Showing papers on "Heterojunction published in 2007"

Single-exciton optical gain in semiconductor nanocrystals

Abstract: Nanocrystal quantum dots have favourable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colours depend on the nanocrystal size—owing to the quantum-confinement effect—and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron–hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant (∼100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons. Semiconductor nanocrystals have very good light-emitting properties, so have potential as optical amplification media that can be easily processed with solution-based techniques: possible applications include optical interconnects in microelectronics, lab-on-a-chip technologies and quantum information processing. The problem with these structures is that at least two excitons (bound electron–hole pairs) need to be present in a nanocrystal before optical gain can be achieved, and this limits performance. In effect, the excitons annihilate each other before optical amplification can occur. This obstacle has now been overcome using nanocrystals with cores and shells made from different semiconductor materials, constructed in such a way that electrons and holes are separated from each other. This makes optical gain based on single excitons possible, significantly enhancing their promise as a practical optical material for laser applications. Semiconductor nanocrystals seem good candidates for 'soft' optical gain media, but optical gain and lasing is hard to achieve owing to a fundamental optical effect, which involves the problem that at least two excitons need to be present in a nanocrystal to achieve gain, and this limits performance. Here the problem is circumvented by designing nanocrystals with cores and shells made from different semiconductor materials, and in such a way that electrons and holes are separated from each other: this makes possible optical gain based on single excitons, thereby significantly enhancing the promise of semiconductor nanocrystals as practical optical materials for a wide range of lasing applications.

Tensile-strained, n-type Ge as a gain medium for monolithic laser integration on Si

Abstract: We analyze the optical gain of tensile-strained, n-type Ge material for Si-compatible laser applications. The band structure of unstrained Ge exhibits indirect conduction band valleys (L) lower than the direct valley (Γ) by 136 meV. Adequate strain and n-type doping engineering can effectively provide population inversion in the direct bandgap of Ge. The tensile strain decreases the difference between the L valleys and the Γ valley, while the extrinsic electrons from n-type doping fill the L valleys to the level of the Γ valley to compensate for the remaining energy difference. Our modeling shows that with a combination of 0.25% tensile strain and an extrinsic electron density of 7.6×1019/cm3 by n-type doping, a net material gain of ~400 cm-1 can be obtained from the direct gap transition of Ge despite of the free carrier absorption loss. The threshold current density for lasing is estimated to be ~6kA cm-2 for a typical edge-emitting double heterojunction structure. These results indicate that tensile strained n-type Ge is a good candidate for Si integrated lasers.

Synthesis and application of highly ordered arrays of TiO2 nanotubes

Abstract: Highly ordered, vertically oriented TiO2 nanotube arrays fabricated by potentiostatic anodization of titanium constitute a material architecture that offers a large internal surface area without a concomitant decrease in geometric and structural order. The precisely oriented nature of the crystalline (after annealing) nanotube arrays makes them excellent electron percolation pathways for vectorial charge transfer between interfaces. Herein are briefly considered their fabrication, as well as their initial application to hydrogen gas sensing, solar generation of hydrogen by water photoelectrolysis, and in heterojunction solar cells.

Solar cells utilizing small molecular weight organic semiconductors

Abstract: In this review, we focus on the field of organic photovoltaic cells based on small molecular weight materials. In particular, we discuss the physical processes that lead to photocurrent generation in organic solar cells, as well as the various architectures employed to optimize device performance. These include the donor–acceptor heterojunction for efficient exciton dissociation, the exciton blocking layer, the mixed or bulk heterojunction, and the stacked or tandem cell. We show how the choice of materials with known energy levels and absorption spectra affect device performance, particularly the open-circuit voltage and short-circuit current density. We also discuss the typical materials and growth techniques used to fabricate devices, as well as the issue of device stability, all of which are critical for the commercialization of low-cost and high-performance organic solar cells. Copyright © 2007 John Wiley & Sons, Ltd.

