Showing papers on "Heterojunction published in 2009"
TL;DR: In this paper, the elastic constants of 2D honeycomb structures from the strain energy and calculate the Poisson's ratio as well as in-plane stiffness values were revealed, and the properties of these honeycomb materials were compared to those of three-dimensional Group IV and Group III-V compounds.
Abstract: Using first-principles plane wave calculations, we investigate two dimensional honeycomb structure of Group IV elements and their binary compounds, as well as the compounds of Group III-V elements. Based on structure optimization and phonon mode calculations, we determine that 22 different honeycomb materials are stable and correspond to local minima on the Born-Oppenheimer surface. We also find that all the binary compounds containing one of the first row elements, B, C or N have planar stable structures. On the other hand, in the honeycomb structures of Si, Ge and other binary compounds the alternating atoms of hexagons are buckled, since the stability is maintained by puckering. For those honeycomb materials which were found stable, we calculated optimized structures, cohesive energies, phonon modes, electronic band structures, effective cation and anion charges, and some elastic constants. The band gaps calculated within Density Functional Theory using Local Density Approximation are corrected by GW0 method. Si and Ge in honeycomb structure are semimetal and have linear band crossing at the Fermi level which attributes massless Fermion character to charge carriers as in graphene. However, all binary compounds are found to be semiconductor with band gaps depending on the constituent atoms. We present a method to reveal elastic constants of 2D honeycomb structures from the strain energy and calculate the Poisson’s ratio as well as in-plane stiffness values. Preliminary results show that the nearly lattice matched heterostructures of these compounds can offer new alternatives for nanoscale electronic devices. Similar to those of the three-dimensional Group IV and Group III-V compound semiconductors, one deduces interesting correlations among the calculated properties of present honeycomb structures. PACS numbers: 73.22.-f, 61.48.De, 63.22.-m, 62.23.Kn
1,686 citations
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17 Feb 2009
TL;DR: Theoretical studies on ZnO-based Dilute Magnetic Semiconductors 5.8 as mentioned in this paper Theoretical Studies on DMS 5.7 Magnetic Interactions in DMS 4.2 p-Type Doping 4.1 Doping with Transition Metals 5.4 A Brief Theory of Magnetization 5.6 Measurements Techniques for Identification of Ferromagnetism
Abstract: Preface 1 General Properties of ZnO 1.1 Crystal Structure 1.2 Lattice Parameters 1.3 Electronic Band Structure 1.4 Mechanical Properties 1.5 Vibrational Properties 1.6 Thermal Properties 1.7 Electrical Properties of Undoped ZnO 2 ZnO Growth 2.1 Bulk Growth 2.2 Substrates 2.3 Epitaxial Growth Techniques 3 Optical Properties 3.1 Optical Processes in Semiconductors 3.2 Optical Transitions in ZnO 3.3 Defects in ZnO 3.4 Refractive Index of ZnO and MgZnO 3.5 Stimulated Emission in ZnO 3.6 Recombination Dynamics in ZnO 3.7 Nonlinear Optical Properties 4 Doping of ZnO 4.1 n-Type Doping 4.2 p-Type Doping 5 ZnO-Based Dilute Magnetic Semiconductors 5.1 Doping with Transition Metals 5.2 General Remarks about Dilute Magnetic Semiconductors 5.3 Classification of Magnetic Materials 5.4 A Brief Theory of Magnetization 5.5 Dilute Magnetic Semiconductor Theoretical Aspects 5.6 Measurements Techniques for Identification of Ferromagnetism 5.7 Magnetic Interactions in DMS 5.8 Theoretical Studies on ZnO-Based Magnetic Semiconductors 5.9 Experimental Results on ZnO-Based Dilute Magnetic Semiconductors 6 Bandgap Engineering 6.1 MgxZn1-xO Alloy 6.2 BexZn1-xO Alloy 6.3 CdyZn1-yO Alloy 7 ZnO Nanostructures 7.1 Synthesis of ZnO Nanostructures 7.2 Applications of ZnO Nanostructures 8 Processing, Devices, and Heterostructures 8.1 A Primer to Semiconductor-Metal Contacts 8.2 Ohmic Contacts to ZnO 8.3 Schottky Contacts to ZnO 8.4 Etching of ZnO 8.5 Heterostructure Devices 8.6 Piezoelectric Devices 8.7 Sensors and Solar Cells Based on ZnO Nanostructures 8.8 Concluding Remarks
865 citations
TL;DR: The epitaxial deposition of a compressive shell onto a soft nanocrystalline core to form a lattice-mismatched quantum dot can dramatically change the conduction and valence band energies of both the core and the shell.
