Improvement of photoelectrochemical and optical characteristics of MEH-PPV using titanium dioxide nanoparticles

The use of bulk heterojunctions can increase the efficiency of exciton dissociation in polymer-based photovoltaics. We prepared and characterized bulk heterojunctions of poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and titanium dioxide nanoparticles deposited by spin coating on indium tin oxide substrates. The surface morphology of the MEH-PPV+TiO₂ composite films revealed that addition of TiO₂ nanoparticles increased the film roughness. The effect of TiO₂ nanoparticles on the photoelectrochemical and optical characteristics of MEH-PPV polymer heterojunctions was studied. Addition of TiO₂ nanoparticles improved the absorbance of MEH-PPV composite films. Moreover, the photocurrent of the composite devices increased with the TiO₂ nanoparticle concentration. These observations provide an insight into new approaches to improve the light collection efficiency in photoconductive polymers.     

Electrical properties of anisotype heterojunctions n-CdZnO/p-CdTe

Anisotype surface-barrier n-Cd_0.5Zn_0.5O/p-CdTe heterojunctions are fabricated by the high-frequency sputtering of a Cd_0.5Zn_0.5O alloy film onto a freshly cleaved single-crystal CdTe surface. The main electrical properties of the heterojunctions are studied and the dominant mechanisms of charge transport are established, namely, the multistage tunnel-recombination mechanism under forward bias, Frenkel-Pool emission, and tunneling under forward bias. The influence of the surface electrically active states at the heterojunction interface is analyzed and their surface concentration is evaluated: N _ss ?? 10¹⁴ cm^?2.     

Charge-transport mechanisms in heterostructures based on TiO2:Cr2O3 thin films

n-TiO₂:Cr₂O₃/p-Si anisotype heterostructures are fabricated by the deposition of a TiO₂: Cr₂O₃ film by electron-beam evaporation onto a polished polycrystalline silicon substrate. Their electrical properties are studied and the dominant charge-transport mechanisms are determined: multistage tunneling-recombination mechanism involving surface states at the TiO₂: Cr₂O₃/Si metallurgical interface under small forward biases and tunneling at biases exceeding 0.8 V. The reverse currents through the heterostructures under study are analyzed in terms of the single-stage tunneling mechanism of charge transport.     

Improved Photocatalytic Performance of Heterojunction by Controlling the Contact Facet: High Electron Transfer Capacity between TiO2 and the {110} Facet of BiVO4 Caused by Suitable Energy Band Alignment

Charge separation at the interface of heterojunctions is affected by the energy band alignments of the materials that compose the heterojunctions. Controlling the contact crystal facets can lead to different energy band alignments owing to the varied electronic structures of the different crystal facets. Therefore, BiVO₄‐TiO₂ heterojunctions are designed with different BiVO₄ crystal facets at the interface ({110} facet or {010} facet), named BiVO₄‐110‐TiO₂ and BiVO₄‐010‐TiO₂, respectively, to achieve high photocatalytic performance. Higher photocurrent density and lower photoluminescence intensity are observed with the BiVO₄‐110‐TiO₂ heterojunction than those of the BiVO₄‐010‐TiO₂ heterojunction, which confirms that the former possesses higher charge carrier separation capacity than the latter. The photocatalytic degradation results of both Rhodamine B and 4‐nonylphenol demonstrate that better photocatalytic performance is achieved on the BiVO₄‐110‐TiO₂ heterojunction than the BiVO₄‐010‐TiO₂ heterojunction under visible light (≥422 nm) irradiation. The higher electron transfer capacity and better photocatalytic performance of the BiVO₄‐110‐TiO₂ heterojunction are attributed to the more fluent electron transfer from the {110} facet of BiVO₄ to TiO₂ caused by the smaller interfacial energy barrier. This is further confirmed by the selective deposition of Pt on the TiO₂ surface as well as the longer lifetime of Bi⁵⁺ in the BiVO₄‐110‐TiO₂ heterojunction.     