Analysis of the failure mechanism for a stable organic photovoltaic during 10 000 h of testing

Abstract: The degradation and failure mechanisms of a stable photovoltaic device comprising a bilayer heterojunction formed between poly(3-carboxythiophene-2,5-diyl-co-thiophene-2,5-diyl) (P3CT) and Buckminsterfullerene (C 60 ) sandwiched between indium tin oxide (ITO) and aluminium (Al) electrodes were elucidated by the time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis in conjunction with isotopic labelling using 18 O 2 after a total testing time of 13 000 h. This experiment allowed us to understand the chemistry that takes place in three dimensions during degradation and failure of the device under accelerated testing conditions. The cell was subjected to continuous illumination with an incident light intensity of 1000 Wm -2 (AM1-5) at 72±2°C under a vacuum of <10 -6 mBar. During the illumination period, IV-curves were recorded at regular intervals and the short circuit current of the device was monitored every 10s for 10 760 h. The total illumination time was 12 200 h. During this period of time, the device performance degraded and the device was finally left in the dark at 25°C in an atmosphere where the oxygen had been replaced with the isotope 18 O 2 . After 800 h in this atmosphere in the dark, the final IV-curves in the dark and under illumination were recorded. The main purpose of this work was the analysis using TOF-SIMS imaging and depth profiling of the degraded cell. The combined analyses correspond to the three-dimensional chemical imaging of the device showing specifically where the oxygen had reacted during exposure. Several general findings were made that are applicable to similar devices. It was found that the oxygen diffuses into the device through the Al electrode in between the Al grains and through microscopic holes in the Al electrode. Once inside the device the oxygen diffuses in the lateral and vertical plane until the counter electrode is reached. C 60 was found to be susceptible to the incorporation of 18 O but P3CT was not under the conditions in question. The other prominent degradation pathway was found to be the diffusion of electrode materials into the device. Both electrode materials diffuse through the entire device to the counter electrode.

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures for photovoltaic applications.

Abstract: Although ZnO and ZnS are abundant, stable, and environmentally benign, their band gap energies (3.44, 3.72 eV, respectively) are too large for optimal photovoltaic efficiency. By using band-corrected pseudopotential density functional theory calculations, we study how the band gap, optical absorption, and carrier localization can be controlled by forming quantum-well-like and nanowire-based heterostructures of ZnO/ZnS and ZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can be synthesized using existing methods, we obtain a band gap of 2.07 eV, which corresponds to a Shockley−Quiesser efficiency limit of 23%. On the basis of these nanowire results, we propose that ZnO/ZnS core/shell nanowires can be used as photovoltaic devices with organic polymer semiconductors as p-channel contacts.

Double and triple junction polymer solar cells processed from solution

Abstract: Multiple junction solar cells incorporating polymer:fullerene bulk heterojunctions as active layers and solution processed electron and hole transport layers are presented. The recombination layer, deposited between the active layers, is fabricated by spin coating ZnO nanoparticles from acetone, followed by spin coating neutral pH poly(3,4-ethylenedioxythiophene) from water and short UV illumination of the completed device. The key advantage of this procedure is that each step does not affect the integrity of previously deposited layers. The open-circuit voltage (Voc) for double and triple junction solar cells is close to the sum of the Voc’s of individual cells.

Photocatalytic Activities of Heterojunction Semiconductors Bi2O3/BaTiO3: A Strategy for the Design of Efficient Combined Photocatalysts

Abstract: The heterojunction semiconductors Bi2O3/BaTiO3 were prepared by a milling-annealing method. The powders were characterized by X-ray diffraction (XRD), the Brunauer−Emmett−Teller (BET) method, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and UV−vis diffuse reflection spectroscopy (DRS). Their UV-induced photocatalytic activities were evaluated by the degradations of methyl orange and methylene blue. The results generally show that the heterojunction semiconductors Bi2O3/BaTiO3 exhibit better photocatalytic properties than the single-phase BaTiO3 or Bi2O3. The obviously increased performance of Bi2O3/BaTiO3 is ascribed mainly to the electric-field-driven electron−hole separations both at the interface and in the semiconductors. A strategy for the design of efficient heterojunction photocatalysts was proposed. That is, an electron-accepting semiconductor and a hole-accepting semiconductor with matching band potentials, which respectively possess high electron and hole c...

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

Abstract: One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.

Electrochemically constructed p-Cu2O/n-ZnO heterojunction diode for photovoltaic device

Abstract: Polycrystalline n-ZnO/p-Cu2O heterojunctions have been fabricated by low-temperature eletrodepositions of ZnO and Cu2O layers in aqueous solutions. The condition for forming the Cu2O layer significantly reflected the electrical rectification characteristic and the photovoltaic performance, and the heterojunction fabricated under optimized conditions showed an excellent electrical rectification characteristic and a photovoltaic performance of 1.28% in conversion efficiency under an AM 1.5 illumination.

