C. K. Suman

Study of Schottky contact in binary and ternary hybrid CdSe quantum dot solar cells

Hybrid binary and ternary organic solar cell devices were fabricated from P3HT–PCBM with CdSe quantum dot materials. The binary and ternary structured solar cells were designed with the combinations of P3HT–PCBM (device A), P3HT–CdSe (device B) and P3HT–CdSe–PCBM (device C). The absorption spectra of P3HT, CdSe and PCBM were analyzed in the wavelength range from 350 nm to 800 nm. The current density–voltage characteristics of the devices were performed in the dark and under illumination to study the conduction process and solar cell performance. The ideality factor of all the devices is more than one. The capacitance–voltage analysis of the devices shows that the depletion width of the binary device (WA and B ∼ 39 and 30.8 nm) is lower than the ternary device (Wc ∼ 46.8 nm). The carrier densities of device A, B and C are 1.49 × 1023 m−3, 0.52 × 1023 m−3 and 2.6 × 1023 m−3, respectively. The device can be modeled as the combination of two RC parallel circuits (one for the active layer, and the other for the interface) in series with contact resistance Rs ∼ 0 ohm for device A, B and 60 ohm for device C.

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.

Improved Performance of Organic LEDs with Modified Metal-Organic Interface

Incorporation of thin layers of strong acceptor tetrafluro-tetracyanoquinodi methane (F4TCNQ) over conducting substrate Indium-Tin-Oxide (ITO) modified its work function. The enhancement depended on the thickness of F4TCNQ film. These modified ITO substrates were used as hole injection contacts in OLEDs which show enhanced device current, lower operating voltages and enhanced current efficiency.

AC Impedance Spectroscopy Studies of PtPc Doped Alq 3 Thin Film

The Platinum phthalocyanine (PtPc) was doped in Tris (8-hydroxyquinolinato) aluminum (Alq3) for tailoring the electrical and optical properties. The electrical properties of the optimally doped Alq3 thin film based on different device application were studied extensively by AC impedance spectroscopy. The absorption peak of the thin film was observed at 381 nm and 598 nm that correspond to the absorption of Alq3 and PtPc, respectively. The photo-luminescence intensity (PL) peak of thin film excited with wavelength 380 nm was found at 510 nm. The electron mobility calculated from trap-free space-charge limited region (SCLC) of current-voltage characteristics is 0.3 × 10−7 cm2 V−1s−1. The frequency dependent impedance spectra show single relaxation process. The Cole-Cole plots indicates that the PtPc doped Alq3 thin film can be represented by a single parallel resistance R P and capacitance C P network with a series resistance R S . The values of R P decreases with increase of applied bias. The variation of AC conductivity (σ ac ) with frequency shows two different region viz frequency dependent and frequency independent.

A vertically stacked phosphorescent multilayer organic light emitting transistor

​The fabricated VOLETs has working voltage of less than 5 V with a luminance of 3450 Cd m−2. The charge injection with a gate electric field is mainly governed by the choice of organic materials and rough source electrode.

Purification method dependent fluorescence from nitrogen-vacancy (NV) centers of nano-diamonds

Fluorescent nanodiamonds (FNDs) with high photo stability at a subwavelength scale are highly desirable for nano-photonics and bio-imaging applications. Nanodiamonds (NDs) with embedded fluorescent color centers made by ion-implantation need to be purified to remove the sp2 layer on their surfaces which significantly degrades the optical properties. In this work, we discuss the structural and photo physical properties of NDs containing nitrogen-vacancy (NV) centers prepared by two different purification methods; chemical etching (H2SO4:HNO3) and air oxidation (450 °C). Chemically etched NDs show better uniformity in their shape, de-aggregation and higher dispersibility in water as compared to air oxidized ones. On the other hand it is observed that air oxidation is more effective in removing the sp2 layer and allows a higher fluorescence photon flux. Therefore, we suggest that air oxidation is more appropriate for bright fluorescent sources, and chemical etching is more appropriate for fluorescent markers in bio-imaging applications with high uniformity in shape and good dispersibility.

Optical and electrical properties of TiOPc doped Alq3 thin films

The Titanyl phthalocyanine (TiOPc) was doped in Tris (8-hydroxyquinolinato) aluminum (Alq3) with different concentration. The thin film of optimized doping concentration was studied extensively for optical and electrical properties. The optical properties, studied using ellipsometry, absorption and photoluminescence. The absorption peak of Alq3 and TiOPc was observed at 387 nm and 707 nm and the photo-luminescence intensity (PL) peak of doped thin film was observed at 517 nm. The DC and AC electrical properties of the thin film were studied by current density-voltage (J-V) characteristics and impedance over a frequency range of 100 Hz - 1 MHz. The electron mobility calculated from trap-free space-charge limited region (SCLC) is 0.17×10−5 cm2/Vs. The Cole-Cole plots shows that the TiOPc doped Alq3 thin film can be represented by a single parallel resistance RP and capacitance CP network with a series resistance RS (10 Ω). The value of RP and CP at zero bias was 1587 Ω and 2.568 nF respectively. The resista...

Study of Schottky Barrier Contact in Hybrid CdSe Quantum Dot Organic Solar Cells

The hybrid organic solar cell devices are fabricated from P3HT:PCBM with Quantum dot CdSe materials.The Schottky barrier built-in potential of 0.45 V was calculated from C–V measurements. The photo current (JLight − JDark) equals to zero at compensation voltage of 0.61 V. The depletion width (W) of an abrupt Schottky junction is calculated using carrier density. The Cole–Cole plot of the device is also determined from different bias voltage in the range of 10–1 MHz. The device can be modeled as the combination of RC parallel circuit.