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Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE

Date 3-8 June 2012

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Displaying Results 1 - 25 of 751
  • High-penetration PV deployment in the Arizona Public Service System, Phase 1 update

    Page(s): 000001 - 000004
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (723 KB) |  | HTML iconHTML  

    In an effort to better understand the impacts of high penetrations of photovoltaic generators on distribution systems, Arizona Public Service and its partners have begun work on a multi-year project to develop the tools and knowledge-base needed to safely and reliably integrate high penetrations of utility- and residential-scale photovoltaics (PV). Building upon the APS Community Power Project - Flagstaff Pilot, this project will analyze the impact of PV on a representative feeder in northeast Flagstaff. To quantify and catalog the effects of the estimated 1.3 MW of PV that will be installed on the feeder (both smaller units at homes as well as large, centrally located systems), high-speed weather and electrical data acquisition systems and digital “smart” meters are being designed and installed to facilitate monitoring and to build and validate comprehensive, high-resolution models of the distribution system. These models will be used to analyze the impacts of the PV on distribution circuit protection systems (including anti-islanding), predict voltage regulation and phase balance issues, and develop volt/var control schemes. This paper continues from a paper presented at the 2011 IEEE PVSC conference that introduces the project and describes some of the preliminary consideration, as well as project plans and early results. This paper gives a status update of the project and presents selected results from Phase 2 of the project. It discusses baseline feeder modeling, load allocation, data acquisition, utility-scale PV integration, preliminary model validation, and plans for future phases. View full abstract»

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  • Thin film a-Si/c-Si1−xGex/c-Si heterojunction solar cells with Ge content up to 56%

    Page(s): 000005 - 000008
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    Thin film a-Si(n+)/c-Si1-xGex(p)/c-Si(p+) heterojunction solar cells are fabricated with Ge content up to 56 atomic percent. Solar cells with junction layers consisting of Si, Si0.75Ge0.25, Si0.59Ge0.41, and Si0.44Ge0.56 are compared to study the effect of increasing Ge concentration. The measured short-circuit current (Jsc) increases from ~14 mA/cm2 for Si cells to 21 mA/cm2 for the Si0.44Ge0.56 cells, for one light pass and a 2 μm-thick SiGe layer. The results show an open-circuit voltage (Voc) of 0.61 V for Si cells, dropping to 0.32 V for Si0.44Ge0.56, consistent with the reduction in band-gap. Quantum efficiency measurements highlight the improved spectral response for higher Ge percentages. Physics based TCAD simulations combined with the experimental results are used to extract lifetime and interface velocity. View full abstract»

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  • Luminescent down-shifting for CdTe solar cells: A review of dyes and simulation of performance

    Page(s): 000009 - 000014
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    The luminescent materials that have been used for luminescent down-shifting (LDS) range from organic molecules and organometallic complexes to quantum dots (QDs), each of them with certain strengths and weaknesses. In this work, we introduce five figures of merit to quantitatively evaluate how the properties of these materials compare with the desirable properties of an optimum LDS layer, focusing our attention in the cadmium sulfide/cadmium telluride (CdS/CdTe) PV technology. In order to elaborate a quantitative ranking with the candidate materials, we have used a ray-tracing simulation software to calculate the expected enhancement in a CdTe module conversion efficiency. Finally, we briefly discuss the issues that directly affect the industrial application of these dyes for LDS, such as the amount of material required, its cost and its stability under the sunlight. View full abstract»

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  • Plasmonic effect in dye-sensitized solar cells

    Page(s): 000015 - 000020
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    Improvement of the absorption of the dye-sensitized solar cells (DSSCs) through the plasmonic effect is studied by simulations and experiments. Silver nanoparticles (NPs) having different size, density, embedded in different host materials were optically simulated first. The sputtered NPs were optically characterized to prove the presence of the plasmonic effect and the shift of the resonance frequencies. Finally the NPs were integrated in the DSSCs in a compact TiO2 layer at the TCO-active layer interface. The presence of NPs increased the quantum efficiency (QE) around 350 nm and decreased QE around main absorption peak at 550 nm. View full abstract»