Current status of AlInN layers lattice-matched to GaN for photonics and electronics

Abstract: We report on the current properties of Al1-x InxN (x approximate to 0.18) layers lattice- matched ( LM) to GaN and their specific use to realize nearly strain- free structures for photonic and electronic applications. Following a literature survey of the general properties of AlInN layers, structural and optical properties of thin state- of- the- art AlInN layers LM to GaN are described showing that despite improved structural properties these layers are still characterized by a typical background donor concentration of ( 1 - 5) x 10(18) cm(-3) and a large Stokes shift (similar to 800 meV) between luminescence and absorption edge. The use of these AlInN layers LM to GaN is then exemplified through the properties of GaN/ AlInN multiple quantum wells ( QWs) suitable for near- infrared intersubband applications. A built- in electric field of 3.64MVcm(-1) solely due to spontaneous polarization is deduced from photoluminescence measurements carried out on strain- free single QW heterostructures, a value in good agreement with that deduced from theoretical calculation. Other potentialities regarding optoelectronics are demonstrated through the successful realization of crack- free highly reflective AlInN/ GaN distributed Bragg reflectors ( R > 99%) and high quality factor microcavities ( Q > 2800) likely to be of high interest for short wavelength vertical light emitting devices and fundamental studies on the strong coupling regime between excitons and cavity photons. In this respect, room temperature ( RT) lasing of a LM AlInN/ GaN vertical cavity surface emitting laser under optical pumping is reported. A description of the selective lateral oxidation of AlInN layers for current confinement in nitride- based light emitting devices and the selective chemical etching of oxidized AlInN layers is also given. Finally, the characterization of LM AlInN/ GaN heterojunctions will reveal the potential of such a system for the fabrication of high electron mobility transistors through the report of a high two- dimensional electron gas sheet carrier density ( n(s) similar to 2.6 x 10(13) cm(-2)) combined with a RT mobility mu(e) similar to 1170 cm(2) V-1 s(-1) and a low sheet resistance, R similar to 210 Omega square.

Model for a-Si: H/c-Si interface recombination based on the amphoteric nature of silicon dangling bonds

Abstract: The performance of many silicon devices is limited by electronic recombination losses at the crystalline silicon c-Si surface. A proper surface passivation scheme is needed to allow minimizing these losses. The surface passivation properties of amorphous hydrogenated silicon a-Si:H on monocrystalline Si wafers are investigated here. We introduce a simple model for the description of the surface recombination mechanism based on recombination through amphoteric defects, i.e. dangling bonds, already established for bulk a-Si:H. In this model, the injection-dependent recombination at the a-Si:H/c-Si interface is governed by the density and the average state of charge of the amphoteric recombination centers. We show that with our surface recombination model, we can discriminate between the respective contribution of the two main mechanisms leading to improved surface passivation, which is achieved by a the minimization of the density of recombination centers and b the strong reduction of the density of one carrier type near the interface by field effect. We can thereafter reproduce the behaviors experimentally observed for the dependence of the surface recombination on the injection level on different wafers, i.e., of both p and n doping type as well as intrinsic. Finally, we are able to exploit the good surface passivation properties of our a-Si:H layers by fabricating flat heterojunction solar cells with open-circuit voltages exceeding 700 mV.

Internal photoemission at interfaces of high-κ insulators with semiconductors and metals

Abstract: Internal photoemission spectroscopy provides the most straightforward way to characterize the relative energies of electron states at interfaces of insulators with metals and semiconductors by measuring the spectral onset of electron/hole photoemission from one solid into another. The article reviews the application of this technique for characterization of advanced nanometer-thin insulators prospected to be used in microelectronic devices. Fundamental aspects and technical features of the internal photoemission experiments are discussed together with basic electronic properties of a number of investigated high-permittivity insulating films and their interfaces in semiconductor heterostructures. Significant differences are found in the electronic properties of nanometer-thin amorphous insulating layers as compared to the known bulk phase characteristics. The band alignment at the interfaces of these insulators with metals is found to be highly sensitive to the surface preparation procedures. By contrast, ...

Effect of the Molecular Weight of Poly(3-hexylthiophene) on the Morphology and Performance of Polymer Bulk Heterojunction Solar Cells

Abstract: The morphology and performance of bulk heterojunction solar cells comprised of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) have been investigated using P3HT with different molecular weights (MWs). It is concluded that the optimum annealing temperature for the bulk heterojunction material is related to the MW of P3HT. The best performance is obtained by using P3HT with an optimum ratio between high MW and low MW components. The corresponding 'ideal morphology' is comprised of highly ordered crystalline regions formed by low MW P3HT embedded and interconnected by a high MW P3HT matrix.

