Characteristics Of Heterojunctions Research Articles

Transport properties of Pr 0.7Ca 0.3MnO 3/Nb:SrTiO 3 heterojunctions

Current–voltage ( J– V) characteristics of epitaxial hetero-junctions composed of Pr 0.7Ca 0.3MnO 3 and Nb:SrTiO 3 were studied under forward and reversed bias conditions. Detailed analysis showed that the J– V characteristics of these heterojunctions can be well-fitted by the thermally-assisted tunnelling model. While the dielectric constant of Nb:SrTiO 3 extracted under the forward bias was about one order of magnitude smaller than that of bulk SrTiO 3, the value obtained under reverse bias was very close to that of the bulk SrTiO 3. The result can be explained by the existence of an interface layer on the Nb:SrTiO 3 substrate with a smaller effective dielectric constant. The current finding suggested that the properties of interface layer should be taken into account in order to accurately simulate the J– V characteristics of such heterojunctions.

Ideal diode equation for organic heterojunctions. II. The role of polaron pair recombination

In paper I [N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest, Phys. Rev. B 82, 155305 (2010)], we proposed that current transport in a donor-acceptor heterojunction (HJ) depends on the balance of polaron pair (PP) dissociation and recombination. Here, we directly investigate these processes in archetype planar copper phthalocyanine $(\text{CuPc})/{\text{C}}_{60}$ and boron subpthalocyanine chloride $(\text{SubPc})/{\text{C}}_{60}$ HJs. Using intensity-modulated photocurrent spectroscopy (IMPS) along with emission from interfacial $\text{Pc}/{\text{C}}_{60}$ exciplex states, we monitor the geminate PP density at the HJ as a function of bias and illumination intensity. We find that the $\text{SubPc}/{\text{C}}_{60}$ PP density is limited by the dynamics of dissociation, where it increases from short circuit, and peaks at open circuit. In contrast, that of $\text{CuPc}/{\text{C}}_{60}$ is dominated by faster recombination kinetics and declines monotonically over the same voltage domain. We conclude that the PP recombination rate depends on electric field, and propose a simple expression that qualitatively explains the observed exciplex luminescence and IMPS behavior for these HJs. Our results provide insight into polaron pair recombination, which governs the current-voltage characteristics of organic heterojunctions in the dark and under illumination.

Ideal diode equation for organic heterojunctions. I. Derivation and application

The current-voltage characteristics of organic heterojunctions (HJs) are often modeled using the generalized Shockley equation derived for inorganic diodes. However, since this description does not rigorously apply to organic semiconductor donor-acceptor (D-A) HJs, the extracted parameters lack a clear physical meaning. Here, we derive the current density-voltage $(J\text{\ensuremath{-}}V)$ characteristic specifically for D-A HJ solar cells and show that it predicts the general dependence of dark current, open-circuit voltage $({V}_{oc})$, and short-circuit current $({J}_{sc})$ on temperature and light intensity as well as the maximum ${V}_{oc}$ for a given D-A material pair. We propose that trap-limited recombination due to disorder at the D-A interface leads to the introduction of two temperature-dependent ideality factors and show that this describes the dark current of copper phthalocyanine/${\text{C}}_{60}$ and boron subphthalocyanine/${\text{C}}_{60}$ cells at low temperature, where fits to the generalized Shockley equation break down. We identify the polaron pair recombination rate as a key factor that determines the $J\text{\ensuremath{-}}V$ characteristics in the dark and under illumination and provide direct measurements of this process in our companion paper II [N. C. Giebink, B. E. Lassiter, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest, Phys. Rev. B 82, 155306 (2010)]. These results provide a general physical framework for interpreting the $J\text{\ensuremath{-}}V$ characteristics and understanding the efficiency of both small molecule and polymer organic, planar and bulk HJ solar cells.