Investigation of field-effect transistors fabricated by metal-ion-doped nano-titania using sol-gel technique

In our present investigations, field-effect transistors (FETs) based on nano-TiO₂ and metal-ion-doped TiO₂ particles were fabricated and their characteristics were studied. Semiconductor characteristics of nano-TiO₂/metal-ion-doped TiO₂ films, which perform as a diode, were used in the fabrication of the FET device. The performance of the FET device was evaluated by analyzing the data obtained from source-drain current vs. voltage (I_ds-V_ds) by controlling the gate voltage (V_g). The on/off conditions of the transistor were feasibly performed and the threshold voltage (V_t) were determined by adjusting V_g for different types of the FET design. In addition, the influential factors, such as type of metal ions, and positions of TiO₂ and metal-ion-doped TiO₂ layers, on the performance of the FET device were also discussed in this study.     

Temperature dependences of the electrical parameters of anisotype NiO/CdTe heterojunctions

The I–V characteristics of NiO/CdTe heterostructures fabricated by reactive magnetron sputtering are measured at different temperatures. It is established that current transport through the NiO/CdTe heterojunction is mainly controlled via generation–recombination and tunneling under forward bias and via tunneling under reverse bias. The investigated heterostructures generate an open-circuit voltage of V _oc = 0.26 V and a short-circuit current density of I _sc = 58.7 μA/cm² at an illumination intensity of 80 mW/cm².     

Water-soluble poly(3,4-ethylenedioxythiophene)/nano-crystalline TiO2 heterojunction solar cells

Nanocrystalline titanium dioxide (TiO₂) thin films were prepared by the sol-gel method and were then used to fabricate an indium-tin oxide (ITO)/nano-crystalline TiO₂/poly(3,4-ethylenedioxythiophene) (PEDOT)/Au device. The junction thus obtained shows a rectifying behavior. Their current-voltage (I-V) characteristics in dark indicate that a heterojunction at the nano-crystalline TiO₂/PEDOT interface has been created. The measured open-circuit voltage (V_oc) and short-circuit current (I_sc) for the device under illumination with 50 mW/cm² light intensity under AM 1.5 conditions (device dimension was 1 cm²) are V_oc=0.39 V, I_sc=54.9 μA/cm², the filling factor (FF)=0.429 and the energy conversion efficiency (η)=0.03%.     

Sb2(SxSe1−x)3 sensitized solar cells prepared by solution deposition methods

Solid state semiconductor sensitized solar cells are a very active research subject in emerging photovoltaic technologies. In this work, heterojunctions of antimony sulfide-selenide (Sb₂(S_xSe_1−x)₃) solid solution as the absorbing material and cadmium sulfide coated titanium dioxide (TiO₂/CdS) as the electron conductor have been developed with solution deposition methods such as spin-coating, successive ionic layer adsorption and reaction (SILAR), and chemical bath deposition. In particular, CdS has been deposited on mesoporous TiO₂ layers by SILAR deposition, followed by the chemical deposition of Sb₂(S_xSe_1−x)₃. It was found that by increasing the number of CdS SILAR deposition, both the open circuit voltage V_oc and the short circuit current density J_sc of the Sb₂(S_xSe_1−x)₃ sensitized solar cells had been increased from 153 to 434 mV and 0.77–9.73 mA/cm², respectively. This improvement was attributed to the fact that the presence of the CdS on TiO₂ surface reduces the formation of undesired Sb₂O₃ and promotes a better nucleation of the Sb₂(S_xSe_1−x)₃ during the chemical bath deposition. The best result was obtained for the solar cell with 30 cycles of CdS which produced a V_oc of 434 mV, a J_sc of 9.73 mA/cm², and a power conversion efficiency of 1.69% under AM1.5 G solar radiation.     

An Interface Engineered Multicolor Photodetector Based on n‐Si(111)/TiO2 Nanorod Array Heterojunction

A multicolor photodetector based on the heterojunction of n‐Si(111)/TiO₂ nanorod arrays responding to both ultraviolet (UV) and visible light is developed by utilizing interface engineering. The photodetector is fabricated via a consecutive process including chemical etching, magnetron sputtering, hydrothermal growth, and assembling. Under a small reverse bias (from 0 to ≈?2 V), only the photogenerated electrons in TiO₂ are possible to tunnel through the low barrier of ΔE_C, and the device only responses to UV light; as the reverse bias increases, the photogenerated holes in Si also begin to tunnel through the high barrier of ΔE_V. As a result, the device is demonstrated to have the capacity to detect both UV and visible lights, which is useful in the fields of rapid detection and multicolor imaging. It has been also observed that the crystal orientation of Si affects the characteristics of bias‐controlled spectral response of the n‐Si/TiO₂ heterojunctions.     

