal filaments and molecular motors. We argue that the stall force generated by a group of filaments or motors is additive, that is, the stall force of N number of filaments (motors) is N times the stall force of one filament (motor), when the system is reversible at stall. Conversely, we show that this additive property typically does not hold true when the system is irreversible at stall. We thus present a novel and unified understanding of the existing models exhibiting such non-addivity, and generalise our arguments by developing new models that demonstrate this phenomena. We also propose a quantity similar to thermodynamic efficiency to easily predict this deviation from stall-force additivity for filament and motor collectives.

High-Performance ZnPc Thin Film-Based Photosensitive Organic Field-Effect Transistors: Influence of Multilayer Dielectric Systems and Thin Film Growth Structure.

The key impact and significance of a multilayer polymer-based dielectric system on the remarkable photoresponse properties of zinc phthalocyanine (ZnPc)-based photosensitive organic field-effect transistors (PS-OFETs) have been systematically analyzed at various incident optical powers. A combination of inorganic aluminum oxide (Al2O3) and organic nonpolar poly(methyl methacrylate) (PMMA) is used as the bilayer dielectric configuration, whereas in the trilayer dielectric system, a bilayer polymer dielectric, consisting of PMMA, as the low-k dielectric polymer, on top of poly(vinyl alcohol) (PVA), the high-k polar dielectric, has been fabricated along with Al2O3 as the third layer. Before fabricating the OFETs, a systematic optimization of the nature of growth of the ZnPc molecules, deposited on PMMA-coated glass substrates at different substrate temperatures (T s) was performed and examined by atomic-force microscopy, field-emission scanning electron microscopy, X-ray diffraction, and Raman analysis. At 90 °C, the fabricated PS-OFETs with the Al2O3/PVA/PMMA trilayer dielectric configuration showed the best p-channel behavior, with an enhanced and remarkable photoresponsivity of R ∼ 9689.39 A W-1 compared to that of the Al2O3/PMMA bilayer dielectric system (R ∼ 2679.40 A W-1) due to the polarization of the dipoles inside the polar PVA dielectric, which increases the charge transport through the channel. The charge carrier mobility of the device also improved by one order (µh ∼ 1.3 × 10-2 cm2 V-1 s-1) compared to that of the bilayer dielectric configuration (µh ∼ 3.5 × 10-3 cm2 V-1 s-1). The observed specific detectivity (D*) and NEP values of the bilayer dielectric system were 6.01 × 1013 Jones and 2.655 × 10-17 W Hz-1/2, whereas for the trilayer dielectric system, the observed D* and NEP values were 5.13 × 1014 Jones and 1.043 × 10-17 W Hz-1/2, respectively. Additionally, the operating voltage of each of the fabricated devices was also very low (-10 V) due to the influence of the inorganic high-k Al2O3 dielectric layer. The electrical stability of all of the fabricated devices was also investigated by bias stress analysis under both light and dark conditions in vacuum. To the best of our knowledge, the photoresponsivity (R) reported here with an Al2O3/PVA/PMMA trilayer dielectric configuration is the highest reported value for thin film-based PS-OFETs, at a remarkably low operating voltage of -10 V, on low-cost glass substrates without indium tin oxide or/and Si/SiO2.

Photosensitive organic field effect transistors: the influence of ZnPc morphology and bilayer dielectrics for achieving a low operating voltage and low bias stress effect.

Photosensitive-organic field effect transistors (PS-OFETs) based on a morphology controlled zinc phthalocyanine (ZnPc) layer, with an inorganic-organic bilayer gate dielectric system, fabricated on a glass substrate showed remarkable efficiency as light sensors at various incident optical powers. The indium tin oxide (ITO) and Si/SiO2 free low-cost OFET devices show low bias stress and a reduced operating voltage with aluminum oxide and poly(methyl methacrylate) (Al2O3/PMMA) as bilayer gate dielectrics and copper (Cu) as a top contact. They exhibit excellent p-channel behavior with a remarkable photo-responsivity of 2679.40 A W-1 and a photo-ON/OFF current ratio of 933.56 with a very low operating voltage (0 to -8 V), which have not been observed previously. The bias stress effect of the device was investigated under both light and dark conditions in a vacuum. It was observed that the effect of the stress is extremely small in the presence of light (a decay of IDS of ∼ 20% after 30 min) compared to the dark, with a characteristic carrier relaxation time τ' ∼ 104 s. This device with high electrical stability under ambient conditions and a low threshold voltage under constant electrical bias stress is expected to have potential applications in optoelectronic devices and energy efficient sensors.

White polymer light emitting diodes based on PVK: the effect of the electron injection barrier on transport properties, electroluminescence and controlling the electroplex formation.

The effects of the electron injection barrier on the charge transport, brightness and the electroluminescence (EL) properties of polymer light emitting diodes (PLEDs) with poly(9-vinylcarbazole) (PVK) as an emissive layer have been studied. By using Al and LiF/Al as the cathode in single layer PLEDs and diverse electron transporting layers (ETLs) such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (BPhen) and 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) in the case of multilayer PLEDs, the charge transport, brightness, color tuning and the EL properties of the devices were drastically modified. The energy barrier for electrons affects the electron current flowing through the device, thereby affecting the operating voltage and the brightness of the PLEDs. The PLEDs with TPBi as the ETL possess the lowest injection barrier and give the maximum brightness of 426.24 cd m-2. The electron injection barrier is also found to play a major role in defining the EL spectra of the PLEDs. A larger injection barrier gives rise to electroplex formation in the EML-ETL interface of the PLEDs and an additional peak at ∼605 nm was observed in the EL spectrum. As a result, a near white emission with CIE coordinates of (0.30, 0.30) and (0.25, 0.23) at 20 V was obtained from devices with BCP and BPhen as ETLs. Furthermore, PVK doped with 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) at 10, 20 and 30 wt% ratios modified the electron transport nature of PVK and had a remarkable influence on the aforesaid properties, especially on the electroplex formation.