Modification of electrical properties of the Au/1,1′ dimethyl ferrocenecarboxylate/n-Si Schottky diode

The electrical and interface state density properties of the Au/1,1′ dimethyl ferrocenecarboxylate (DMFC)/n-Si structure have been investigated by current–voltage, capacitance–voltage and conductance–frequency methods. The Au/DMFC/n-Si structure exhibits a rectifying behavior with a non-ideal I–V behavior with an ideality factor greater than unity. The ideality factor and barrier height of the Au/DMFC/n-Si Schottky diode is lower than that of Au/n-Si Schottky diode. The interface state density of the diode was determined from G/ω–f plots and was of order of 5.61 × 10¹² eV^?1 cm^?2. It is evaluated that the electrical properties of Au/n-Si diode is controlled using 1′ dimethyl ferrocenecarboxylate organic layer and in turn, Au/DMFC/n-Si structure gives new electronic parameters.     

Electrical and photovoltaic properties of FeTPPCl/p-Si heterojunction

Hybrid heterojunction cell based on thermally evaporated 5,10,15,20-tetraphenyl-21H, 23H-porphine iron (III) chloride (FeTPPCl) as the organic semiconductor and p-Si wafer as the inorganic semiconductor have been investigated. This device showed rectification behaviour like diode. The conduction mechanisms and the diode parameters have been studied using current–voltage (I–V) characteristics in the temperature range (298–373 K) and capacitance–voltage (C–V) characteristics at room temperature. This cell exhibited photovoltaic characteristics with a short-circuit current (I_sc) of 2.8 mA, an open-circuit voltage (V_oc) of 0.475 V, a fill factor FF = 32%.     

Inhomogeneous transport property of Alq3 thin films: Local order or phase separation?

Steady-state current–voltage (I–V) and impedance–voltage (Z–V) measurements were performed on in situ (UHV) prepared metal (Ag, Al)/Alq₃/indium-tin oxide (ITO) devices after exposure to air. When increasing the positive bias on the top metal electrode to a relatively well-defined critical value, a transition from semiconducting to semi- or even insulating behavior of the contacted Alq₃ thin film is observed by means of I–V measurements. The final insulating state remains stable when applying negative bias to the Ag electrode. In the case of the Al electrode, there is a voltammetric current wave under a well-defined negative bias indicating a redox reaction of mobile ions at the Al electrode.The Z–V measurements reveal a peculiar feature of ac transport through the Alq₃ thin films, namely the equivalent series capacitance is equal to its parallel counterpart in the frequency range from 100 to 1 MHz and amounts to only a fraction (0.3–0.5) of the expected geometrical capacitance of the device. An equivalent electrical circuit has been developed, based on the existence of two parallel transport paths: an insulating (amorphous) Alq₃-phase shunted by a semiconducting (semi-insulating) one, both running into the impedance of the back contact. The equivalent circuit model composed exclusively of frequency independent elements is useful for predicting the maximum frequency for retaining the full geometrical capacitance. Even though the model is capable of describing the bias dependence of the impedance correctly, it does not shine light on the nature of the (ordered) phase or domain responsible for the dielectric loss. The possibility of local order connected with dipole–dipole interaction in the metal/Alq₃ interface zone is discussed. In any case, the ordered portion of the organic material seems to form the huge interface dipole of about 1 eV with Ag or Al [M.A. Baldo, S.R. Forrest, Phys. Rev. B 64 (2001) 085201], the direction of the dipole promoting electron injection to Alq₃. Then the semiconductor-to-insulator transition could be initiated by a damage of the interface dipole under a critical positive dc bias of the metal, preventing the flow of both dc and the real component of low-frequency ac current. The transition is not accompanied by any significant change in the impedance of the back contact common to both phases.     