Electrical and photoelectric properties of anisotype n-TiN/p-Si heterojunctions

Photosensitive n-TiN/p-Si heterojunctions are fabricated by the reactive magnetron sputtering of a thin titanium-nitride film with n-type conductivity onto polished polycrystalline p-Si wafers. The I–V characteristics of the heterostructures are measured at different temperatures. The temperature dependences of the potential-barrier height and series resistance of the n-TiN/p-Si heterojunction are studied. The dominant mechanisms of current transport through the heterojunction in the cases of forward and reverse bias are established. The heterostructures generate the open-circuit voltage V _oc = 0.4 V and the short-circuit current I _sc = 1.36 mA/cm² under illumination with a power density of 80 mW/cm².     

Tunnelling current in PbTe-Pb0.8Sn0.2Te heterojunctions

A study was made of the forward current characteristics of (n)PbTe-(p)Pb_0.8Sn_0.2Te heterojunctions as a function of temperature between 90 and 20 K. The heterojunctions were formed from layers grown by liquid phase epitaxy on Pb_0.8Sn_0.2Te substrates. The forward current is found to be proportional to I_o(T) exp (AV), where A is a constant, essentially independent of temperature, and I_o(T) is a relatively weak function of temperature. This behavior is consistent with a current due to a tunneling mechanism. The tunneling current is probably related to crystallographic defects generated near the PbTe-Pb_0.8Sn_0.2Te interface during the epitaxial growth. Comparison between theory and experiment is made in terms of a multistep recombination-tunneling model.     

Heterojunction performance of dip coated n-Cd0.5Zn0.5S thin films on different metal sulfide substrates

In this work, n-type cadmium zinc sulfide (n-Cd_0.5Zn_0.5S) films were grown by a dip coating technique on different p-type metal sulfide substrates. The morphology, structure, and composition of the yielded materials have been con?rmed by scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray analysis, respectively. Using the absorption measurements, the direct allowed band gap energies for Cd_0.5Zn_0.5S, PbS, CuS, CuInS₂, were found to be 2.93, 1.83, 1.98 and 2.5 eV at room temperature, respectively. The substrate dependence of the current density–voltage (J–V) characteristics of n-Cd_0.5Zn_0.5S/p-PbS, n-Cd_0.5Zn_0.5S/p-CuS, n-Cd_0.5Zn_0.5S/p-Cu_0.8In_1.2S₂, n-Cd_0.5Zn_0.5S/p-Cu_0.9In_1.1S₂, n-Cd_0.5Zn_0.5S/p-CuInS₂, n-Cd_0.5Zn_0.5S/p-Cu_1.1In_0.9S₂ and n-Cd_0.5Zn_0.5S/p-Cu_1.2In_0.8S₂ heterojunctions were measured at room temperature (～300 K). These characteristics showed a rectifying behavior consistent with a potential barrier formed at the interface for all the studied devices. The forward current density–voltage characteristics under low voltage bias were explained on the basis of thermionic emission mechanism. Heterojunction parameters such as ideality factor, n, series resistance, R_S and barrier height, Φ_b were obtained from J–V measurements using Cheung's method. The heterojunctions show non-ideal J–V behavior with an ideality factor greater than unity. Analysis of the experimental data under reverse voltage bias suggests that Schottky effect is the dominant mechanism. The dark capacitance–voltage characteristics of the heterojunctions were studied at 1 MHz. High value of built-in potential of 0.58 V was obtained for n-Cd_0.5Zn_0.5S/p-Cu_0.9In_1.1S₂ heterojunction as compared to the other studied heterojunctions. The photovoltaic characteristics were analyzed for the heterojunctions under illumination of 100 mW/cm².     