Abstract: Lattice strain is a structural parameter that has been exploited in microelectronic devices with great success, but its role in colloidal nanocrystals is still poorly understood. Here we have developed strain-tunable colloidal nanocrystals by using lattice-mismatched heterostructures that are grown by epitaxial deposition of a compressive shell (e.g., ZnSe or CdS) onto a soft and small nanocrystalline core (e.g., CdTe). This combination of a “squeezed” core and a “stretched” shell causes dramatic changes in both the conduction and valence band energies. As a result, we show that core-shell QDs with standard type-I behavior are converted into type-II nanostructures, leading to spatial separation of electrons and holes, extended excited state lifetimes, and giant spectral shifting. This new class of strain-tunable QDs exhibits narrow light emission with high quantum yield across a broad range of visible and near-infrared wavelengths (500 nm to 1050 nm).
699 citations
TL;DR: Results show that soluble molecular donors can lead to BHJ cells that combine high conversion efficiency with the distinct advantages of working with single molecules, including structural definition, synthesis, purification, and reproducibility.
Abstract: The predicted exhaustion of fossil energy resources and the pressure of environmental constraints are stimulating an intensification of research on renewable energy sources, in particular, on the photovoltaic conversion of solar energy. In this context, organic solar cells are attracting increasing interest that is motivated by the possibility of fabricating large-area, lightweight, and flexible devices using simple techniques with low environmental impact. Organic solar cells are based on a heterojunction resulting from the contact of a donor (D) and an acceptor (A) material. Absorption of solar photons creates excitons, Coulombically bound electron−hole pairs, which diffuse to the D/A interface, where they are dissociated into free holes and electrons by the electric field. D/A heterojunctions can be created with two types of architectures, namely, bilayer heterojunction and bulk heterojunction (BHJ) solar cells. BHJ cells combine the advantages of easier fabrication and higher conversion efficiency due...
671 citations
TL;DR: In this paper, the electron affinity and ionization energy of vacuum-deposited molybdenum trioxide (MoO3) and of a typical MoO3/hole transport material (HTM) interface were determined via ultraviolet and inverse photoelectron spectroscopy.
Abstract: The electronic structures of vacuum-deposited molybdenum trioxide (MoO3) and of a typical MoO3/hole transport material (HTM) interface are determined via ultraviolet and inverse photoelectron spectroscopy. Electron affinity and ionization energy of MoO3 are found to be 6.7 and 9.68 eV, more than 4 eV larger than generally assumed, leading to a revised interpretation of the role of MoO3 in hole injection in organic devices. The MoO3 films are strongly n-type. The electronic structure of the oxide/HTM interface shows that hole injection proceeds via electron extraction from the HTM highest occupied molecular orbital through the low-lying conduction band of MoO3.
640 citations
TL;DR: In this article, the authors reported the fabrication and application of hybrid bulk heterojunction photodiodes containing PbS nanocrystalline quantum dots as sensitizers for near-infrared detection up to 1.8 µm.
Abstract: Solution-processed photodiodes with infrared sensitivities at wavelengths beyond the bandgap of silicon (corresponding to a wavelength of ∼1,100 nm) would be a significant advance towards cost-effective imaging. Colloidal quantum dots are highly suitable as infrared absorbers for photodetection, but high quantum yields have only been reported with photoconductors1,2,3. For imaging, photodiodes are required to ensure low-power operation and compatibility to active matrix backplanes4. Organic bulk heterojunctions5 are attractive as solution-processable diodes, but are limited to use in the visible spectrum. Here, we report the fabrication and application of hybrid bulk heterojunction photodiodes containing PbS nanocrystalline quantum dots as sensitizers for near-infrared detection up to 1.8 µm, with rectification ratios of ∼6,000, minimum lifetimes of one year and external quantum efficiencies of up to 51%. By integration of the solution-processed devices on amorphous silicon active matrix backplanes, we demonstrate for the first time near-infrared imaging with organic/inorganic hybrid photodiodes. Near-infrared imaging with solution-processed organic–inorganic hybrid photodiodes is demonstrated for the first time. The hybrid bulk-heterojunction photodiodes contain PbS nanocrystalline quantum dots as sensitizers for the detection of light of up to 1.8 µm in wavelength, have a minimum lifetime of one year, and external quantum efficiencies of up to 51%.
