基于Berreman矩阵的双折射薄膜光谱响应特性计算

采用Berreman特征矩阵方法，通过数值计算研究了双折射薄膜的反射、透射等光谱响应特性。依据电磁场理论的电场分量、磁场分量的界面连续条件，推导了光波在各向异性双轴薄膜中的Berreman转移矩阵，用以分析含有各向异性介质层的复杂薄膜系统的光学性质。这些矩阵递推关系包含了界面处的多点反射，适用于一般的各向异性的多层膜系统，包括入射媒质或基底为各向异性的情况。在文中给出了各向同性入射媒质双轴各向异性膜层一各向同性基底薄膜系统的计算结果，验证了该计算方法的可行性，以此作为进一步研究各向异性薄膜和相关光学薄膜器

Effect of urban climate on building integrated photovoltaics performance

It is generally recognized that BIPV (building integrated photovoltaics) has the potential to become a major source of renewable energy in the urban environment. The actual output of a PV module in the field is a function of orientation, total irradiance, spectral irradiance, wind speed, air temperature, soiling and various system-related losses. In urban areas, the attenuation of solar radiation due to air pollution is obvious, and the solar spectral content subsequently changes. The urban air temperature is higher than that in the surrounding countryside, and the wind speed in urban areas is usually less than that in rural areas. Three different models of PV power are used to investigate the effect of urban climate on PV performance. The results show that the dimming of solar radiation in the urban environment is the main reason for the decrease of PV module output using the climatic data of urban and rural sites in Mexico City for year 2003. The urban PV conversion efficiency is higher than that of the rural PV system because the PV module temperature in the urban areas is slightly lower than that in the rural areas in the case. The DC power output of PV seems to be underestimated if the spectral response of PV in the urban environment is not taken into account based on the urban hourly meteorological data of Sao Paulo for year 2004. © 2006 Elsevier Ltd. All rights reserved.

Interactions between Building Integrated Photovoltaics and microclimate in urban environments

BIPV(Building Integrated Photovoltaics) has progressed in the past years and become an element to be considered in city planning. BIPV has influence on microclimate in urban environments and the performance of BIPV is also affected by urban climate. The effect of BIPV on urban microclimate can be summarized under the following four aspects. The change of absorptivity and emissivity from original building surface to PV will change urban radiation balance. After installation of PV, building cooling load will be reduced because of PV shading effect, so urban anthropogenic heat also decreases to some extent. Because PV can reduce carbon dioxide emissions which is one of the reasons for urban heat island, BIPV is useful to mitigate this phenomena. The anthropogenic heat will alter after using BIPV, because partial replacement of fossil fuel means to change sensible heat from fossil fuel to solar energy. Different urban microclimate may have various effects on BIPV performance that can be analyzed from two perspectives. Firstly, BIPV performance may decline with the increase of air temperature in densely built areas because many factors in urban areas cause higher temperature than that of the surrounding countryside. Secondly, the change of solar irradiance at the ground level under urban air pollution will lead to the variation of BIPV performance because total solar irradiance usually is reduced and each solar cell has a different spectral response characteristic. The thermal model and performance model of ventilated BIPV according to actual meteorologic data in Tianjin(China) are combined to predict PV temperature and power output in the city of Tianjin. Then, using dynamic building energy model, cooling load is calculated after BIPV installation. The calculation made based in Tianjin shows that it is necessary to pay attention to and further analyze interaction between them to decrease urban pollution, improve BIPV Performance and reduce colling load. Copyright © 2005 by ASME.

Stability and photoemission characteristics for GaAs photocathodes in a demountable vacuum system

The stability and photoemission characteristics for reflection-mode GaAs photocathodes in a demountable vacuum system have been investigated by using spectral response and x-ray photoelectron spectroscopy measurements at room temperature. We find that the shape of the spectral response curve for the cathode changes with time in the vacuum system, but after applying fresh cesium to the degraded cathode, the spectral response can almost be restored. The change and restoration of curve shape are mainly attributed to the evolution of the surface barrier. We illustrate the evolution and analyze the influence of the barrier on the spectral response of the cathode. (C) 2008 American Institute of Physics.

SHORT PERIOD InAs/GaSb SUPERLATTICE INFRARED DETECTOR ON GaAs SUBSTRATES

Two type II superlattices (SLs) InAs(2ML)/GaSb(8ML) and InAs(8ML)/GaSb(8ML) were grown on GaAs substrates by molecular-beam epitaxy. High resolution X-ray diffraction showed the periods of the two SLs were 31.2 angstrom and 57.3 angstrom, respectively. Room-temperature optical transmittance spectra showed that there were clear absorption edges at 2.1 mu m and 5 mu m for the two SLs. The SWIR and MWIR photoconductor devices were fabricated by standard lithography and etched by tartaric acid solution. The spectral response and blackbody tests were carried out at low and room temperatues. The results show that the 50% cutoff wavelengths of the two photoconductors are 2.1 mu m and 5.0 mu m respectively and D-bb* is above 2 x 10(8) cmHz(1/2)/W for two kinds of photoconductors at 77K. D-bb* is above 10(8) cmHz(1/2)/W for SWIR photoconductor at room temperature.

Simulation of light coupling enhancement and localization of transmission field via subwavelength metallic gratings

Surface plasmons(SPs) generated in nano metallic gratings on medium layer can greatly enhance the transmission field through the metallic gratings. The enhancement effect is achieved from lambda = 500 nm to near-infrared domain. The enhancement rate is about 110 % at the wavelength of about 6 10 nm and about 180 % at lambda = 700 nm and 740 nm where most kinds of thin film solar cells have a high spectral response. These structures should provide a promising way to increase the coupling efficiency of thin film solar cells and optical detectors of different wavelength response.