Temperature dependence of the electrical and interface states of the Sn/Rhodamine-101/p-Si Schottky structures

In this paper, the current–voltage (I–V) characteristics of Sn/Rhodamine-101/p-Si/Al contacts have been measured at temperatures ranging from 80 to 400 K at 30 K intervals. The nonpolymeric organic compound Rhodamine-101 (Rh101) film on a p-type Si substrate has been formed by means of the evaporation process and the Sn/Rhodamine-101/Si contacts have been fabricated. The current–voltage characteristics of the diode show rectifying behaviour consistent with a potential barrier formed at the interface. The obtained I–V barrier heights (Φ_b) were in the range of 0.208–0.940 eV with ideality factors (n) of 14.37–2.72. The high values of ideality factor (n) may be ascribed to decrease the exponentially increase rate in current due to space-charge injection into Rh101 thin film at higher voltage. Temperature dependence of the energy distribution of interface states density profile was determined from the forward bias I–V characteristics. It is shown that organic semiconductor layer (Rhodamine-101) controls electrical charge transport properties of Sn/p-Si Schottky structure by excluding effects.     

Kinetics of interface state-limited hole injection in α-naphthylphenylbiphenyl diamine (α-NPD) thin layers

Injection-limited operation is identified in thin-film, α-NPD-based diodes. A detailed model for the impedance of the injection process is provided which considers the kinetics of filling/releasing of interface states as the key factor behind the injection mechanism. The injection model is able to simultaneously account for the steady-state, current–voltage (J–V) characteristics and impedance response, and is based on the sequential injection of holes mediated by energetically distributed surface states at the metal–organic interface. The model takes into account the vacuum level offset caused by the interface dipole, along with the partial shift of the interface level distribution with bias voltage. This approach connects the low-frequency (～1 Hz) capacitance spectra, which exhibits a transition between positive to negative values, to the change in the occupancy of interface states with voltage. Simulations based on the model allow to derive the density of interface states effectively intervening in the carrier injection (～5 × 10¹² cm^?2), which exhibit a Gaussian-like distribution. A kinetically determined hole barrier is calculated at levels located ～0.4 eV below the contact work function.     

Electrical and impedance spectral characterisation of ITO/DAG/In device

A Schottky device, with configuration ITO/DAG/In is fabricated using diazopheny diamino glyoxime (DAG) as an n-type organic material. Current–voltage characteristics and impedance spectroscopy measurements were carried out, which reveals that the injection and transport properties are dominated by negative charge carriers. Space charge limited current theory with an exponential distribution of traps is very well followed by observations resulted through current–voltage characteristics at high voltage region. This gives a traps density N_t of about 4.5×10²¹ m⁻³ and mobility of electron is about 3.9×10⁻¹⁰ m² V⁻¹ s⁻¹. It is found that DAG behaves as an n-type materials as it forms Schottky barrier with ITO (high work function electrode) and conduction is governed by majority carriers, i.e. electrons. Using temperature and bias dependence of impedance spectral characteristics in a broad frequency range, i.e. 40 Hz to 100 kHz, it is found that the ac behaviour of In/DAG/ITO device shows several features, described by the simple double RC circuit representing a depleted junction region and an undepleted bulk region. From the large frequency range of impedance spectroscopy two distinct processes were identified, corresponding to bulk DAG layer and junction region. The activation energy of the relaxation times coincides well with the results obtained from the temperature dependent dc conductivity. The temperature dependent capacitance–voltage measurements were analysed at a frequency of 40 Hz. The obtained inherent donor concentration from 1/C²–V plots varies from 1.8×10²³ m⁻³ at room temperature to 5.9×10²³ m⁻³ at 360 K.     

Current–voltage characteristics of Al/Rhodamine-101/n-GaAs and Cu/Rhodamine-101/n-GaAs rectifier contacts

The organic compound Rhodamine-101 (Rh101) film on an n-type GaAs substrate with carrier concentration of 7.3 × 10¹⁵ cm⁻³ has been formed by means of the evaporation process, and thus Al/Rh101/n-GaAs and Cu/Rh101/n-GaAs contacts have been fabricated. Our aim is to realize a modification of Schottky barrier height (SBH) of the devices using a thin non-polymeric organic compound layer. The Al/Rh101/n-GaAs and Cu/Rh101/n-GaAs contacts have behaved like rectifying contact with the SBH values of 0.68 eV and 0.72 eV, and with ideality factor values of 2.61 and 2.60 obtained from their forward bias current–voltage (I–V) characteristics at the room temperature, respectively. It has seen that the SBH values obtained for these devices are significantly different from those obtained for the conventional Al/n-GaAs or Cu/n-GaAs Schottky diodes. Furthermore, it has been demonstrated that the trapped-charge-limited current is the dominant transport mechanism at large forward bias voltage.     