597 citations
TL;DR: It is observed that highly dense Si/TiO(2) core/shell nanowire arrays enhanced the photocurrent by 2.5 times compared to planar Si/ TiO( 2) structure due to their low reflectance and high surface area.
Abstract: There are currently great needs to develop low-cost inorganic materials that can efficiently perform solar water splitting as photoelectrolysis of water into hydrogen and oxygen has significant potential to provide clean energy. We investigate the Si/TiO2 nanowire heterostructures to determine their potential for the photooxidation of water. We observed that highly dense Si/TiO2 core/shell nanowire arrays enhanced the photocurrent by 2.5 times compared to planar Si/TiO2 structure due to their low reflectance and high surface area. We also showed that n-Si/n-TiO2 nanowire arrays exhibited a larger photocurrent and open circuit voltage than p-Si/n-TiO2 nanowires due to a barrier at the heterojunction.
521 citations
TL;DR: In this article, the photovoltaic effect in ferroelectric BiFeO3 thin films was reported and the all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes were characterized by open-circuit voltages ∼08-09V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light.
Abstract: We report a photovoltaic effect in ferroelectric BiFeO3 thin films The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼08–09 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 15) thus far reported for ferroelectric-based devices The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface
501 citations
TL;DR: After annealing treatment, genuine 3D nanoscale networks are formed with high crystalline order and favorable concentration gradients of both P3HT and PCBM through the thickness of the photoactive layer, which accounts for a considerable increase of the power conversion efficiency in corresponding solar cell devices.
Abstract: In this study, the three-dimensional (3D) nanoscale organization in the photoactive layers of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) is revealed by electron tomography. Morphologies suggested by previous experimental evidence were, for the first time, observed directly with a nanometer resolution and studied in detail. After annealing treatment, either at elevated temperature or during slow solvent evaporation, genuine 3D nanoscale networks are formed with high crystalline order and favorable concentration gradients of both P3HT and PCBM through the thickness of the photoactive layer. These favorable morphological changes account for a considerable increase of the power conversion efficiency in corresponding solar cell devices.
493 citations
TL;DR: The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented in this paper.
Abstract: The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.
457 citations
TL;DR: The first successful application of an ordered bicontinuous gyroid semiconducting network in a hybrid bulk heterojunction solar cell is reported, a major step toward realizing the full potential of self-assembly in the next generation of device technologies.
Abstract: We report the first successful application of an ordered bicontinuous gyroid semiconducting network in a hybrid bulk heterojunction solar cell. The freestanding gyroid network is fabricated by electrochemical deposition into the 10 nm wide voided channels of a self-assembled, selectively degradable block copolymer film. The highly ordered pore structure is ideal for uniform infiltration of an organic hole transporting material, and solid-state dye-sensitized solar cells only 400 nm thick exhibit up to 1.7% power conversion efficiency. This patterning technique can be readily extended to other promising heterojunction systems and is a major step toward realizing the full potential of self-assembly in the next generation of device technologies.
TL;DR: The design, fabrication, and characterization of colloidal PbSe nanocrystal (NC)-based photovoltaic test structures that exhibit an excitonic solar cell mechanism are reported, suggesting a charge separation mechanism that is distinct from previously reported Schottky devices and consistent with signatures of excited solar cells.
Abstract: We report the design, fabrication, and characterization of colloidal PbSe nanocrystal (NC)-based photovoltaic test structures that exhibit an excitonic solar cell mechanism. Charge extraction from the NC active layer is driven by a photoinduced chemical potential energy gradient at the nanostructured heterojunction. By minimizing perturbation to PbSe NC energy levels and thereby gaining insight into the "intrinsic" photovoltaic properties and charge transfer mechanism of PbSe NC, we show a direct correlation between interfacial energy level offsets and photovoltaic device performance. Size dependent PbSe NC energy levels were determined by cyclic voltammetry and optical spectroscopy and correlated to photovoltaic measurements. Photovoltaic test structures were fabricated from PbSe NC films sandwiched between layers of ZnO nanoparticles and PEDOT:PSS as electron and hole transporting elements, respectively. The device current-voltage characteristics suggest a charge separation mechanism that is distinct from previously reported Schottky devices and consistent with signatures of excitonic solar cells. Remarkably, despite the limitation of planar junction structure, and without film thickness optimization, the best performing device shows a 1-sun power conversion efficiency of 3.4%, ranking among the highest performing NC-based solar cells reported to date.
