Priyanka Tyagi

Energy transfer process between exciton and surface plasmon: Complete transition from Forster to surface energy transfer

The energy transfer process between surface plasmons and excitons was studied by varying the filling fraction of gold (Au) nano-clusters (NCs) and by placing a spacer of different thickness between Au NC and organic semiconductor layer. The intensity enhancement has occurred for 10%-50% filling fractions and 4-14 nm spacer thicknesses. Energy transfer mechanism was found to switch from Forster type to surface type by increase in filling fraction. Transverse electric field for Au NCs was simulated and we observed that for filling fraction 60%, they behave like 2-dimensional dipoles.

Effect of doping of 8-hydroxyquinolinatolithium on electron transport in tris(8-hydroxyquinolinato)aluminum

Effect of doping of 8-hydroxyquinolinatolithium (Liq) on the electron transport properties of tris(8-hydroxyquinolinato)aluminum (Alq3) has been investigated as a function of temperature and doping concentration by fabricating electron only devices. It has been observed that current density in the devices increases with the doping of Liq up to a doping concentration of 33 wt. % and then decreases. Current density-voltage (J-V) characteristics of 0, 15, and 33 wt. % Liq doped Alq3 devices were found to be bulk limited and analyzed on the basis of trap charge limited conduction model. The J-V characteristics of 50 and 100 wt. % Liq doped Alq3 devices were found to be injection limited and were analyzed using the Fowler-Nordheim model. The increase in current density with doping up to 33 wt. % was found to be due to an increase in electron mobility upon doping, whereas the decrease in current density above 33 wt. % was due to the switching of transport mechanism from bulk limited to injection limited type du...

Exciton quenching by diffusion of 2,3,5,6-tetrafluoro-7,7',8,8'-tetra cyano quino dimethane and its consequences on joule heating and lifetime of organic light-emitting diodes.

In this Letter, the effect of F(4)-TCNQ insertion at the anode/hole transport layer (HTL) interface was studied on joule heating and the lifetime of organic light-emitting diodes (OLEDs). Joule heating was found to reduce significantly (pixel temperature decrease by about 10 K at a current density of 40 mA/cm(2)) by this insertion. However, the lifetime was found to reduce significantly with a 1 nm thick F(4)-TCNQ layer, and it improved by increasing the thickness of this layer. Thermal diffusion of F(4)-TCNQ into HTL leads to F(4)-TCNQ ionization by charge transfer, and drift of these molecules into the emissive layer caused faster degradation of the OLEDs. This drift was found to reduce with an increase in the thickness of F(4)-TCNQ.

Enhanced carrier transport in tris(8-hydroxyquinolinate) aluminum by titanyl phthalocyanine doping

The effect of doping titanyl phthalocyanine (TiOPc) into tris(8-hydroxyquinolinate) aluminum (Alq3) (Alq3:T; where T represents TiOPc), used as an electron transport layer (ETL) for organic light emitting diodes (OLEDs), was investigated. The surface roughness of the doped thin films increases with the doping concentration as a result of a needle like 3D growth of TiOPc in Alq3. The electron mobility depends on the doping concentration. The electron mobility calculated in the trap-free space-charge limited region (SCLC) for a 2% doped Alq3 thin film was found to be 0.17 × 10−5 cm2 V−1 s−1 which is four orders of magnitude greater than that for pristine Alq3. The electroluminescence at a constant current density of 10 mA cm−2 is 3098, 4700, 7800, 3600 and 520 cd m−2 for Alq3 to Alq3:T(1%, 2%, 3% and 5%) devices, respectively. Similarly the power efficiency at a constant current density of 10 mA cm−2 is 2.1, 2.7, 4.2, 1.3 and 0.48 lm W−1 for the different doped devices Alq3 to Alq3:T(1%, 2%, 3% and 5%), respectively. The OLEDs based on the optimized 2% doped TiOPc in Alq3 show a four times increase in the electroluminescence as well as an almost doubling of the power efficiency. There are interfacial charges near the doped layer. The Cole–Cole plot indicates the device can be modeled as the combination of three parallel resistance–capacitance (R–C) equivalent circuits.

