Search

In this study, we fabricated n-type bifacial solar cells by liquid phase deposited (LPD) SiO2 films as surface passivation layers. We have found that the growth conditions of LPD SiO2 films have great influence on the deposition rate of the LPD SiO2 films. Besides, the surface passivation effects of LPD SiO2 films on both p-type and n-type silicon wafers are enormously improved after annealing at a temperature higher than 700 °C. Finally, the optimized LPD SiO2 films have been successfully applied to the n-type bifacial silicon solar cells as the surface passivation layers, achieving a conversion efficiency of 19.06% on a large size (156 mm×156 mm) solar cell.

Composite photoanode comprising nanoparticles and one-dimensional (1D) nanostructure is a promising alternative to conventional photoanode for dye-sensitized solar cells (DSCs). Besides fast electron transport channels, the 1D nanostructure also plays as light scattering centers. Here, we theoretically investigate the light scattering properties of capsule-shaped 1D nanostructure and their influence on the light collection of DSCs. It is found that the far-field light scattering of a single capsule depends on its volume, shape, and orientation: capsules with bigger equivalent spherical diameter, smaller aspect ratio, and horizontal orientation demonstrate stronger light scattering especially at large scattering angle. Using Monte Carlo approach, we simulated and optimized the light harvesting efficiency of the cell. Two multilayer composite photoanodes containing orderly or randomly oriented capsules are proposed. DSCs composed of these two photoanodes are promising for higher efficiencies because of their efficient light collection and superior electron collection. These results will provide practical guidance to the design and optimization of the photoanodes for DSCs. [ABSTRACT FROM AUTHOR]

Dark current mechanisms of terahertz quantum-well photodetectors (THz QWPs) are systematically investigated experimentally and theoretically by measuring two newly designed structures combined with samples reported previously. In contrast to previous investigations, scattering-assisted tunneling dark current is found to cause significant contributions to total dark current. A criterion is also proposed to determine the major dark current mechanism at different peak response frequencies. We further determine background limited performance (BLIP) temperatures, which decrease both experimentally and theoretically as the electric field increases. This work gives good description of dark current mechanism for QWPs in the THz region and is extended to determine the transition fields and BLIP temperatures with response peaks from 3 to 12 THz. [ABSTRACT FROM AUTHOR]

We present a simple assembly strategy of single nanowires (NWs) to form half-coaxial nanowire arrays (NWAs) which can be easily realized in large size by standard pattering and deposition techniques. Through the finite-difference time-domain simulation, we show that the proposed half-coaxial NWAs effectively preserve the leaky modes resonances within single NWs and consequently achieve strong absorption enhancement under optimization of various structural factors. The best half-coaxial NWAs with 100 nm thick absorbing shell offer equivalent light absorption of more than 400 nm thick planar film. Benefiting from the >75% cut of the required thickness of the absorbing layer, the performances of the demonstrated half-coaxial NWAs based a-Si thin film solar cell also gain significant improvement. [ABSTRACT FROM AUTHOR]

Theoretical studies and numerical simulations on design, performance, and improvements of terahertz quantum-well photodetector (THz QWP) are presented. In the first part of this paper, we discuss the device band structure resulting from a self-consistent solution and simulation results. First, the temperature dependence of device characteristics is analyzed. Next, we deduce the condition of optimal doping concentration for maximizing dark current limited detectivity Ddet* when QWP is lightly doped. Accordingly, unlike in previously published reports, doping concentration is not fixed and is selected by the above condition. In the second part of this paper, we propose two schemes for improving operation temperature. The first is to incorporate an optical antenna which focuses incident THz wave. Numerical results show that the QWP with peak frequency higher than 5.5 THz is expected to achieve background-noise-limited performance at 77 K or above when employing a 106 times enhancement antenna. The second scheme is to use a laser as the signal source to achieve photon-noise-limited performance (PLIP) at high temperatures. Simulations show that when operating below critical temperature QWPs in the range of 1 ∼ 7 THz can reach PLIP under practical illumination intensities. [ABSTRACT FROM AUTHOR]