<Emphasis Type="Italic">C-V</Emphasis> and <Emphasis Type="Italic">I-V</Emphasis> characteristics of ultrathin-oxide MOS structures: Identification and analysis

For an NMOS structure with 3.7-nm-thick oxide, dynamic I-V characteristics are digitally measured by applying an upward and a downward gate-voltage ramp. An averaging procedure is employed to deduce the tunneling (active) current component and the quasi-static C-V characteristic (CVC). Analyzing the depletion segment of the CVC provides reliable values of the semiconductor doping level, the oxide capacitance and thickness, and the sign and density of oxide-fixed charge, as well as estimates of the dopant concentration in the poly-Si region. These data are used to identify the Ψ_s(V _g), V _i(V _g), and I _t(V _i) characteristics, where Ψ_s is the n-Si surface potential, V _i is the voltage drop across the oxide, V _g is the gate voltage, and I _t is the tunneling current; the gate-voltage range explored extends to prebreakdown fields (～13 MV cm^?1). The results are obtained without recourse to fitting parameters and without making any assumptions as to the energy spectrum of electrons tunneling from the n-Si deep-accumulation region through the oxide. It is believed that experimental I _t-V _i and Ψ_s-V _g characteristics will provide a basis for developing a theory of tunneling covering not only the degeneracy and size quantization of the electron gas in the semiconductor but also the nonclassical profile of the potential barrier to electron tunneling associated with the oxide-fixed charge.     

Efficient organic photovoltaics using solution-processed,annealing-free TiO2 nanocrystalline particles as an interface modification layer

A solution-processed, annealing-free TiO₂ nanocrystalline particles (TiO₂ NPs) as an interface modification layer was inserted in organic photovoltaics (OPVs), in which the widely used polymer poly (3-hexyl thiophene) (P3HT), a low band gap alkoxylphenyl substituted [1,2-b:4,5-b′] dithiophene-based polymer (PBDTPO-DTBO), and a soluble small molecule benzodithiophene derivative (TIBDT) were used as the donor material, respectively. The annealing-free TiO₂ NPs could be easily spin-coated upon the surface of organic active layers, and showed comparable properties to thermal-annealed ones. The power conversion efficiencies (PCEs) of OPV devices could be enhanced dramatically with inserting an annealing-free TiO₂ NPs layer. The PCEs of OPV devices based on P3HT:PC₆₁BM, PBDTPO-DTBO:PC₇₁BM and TIBDT:PC₆₁BM bulk heterojunctions were improved by 28%, 15% and 27%, respectively, with an annealing-free TiO₂ NPs layer, in which the highest PCE of 5.76% was achieved in PBDTPO-DTBO:PC₇₁BM OPVs. The solution-processed, annealing-free TiO₂ NPs thin films show great potential applications in the fabrication of large-area OPVs by printing or coating techniques on flexible polymer substrates. In particularly, it would promote to fabricate solution-processed, annealing-free OPV devices with suitable hole transport layer and organic/polymer active materials.     

Quantum supercurrent and Andreev reflection in silicon nanostructures

The tunneling spectroscopy is used for studying the hole transport in a sandwich nanostructure of the superconductor-ultra-narrow-self-assembled-p-silicon-quantum-well (Si-QW)-superconductor type on the n-Si (100) surface; the quantum well’s width is less than the coherence length and the Fermi wave-length. The high-resolution tunneling I–V characteristics display the supercurrent quantization, the characteristics of which depend on the positions of the dimensional-quantization levels for holes in the Si-QW. The correlation in the tunneling of single holes and Cooper pairs manifests itself in identical oscillations of the I–V characteristics for the supercurrent at T < T _c and the conductivity at T > T _c . In addition to the Josephson effect, the forward and reverse I–V characteristics for the first time reveal the processes of multiple Andreev reflection of two-dimensional holes in the Si-QW, which cause the microscopic mechanism responsible for the superconducting proximity effect. The investigation of the two-dimensional hole’s conductivity in the Si-QW plane indicates the presence of coherent tunneling under conditions of the spin-dependent multiple Andreev reflection between the superconducting δ] barriers confining it.     

Analysis of the Temperature Dependence of the Capacitance–Voltage and Conductance–Voltage Characteristics of Au/TiO2(rutile)/n-Si Structures

The capacitance–voltage–temperature (C–V–T) and the conductance/angular frequency–voltage–temperature (G/ω–V–T) characteristics of Au/TiO₂(rutile)/n-Si Schottky barrier diodes (SBDs) were investigated over the temperature range from 200 K to 380 K by considering the series resistance effect. Titanium dioxide (TiO₂) was deposited on n-type silicon (Si) substrate using a direct-current (DC) magnetron sputtering system at 200°C. To improve the crystal quality, the deposited film was annealed at 900°C to promote a phase transition from the amorphous to rutile phase. The C ^?2 versus V plots gave a straight line in the reverse-bias region. The main electrical parameters, such as the doping concentration (N _D), Fermi energy level (E _F), depletion layer width (W _D), barrier height (ф _CV), and series resistance (R _S), of Au/TiO₂(rutile)/n-Si SBDs were calculated from the C–V–T and the G/ω–V–T characteristics. The obtained results show that ф _CV, R _S, and W _D values decrease, while E _F and N _D values increase, with increasing temperature.     