Study of 2,3,5,6-tetrafluoro-7,7′,8,8′- tetracyano quinodimethane diffusion in organic light emitting diodes using secondary ion mass spectroscopy

In this work, 2,3,5,6 – tetrafluoro – 7,7′,8,8′- tetracyano quinodimethane (F4-TCNQ) diffusion has been studied using secondary ion mass spectroscopy (SIMS). SIMS depth profiling has been performed in dual beam mode, in which, a low energy oxygen beam has been used for etching and a high energy Bi1+ ion beam has been used for analysis. F4-TCNQ has been identified by the distinguishable presence of fluorine in organic layers. For this study, organic light emitting diodes (OLEDs) were fabricated with F4-TCNQ as a hole injection layer with 1, 2.5 and 5.5 nm thicknesses. The diffusion length and depth were measured to be 13, 19, 19.1 nm and 27, 28, 29 nm for 1, 2.5, 5.5 nm thicknesses of F4-TCNQ, respectively. The diffusion of F4-TCNQ into the hole transport layer leads to the ionization of F4-TCNQ molecules and p-type doping of the hole transport material. The effect of the electric field on the diffusion was also studied by performing the depth profiling on electric field applied OLEDs and it was observed that the application of the electric field has increased both the diffusion length and depth of F4-TCNQ. This effect was found to be more pronounced for the OLED with 1 nm thickness of F4-TCNQ in comparison to the OLEDs with 2.5 and 5.5 nm thicknesses of F4-TCNQ. The field affected diffusion length and depth were found to be 14.5, 19.5, 19.6 and 35, 28.6, 29.6 nm for OLEDs with 1, 2.5, 5.5 nm thicknesses of F4-TCNQ hole injection layer. The decrease in the field effect has been ascribed as being due to the increase in the cluster density and size. Further, the effect of F4-TCNQ diffusion on OLEDs has been studied by capturing the optical images at different instances of time. OLEDs with 1 nm F4-TCNQ layer were found to be the most unstable. This effect has been ascribed as being due to diffusion of F4-TCNQ into the emissive layer which leads to the dissociation of excitons formed inside the emissive layer.

Application of 2D-MoO3 nano-flakes in organic light emitting diodes: effect of semiconductor to metal transition with irradiation

The current work demonstrates efficient utilization of 2D-MoO3 nano-flakes as a hole injection layer (HIL) in organic light emitting diodes (OLEDs). Nano-flakes are synthesized using an organic solvent-assisted grinding and sonication method of liquid exfoliation for MoO3, and 8–16 nm thick flakes are obtained. The effect of solar illumination on the hole injection properties of these nano-flakes is then studied by exposing the nano-flakes for 0, 15, 30, 45, 60 and 120 min and using them as HIL in green OLED. The device results are then compared with the OLED having bulk MoO3 as HIL. OLEDs with nano-flakes as the HIL have shown better performance than the OLED with bulk MoO3 as the HIL due to the better semiconducting properties in the nano-flake phase. The luminous intensity is increased by increasing the duration of irradiation and was found to be optimum in case of nano-flakes irradiated for 30 or 45 min and then started to decrease with the increase of duration of irradiation. The current density in the OLEDs with nano-flakes as the HIL shows a switching from high resistance to low resistance; however, the sequential pattern of switching voltage was missing with the duration of irradiation. The current density also decreased for nano-flakes with 60 and 120 min of irradiation. Transition from the semiconducting to metal nature of nano-flakes by solar irradiation is suggested to be the reason behind this decrease in current density and luminous intensity with a longer duration of illumination.

Elucidation on Joule heating and its consequences on the performance of organic light emitting diodes

Current work presents a quantitative analysis of Joule heating by temperature measurements using infrared thermography and heat estimation of organic light emitting diodes (OLEDs) and their correlation with device life time. These temperature measurements were performed at 10, 20, 30, 40, and 50 mA/cm2 current densities and studied with operational time. The temperature rise of the device has increased from 9.8 to 16.6 °C within 168 h at an operating current density of 40 mA/cm2. This has been ascribed as due to the external contamination by water, oxygen, and dust particles as well as by internal heat generation. Encapsulation of the device avoids external degradation of OLEDs by preventing the destruction caused by these external contaminations. In this way, encapsulation has led to the decreased temperature rise of 12.4 °C within the duration of 168 h, which reflects the improved stability of the device. The temperature measured has been used to calculate the heat generated inside the device by solving...