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  • Intermediate band solar cell with non-ideal band structure under AM1.5 spectrum

    Page(s): 000021 - 000024
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    The limiting efficiency of an intermediate band solar cell with a non-ideal band structure is analyzed using the principle of detailed balance. Firstly, the impact of finite band width on the optimum band gap design for AM1.5 spectrum is examined. It is found that the band width may be a determining factor in the optimum band gap arrangement, but that the degradation in efficiency due to the band width up to ~ 200 meV is moderate. It is also found that the band width can determine which band gap combination gives the highest global limiting efficiency. Further to the intermediate band width modification, the inclusion of band tails, analogous to those present in amorphous materials is discussed in terms of realizing an intermediate band using quantum dot arrays. In this model the worst case scenario is assumed, that of low absorption, but maximum emissivity at the band tail edge of the intermediate band. Results show that, with multiple band gap combinations giving several local peaks, the one giving the global maximum efficiency changes with band width. When band tails are included in a materials system proposed for implementing an intermediate band solar cell via quantum dot miniband formation significant drop off in efficiency is seen. The results taken together suggest the intermediate band width and any band tails should be considered in designs for intermediate band solar cells. View full abstract»

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  • The steady state occupancy and Effective Fermi Level of P-N junction

    Page(s): 000025 - 000028
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    Explicit difference between occupation probability of the transition Gibbs free energy level (TGFEL) of semiconductor under equilibrium and that under steady state is pointed out. Our own computer simulation results are presented to show the detailed steady state occupancy of TGFELs under forward and reverse bias cases.. The result shows that Quasi Fermi levels and capture cross section of defects which is due to columbic interaction or other effects determine the different occupation probability in the region where Quasi Fermi levels are present. An Effective Fermi Level EFeff is defined for these two cases of the semiconductor P-N junction, with the presence of two Quasi Fermi levels. View full abstract»

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  • High efficiency nano-structured photovoltaic cell

    Page(s): 000029 - 000031
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (411 KB) |  | HTML iconHTML  

    The Qsolar Nano-Structure (QNS) cell is a three dimensional nano-structured silicon PV cell that merges well-known semiconductor processing technology and existing PV technology in a cost-effective way. The QNS cell structure significantly boosts energy output 1.8 times over conventional silicon solar cells by increasing junction area, maximizing light absorption, maximizing carrier capture, and minimizing contact resistance. View full abstract»

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  • Hot Carrier solar cell absorbers: Superstructures, materials and mechanisms for slowed carrier cooling

    Page(s): 000032 - 000035
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    The Hot Carrier solar cell is a Third Generation device that aims to tackle the carrier thermalisation loss after absorption of above band-gap photons. It is theoretically capable of efficiencies very close to the maximum thermodynamic limit. It relies on slowing the rate of carrier cooling in the absorber from ps to ns. This challenge can be addressed through nanostructures and modulation of phonon dispersions. The mechanisms of carrier cooling are discussed and methods to interrupt this process investigated to give a list of properties required of an absorber material. Quantum well or nano-well structures and large mass difference compounds with phonon band gaps are discussed in the context of enhancing phonon bottleneck and hence slowing carrier cooling. Materials for these structures are discussed and potential combined structures to maximize phonon bottleneck and slow carrier cooling are suggested. View full abstract»

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  • Raman measurements on GaN thin films for PV - purposes

    Page(s): 000036 - 000038
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    Raman scattering (RS) is a very important experimental tool to characterize the optical modes and another elementary excitations of materials. Among other issues it can determine for example the degree of crystalline quality and point defects like local modes. Therefore GaN - thin films and related compounds for photovoltaic purposes and as processed by several systems have been measured by this technique. The films were grown by Molecular Beam Epitaxy (MBE), Close Spaced Vapor Transport (CSVT) and Laser Ablation (LA) with the use of optimal growth parameters and substrates. Gallium nitride crystallizes in the wurtzite structure with 4 atoms in the unit cell and presents 7 allowed Raman modes of A1, E1, and E2 symmetries. In this work we present and discussed our Raman experiments where particularly the detection of the E2 and A1 modes are illustrated in these nitride semiconductor compounds. View full abstract»

