Faculty Publications
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Item Investigation of hole transport in ?-NPD using impedance spectroscopy with F4TCNQ as hole-injection layer(Academic Press, 2015) Fernandes, J.M.; Raveendra Kiran, M.R.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.The charge carrier transport is studied in N,N?-di(1-naphthyl)-N,N?-diphenyl-(1,1?-biphenyl)-4,4?-diamine (?-NPD) with the incorporation of sequentially doped p-type dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) as hole-injection layer in hole-only device structures. The field dependent mobility of the charge carriers is determined using frequency dependent capacitance, conductance and impedance methods by varying the thickness of ?-NPD. The Poole-Frenkel zero-field mobility and the Poole-Frenkel coefficient thus obtained for each device in all the three methods is found to be almost constant. © 2015 Elsevier Ltd. All rights reserved.Item Investigation of charge transport in Vanadyl-phthalocyanine with molybdenum trioxide as a buffer layer: Impedance spectroscopic analysis(Elsevier Ltd, 2015) Raveendra Kiran, M.R.; Ulla, H.; Krishnamanohara; Satyanarayan, M.N.; Umesh, G.Charge transport in organic materials is one of the intrinsic properties, which governs the device performance. In this paper, we report the fabrication and electrical characterization of two diodes ITO/VOPc/MoO3/Al and ITO/VOPc/Al. We investigate the electrical conduction of Vanadyl phthalocyanine (VOPc) in both the devices and also the effect of MoO3 as a buffer layer. Improvement of current density through the device is estimated using current density - voltage characteristics and capacitance - voltage characteristics. Space charge limited current (SCLC) conduction with an exponential trap distribution is observed from Impedance measurements. The dominant hopping charge transport is discussed based on ac conductivity measurements and by adopting Correlated barrier hopping (CBH) model. © 2015 Elsevier B.V. All rights reserved.Item Effect of deposition rate on the charge transport in Vanadyl-phthalocyanine thin films(Elsevier Ltd, 2017) Raveendra Kiran, M.R.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.We report fabrication of Vanadyl phthalocyanine (VOPc) based diodes with different deposition rates (0.1, 1 and 5 Å/s) in hole only device configuration: ITO/MoO3/VOPc/MoO3/Al. The dc and ac electrical conductivity of Vanadyl phthalocyanine based devices is investigated by employing Impedance spectroscopy measurements. The frequency dependence of conductivity indicates that the dominant mechanism for charge transport is the hopping type. Further, the dependence of conductivity on temperature and bias voltage clearly indicates that the hopping mechanism is described by the correlated barrier hopping (CBH) model. The thin layer (3 nm) of MoO3 in our devices is seen to enhance the electrical conductivity. J-V measurements indicate that the current density J as well as the charge carrier mobility are higher for the devices fabricated at a relatively lower deposition rate (0.1 Å/s). Our results suggest that the VOPC films deposited at lower rates are more appropriate for the optoelectronic device applications. © 2016 Elsevier B.V.Item Optoelectronic properties of novel alkyl-substituted Triphenylamine derivatives(Elsevier B.V., 2017) Fernandes, J.M.; Swetha, C.; Appalnaidu, E.; Navamani, K.; Rao, V.J.; Satyanarayan, M.N.; Umesh, G.Hole transport characteristics in three new organic compounds based on triphenylamine (TPA) moiety are presented. The effect on electrical and optical properties of TPA, attached with methyl or tert-butyl side groups, has been investigated through measurement of current density versus voltage (J-V), capacitance versus voltage (C-V), frequency dependent capacitance, ac conductivity, Impedance spectroscopy, UV-Vis spectroscopy, Photoluminescence (PL) spectroscopy and X-Ray Diffraction (XRD) studies. These measurements reveal that, the attachment of methyl or tert-butyl group in the para-position of the TPA moiety leads to improved optoelectronic properties and greater molecular stability. XRD analysis of the samples indicates that the inter-molecular distance is the lowest for TPA with tert-butyl side group (3.43 Å) as compared to pure TPA (3.57 Å). This leads to stronger inter-molecular interaction as evidenced by the UV-Vis spectra. PL studies indicate significant Quantum Efficiency (?30%) for alkyl attached TPA. In order to get a better understanding of the charge transport phenomena, the effect of molecular structure dynamics on charge transfer kinetics is analyzed by evaluating the charge carrier hopping rate coefficient and dynamic state factor. The dynamic state factor b has higher value for lower bias voltage, corresponding to dc conductivity, whereas, at higher bias, the value of b is smaller, indicating the dominance of ac conductivity. Hopping conductivity is seen to be highest for the device with tert-butyl substitution in TPA moiety. Our experiments indicate an order of magnitude enhancement in charge carrier mobility for alkyl-substituted TPA. © 2017 Elsevier B.V.Item Optoelectronic properties of hybrid diodes based on vanadyl-phthalocyanine and zinc oxide(Academic Press, 2017) Raveendra Kiran, M.R.