TL;DR: It is shown that non-planar phthalocynanines have appealing absorption characteristics but also have reduced charge carrier transport, and a method for increasing the exciton diffusion length by converting singlet excitons into long-lived triplets is proposed.
Abstract: Thin-film blends or bilayers of donor- and acceptor-type organic semiconductors form the core of heterojunction organic photovoltaic cells. Researchers measure the quality of photovoltaic cells based on their power conversion efficiency, the ratio of the electrical power that can be generated versus the power of incident solar radiation. The efficiency of organic solar cells has increased steadily in the last decade, currently reaching up to 6%. Understanding and combating the various loss mechanisms that occur in processes from optical excitation to charge collection should lead to efficiencies on the order of 10% in the near future. In organic heterojunction solar cells, the generation of photocurrent is a cascade of four steps: generation of excitons (electrically neutral bound electron-hole pairs) by photon absorption, diffusion of excitons to the heterojunction, dissociation of the excitons into free charge carriers, and transport of these carriers to the contacts. In this Account, we review our recent contributions to the understanding of the mechanisms that govern these steps. Starting from archetype donor-acceptor systems of planar small-molecule heterojunctions and solution-processed bulk heterojunctions, we outline our search for alternative materials and device architectures. We show that non-planar phthalocynanines have appealing absorption characteristics but also have reduced charge carrier transport. As a result, the donor layer needs to be ultrathin, and all layers of the device have to be tuned to account for optical interference effects. Using these optimization techniques, we illustrate cells with 3.1% efficiency for the non-planar chloroboron subphthalocyanine donor. Molecules offering a better compromise between absorption and carrier mobility should allow for further improvements. We also propose a method for increasing the exciton diffusion length by converting singlet excitons into long-lived triplets. By doping a polymer with a phosphorescent molecule, we demonstrate an increase in the exciton diffusion length of a polymer from 4 to 9 nm. If researchers can identify suitable phosphorescent dopants, this method could be employed with other materials. The carrier transport from the junction to the contacts is markedly different for a bulk heterojunction cell than for planar junction cells. Unlike for bulk heterojunction cells, the open-circuit voltage of planar-junction cells is independent of the contact work functions, as a consequence of the balance of drift and diffusion currents in these systems. This understanding helps to guide the development of new materials (particularly donor materials) that can further boost the efficiency of single-junction cells to 10%. With multijunction architectures, we expect that efficiencies of 12-16% could be attained, at which point organic photovoltaic cells could become an important renewable energy source.
TL;DR: Hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated.
Abstract: When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D/A interface can be higher than that to CT(1s) as expected from energy resonance arguments. We suggest a design principle in organic heterojunction solar cells: there must be strong electronic coupling between molecular excitons in the donor and hot CT excitons across the D/A interface.
TL;DR: A network-structured SnO(2)/ZnO heterojunction nanocatalyst with high photocatalytic activity was successfully synthesized through a simple two-step solvothermal method and is characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscope, N(2) physical adsorption, and UV-vis spectroscopy.
Abstract: A network-structured SnO2/ZnO heterojunction nanocatalyst with high photocatalytic activity was successfully synthesized through a simple two-step solvothermal method. The as-synthesized samples are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, N-2 physical adsorption, and UV-vis spectroscopy. The results show that the SnO2/ZnO sample with a molar ratio of Sn/Zn = 1 is a mesoporous composite material composed of SnO2 and ZnO. The photocatalytic activity of SnO2/ZnO heterojunction nanocatalysts for the degradation of methyl orange is much higher than those of solvothermally synthesized SnO2 and ZnO samples, which can be attributed to the SnO2-ZnO heterojunction, the pore structure, and higher Brunauer-Emmeff-Teller (BET) surface area of the sample: (1) The SnO2-ZnO heterojunction improves the separation of photogenerated electron-hole pairs due to the potential energy differences between SnO2 and ZnO, thus enhancing the photocatalytic activity. (2) The SnO2/ZnO sample might possess more surface reaction sites and adsorb and transport more dye molecules due to the higher BET surface area and many pore channels, also leading to higher photocatalytic activity.