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  • Investigation of carrier escape mechanism in InAs/GaAs quantum dot solar cells

    Page(s): 000039 - 000044
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    In order to enhance understanding of the short circuit improvement in InAs/GaAs quantum dot (QD) solar cells, the thermally assisted and tunneling mechanisms of carrier escape from the QD quantum confinement are investigated. The dependence of voltage biased spectral responsivity for QD solar cells at room temperature is studied to analyze carrier extraction through tunneling. Photoexcited carrier confinement and escape were also studied by means of temperature dependent spectral response (TDSR) and temperature dependent photoluminescence (TDPL). Energy required to move a carrier from the ground state to the first excited state, thermal activation energy (Ea), in a quantum dot is calculated from TDPL to be 114 meV. It is found that at room temperature carrier escape from the quantum dot confinement is affected by both thermal assisted escape and tunneling while at low temperature tunneling is the dominant in carrier escape from both wetting layer and QDs. For all temperature ranges, carrier exchange between ground states and excited states and carrier escape from ground states (GS) is first thermal escape to excited states (ES) then tunneling. View full abstract»

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  • Intermediate band solar cells using in-plane ultrahigh-density InAs/GaAsSb quantum-dot sheets

    Page(s): 000045 - 000047
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    In-plane ultrahigh-density InAs QDs with 5×1011cm-2 were successfully grown on the GaAsSb/GaAs(001). The QD sheets with type-1 and type-2 band structures were inserted into the pn-GaAs solar cells. In spite of only one QD sheet, the additional quantum efficiency was clearly observed at a long wavelength region of 900~1200 nm. The additional quantum efficiency for the type-2 QD cell was obtained at a wide wavelength region (1400 nm) and was higher than that for the type-1 QD cell. However, the open circuit voltage for the type-2 QD cell decreased as compared with that for type-1 QD cell. In addition, a separation distance from the p-layer to the type-1 QD sheet influenced on the cell performance. View full abstract»

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  • Modeling down-conversion and down-shifting for photovoltaic applications

    Page(s): 000048 - 000052
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    The efficiency improvements achieved by adding idealized, top-mounted, down-conversion (DC) and luminescent down-shifting (LDS) layers to a commercial grade silicon solar cell are studied. A comparison is then made to silicon nanocrystals (Si-NC) LDS layer coupled to a silicon solar cell, where the optical properties of the Si-NC are based on measured data. Since the modeled DC and LDS layers are electrically isolated from the solar cell, the devices are studied by modifying the incident AM1.5G spectrum according to the bandgap, absorption and emission profiles, and global efficiency of the DC and LDS layers. Simulation results indicate that a minimum DC/LDS efficiency of 1% is required to enhance the solar cell efficiency, and that this threshold rises to 38% for a Si-NC based LDS layer. Additionally, the incorporation of an optimal, perfectly efficient DC layer (200%) is shown to enhance the photovoltaic efficiency from 14.1% to 16.6% as opposed to 16.3% for a perfect LDS layer (100%). View full abstract»

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  • Fabrication and characterization of c-si solar cells integrated with ordered metallic nanostructure arrays

    Page(s): 000053 - 000056
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    In this study we present the fabrication process and characterization techniques to integrate metallic nanostructures on the surface of crystalline Si (c-Si) solar cells. These structures are designed to enhance light trapping at particular wavelengths [2]. Tuning the wavelength to maximize the efficiency can be achieved by changing the sizes and shapes of these structures. A special design of these structures atop c-Si solar cell is expected to affect the absorption near the band gap. These structures have been fabricated by electron beam lithography on c-Si substrates. The processing integration and preliminary characterization results of metallic nanostructures on c-Si solar cells are reported. View full abstract»