An n-type, new emerging luminescent polybenzodioxane polymer for application in solution-processed green emitting OLEDs

Herein, we report polybenzodioxane polymer (PIM-1), a multifunctional n-type emitter with strong green luminescence, and its suitability as an electron transport layer for OLEDs devices. The Brunauer–Emmett–Teller (BET) test and photo-electrical properties of as-synthesized PIM-1 confirm the presence of large microporosity and excellent electron mobility. The photoluminescence (PL) spectroscopy shows the intense green emission at 515 nm upon 332 nm excitation wavelength. Moreover, the Hall effect study reveals the negative Hall resistivity, which indicates that PIM-1 possesses n-type semiconductor characteristics. It enables the highly-efficient polymer-based green LEDs with configuration; ITO (120 nm)/PEDOT:PSS (30 nm)/PIM-1 (100 nm)/LiF (1 nm)/Al (150 nm), which are fabricated by the sequential solution-processing method. The OLED incorporating PIM-1 thin layer achieves maximum current efficiency of 1.71 Cd A−1 and power efficiency of 0.49 lm W−1. Additionally, the electron mobility is found to be 4.4 × 10−6 cm2 V−1 s−1. Hence, these results demonstrate that PIM-1 could be an ultimate choice as an n-type emitter for the next generation of advanced electronic devices.

Optimization, characterization, and efficacy evaluation of 2% chitosan scaffold for tissue engineering and wound healing

Objective: To develop a chitosan-based scaffold and carry out a complete comprehensive study encompassing optimization of exact chitosan strength, product characterization, toxicity evaluation, in vitro validation in cell culture experiments, and finally in vivo efficacy in animal excision wound model. Materials and Methods: Developed chitosan scaffolds (CSs) were optimized for tissue engineering and wound healing efficacy by means of microstructure, toxicity, and biocompatibility evaluation. Results: Scanning electron microscope (SEM) studies revealed that porosity of CS decreased with increase in chitosan concentration. Chemical stability and integrity of scaffolds were confirmed by Fourier transform infrared studies. Highest swelling percentage (SP) of 500% was observed in 2%, while lowest (200%) was observed in 1% CS. Reabsorption and noncytotoxic property of optimized scaffold were established by enzymatic degradation and MTT assay. Enzymatic degradation suggested 20–45% of weight loss (WL) within 14 days of incubation. Cytotoxicity analysis showed that scaffolds were noncytotoxic against normal human dermal fibroblast human dermal fibroblast cell lines. Significant cellular adherence over the scaffold surface with normal cellular morphology was confirmed using SEM analysis. In vivo efficacy evaluation was carried out by means of reduction in wound size on Sprague-Dawley rats. Sprague-Dawley rats treated with optimized scaffold showed ~ 100% wound healing in comparison to ~80% healing in betadine-treated animals within 14 days. Histological examination depicted advance re-epithelization with better organization of collagen bundle in wound area treated with 2% CS in comparison to conventional treatment or no treatment. Conclusion: This study, thus, reveals that 2% CSs were found to have a great potential in wound healing.

Synthesis and electroluminescence properties of tris-[5-choloro-8-hydroxyquinoline] aluminum Al(5-Clq)3

A new electroluminescent material tris-[5-choloro-8-hydroxyquinoline] aluminum has been synthesized and characterized. Solution of this material Al(5-Clq) 3 in toluene showed absorption maxima at 385 nm which was attributed to the moderate energy (π-π * ) transitions of the aromatic rings. The photoluminescence spectrum of Al(5-Clq) 3 in toluene solution showed a peak at 522 nm. This material shows thermal stability up to 400 ℃. The structure of the device is ITO/0.4 wt%F4-TCNQ doped α-NPD (35 nm) / Al(5-Clq) 3 (30 nm)/ BCP (6 nm)/ Alq 3 (30 nm)/ LiF (1 nm)/Al(150 nm). This device exhibited a luminescence peak at 585 nm (CIE coordinates, x = 0.39, y = 0.50). The maximum luminescence of the device was 920 Cd/m 2 at 25 V. The maximum current efficiency of OLED was 0.27 Cd/A at 20 V and maximum power efficiency was 0.04 lm/W at 18 V.