TL;DR: The most efficient organic solar cells produced to date are bulk heterojunction (BHJ) photovoltaic devices based on blends of semiconducting polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) with fullerene derivatives such as [6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM) as mentioned in this paper.
Abstract: The most efficient organic solar cells produced to date are bulk heterojunction (BHJ) photovoltaic devices based on blends of semiconducting polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) with fullerene derivatives such as [6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM). The need for blending the two components is based on the idea that the exciton diffusion length in polymers like P3HT is only ∼10 nm, so that the polymer and fullerene components must be mixed on this length scale to efficiently split the excitons into charge carriers. In this paper, we show that the BHJ geometry is not necessary for high efficiency, and that all-solution-processed P3HT/PCBM bilayer solar cells can be nearly as efficient as BHJ solar cells fabricated from the same materials. We demonstrate that o-dichlorobenzene (ODCB) and dichloromethane serve nicely as a pair of orthogonal solvents from which sequential layers of P3HT and PCBM, respectively, can be spin-cast. Atomic force microscopy, various optical spectrosco...
TL;DR: A comprehensive review of the state-of-the-art research activities in ZnO and ZnS nanostructures, including their syntheses and po... as discussed by the authors.
Abstract: ZnO and ZnS, well-known direct bandgap II–VI semiconductors, are promising materials for photonic, optical, and electronic devices. Nanostructured materials have lent a leading edge to the next generation technology due to their distinguished performance and efficiency for device fabrication. As two of the most suitable materials with size- and dimensionality-dependent functional properties, wide bandgap semiconducting ZnO and ZnS nanostructures have attracted particular attention in recent years. For example, both materials have been assembled into nanometer-scale visible-light-blind ultraviolet (UV) light sensors with high sensitivity and selectivity, in addition to other applications such as field emitters and lasers. Their high-performance characteristics are particularly due to the high surface-to-volume ratios (SVR) and rationally designed surfaces. This article provides a comprehensive review of the state-of-the-art research activities in ZnO and ZnS nanostructures, including their syntheses and po...
TL;DR: Real-time imaging of growth kinetics reveals that a low solubility of Si and Ge in the solid particle accounts for the interfacial abruptness, and single interfaces that are defect-free and close to atomically abrupt are demonstrated.
Abstract: We have formed compositionally abrupt interfaces in silicon-germanium (Si-Ge) and Si-SiGe heterostructure nanowires by using solid aluminum-gold alloy catalyst particles rather than the conventional liquid semiconductor-metal eutectic droplets. We demonstrated single interfaces that are defect-free and close to atomically abrupt, as well as quantum dots (i.e., Ge layers tens of atomic planes thick) embedded within Si wires. Real-time imaging of growth kinetics reveals that a low solubility of Si and Ge in the solid particle accounts for the interfacial abruptness. Solid catalysts that can form functional group IV nanowire-based structures may yield an extended range of electronic applications.
TL;DR: In this paper, the currentvoltage characteristics of single nanowires were measured in the dark and under various illumination conditions including 1.5 AM, and the total efficiency was 4.5%.
Abstract: Gallium arsenide p-i-n radial junctions were fabricated by molecular beam epitaxy. The current-voltage characteristics of single nanowires were measured in the dark and under various illumination conditions including 1.5 AM. The total efficiency was 4.5%. Spatially resolved and power dependent photocurrent measurements indicate that the p-i-n junction is homogeneous along the nanowire. Electroluminescence measurements show an emission peak at about 1.4 eV, further corroborating the good quality of the nanowire. These results constitute an important progress for the use of nanowires in photovoltaic applications.
TL;DR: The external quantum efficiency and photoresponsivity profiles of the device showed a broad spectral response from the visible to the infrared region, indicating potential applications as a broad band photovoltaic cell or a visible-infrared dual-band photodetector.