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  • Single wall carbon nanotube electrodes for hydrogenated amorphous silicon solar cells

    Page(s): 000057 - 000061
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    Carbon single wall nanotube (SWNT) films were applied as electrodes to replace the p-layer in hydrogenated amorphous silicon (a-Si:H) solar cells. Devices were fabricated by transferring vacuum-filtered SWNT films of varying thickness onto a-Si:H layers grown by plasma enhance chemical vapor deposition on Pilkington TEC 15 glass substrates. Cells incorporating SWNTs were illuminated from each side (glass / SWNT). A cell illuminated through a 25 nm thick SWNT film yielded short circuit current density, open circuit voltage, and efficiency of 5.47 mA/cm2, 0.793 V, and 1.46%, respectively. Maximum quantum efficiency of 48% was measured at 475 nm for the same device. View full abstract»

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  • Limiting efficiencies over 50% using multijunction solar cells with multiple exciton generation

    Page(s): 000062 - 000067
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    The achievement of solar cells over 50% is a critical goal for photovoltaics. Multijunction solar cells over 5 junctions allow such efficiencies, but are severely limited by material constraints and growth requirements for lattice matching. Nanostructured approaches such as multiple exciton generation (MEG) potentially offer a route to higher efficiency but still require high values of sunlight concentration and large quantum yields. We show an approach that allows for higher efficiencies based on including MEG in a multijunction solar cell. We also present a thermodynamic model for multijunction solar cells with MEG that demonstrates possible improvements. View full abstract»

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  • Theoretical analysis for intermediate band and tandem hybrid solar cell materials

    Page(s): 000068 - 000072
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    The efficiency limit of an intermediate band (IB) solar cell can be increased by a “tandem” configuration of multiple intermediate band devices. Thermodynamic models show that the efficiency of a two-stack tandem of IB devices achieves the efficiency of a six junction series connected solar cell. The efficiency of an IB in conjunction with a single or double stack tandem has similar efficiency advantages. Further, analysis of the materials which can be used to implement IB solar cells in a tandem configuration shows advantages relating to the ability to implement IB materials with quantum wells or quantum dots. For a single IB solar cell, a key difficulty is identifying materials for the barrier and the quantum well which have a small valence band offset and large conduction band offset (or the reverse). The use of an IB solar cell as the bottom solar cell of a tandem allows a larger range of materials with suitable barrier band gaps and a smaller ideal conduction band offset. A further theoretical advantage of such a structure is that it avoids the extremely low open circuit voltages achieved from pn junctions in low bandgap materials; for example, the thermodynamic optimum for a 6 junction tandem solar cell has its lowest bandgap below 0.4 eV. We present a thermodynamic model for IB hybrid tandem configurations which does not assume spectral selectivity among the different solar cells and predicts that a barrier/quantum dot structure can have an efficiency as high as 60 to 70 percent at 1000X blackbody radiation. View full abstract»

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  • Decoupling spectral overlap of intermediate band solar cells using low-high state filling

    Page(s): 000073 - 000077
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    The high efficiency predicted for solar cells based on multi-photon processes relies on the assumption of optical transitions with non-overlapping spectral bands. Spectral overlap reduces the conversion efficiency due to energy loss associated with improper usage of higher energy photons for lower energy optical transitions. In this study, device structures with non-uniform occupation of intermediate electronic states are proposed to reduce the dependence of conversion efficiency on spectral overlap. Solar cell conversion efficiencies are calculated for structures where absorption bands are spatially decoupled due to defined occupation of intermediate states. Conversion efficiencies for a two section and three section device with spectral overlap of 4 eV are determined to be 52.8 % and 61.5 %, respectively, and are significantly larger than the calculated efficiency of 35.1% for the case of a standard multi-photon cell with uniform half-filled intermediate states The spectrally-decoupled device provides a means to achieve high theoretical efficiency independent of spectral overlap that approaches the detailed balance efficiency limit of 63.2 % for intermediate state devices without spectral overlap and 63.8 % for unconstrained triple-junction tandem cells. View full abstract»