Electrical admittance studies of polymer photovoltaic cells

We investigate the electrical transport properties of ITO/conjugated polymer–fullerene/Al solar cells and the role of defect states with the help of admittance spectroscopy and C(ω)–V measurements in the dark. A characteristic step in the admittance spectrum can be observed in the temperature range 40–320 K, originating from the electrically active defects. The activation energy determined from an Arrhenius plot was found to be 34 meV. The position of the step does not depend on the dc bias voltage which may indicate the pinning of the hole Fermi-level at the interface due to the high density of defect states. The diode capacitance as a function of the reverse bias depends strongly on the device preparation conditions and correlates with the device I–V characteristics. We found a completely bias independent capacitance under reverse bias for the cells with low-power efficiency and almost no rectification under illumination. On the other hand, the high-efficiency cells with good rectification under illumination demonstrate an increase of the capacitance with ac bias even at frequencies above 100 kHz. The former devices are found to be fully depleted, whereas in the latter, a formation of the space charge region can be assumed.     

Carrier transport properties of aluminum oxide/polypyrrole films doped with naphthalene-1,5-disulfonic acid

The electrical structure of the Al/Al₂O₃/PPy-NDSA/Au has been investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements, in a temperature range of 90–350 K. The forward C–V measurements have been carried out in the range of frequency of 1 kHz to 20 MHz. The effects of series resistance, interfacial layer and interface states on I–V and C–V characteristics are investigated. At high current densities in the forward direction, the series resistance effect has been observed for voltages greater than 0.7 V. The analysis of I–V characteristics based on the thermionic emission mechanism has been explained by the assumption of a Gaussian distribution of barrier heights, due to barrier height inhomogeneities that prevail at the interface. It has been observed that the forward C–V plot exhibits a peak, whose position shifts towards lower voltages and that decreases with increasing frequency. The non-linearity of 1/C² versus V plot at high frequency was explained with the assumption that only some of the interface states follow the applied ac signal. Surface analysis by X-ray photoelectron spectroscopy (XPS) was performed in order to evaluate the chemical states of the constituents of the Al/Al₂O₃/PPy-NDSA/Au sample.     

Photovoltaic properties of a single layer poly [3-(4-octylphenyl)-2,2′-bithiophene] (PTOPT)

A Schottky contact is made from a single layer polymer poly [3-(4-octylphenyl)-2,2′-bithiophene] (PTOPT) in its neutral state and a low work function metal (Al). The electrical and optical properties have been investigated by means of I–V measurements in the dark and under illumination. Various parameters such as the reverse saturation current density, barrier height, and diode quality factor were determined from the I–V curves in the dark of Al/PTOPT/ITO sandwich structure using thermionic emission theory. Spectral response of the device was measured at various wavelengths giving a peak at 500 nm. The IPCE% was obtained for illumination through both the Al and ITO sides. By illuminating the diode with a monochromatic light of wavelength 500 nm, the open-circuit voltage, short-circuit current density, power conversion efficiency, and fill-factor (FF) were obtained. The dependence of photocurrent on light intensity was also recorded and analyzed.     

The calculation of electronic parameters of an Ag/chitin/n-Si Schottky barrier diode

We have formed polymeric organic compound chitin film on n-Si substrate by adding a solution of polymeric compound chitin in N,N-dimethylacetamide and lithium chloride on top of an n-Si substrate and then evaporating solvent. It has been seen that the chitin/n-Si contact has demonstrated clearly rectifying behavior and the reverse curves exhibit a weak bias voltage dependence by the current–voltage (I–V) curves studied at room temperature. The barrier height and ideality factor values of 0.959 eV and 1.553, respectively, for this structure have been obtained from the forward bias I–V characteristics. Furthermore, the energy distribution of the interface state density located in the semiconductor band gap at the chitin/n-Si substrate in the energy range from (E_c − 0.897) to (E_c − 0.574) eV have been determined from the I–V characteristics. The interface state density, N_ss, ranges from 5.965 × 10¹² cm⁻² eV⁻¹ in (E_c − 0.897) eV to 1.706 × 10¹³ cm⁻² eV⁻¹ in (E_c − 0.574) eV and has an exponential rise with bias this energy range.     