The selective detection of C2H5OH using SnO2–ZnO thin film gas sensors prepared by combinatorial solution deposition

Abstract: Sensing materials for selective detection of C2H5OH were designed using combinatorial solution deposition of SnO2–ZnO thin films. The SnO2–ZnO composite sensor prepared by alternate deposition of 10 droplets of SnO2 and ZnO sols (S50Z50 sensor) showed a high response to 200 ppm C2H5OH (S(ethanol) = Ra/Rg = 4.69, Ra: resistance in air, Rg: resistance in gas) at 300 °C, while the gas responses to 100 ppm C3H8, 100 ppm CO, 200 ppm H2, and 5 ppm NO2 ranged from 1.11 to 1.19. The S(ethanol) value of the S50Z50 sensor was twice that to 200 ppm CH3COCH3 (S(acetone)). In contrast, the S(ethanol) and S(acetone) of pure SnO2 and ZnO thin films were similar to each other. The heterostructure between SnO2 and ZnO was suggested as one of the probable reasons for the successful discrimination between C2H5OH and CH3COCH3.

A fullerene–single wall carbon nanotube complex for polymer bulk heterojunction photovoltaic cells

Abstract: A novel immobilized fullerene–single wall carbon nanotube (C60–SWCNT) complex was synthesized via a microwave induced functionalization approach. It has been used as a component of the photoactive layer in a bulk heterojunction photovoltaic cell. As compared to a control device with only C60, the addition of SWCNTs resulted in an improvement of both the short circuit current density JSC and the fill factor (FF). This device takes advantage of the electron accepting feature of C60 and the high electron transport capability of SWCNTs. The results indicate that C60 decorated SWCNTs are promising additives for performance enhancement of polymer photovoltaic cells.

High efficiency double heterojunction polymer photovoltaic cells using highly ordered TiO2 nanotube arrays

Abstract: Vertically oriented TiO2 nanotube arrays formed by anodization offer a highly ordered material architecture for efficient charge generation and collection in photoelectrochemical devices. A blend of regioregular poly(3-hexylthiophene) and a methanofullerene (phenyl C71-butyric acid methyl ester) was infiltrated into transparent TiO2 nanotube films. The heterojunction poly(3-hexylthiophene) (P3HT)-([6,6]-phenyl-C71-butyric acid methyl ester) and P3HT-TiO2 interfaces both result in charge separation. The resulting solid state solar cells show a short-circuit current density of 12.4mA∕cm2, 641mV open circuit potential, and a 0.51 fill factor, yielding power conversion efficiencies of 4.1% under AM 1.5 sun.

Low Band Gap Donor-Acceptor Conjugated Polymers toward Organic Solar Cells Applications

Abstract: Mixtures of conjugated polymers and fullerenes command considerable attention for application in organic solar cells. To increase their efficiency, the design of new materials that absorb at longer wavelengths is of substantial interest. We have prepared such low band gap polymers using the donor-acceptor route, which is based on the concept that the interaction between alternating donors and acceptors results in a compressed band gap. Furthermore, for application in photovoltaic devices, sufficient polymer solubility is required. We have prepared four low band gap conjugated polymers, with a bis(1-cyano-2-thienylvinylene)phenylene base structure, and achieved an excellent solubility by the introduction of long alkoxy and alkyl side chains. The polymers were synthesized via an oxidative polymerization. Their electronic properties were determined from electrochemical and optical measurements, which confirm that they indeed have a low band gap. In the blend of such a low band gap polymer with PCBM, evidence for efficient charge transfer was obtained from PL and EPR measurements. However, bulk heterostructure solar cells made of such blends display only low efficiencies, which is attributed to low charge carrier mobilities.

TiO2−Multiwalled Carbon Nanotube Heterojunction Arrays and Their Charge Separation Capability

Abstract: Aligned multiwalled carbon nanotubes (MWNTs) coated with TiO2 nanoparticles were fabricated on a titanium foil by atmospheric pressure chemical vapor deposition (CVD). Their morphology was characte...