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  • A unified mathematical framework for intermediate band solar cells

    Page(s): 000078 - 000082
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    The modeling of intermediate band solar cell has been developed since 90's and continued effort is made to facilitate the realization of this novel device. Two formulation has been used to model the generation recombination rate of IBSC including conventionally available modified-Shockley-Reed-Hall formulation or later proposed IBSC formulation (Luque and Marti, PRL 78 5014). This paper proves that these two formulations are actually mathematically equivalent and actually one can be derived from the other. A unified mathematical framework can thus be established and the conventional drift-diffusion model can thus be employed for modeling novel IBSC with the inclusion of new model for intermediate band carrier transport. The debate whether the addition of impurity atoms would decrease the efficiency by shorter recombination lifetime or increase the efficiency by more absorption is studied, and results confirm that the efficient removal of photo-generated carriers from valence and conduction bands and solar concentration is the key to the success of subbandgap photovoltaics. View full abstract»

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  • Analysis of the energy structure of nitrogen δ-doped GaAs superlattices for high efficiency intermediate band solar cells

    Page(s): 000083 - 000086
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    Nitrogen δ-doped GaAs superlattices were fabricated and their energy structures were investigated. Several transitions related to E+ band of nitrogen δ-doped regions were observed in photoreflectance (PR) spectra at energies ranging 1.5-1.7 eV for the superlattices at which no transitions were observed for uniformly doped GaAsN. The PR signal intensity of E+ related band transitions is significantly higher than those observed in uniformly doped GaAsN. This enhancement of E+ related band transitions is advantageous as an intermediate band material, and thus, nitrogen δ-doped GaAs superlattice structures are expected to be an excellent alternative for the use of intermediate band solar cells. View full abstract»

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  • Optical design of selectively scattering nanostructures for angle sensitive semi-transparent photovoltaics

    Page(s): 000087 - 000091
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    Semi-transparent photovoltiacs are of interest for improving integration of solar energy harvesting with architecture. However, the competing requirements of optical transparency and efficient absorption of the incident spectrum severely limit performance. To address this tradeoff, we propose an angle selective organic photovoltaic window structure, structured such that normally incident light is transmitted to maintain window-quality transparency, while direct sunlight at an elevated angle is targeted for absorption. The localized surface plasmon resonance properties of metal nanorods are employed for angle and spectrally dependant scattering. The optical interference patterns arising when light propagates through subwavelength planar dielectric stacks are engineered to optimize the optical mode created by the metal scatterers via an evolutionary algorithm. We numerically model the transmission and absorption performance of a thin semi-transparent organic photovoltiac film under angled solar illumination to evaluate the potential for the proposed design. An optimized selective structure can maintain 70% optical transparency at normal incidence while improving total absorbed power by a factor of 2.3 vs. a lone semi-transparent cell of comparable transparency. View full abstract»

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  • Quantum efficiency measurements of down-shifting using silicon nanocrystals for photovoltaic applications

    Page(s): 000092 - 000096
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1164 KB) |  | HTML iconHTML  

    Silicon nanocrystal (Si-NC) luminescent down-shifting materials for photovoltaic (PV) applications were fabricated by ion implantation and plasma-enhanced chemical vapor deposition (PECVD). The absolute optical conversion efficiency of the Si-NC-emitted photoluminescence was measured using conventional methods, and an optical set-up involving an integrating sphere. Modeling shows that down-shifting the light incident on a single-junction silicon cell (SJSC) can improve the cell performance if the optical conversion efficiency is sufficiently high. The measured conversion efficiency of the Si-NCs in a fused silica host was found to range from 0.8% to 1.84% and was compared with the efficiency required to maintain the performance of a SJSC. View full abstract»

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  • Theoretical efficiency of intermediate band solar cells with overlapping absorption coefficients for various combinations of band gaps