Effect of Au deposition rate on the performance of top-contact pentacene organic field-effect transistors

A study of the influence of the deposition rate of top-contact Au source and drain electrodes deposited by electron-beam evaporation on the electrical performance of pentacene organic field-effect transistors (OFETs) is presented. By adjusting the deposition rate of the Au electrodes to minimize metal diffusion into the semiconductor pentacene layer, the source/drain contact resistance could be reduced. At a Au deposition rate of 10 Å/s, high-performance pentacene p-channel OFETs were obtained with a field-effect mobility of 0.9 cm²/Vs and a normalized channel width resistance of 23 kΩ cm in a device with a channel length of 25 μm.     

ZnO ultraviolet photodiodes with Pd contact electrodes

ZnO epitaxial films were grown on sapphire (0 0 0 1) substrates by using RF plasma-assisted molecular beam epitaxy (MBE). Metal–semiconductor–metal (MSM) ZnO photodiodes with palladium contact electrodes were then fabricated. With an incident wavelength of 370 nm and an applied bias of 1 V, it was found that maximum responsivity of the Pd/ZnO/Pd MSM photodetectors was 0.051 A W⁻¹, which corresponds to a quantum efficiency of 11.4%. Furthermore, it was found that the time constant of our photodiodes was 24 ms with a three-order decay exponential function. For a given bandwidth of 100 Hz and an applied bias of 1 V, we found that noise equivalent power and corresponding detectivity D¹ were 1.13 × 10⁻¹² W and 6.25 × 10¹¹ cm Hz^0.5 W⁻¹, respectively.     

Extraction of the device parameters of Al/P3OT/ITO organic Schottky diode using J–V and C–V characteristics

Thin film of poly(3-octylthiophene) (P₃OT) was successfully prepared using dip coating technique. The morphology and the crystal structure of the prepared thin film were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. A study on interface states density distribution and characteristic parameters of the Al/P₃OT/ITO device capacitor have been made. The diode parameters such as ideality factor, series resistance and barrier height were extracted from the forward biasing J–V characteristics. The energy distribution of the interface state density D_it was determined from the forward bias J–V characteristics by taking into account the bias dependence of the effective barrier height. The C–V and G/ω–V characteristics were measured in the frequency range from 10 kHz to 1 MHz and dc biasing voltage swept from ?4 V to +4 V at room temperature (300 K). The non-ideal behavior of J–V and C–V characteristics can be attributed to the presence of the interface and the series resistance.     

Controlling of silicon–insulator–metal junction by organic semiconductor polymer thin film

Electrical and photovoltaic properties of a metal–semiconductor–insulator–polymer–metal diode were investigated. The n-Si/SiO₂/MEH-PPV/Al diode shows a rectifying behavior with the rectification ratio of 2.22 × 10⁵ at ±5 V and exhibits a non-ideal behavior due to the series resistance and oxide-organic layers. The organic semiconductor makes a contribution to the I–V characteristics of the diode and the trap-charge limited space charge and space charge limited current mechanisms were observed for the diode. The current–voltage characteristics of the n-Si/SiO₂/MEH-PPV/Al diode under different illumination intensities give an open circuit voltage (V_oc) along with a short circuit current (I_sc). This suggests that the n-Si/SiO₂/MEH-PPV/Al diode is a photovoltaic device with V_oc = 0.456 V and J_sc = 7.89 × 10^?8 A/cm² values under 100 mW/cm² illumination intensity. The photoconductivity mechanism of the diode is controlled by monomolecular recombination. The interface state density D_it values with time constant τ_it of the diode under dark and illumination conditions were found to be 2.53 × 10¹⁰ eV^?1 cm^?2 with 5.09 × 10^?5 s and 2.50 × 10¹⁰ eV^?1 cm^?2 with 8.27 × 10^?5 s, respectively. The obtained results indicate that the n-Si/SiO₂/MEH-PPV/Al diode is a photo-sensitive diode.     