Abstract: Catalyst-free, direct heteroepitaxial growth of vertical InAs nanowires on Si(111) substrates was accomplished over a large area by metal-organic chemical vapor deposition Nanowires showed very uniform diameters and a zinc blende crystal structure The heterojunctions formed at the interface between the n-type InAs nanowires and the p-type Si substrate were exploited to fabricate vertical array photodiode devices which showed an excellent rectification ratio and low reverse leakage current Temperature-dependent current transport across the heterojunctions was studied theoretically and experimentally in the dark and under AM 15 illumination When operated in photovoltaic mode, the open-circuit voltage was found to increase linearly with decreasing temperature while the energy conversion efficiency changed nonmonotonically with a maximum of 25% at 110 K Modeling of the nanowire/substrate heterojunctions showed good agreement with the experimental observations, and allowed determining the conduction band offset between the InAs nanowires and Si to be 010-015 eV The external quantum efficiency and photoresponsivity profiles of the device showed a broad spectral response from the visible to the infrared region, indicating potential applications as a broad band photovoltaic cell or a visible-infrared dual-band photodetector
TL;DR: The suppression of recombination at P3HT/TiO2 nanorod interfaces by the attachment of effective ligand molecules substantially improves device performance and provides a new route for fabricating low-cost, environmentally friendly polymer/inorganic hybrid bulk heterojunction photovoltaic devices.
Abstract: This work presents polymer photovoltaic devices based on poly(3-hexylthiophene) (P3HT) and TiO2 nanorod hybrid bulk heterojunctions. Interface modification of a TiO2 nanorod surface is conducted to yield a very promising device performance of 2.20% with a short circuit current density (Jsc) of 4.33 mA/cm2, an open circuit voltage (Voc) of 0.78 V, and a fill factor (FF) of 0.65 under simulated A.M. 1.5 illumination (100 mW/cm2). The suppression of recombination at P3HT/TiO2 nanorod interfaces by the attachment of effective ligand molecules substantially improves device performance. The correlation between surface photovoltage and hybrid morphology is revealed by scanning Kelvin probe microscopy. The proposed method provides a new route for fabricating low-cost, environmentally friendly polymer/inorganic hybrid bulk heterojunction photovoltaic devices.
TL;DR: In this article, a coupled WO3/BiVO4 thin film has been deposited on an FTO substrate by a spin coating method from precursor solutions, and the composite films were characterized by AFM, SEM, XPS and XRD techniques.
TL;DR: In the case of parallel configurations of the molecules at the pentacene/C(60) interface, the decay of the lowest charge-transfer state to the ground state is calculated to be very fast; as a result, it can compete with the dissociation process into mobile charge carriers.
Abstract: The exciton-dissociation and charge-recombination processes in organic solar cells based on pentacene/C60 heterojunctions are investigated by means of quantum-mechanical calculations. The electronic couplings and the rates of exciton dissociation and charge recombination have been evaluated for several geometrical configurations of the pentacene/C60 complex, which are relevant to bilayer and bulk heterojunctions. The results suggest that, irrespective of the actual pentacene−fullerene orientation, both pentacene-based and C60-based excitons are able to dissociate efficiently. Also, in the case of parallel configurations of the molecules at the pentacene/C60 interface, the decay of the lowest charge-transfer state to the ground state is calculated to be very fast; as a result, it can compete with the dissociation process into mobile charge carriers. Since parallel configurations are expected to be found more frequently in bulk heterojunctions than in bilayer heterojunctions, the performance of pentacene/C6...
TL;DR: A new type of excitonic solar cell based on planar heterojunctions between PbSe semiconductor nanocrystals and thin ZnO films is reported, which generates large photocurrents and higher photovoltages compared to Schottky cells assembled with similar nanocrystal films.
Abstract: We report a new type of excitonic solar cell based on planar heterojunctions between PbSe semiconductor nanocrystals and thin ZnO films. These solar cells generate large photocurrents and higher photovoltages compared to Schottky cells assembled with similar nanocrystal films. When illuminated with 100 mW/cm2 simulated AM1.5 spectrum, these solar cells exhibit short-circuit currents between 12 and 15 mA/cm2, open-circuit voltages up to 0.45 V, and a power conversion efficiency of 1.6%. The photovoltage depends on the size of the nanocrystals, increasing linearly with their effective band gap energy.
TL;DR: The results showed that the prepared nanocomposite catalysts exhibited strong photocatalytic activity for decomposition of benzene under visible light irradiation with high photochemical stability.
Abstract: A nanocrystal heterojunction LaVO4TiO2 visible light photocatalyst has been successfully prepared by a simple coupled method. The catalyst was characterized by powder X-ray diffraction, nitrogen adsorption-desorption, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectra, photoluminescence, and electrochemistry technology.The results showed that the prepared nanocomposite catalysts exhibited strong photocatalytic activity for decomposition of benzene under visible light irradiation with high photochemical stability. The enhanced photocatalytic performance of LaVO4/TiO2 may be attributed to not only the matched band potentials but also interconnected heterojunction of LaVO4 and TiO2 nanoparticles.