Films of degenerate intrinsic oxides of InSe and In4Se3 semiconductor crystals

The results of investigation of electric properties of oxide-p-InSe and oxide-p-In4Se3 heterojunctions, the front layer in which is obtained by the thermal oxidation of crystalline substrates, are presented. It is established that the forward portions of I–V characteristics of heterojunctions are similar to the forward portions of the I–V characteristics of metal-semiconductor diodes in which the majority carriers are involved in the charge transport. Therefore, the electric properties of heterojunctions are interpreted under the assumption that the intrinsic oxide is a degenerate semiconductor. For confirmation of the assumption, the data were compared to the results of the investigation carried out on the ITO-GaTe heterojunctions in which the ITO film was intentionally made degenerate.

Toward Stabilization of Domains in Polymer Bulk Heterojunction Films

With the purpose of studying the effect of stabilizing film morphology on polymer photovoltaic cell performance, the morphology and characteristics of bulk heterojunction devices fabricated using b...

Special electronic structures and quantum conduction of B/P co-doping carbon nanotubes under electric field using the first principle

Boron (B)/phosphorus (P) doped single wall carbon nanotubes (B-PSWNTs) are studied by using the First- Principle method based on density function theory (DFT). Mayer bond order, band structure, electrons density and density of states are calculated. It concludes that the B-PSWNTs have special band structure which is quite different from BN nanotubes, and that metallic carbon nanotubes will be converted to semiconductor due to boron/phosphorus co-doping which breaks the symmetrical structure. The bonding forms in B-PSWNTs are investigated in detail. Besides, Mulliken charge population and the quantum conductance are also calculated to study the quantum transport characteristics of B-PSWNT hetero-junction. It is found that the position of p-n junction in this hetero-junction will be changed as the applied electric field increase and it performs the characteristics of diode.

Insights gained from computer modeling of heterojunction with instrinsic thin layer “HIT” solar cells

The detailed electrical–optical simulator ASDMP is used to model the measured output characteristics of heterojunction with intrinsic thin layer “HIT” cells on P- and N-type c-Si substrates. The parameters extracted from modeling are used to understand the relative performances of the two types of HIT structures, with particular emphasis on the defect states on the surface of the c-Si wafer ( N ss), the minority carrier lifetime in the c-Si absorber, the recombination speeds at the contacts and the band discontinuities at the amorphous/crystalline interface. Except, where the band discontinuities are varied, we apportion one-third of the band offset to the conduction band and two-thirds to the valence band. We find that N ss on the front surface (facing the incoming light) of the P-c-Si wafer strongly influence the open-circuit voltage ( V oc) and to some extent the fill factor (FF), while those on the rear face reduce mainly the current and FF in P-c-Si HIT cells. In N-c-Si HIT cells, the defect density on the front face of the c-Si wafer strongly affects cell performance, those on the rear face having little influence. The lifetime of the minority carriers in the c-Si wafer, also has some influence on cell performance. However, the surface recombination speeds (SRS) of the contacts have little influence down to 10 3 cm/s, except for the SRS of the holes at the contact that is the hole-collector. The amorphous/crystalline (a–c) valence band offset is influential in all cases; and in particular leads to S-shaped characteristics in N–c-Si HIT, except when this offset is reduced by a lower P-a-Si:H band gap or tunneling of holes takes place. Finally unlike N-type HIT, the transition from front to double HIT on P-c-Si, does not appreciably improve the solar cell output.

Electron transport characteristics of one-dimensional heterojunctions with multi-nitrogen-doped capped carbon nanotubes

We present a systematic analysis of electron transport characteristics for one-dimensional heterojunctions with two multi-nitrogen-doped (multi-N-doped) capped carbon nanotubes (CNTs) facing one another at different numbers of nitrogen atoms and conformations. Our results show that the modification of the molecular orbitals by the nitrogen dopants generates conducting channels in the designed heterojunctions inducing multi-switching behavior with sequential negative differential resistance (NDR). The NDR behavior significantly depends on the doping site and conformation of doped nitrogen atoms. Furthermore, we provide a clear interpretation for the NDR behavior by a rigid shift model of the HOMO- and LUMO-filtered energy levels in the left and right electrodes under the applied biases. We believe that our results will give an insight into the design and implementation of various electronic logic functions based on CNTs for applications in the field of nanoelectronics.