Characterization of graphene-based supercapacitors fabricated on Al foils using Au or Pd thin films as interlayers

We demonstrate that an Au or Pd interlayer between graphene and an Al-foil current collector plays an important role in enhancing supercapacitor performance. Graphene was prepared by scalable chemical exfoliation and mild thermal reduction (～150 °C) processes. Working electrodes were prepared by coating the graphene on Au/Al or Pd/Al by drop-dry method. The graphene deposited on the noble metals Au and Pd demonstrated excellent supercapacitor performance. Estimated specific energy and specific power of supercapacitors were ～40 Wh/kg and ～40 kW/kg at the current density of ～33 A/g, when operated in organic solution. Altogether, we demonstrate that employing noble metals in the fabrication of graphene-based supercapacitors can lead to excellent performance, and this could be a critical basis for further development of graphene-based supercapacitors.     

The calculation of electronic properties of an Ag/chitosan/n-Si Schottky barrier diode

In this study, the film of chitosan by adding the solution of chitosan being a polymeric compound on the top of an n-Si substrate and then by evaporating solvent was formed. It was seen that the chitosan/n-Si contact demonstrated clearly rectifying behavior and the reverse curves exhibit a weak bias voltage dependence by the current–voltage (I–V) curves studied at room temperature. Average barrier height and ideality factor values for this structure were determined as 0.94 eV and 1.81, respectively. Furthermore, the energy distribution of the interface state density located in the semiconductor band gap at the chitosan/n-Si substrate in the energy range (E_c−0.785) to (E_c−0.522) eV have been determined from the I–V characteristics. The interface state density N_ss ranges from 5.39 × 10¹² cm⁻² eV⁻¹ in (E_c−0.785) eV to 1.52 × 10¹³ cm⁻² eV⁻¹ in (E_c−0.522) eV. The interface state density has an exponential rise with bias from the midgap towards the bottom of the conduction band.     

The effect of channel length on turn-on voltage in pentacene-based thin film transistor

We report on the influence of channel length on transfer characteristics of pentacene-based thin film transistor (TFT). As the channel length is reduced from 50 to 5 μm, turn-on voltage (V_turn-on) is shifted to more positive values, regardless of surface treatments and gate insulators. Especially in case of relatively short channel TFTs having the channel length of below 10 μm, multi-channel operation behavior has been observed and resulted in hump-shaped transfer characteristics. The positive shift behavior of V_turn-on is similar to the threshold voltage reduction behavior by drain induced barrier lowering in Si devices and multi-channel operation can be explained by the channel length variation inevitably obtained after lift-off patterning of Au/Ti source–drain metal in relatively short channel TFTs.     

Effects of annealing of poly(3-hexylthiophene) film on the performance of double-layered EL devices of ITO/polymer/Alq3/Mg–Ag

Double layer devices with a structure of ITO/pHT/Alq3/Mg–Ag (ITO = indium tin oxide, pHT = regio-regular or random poly(3-hexylthiophene), Alq3 = tris(8-hydroxyquinoline)aluminium) were fabricated. The device with a random pHT film emitted a green-yellow light in all voltage region, while that having a regio-regular pHT film exhibited a color change from green to red by applying the bias voltage higher than 15 V. Annealing the pHT films prepared on ITO at 200 °C for 1 h in nitrogen, prior to vapor-deposition of the Alq3 layer, improved the device performance with lowering the onset bias voltage by 2–3 V. The EL colors and spectra were also affected by annealing. X-ray reflectivity measurements before and after annealing the pHT film on ITO indicated increased density of the pHT layer and structural changes in the pHT/ITO interface by annealing, which seems to be responsible for the improved EL device performance.     

The effect of a novel organic compound chiral macrocyclic tetraamide-I interfacial layer on the calculation of electrical characteristics of an Al/tetraamide-I/p-Si contact

The Al/tetraamide-I/p-Si Schottky barrier diode (SBD) has been prepared by adding a solution of a novel nonpolymeric organic compound chiral macrocylic tetraamide-I in chloroform on top of a p-Si substrate and then evaporating the solvent. It has been seen that the forward-bias current–voltage (I–V) characteristics of Al/tetraamide-I/p-Si SBD with a barrier height value of 0.75 eV and an ideality factor value of 1.77 showed rectifying behaviour. The energy distribution of the interface state density determined from I–V characteristics increases exponentially with bias from 5.81 × 10¹² cm⁻² eV⁻¹ at (0.59-E_v) eV to 1.02 × 10¹³ cm⁻² eV⁻¹ at (0.40-E_v) eV. It has showed that space charge limited current (SCLC) and trap charge limited current (TCLC) are the dominant transport mechanisms at large forward-bias voltages.