    Page(s): 000097 - 000100
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    The most commonly used criteria for evaluation of the suitability of different intermediate band materials for use in intermediate band solar cells are their band gaps. One often sees that such an evaluation is made based on theoretical efficiency limits with non-overlapping absorption coefficients. In this work the theoretical efficiency limits for various degrees of overlap are calculated for relevant combinations of band gaps. It is found that the optimal position of the intermediate band moves towards the middle of the band gap when the overlap increases. It is also shown that overlap between the absorption coefficients can increase the theoretical efficiency for some band gap combinations. This work aims to serve as a rough guide for determining whether a combination of band gaps is promising for use in intermediate band solar cells or not. View full abstract»

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  • Carriers multiplication in neighboring surfactant-free silicon nanocrystals produced by 3D-surface engineering in liquid medium.

    Page(s): 000101 - 000104
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    Carriers multiplication in silicon nanocrystals (Si-ncs) in a one promising eefect to considerably enhance conversion efficiency of solar cells that can overcome theoretical limits. A close proximity of Si-ncs is an essential factor for carrier multiplication due to the separated quantum cutting effect. In this study we present results on investigation of 3-dimensional (3D) surface engineering of Si-ncs directly in water. Thus at the same time allow close proximity Si-ncs without of using any surfactant. The approach is based on ns laser treatment of Si-ncs dispersed in liquid solution. We explore the excitation wavelength dependence of photoluminescence quantum yield (ratio of the number of emitted and absorbed photons) for Si-ncs as prepared and surface engineered by ns laser processing. Our results suggest that close proximity of Si-ncs in spherical particles induced by laser processing might enhance also carriers multiplication. View full abstract»

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  • Thin-film InAs/GaAs quantum dot solar cells layer-transferred onto Si substrates and flexible plastic films

    Page(s): 000105 - 000108
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    Thin-film InAs/GaAs quantum dot solar cells on mechanically flexible plastic films are fabricated. A 4.1-μm-thick compound semiconductor photovoltaic layer was grown on a GaAs substrate, and then transferred onto a plastic film through a bonding technique. Our bonding scheme is mediated by a metal-epoxy agent for the realization of bonding at low temperatures (below 200 °C), enabling the use of plastic materials as support substrates, as well as preventing the degradation of the semiconductor photovoltaic layers including quantum dots. We also fabricated thin-film InAs/GaAs quantum dot solar cells on Si substrates, as alternative low-cost, lightweight, robust substrates, using the same layer-transfer scheme. The open-circuit voltages of the transferred thin-film cells are equal to that of the as-grown bulk cell on a GaAs substrate, indicating that no material degradation occurs during our bond-and-transfer process. Furthermore, our transferred thin-film cells exhibit larger photocurrents than the bulk reference and thus higher efficiencies because of the efficient carrier collection in the thin-film photovoltaic layers and enhanced optical path length due to the metallic back reflectors implemented on the support substrates. Our successful fabrication of thin-film quantum dot solar cells on both plastic films and Si substrates is a strong demonstration of the validity of our bond-and-transfer scheme for the formation of thin-film photovoltaics on any kind of support plate or film without degradation, and thus provides a pathway for the production of lightweight, mechanically flexible, low-cost and highly efficient quantum dot solar cells based on ultrathin single-crystalline III-V semiconductors. View full abstract»

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  • Synthesis and optical properties of ZnTe1−xOx highly mismatched alloys for intermediate band solar cells

    Page(s): 000109 - 000112
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    Highly mismatched ZnTe1-xOx (ZnTeO) alloys have been grown by molecular beam epitaxy. X-ray diffraction (XRD) analyses showed that a single-phase ZnTeO layer were grown with a substitutional O composition x up to 1.34% on ZnTe(001) substrate in this experiments. Optical transitions associated with the lower (E-) and upper (E+) conduction subbands resulting from the anticrossing interaction between the localized O states and the extended conduction states of ZnTe were clearly observed, and the dependence of the energy position of these bands on the O composition was consistent with the band anticrossing model. The photovoltaic activities of solar cells using ZnTeO layers are also reported. View full abstract»

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