Ultrafast Electron Transfer and Decay Dynamics in a Small Band Gap Bulk Heterojunction Material

Abstract: Bulk heterojunction materials comprising bicontinuous networks of electron donor and acceptor components sandwiched between electrodes with different work functions (e.g., indium-tin oxide, ITO, and a lower work-function metal such as Al) have yielded the best results to date for organic polymer-based solar cells. In such bulk heterojunction materials, ultrafast photoinduced electron transfer occurs at the interface between the polymer (donor) and the fullerene (acceptor) and results in efficient charge separation. Provided that the back transfer rate is sufficiently slow, the photo-generated mobile positive and negative carriers can be collected at the electrodes; electrons at the lower work function electrode and holes at the higher work function electrode. Using this approach, power conversion efficiencies of 5 % have been demonstrated in bulk heterojunction materials comprised of poly(3-hexylthiophene), P3HT and the [6,6]phenyl-C61 butyric acid methyl ester fullerene derivative, PCBM. Although the quantum efficiency is high within the absorption spectrum, the band gap of this conjugated polymer is too large (the solar radiation spectrum extends into the infrared). Thus, the quest for higher solar cell efficiencies has focused on smaller band gap polymers designed and synthesized to improve the harvesting of solar radiation. Here we describe the results of time-resolved spectroscopic studies of the small band gap semiconducting copolymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT, made of alternating electron-rich and electron-deficient units and in bulk heterojunction materials comprising PCPDTBT and PCBM. The molecular structures of PCPDTBT and PCBM are shown in Scheme 1. Because carrier recombination prior to collection at the electrodes would limit the photovoltaic power conversion efficiency, the goal of our stud-

Impact of epitaxial growth at the heterointerface of a-Si:H∕c-Si solar cells

Abstract: The authors have demonstrated that interface structures of heterojunction solar cells consisting of hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) have quite large impact on the solar cell performance. In particular, unintentional epitaxial growth was found to occur during an intended a-Si:H i-layer growth on c-Si in plasma-enhanced chemical vapor deposition (PECVD). By the formation of the epitaxial layer at the interface, the solar cell efficiency decreases significantly. Their result shows that the epitaxial layer is formed rather easily in PECVD, even without the presence of H2 gas, and may have affected many previous studies on a-Si:H∕c-Si solar cells seriously.

Simulation of Interfacial Phonon Transport in Si–Ge Heterostructures Using an Atomistic Green’s Function Method

Abstract: An atomistic Green 's function method is developed to simulate phonon transport across a strained germanium (or silicon) thin film between two semi-infinite silicon (or germanium) contacts. A plane-wave formulation is employed to handle the translational symmetry in directions parallel to the Interfaces. The phonon transmission function and thermal conductance across the thin film are evaluated for various atomic configurations. The contributions from lattice straining and material heterogeneity are evaluated separately, and their relative magnitudes are characterized. The dependence of thermal conductance on film thickness is also calculated, verifying that the thermal conductance reaches an asymptotic value for very thick films. The thermal boundary resistance of a single Si/Ge interface is computed and agrees well with analytical model predictions. Multiple-interface effects on thermal resistance are investigated, and the results indicate that the first few interfaces have the most significant effect on the overall thermal resistance.

Excitonic ultraviolet lasing in ZnO-based light emitting devices

Abstract: The authors have fabricated ultraviolet (UV) laser diodes based on ZnO∕BeZnO films The devices have p-n heterojunction structures with a multiple quantum well (MQW) active layer sandwiched between guide-confinement layers The MQW active layer comprises undoped ZnO and BeZnO, while the two guide-confinement layers were As-doped p-type ZnO∕BeZnO and Ga-doped n-type BeZnO∕ZnO films, respectively The exciton binding energy in the MQW region is exceptionally large (263meV) Exciton-related lasing was observed by optically pumping the MQWs ZnO∕BeZnO-based diodes showed laser action by current injection at room temperature The lasing mechanism is inelastic exciton-exciton collision

The morphology of axial and branched nanowire heterostructures.

Abstract: We present an extensive investigation of the epitaxial growth of Au-assisted axial heterostructure nanowires composed of group IV and III-V materials and derive a model to explain the overall morphology of such wires. By analogy with 2D epitaxial growth, this model relates the wire morphology (i.e., whether it is kinked or straight) to the relationship of the interface energies between the two materials and the particle. This model suggests that, for any pair of materials, it should be easier to form a straight wire with one interface direction than the other, and we demonstrate this for the material combinations presented here. However, such factors as kinetics and the use of surfactants may permit the growth of straight double heterostructure nanowires. Finally, we demonstrate that branched nanowire heterostructures, also known as nanotrees, can be successfully explained by the same model.

Oxygen vacancies in high dielectric constant oxide-semiconductor films.