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.We report an investigation of the optoelectronic properties of a hybrid p-n diode device fabricated using ZnO film prepared by sol-gel technique on which a VOPc organic film is deposited by vacuum evaporation. The charge transport properties of devices having the configurations ITO/ZnO/Al and ITO/ZnO/VOPc/MoO3/Al were investigated at different annealing temperatures (150 °C, 250 °C, 350 °C and 450 °C) by Impedance Spectroscopy (IS). The structural, morphological, optical and electrical properties were also studied at different annealing temperatures. The parameters related to the ITO/ZnO and ZnO/VOPc interfaces such as ideality factor (n), barrier height (q?B) and rectification ratio (RR) of the diodes were determined from current density-voltage (J-V) characteristics. IS measurements suggest that the large photocurrent generated is due to the decrease in bulk resistance of the device on account of the generation of electron-hole pairs in the organic active layer when exposed to light. The RR and the photocurrent responsivity (Rph) values obtained from the J-V characteristics compare well with those obtained from the IS measurements. It was observed that the absolute value of Rph (470 mA/W) for the p-n diode with ZnO annealed at 350 °C is high compared to that of diodes with different ZnO annealing temperatures. These values also agree well with the values obtained for p-n diodes of other phthalocyanines. Our studies clearly demonstrate that a p-n diode with ZnO film annealed at 350 °C exhibits much better optoelectronic characteristics on account of increased grain size, improved charge injection due to the reduction of barrier height and hence higher (up to 5 orders) charge carrier mobility. © 2017 Elsevier LtdItem Capacitance and impedance spectroscopy studies of polymer light emitting diodes based on MEH-PPV:BT blends(Elsevier Ltd, 2019) K M, N.K.; Sterin, N.S.; Das, P.P.; Umesh, G.; Satyanarayan, M.N.Light emitting polymer poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is blended with a wide bandgap electron transport material benzothiadiazole (BT) and its effect on the electronic properties has been studied by capacitance and impedance spectroscopy (IS) in PLEDs. The impedance data is fitted using equivalent circuit models and the minimum parallel resistance (Rp) at zero bias have been obtained for 1:3 ratio of MEH-PPV:BT blended devices. The negative capacitance (NC) shows the occurrence of the trap-assisted non-radiative recombination mechanism at low frequencies in the unblended MEH-PPV PLEDs. Further, this behavior is seen to be reduced in PLEDs with MEH-PPV:BT blends. This clearly suggests that the blending of MEH-PPV and BT at different weight ratios results in the suppression of trap-assisted recombination. This can be attributed to the elimination of trap states due to the dilution of semiconductor material on account of the addition of wide bandgap host material. Moreover, the blended devices have shown a significant improvement in the conductivity at small bias voltages. © 2019 Elsevier B.V.Item Optoelectronic properties of hybrid diodes based on vanadyl- phthalocyanine and zinc oxide nanorods thin films(Elsevier B.V., 2019) Raveendra Kiran, M.; Ulla, H.; Satyanarayan, M.N.; Umesh, G.Herein, we report the optoelectronic properties of hybrid diodes fabricated using vanadyl phthalocyanine (VOPc) and zinc oxide nanorods (ZNR) with the configuration: ITO/ZNR/VOPc/MoO3/Al. Vertically aligned ZnO nanorods were grown using a simple aqueous solution (AS) method as a function of growth temperature. The correlation between the morphology of ZNR films and the optoelectronic properties of the ZNR/VOPc hybrid devices was investigated. The results show that the hybrid diodes with ZNR films grown at 120 °C offer the best optoelectronic properties. The higher photocurrent responsivity, Rph, (16.28 A/W) was achieved for devices with ZNR films grown at 120 °C. This value is 25 times higher than the Rph value obtained for the devices made with ZnO nanoparticle films that were reported earlier. © 2019