Effects of Electrode Structure on Photoelectrochemical Properties of ZnO Electrodes Modified with Porphyrin−Fullerene Composite Layers with an Intervening Fullerene Monolayer

ZnO nanorods and nanoparticle electrodes have been applied to solar cells possessing characteristics of bulk heterojunction and dye-sensitized devices. First, the ZnO electrodes were covered with fullerene acid molecules to yield the monolayer on the electrodes. Then, zinc porphyrin and fullerene acid molecules were spin coated onto the modified surfaces to give porphyrin−fullerene-modified ZnO electrodes. The porphyrin−fullerene-modified ZnO nanorod devices with the intervening fullerene monolayer exhibited efficient photocurrent generation compared to that of the reference systems without the fullerene monolayer. The significant improvement of the photocurrent generation efficiency by the fullerene monolayer may be associated with efficient charge separation in the porphyrin−fullerene composite layer, followed by electron injection into a conduction band of the ZnO nanorod electrode together with the suppression of charge recombination between the separated charges by the fullerene monolayer. Cell perfo...

Influence of Oxygen Vacancy on Transport Property in Perovskite Oxide Heterostructures

Effect of oxygen vacancy on transport property of perovskite microstructures is studied theoretically. Compared with calculated and measured I–V curves, it is revealed that electron conduction plays an important role in the oxygen nonstoichiometry perovskite heterostructures even with hole-doped or un-doped material due to the oxygen vacancies. In addition, a detailed understanding of the influence of oxygen vacancy concentration and temperature on the conduction characteristics of oxide heterojunction with both forward and reverse biases is obtained by calculation.

Superconductivity induced changes of the characteristics of YBa2Cu3O7−δ/Nb doped SrTiO3 heterojunction

We report the effect of superconductivity on the characteristics of a YBa2Cu3O7−δ/0.7 wt% Nb-doped SrTiO3 heterojunction. This junction shows a good rectifying property in the current–voltage (I–V) characteristics. With the superconducting transition, the temperature dependence of the junction resistance shows a decrease and the diffusion voltage has a flat step. The capacitance and loss of the junction show jumps at the superconducting transition temperature (Tc) and these jumps can be suppressed by a magnetic field. In addition, with an increase in the measuring frequency, the capacitance of the junction decreases and the capacitance jumps at the Tc increase. Analysis of the I–V curves suggests the existence of inhomogeneity in the heterojunction, which can account for the effect of superconductivity on the characteristics of the heterojunction. This work is helpful in understanding the effect of superconductivity on the characteristics of superconductor heterojunctions.

Photoelectric response of Schottky barrier in La0.7Ca0.3MnO3∕Nb:SrTiO3 heterojunctions

Heterojunctions composed of La0.7Ca0.3MnO3 and 0.05wt% Nb-doped SrTiO3 were fabricated using pulse laser deposition. The current-voltage characteristics of such heterojunctions can be described by tunneling with an effective Schottky barrier. These junctions showed significant response to ultraviolet and visible light. Band-to-band and internal photoemission were characterized by photoelectric experiments. A quantum efficiency of about 86% was observed at an incident energy of ∼3.95eV, which corresponds to the band-to-band excitation of electrons in Nb:SrTiO3. From the internal photoemission, the height of Schottky barrier was determined as 1.64eV.