Carrier Transport Mechanism and Band Offsets at the Interface of ZnS/<Emphasis Type="Italic">n</Emphasis>-Si (111) Heterojunctions Fabricated by Vacuum Thermal Evaporation

The electrical properties and band offset of ZnS/n-Si(111) heterojunctions with and without annealing were analyzed. The result showed that the rectifying characteristics of ZnS/n-Si(111) heterojunctions became better and the leakage current increased after annealing. This phenomenon is mostly due to the volatilization of S atoms of ZnS films and leads to defect levels appearing at the interface of the ZnS/n-Si(111) hetrojunctions. The valence band offset (ΔE _V) of the ZnS/n-Si(111) heterojunctions can be calculated to be ?0.7 ± 0.15 eV by means of photoelectron spectroscopy, indicating that the band offsets of ZnS/n-Si(111) heterojunctions show a type-II band alignment.     

Circuital modelling of S-shape removal in the current–voltage characteristic of TiOx inverted organic solar cells through white-light soaking

In this work light activation phenomenon in inverted bulk heterojunction (BHJ) organic solar cells (OSC) has been electrically modelled with a two-diode equivalent circuit. OSC are based on poly(3-hexylthiophene) (P3HT): 1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6) C61 (PCBM) with a titanium oxide (TiO_x) sublayer. Current–voltage (I–V) characteristics show a highly pronounced S-shape that is gradually removed during light activation process. The circuit used to model I–V curves includes two diodes in forward and reverse bias together with two parallel resistances, R_P1 and R_P2. The parallel of the reverse bias diode and its corresponding resistance R_P2 models the electrical behaviour of the TiO_x interlayer. This interlayer has been thermally treated at different temperatures, from 80 °C up to 180 °C, reducing the activation time from 400 s for unbaked devices down to 30 s for devices annealed at temperatures higher than 80 °C. The S-shape shown in the I–V characteristic is completely removed after a few minutes of white-light illumination. I–V curves recorded during the activation process have been fitted with the analytical solution of the two-diode circuit based on W-Lambert function. A decrease of the subcircuit 2 equivalent resistance has been found to be the cause of S-shape removal. This resistance diminishing is in good agreement with the increase of TiO_x conductance with baking temperature and white-light exposure time found by other authors.     

Mechanisms of charge transport in anisotype <Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-CdTe heterojunctions

Surface-barrier anisotype n-TiO₂/p-CdTe heterojunctions are fabricated by depositing thin titanium-dioxide films on a freshly cleaved surface of single-crystalline cadmium-telluride wafers by reactive magnetron sputtering. It is established that the electric current through the heterojunctions under investigation is formed by generation-recombination processes in the space-charge region via a deep energy level and tunneling through the potential barrier. The depth and nature of the impurity centers involved in the charge transport are determined.     

0D/1D Heterojunction Implant with Electro-Mechanobiological Coupling Cues Promotes Osteogenesis

Mimicking the natural bone extracellular matrix containing intrinsic topography and electrical signals is an effective way to modulate bone regeneration. However, simultaneously coupling of the intrinsic mechanobiology and electrical cues of implant to modulate bone regeneration remains ignored. Here, the authors report in situ designation of titanium dioxide (TiO₂) nanocone/bismuth oxide (Bi₂O₃) nanodot heterojunctions on bone implant surface to electro-biomechanically trigger osseointegration at bone/implant interface. TiO₂ nanocone/Bi₂O₃ nanodot heterojunctions exhibit built-in electric field at the nanoscale interface and elastic modulus equivalent to that of bone tissue. The nano-heterojunctions significantly promoted the attachment, spreading, and osteogenic differentiation of bone marrow mesenchymal stem cells in vitro, and the osteogenesis in vivo. The authors also show that the effects of nano-heterojunctions on osteogenesis are mediated by yes-associated protein biomechanical signal pathway and intracellular enrichment induced Phosphatidylinositol 3-kinase signal pathway. Their findings highlight the coupling of topographical and electric parameters of biomaterials for modulating cell behaviors.