Abstract: We provide evidence that the oxygen vacancy is a dominant intrinsic electronic defect in nanometer scaled hafnium oxide dielectric films on silicon, relevant to microelectronics technology. We demonstrate this by developing a general model for the kinetics of oxygen vacancy formation in metal-ultrathin oxide-semiconductor heterostructures, calculating its effect upon the band bending and interfacial oxidation rates and showing good experimental agreement with the predictions.

Self-assembled hybrid polymer-TiO2 nanotube array heterojunction solar cells.

Abstract: Films comprised of 4 microm long titanium dioxide nanotube arrays were fabricated by anodizing Ti foils in an ethylene glycol based electrolyte. A carboxylated polythiophene derivative was self-assembled onto the TiO2 nanotube arrays by immersing them in a solution of the polymer. The binding sites of the carboxylate moiety along the polymer chain provide multiple anchoring sites to the substrate, making for a stable rugged film. Backside illuminated liquid junction solar cells based on TiO2 nanotube films sensitized by the self-assembled polymeric layer showed a short-circuit current density of 5.5 mA cm-2, a 0.7 V open circuit potential, and a 0.55 fill factor yielding power conversion efficiencies of 2.1% under AM 1.5 sun. A backside illuminated single heterojunction solid state solar cell using the same self-assembled polymer was demonstrated and yielded a photocurrent density as high as 2.0 mA cm-2. When a double heterojunction was formed by infiltrating a blend of poly(3-hexylthiophene) (P3HT) and C60-methanofullerene into the self-assembled polymer coated nanotube arrays, a photocurrent as high as 6.5 mA cm-2 was obtained under AM 1.5 sun with a corresponding efficiency of 1%. The photocurrent action spectra showed a maximum incident photon-to-electron conversion efficiency (IPCE) of 53% for the liquid junction cells and 25% for the single heterojunction solid state solar cells.

Flexible conjugated polymer photovoltaic cells with controlled heterojunctions fabricated using nanoimprint lithography

Abstract: The authors describe conjugated polymer-based photovoltaic devices in which the shape and area of the interface between the electron donor and acceptor layers were controllably varied using nanoimprint lithography. The short circuit current is shown to increase with the interfacial area of the heterojunction, without affecting the open circuit voltage. The fill factor and power conversion efficiency are also shown to increase with donor-acceptor interfacial area.

Nanorod Heterostructures Showing Photoinduced Charge Separation

Abstract: Size- and shape-dependent property modifications of semiconductor nanocrystals have been a subject of intense interest because of their potential for future engineering devices. The bandgap and related optical-property tuning of these materials are mainly governed by the nature of their band edges. In addition, fusing one type of nanocrystal over another enables further control of material properties that are dependent on the relative alignments of their energy levels. On a molecular scale, the synthesis of supramolecular compounds has inspired advances in theories for photoinduced charge transfer. Heterostructured nanocrystals potentially provide a nanoscale analog of such systems. A method for preparing heterostructured nanocrystals of complex morphologies showing photoinduced charge separation is presented. It is shown that the energy and lifetime of the charge-transfer photoluminescence band can be tuned by changing the relative alignment of band edges in CdSe/CdTe heterostructure nanorods. The long-lived charge transfer states in these type II semiconductors may make them attractive for photovoltaic applications.

Electro‐Optical Study of Subphthalocyanine in a Bilayer Organic Solar Cell

Abstract: Ultra-thin films of subphthalocyanine (SubPc) were grown onto Si/SiO2 substrates by organic molecular beam deposition and the complex refractive index has been characterized by spectroscopic ellipsometry. The peak maximum in the extinction coefficient is determined to be 1.6 at 590 nm and the dielectric constant equals 3.9 in the limit of long wavelength. These values are extraordinary high when compared to the well-known metal-phthalocyanines and will be beneficial for the performance in a photovoltaic cell. The amorphous SubPc structure on top of indium-tin-oxide (ITO) as well as quartz glass is imaged by atomic force microscopy and scanning electron microscopy and we have characterized the nearly flat surface topology. Next, subphthalocyanine films in combination with buckminsterfullerene (C60) have been studied in a planar bilayer donor/acceptor heterojunction by current density-voltage characterization under AM 1.5 simulated illumination at various light intensities. A power conversion efficiency of 3.0 % under 1 sun was measured. Finally, the external and internal quantum efficiencies demonstrated peak maxima at 590 nm of 46 % and 55 %, respectively. Considering the abrupt junction at the donor/acceptor interface, the electron transfer from SubPc to the acceptor material is thus determined to be highly efficient.