High Photoconductive Vertically Oriented TiO2 Nanotube Arrays and Their Composites with Copper Phthalocyanine

Size-controllable and vertically oriented TiO2 nanotube arrays were fabricated by using anodic oxidation of pure titanium sheets in electrolyte solutions and were characterized by field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), and X-ray diffraction (XRD). A double-layered photoconductive device (Au/copper phthalocyanine/TiO2 nanotubes/titanium) was designed and fabricated based on the TiO2 nanotube arrays. The I−V curves demonstrated typical heterojunction characteristics and remarkable photoconductive performance for the devices. The effect of the thickness of copper phthalocyanine (CuPc) and the diameter of TiO2 nanotubes on the photoconductive property of devices was studied. The current value increased by 2 or 3 orders of magnitude when exposed to light from the dark, depending on the thickness of the CuPc layer and the diameter of TiO2 nanotubes.

Field-dependent carrier generation in bulk heterojunction solar cells

We analyse the current–voltage (J–V) characteristics of bulk heterojunction devices based on two different materials systems: (i) poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]:(6,6)-phenyl-C61-butyric acid methyl ester (MDMO-PPV:PCBM); and (ii) poly[N-(2′-decyltetradecyl)carbazole]-2,7-diyl: bis(2-(1-ethylhexyl-4,5-dicyanoimidazol-2-yl)vinyl) benzo[c]1,2,5-thiadiazole (PCz:EV-BT). The J–V characteristics were recorded in the dark and light using a pulsed technique that ensured the two sets of measurements were obtained at equal temperatures. In each case, the dark current was subtracted from the photocurrent to obtain a corrected photocurrent Jph that excludes the photovoltage-induced injection current and is thus due solely to the flux of incident photons. The Jph–V response of the MDMO-PPV:PCBM was consistent with a model in which all photogenerated excitons dissociate into free carriers irrespective of the internal field strength. The Jph–V response of the PCz:EV-BT device, by contrast, was consistent with a model in which an electric field is required to initiate appreciable charge separation. The different behaviour is attributable to the different energy offsets of the frontier orbitals responsible for charge separation: 1.3 eV for MDMO-PPV:PCBM compared to just 0.4 eV for PCz:EV-BT.

The study of electrical characteristics of heterojunction based on ZnO nanowires using ultrahigh-vacuum conducting atomic force microscopy

The electrical performances of the heterojunction of n-ZnO nanowires with p-Si substrate at the nanometer scale have been characterized using an ultrahigh-vacuum conducting atomic force microscopy. Compared with the expected values of 1.0–2.0 reported in p-n junction in the previous studies, the abnormally high diode ideality factor (⪢2) was obtained. It elucidates that a ZnO–Si p-n junction can be modeled by a series of diodes, the actual ZnO–Si junction diode and two Schottky diodes at the metal/ZnO and metal/Si junctions. The tunneling across p-n junction would also play a role in the externally measured high ideality factor.

Size effect on I– V characteristics of low dimensional metal–semiconductor heterojunctions

Cu–Se heterostructures of diameter 20, 100 and 200 nm were synthesized by electrodeposition within the pores of anodic alumina membranes (AAM) via template synthesis. The morphology of heterostructures was characterized by scanning electron microscope (SEM). The collective I– V characteristics of heterojunctions were measured at room temperature (25 °C) and the characteristics were observed to be like those of resonating tunneling diodes (RTDs) and peak-to-valley current ratio was found to reduce with decreasing diameter of RTDs.

Photosensitivity of heterojunctions formed by deposition of gum on a layered III–VI semiconductor

The list of materials showing promise for the production of photosensitive structures is extended to include an organic material of biological origin known as gum. The current-voltage characteristics of hetero-junctions formed by a gum layer and a layered semiconductor (InSe, GaSe) are studied. An attempt is made to relate some spectral features of the relative quantum efficiency of the structures (for example, nonzero photosensitivity beyond the fundamental absorption edge of the semiconductor material) to the deformation interaction between the condensed gum layer and the layered III–VI semiconductor surface.