TL;DR: Vertically aligned ZnO-ZnS heterojunction nanowire (NW) arrays were synthesized by thermal evaporation in a tube furnace under controlled conditions and applied to convert mechanical energy into electricity when they are deflected by a conductive AFM tip in contact mode.
Abstract: Vertically aligned ZnO−ZnS heterojunction nanowire (NW) arrays were synthesized by thermal evaporation in a tube furnace under controlled conditions. Both ZnO and ZnS are of wurtzite structure, and the axial heterojunctions are formed by epitaxial growth of ZnO on ZnS with an orientation relationship of [0001]ZnO//[0001]ZnS. Vertical ZnS NW arrays have been obtained by selectively etching ZnO−ZnS NW arrays. Cathodoluminescence measurements of ZnO−ZnS NW arrays and ZnS NW arrays show emissions at 509 and 547 nm, respectively. Both types of aligned NW arrays have been applied to convert mechanical energy into electricity when they are deflected by a conductive AFM tip in contact mode. The received results are explained by the mechanism proposed for nanogenerator.
TL;DR: In this paper, the effect of substrate temperature on the growth of thin films of Cu2ZnSnS4 (CZTS) was investigated and X-ray diffraction studies revealed that polycrystalline CZTS films with better crystallinity could be obtained for substrate temperatures in the range 643-683 K.
TL;DR: It is demonstrated that quasi-elastic spin-flip processes with energy loss < or = 200 meV are the dominant microscopic mechanism limiting the spin diffusion length in CuPc.
Abstract: A fundamental prerequisite for the implementation of organic semiconductors (OSCs) in spintronics devices is the still missing basic knowledge about spin injection and transport in OSCs. Here, we consider a model system consisting of a high-quality interface between the ferromagnet cobalt and the OSC copper phthalocyanine (CuPc). We focus on interfacial effects on spin injection and on the spin transport properties of CuPc. Using spin-resolved two-photon photoemission, we have measured directly and in situ the efficiency of spin injection at the cobalt-CuPc interface. We report a spin injection efficiency of 85-90% for injection into unoccupied molecular orbitals of CuPc. Moreover, we estimate an electron inelastic mean free path in CuPc in the range of 1 nm and a 10-30 times higher quasi-elastic spin-flip length. We demonstrate that quasi-elastic spin-flip processes with energy loss < or = 200 meV are the dominant microscopic mechanism limiting the spin diffusion length in CuPc.
TL;DR: These results suggest that delta-doped SrTiO3 provides a model system in which to explore the quantum transport and interplay of both superconducting and normal electrons, and demonstrate that high-quality complex oxide heterostructures can maintain electron coherence on the macroscopic scales probed by transport, as well as on the microscopic scales demonstrated previously.
Abstract: Heavily doped semiconductors can exhibit superconductivity, but their performance is severely limited by extremely large electronic disorder. Similarly, the electron mean free path in low-dimensional superconducting thin films is usually limited by interface scattering or atomic-scale disorder. Kozuka et al. use niobium doping to fabricate a high-quality, two-dimensional superconducting layer within a thin-film heterostructure based on the first known superconducting semiconductor, SrTiO3. This should provide a model system in which to explore the quantum transport and interplay of both superconducting and normal electrons.
TL;DR: Room-temperature EL is observed at the direct gap energy from a Ge/Si p-i-n diode exhibiting the same characteristics of the directgap photoluminescence of Ge, indicating a promising future of tensile-strained Ge-on-Si for electrically pumped, monolithically integrated light emitters on Si.
Abstract: We report what we believe to be the first demonstration of direct bandgap electroluminescence (EL) from Ge/Si heterojunction light-emitting diodes (LEDs) at room temperature. In-plane biaxial tensile strain is used to engineer the band structure of Ge to enhance the direct gap luminescence efficiency by increasing the injected electron population in the direct Γ valley. Room-temperature EL is observed at the direct gap energy from a Ge/Si p-i-n diode exhibiting the same characteristics of the direct gap photoluminescence of Ge. The integral direct gap EL intensity increases superlinearly with electrical current owing to an indirect valley filling effect. These results indicate a promising future of tensile-strained Ge-on-Si for electrically pumped, monolithically integrated light emitters on Si.