We investigate the limit of weak infrared photodetection based on a systematic analysis of three main noise mechanisms, and point out the principles to achieve the ultimate performance. Two key issues should be addressed to realize ultra-sensitive infrared photodetection: suppression of background radiation, selection and optimization of photodetectors. We quantitatively studied the dependence of ultimate performance on the background temperature and emissivity. It is revealed that the background limited infrared performance detectivity for mid-infrared photodetection can be increased from the range of 1010 to 1011 cmHz1/2/W to 1015 cmHz1/2/W or even higher values, when the background radiation temperature decreases from the ambient to liquid nitrogen temperature. Furthermore, we investigate the feasibility of photoconductive infrared photodetectors for ultra-sensitive photodetection. Our simulations shows that ideal quantum well infrared photodetectors (QWIPs) can reach background limited infrared performance at feasible operation temperatures facing even very weak background radiation. For specific and quantitative analyses, QWIPs are used as the examples. However, the conclusions apply to a broader range of cases. [ABSTRACT FROM AUTHOR]

We have performed a detailed structural and optical investigation of hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared by plasma-enhanced chemical vapor deposition. The microstructural properties of these thin films are characterized and interpreted physically based on the growth mechanism. Infrared spectroscopy reveals that the bonded hydrogen in a platelet-like configuration, which is believed to be located at grain boundaries, greatly affects oxygen incursions into nc-Si:H thin films, whereas electron spin resonance observations link these incursions to the introduction of dangling bond defects. Consequently, we propose that in nc-Si:H thin films, high bonded-hydrogen content in grain boundaries is of great importance in forming hydrogen-dense amorphous tissues around the small crystalline grains, i.e., compact grain boundary structures with good passivation. Such structures effectively prevent post-deposition oxidation of grain boundary surfaces, which might lead to the formation of dangling bond defects. [ABSTRACT FROM AUTHOR]

A detailed investigation of temperature-dependent micro-Raman scattering has been carried out on AlInN films with different Al compositions (0-0.53). The observed phonon frequency downshift and linewidth broadening with increasing temperature can be well explained by a model taking into account the contributions of the thermal expansion, the lattice-mismatch-induced strain, and the anharmonic phonon processes. It is found that with increasing Al composition the three-phonon process increases over the four-phonon process, but still is not the obvious prevailing process in the phonon decay of AlInN. We have attributed it to the variation of structural properties and phonon density of states in AlInN. [ABSTRACT FROM AUTHOR]

The top mirror of the resonant cavity enhanced homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR)/terahertz (THz) detector is investigated. Aiming at an existing half-optimized n-GaAs HIWIP FIR/THz detector, two designs of the top mirror are investigated to increase the quantum efficiency. The genuine or equivalent single layer is proved to be unqualified as the top mirror. A design based on the two-dimensional (2D) periodical reversed pyramidal structure of intrinsic GaAs is proposed as the top mirror. The resulted quantum efficiency can be as high as 29.0%. Some elementary thoughts and a tentative fabricating solution of this 2D periodical reversed pyramidal intrinsic GaAs top mirror are suggested. The designing ideas of this kind of top mirror may also be applied to other FIR/THz devices for reference. [ABSTRACT FROM AUTHOR]

We have proposed a comprehensive model based on a Cosserat curve to investigate the strain-driven self-rolling mechanism for anomalous coiling of multilayer nanohelices from the biaxial-torque point of view. Special attention has been paid to the edge effects, which dominate the anomalous coiling and decrease the helicity angles to as small as 0. By quantitatively explaining the experiments, we have demonstrated that the edge effects not only generate the torsion torque, together with the initial stress perpendicular to the preferential winding direction, to turn the nanobelts, but also offset part of the initial uniaxial bent torque. Our derived expressions can be used experimentally to determine the geometry of multilayer nanohelices with all helicity angles. The present work has provided useful information for the future experimental investigation on multilayer nanohelices as well as their application in meta-/quantum-devices. [ABSTRACT FROM AUTHOR]