Morphology and collective I– V characteristics of template synthesized nano metal–semiconductor heterojunctions

Zn–Se heterojunctions were fabricated by electrodeposition within the pores of nanoporous etched ion-track membranes by template synthesis. The morphology of heterojunctions was characterized by scanning electron microscope (SEM) and the structures were observed to be heterojunctions of Zn and Se (metal–semiconductor). The I– V characteristics of heterojunctions were measured at room temperature by leaving the heterostructures embedded in the insulating template membrane and the characteristics were observed to be like those of resonant tunneling diodes (RTDs) along with a significant peak to valley current ratio. This ratio was found to reduce with decreasing diameter of RTDs.

Modeling of Thermoelectric Properties of Semi-Conductor Thin Films With Quantum and Scattering Effects

Several new reduced-scale structures have been proposed to improve thermoelectric properties of materials. In particular, superlattice thin films and wires should decrease the thermal conductivity, due to increased phonon boundary scattering, while increasing the local electron density of states for improved thermopower. The net effect should be increased ZT, the performance metric for thermoelectric structures. Modeling these structures is challenging because quantum effects often have to be combined with noncontinuum effects and because electronic and thermal systems are tightly coupled. The nonequilibrium Green’s function (NEGF) approach, which provides a platform to address both of these difficulties, is used to predict the thermoelectric properties of thin-film structures based on a limited number of fundamental parameters. The model includes quantum effects and electron-phonon scattering. Results indicate a 26–90 % decrease in channel current for the case of near-elastic, phase-breaking, electron-phonon scattering for single phonon energies ranging from 0.2 meV to 60 meV. In addition, the NEGF model is used to assess the effect of temperature on device characteristics of thin-film heterojunctions whose applications include thermoelectric cooling of electronic and optoelectronic systems. Results show the predicted Seebeck coefficient to be similar to measured trends. Although superlattices have been known to show reduced thermal conductivity, results show that the inclusion of scattering effects reduces the electrical conductivity leading to a significant reduction in the power factor (S2σ).

Magnetically tuneable nonlinear electronic properties of ZnO/La(Sr)MnO3 ceramic composites

Polycrystalline, multiphase ceramic composites exhibiting voltage-limiting nonlinear current–voltage (I–V) relations with the nonlinearity coefficient (α) of 220–360 have been realized by co-sintering ZnO and La0.6Sr0.4MnO3. The I–V characteristics are symmetrical on voltage reversal to the negative quadrant. By selecting the sintering temperature (1573–1673 K) and duration (1–4 h), the interdiffusion of Mn into ZnO and of Zn into La0.6Sr0.4MnO3 could be controlled so that La and Sr segregation leads to the formation of p-type La0.6Sr0.4Mn1−yZnyO3 (LSMZO) permeating along the intergrain regions of n-type ZnO1−γ : Mn, thereby preserving interior n–p–n heterojunction characteristics at the grain boundaries. Possible mechanisms proposed for large nonlinearity in the I–V curves are (i) avalanche multiplication of minority carrier through impact ionization of majority carriers or (ii) Zener tunnelling of charge carriers across the depleted Schottky potential barriers prevailing at n-ZnO1−γ : Mn/p-LSMZO (and p-LSMZO/n-ZnO1−γ : Mn) interfaces on the left- and right-hand side of p-LSMZO intergrain phase, wherein the type of conduction mechanism is predominantly dependant on the transport properties and thickness of intergranular LSMZO phase. The I–V characteristics are modified under an applied magnetic field by way of decreasing the turn-on voltage leading to higher currents. Such magnetically tuneable electrical nonlinearity arises from the prevalence of spin-polarized conduction in the ferromagnetic insulating LSMZO which prevails at the intergrain regions of n-ZnO1−γ : Mn. Magnetization studies of bulk LSMZO show the conversion from ferromagnetic metallic (y < 0.03) to ferromagnetic insulating (y = 0.03–0.08) and paramagnetic insulating (y > 0.08) phases on increasing the zinc content. However, the n-ZnO1−γ : Mn grains remain paramagnetic insulator and the magnetically tuneable I–V curves are not dependent on the ferromagnetic ordering therein.