Micro-Raman mapping with a spatial resolution of micrometer and room-temperature visible photoluminescence (PL) has been carried out on phosphorus-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by plasma enhanced chemical vapor deposition. Both the thin film uniformity and structural properties, i.e., the distribution of grain sizes and crystalline volume fraction, with different doping concentrations are revealed and physically interpreted based on the growth mechanism. The observed uniformity has been further confirmed by the PL measurements, and the bandgap engineering of the nc-Si:H thin films has been discussed from the dependence of PL peak energy on doping concentration. These results provide implications to realize control of properties of nc-Si:H by P-doping under optimized growth conditions for solar cell application. [ABSTRACT FROM AUTHOR]

Measurements of the temperature dependence of transmission have been carried out on AlInN thin films grown by reactive radio-frequency magnetron sputtering on (0001) sapphire substrates. By following a detailed procedure developed for analyzing the transmission spectra, we obtained more reliable data for the effects of temperature on the optical properties of AlInN. Two sets of temperature and photon-energy dependence of empirical formulas have been established for the absorption coefficient, bandgap, Urbach bandtail, and refractive index, which not only unify various experimental data reported in the literature but also provide a database of the optical properties of AlInN based on experimental results. It was found that the shift in the temperature dependence of the bandgap increases with the Al content and that the Urbach bandtail parameter is closely related to the structural characteristics of the AlInN thin films. These optical properties provide an experimental basis for further theoretical investigation and the design of AlInN-based devices. [ABSTRACT FROM AUTHOR]

The n-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR) detector is investigated to improve the quantum efficiency by simulation. The main structure, the bottom mirror, and the top mirror for the resonant cavity enhanced detector are optimized step by step. Two designs of the bottom mirror are suggested and compared. One consists of a bottom contact layer and one period of undoped/doped GaAs layers; the other is composed of a bottom contact layer, an undoped GaAs layer, and a gold layer. The results show that both mirrors enhance the quantum efficiency significantly and the gold mirror seems to be a better choice if not considering the technical difficulty. Preliminary study of the top mirror is also conducted. The top mirror shows a satisfactory effect under the condition that the reflectivity of the bottom mirror is high enough. The resulting quantum efficiency can be as high as 29.0% theoretically, which is much higher than p-GaAs and Si HIWIP FIR that was ever reported. [ABSTRACT FROM AUTHOR]

We have demonstrated the effective Raman spectroscopy for the phase transition and phonon dynamics of NiPdSi films annealed at various temperatures. In addition to the appearance of alloy disorder-induced Raman peaks, we have shown that the presence of Pd in NiSi results not only in the retardation of NiSi2 phase transition to about 900 °C, but also in the redshift (∼3–4 cm-1) of the first-order Raman peaks. The observed Raman frequency downshift with increasing temperature can be described well by a model which has taken into account the contributions of the anharmonic, thermal expansion, and strain effects, revealing the dominant role of the phonon decay. Effects of Pd addition and annealing temperature on the Raman frequency and linewidth have been discussed. The introduction of Pd is found to reduce the Raman frequency downshift, demonstrating the favorable use of NiPdSi for microelectronics applications. [ABSTRACT FROM AUTHOR]

We report on the photocurrent response of hydrogenated nanocrystalline silicon (nc-Si:H) thin films under external bias voltages. The band gap transition and internal photoemission photocurrent of the nc-Si:H thin films can be enhanced and controlled by adjusting the depletion and inversion layers in the metal-semiconductor junction through the external bias voltage. The photocurrent response from the internal photoemission is found to be able to extend the photodetection wavelength of the Si material to the optical telecommunication range of 1.3–1.6 μm. [ABSTRACT FROM AUTHOR]

Based on the experimental responsivity of single-period GaN/AlGaN midinfrared (MIR) heterojunction interfacial work-function internal photoemission (HEIWIP) detectors, we have optimized in detail the single- and multiperiod GaN/AlGaN MIR HEIWIPs to enhance the light absorption in the detectors. The yielded parameters of emitter, intrinsic, and bottom contact layers could achieve large improvement of the MIR peak absorption efficiency in the multiperiod GaN/AlGaN MIR HEIWIPs. Employing the concept of photon frequency upconversion, we have further investigated the realization of MIR pixel-less upconversion imaging through the integration of GaN/AlGaN MIR HEIWIP with a GaN/AlGaN violet light-emitting diode (LED). Under the optimized device structures, good imaging characteristics and high upconversion efficiency could be expected in the GaN/AlGaN HEIWIP-LED MIR upconverter. [ABSTRACT FROM AUTHOR]

We report on the photocurrent properties of the hydrogenated nanocrystalline silicon (nc-Si:H) thin film/crystalline silicon (c-Si) n-p heterostructure. By comparison with the c-Si n-p homojunction, two Gaussian-type photocurrent peaks are observed in the nc-Si:H/c-Si heterostructure and attributed to be transitions from a tail band or discrete levels in quantum dots with localized states, and a miniband with extended states associated with the embedded nanometer crystallites in the amorphous boundaries of the nc-Si:H thin film. The observed strong photocurrent signals and temperature dependency have revealed the unique electronic states of the miniband in the nc-Si:H thin film. Our investigations into the photocurrent properties may help to realize nc-Si:H/c-Si heterostructure-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

We have studied the electrical properties of hydrogenated nanocrystalline silicon/crystalline silicon heterojunction diode, focusing on the band offsets and electron transport mechanisms. Capacitance-voltage (C-V) analysis reveals that the band discontinuity mainly exists on the valence-band side, and an interface charge density on the order of 1011 cm-2 is estimated via the numerical C-V matching technique. Temperature- and bias-dependent transport mechanisms have been clarified by dark current-voltage-temperature measurements, and the extracted parameters indicate a transition from nontunneling to tunneling dominant transport from 350 to 20 K. [ABSTRACT FROM AUTHOR]

We report on the optoelectronic properties of the hydrogenated nanocrystalline silicon (nc-Si:H) thin film containing large density of nanometer grains and voids. By comparison with the bulk silicon, strong optical absorption and high photocurrent are found in the nc-Si:H thin film and attributed to the enhancement of the optical absorption cross section and good carrier conductivity in the nanometer grains and voids. The observed strong photocurrent signals can be well described by the extended diffusion-recombination model. The high photocurrent response may facilitate the fabrication of infrared photodetector by a single layer of nc-Si:H thin film on a glass substrate, which shows superiority to the traditional amorphous Si photodetector with a diode or Schottky-barrier structure constructed by multilayer films on the crystalline Si substrate. [ABSTRACT FROM AUTHOR]

We report on the optical properties and local bonding configurations of both as-deposited and postannealed hydrogenated amorphous silicon nitride (a-SiNx:H) thin films grown on crystalline Si substrates with x approximately 1.2±0.1. Ultraviolet optical reflection and infrared (IR) absorption measurements were applied to characterize the films. A method simply based on optical reflection spectra is proposed for accurate determination of the optical band gap, band tail, wavelength-dependent refractive index and extinction coefficient, as well as the film thickness, suggesting that the Tauc-Lorentz [G. E. Jellison, Jr. and F. A. Modine, Appl. Phys. Lett. 69, 371 (1996); 69, 2137 (1996)] model with the inclusion of Urbach tail is the optimal one to describe the optical response of a-SiNx:H films. The yielded optical parameters can be related well to the film microstructure as revealed by the IR absorption analysis. These results have implications for future deposition controlling and device applications. [ABSTRACT FROM AUTHOR]