The diphasic nc-Si/a-Si : H thin film with improved medium-range order

A series of silicon film samples were prepared by plasma enhanced chemical vapor deposition (PECVD) near the threshold from amorphous to nanocrystalline state by adjusting the plasma parameters and properly increasing the reactions between the hydrogen plasma and the growing surface. The microstucture of the films was studied by micro-Raman and Fourier transform infrared (FTIR) spectroscopy. The influences of the hydrogen dilution ratio of silane (R-H = [H-2]/[SiH4]) and the substrate temperature (T-s) on the microstructural and photoelectronic properties of silicon films were investigated in detail. With the increase of RH from 10 to 100, a notable improvement in the medium-range order (MRO) of the films was observed, and then the phase transition from amorphous to nanocrystalline phase occurred, which lead to the formation of diatomic hydrogen complex, H-2* and their congeries. With the increase of T-s from 150 to 275 degreesC, both the short-range order and the medium range order of the silicon films are obviously improved. The photoconductivity spectra and the light induced changes of the films show that the diphasic nc-Si/a-Si:H films with fine medium-range order present a broader light spectral response range in the longer wavelength and a lower degradation upon illumination than conventional a-Si:H films. (C) 2004 Elsevier B.V. All rights reserved.

Characterization of polymorphous silicon thin film and solar cells

Polymorphous silicon (pm-Si:H) films have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the pm-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. There are a blue shift for the stretching mode of IR spectra and a red shift for the wagging mode. The shifts are attributed to the variation of the microstructure. By using pm-Si:H film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100mw/cm(2)) at room temperature (T-R).

Computer simulation of a-Si : H/mu c-Si : H diphasic silicon solar cells

Based on our experimental research on diphasic silicon films, the parameters such as absorption coefficient, mobility lifetime product and bandgap were estimated by means of effective-medium theory. And then computer simulation of a-Si: H/mu c-Si: H diphasic thin film solar cells was performed. It was shown that the more crystalline fraction in the diphasic silicon films, the higher short circuit density, the lower open-circuit voltage and the lower efficiency. From the spectral response, we can see that the response in long wave region was improved significantly with increasing crystalline fraction in the silicon films. Taking Lambertian back refraction into account, the diphasic silicon films with 40%-50% crystalline fraction was considered to be the best intrinsic layer for the bottom solar cell in micromorph tandem.

Characterization of silicon carbide thin films and their use in colour sensor

A series of hydrogenated amorphous silicon carbide (a-Si1-xCx:H) films were prepared by plasma-enhanced chemical vapour deposition (PECVD) using a gas mixture of silane, methane, and hydrogen as the reactive source. The previous results show that a high excitation frequency, together with a high hydrogen dilution ratio of the reactive gases, allow an easier incorporation of the carbon atoms into the silicon-rich a-Si1-xCx:H film, widen the valence controllability. The data show that films with optical gaps ranging from about 1.9 to 3.6 eV could be produced. In this work the influence of the hydrogen dilution ratio of the reactive gases on the a-Si1-xCx:H film properties was investigated. The microstuctural and photoelectronic properties of the silicon carbide films were characterized by Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERDA), and FT-IR spectrometry. The results show that a higher hydrogen dilution ratio enhances the incorporation of silicon atoms in the amorphous carbon matrix for carbon-rich a-Si1-xCx:H films. One pin structure was prepared by using the a-Si1-xCx:H film as the intrinsic layer. The light spectral response shows that this structure fits the requirement for the top junction of colour sensor. (c) 2004 Elsevier B.V. All rights reserved.

Comparison between gradient-doping GaAs photocathode and uniform-doping GaAs photocathode

We compared two reflection-mode negative electron affinity (NEA) GaAs photocathode samples that are grown by molecular beam epitaxy with p-type beryllium doping. One sample is uniform doping, and another is gradient doping. Experimental curves of spectral response sensitivity and quantum efficiency are obtained. The thicknesses of the two cathodes are both 2.6 mu m. The integrated sensitivity of the uniform doping one is 1966 mu A/lm, and that of the gradient-doping one is 2421 mu A/lm. The escape probability and diffusion length are fitted from the spectral response curves. For the uniform-doping sample, the escape probability is 0.45 and the diffusion length is 5 mu m. For the gradient-doping sample, the escape probability is 0.55 and the diffusion length is 5.5 mu m. (c) 2007 Optical Society of America.

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (sigma(ph)), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51 % and a stabilized efficiency of 8.01% (AM 1.5, 100 mw/cm(2)) at room temperature. (c) 2006 Published by Elsevier B.V.

The effect of Be-doping structure in negative electron affinity GaAs photocathodes on integrated photosensitivity

A new structure of GaAs photocathode was introduced. The Be-doping concentration is variable in the new structure compared with the constant concentration of Be in the normal photocathode. Negative electron affinity GaAs photocathodes were fabricated by alternate input of Cs and O. The spectral response results measured by the on-line spectral response measurement system show that the integrated photosensitivity of the photocathodes with the new structure is enhanced by at least 50% as compared to those with the monolayer structure. Accordingly, two main factors leading to the enhanced photosensitivity of the photocathodes were discussed. (c) 2005 Elsevier B.V. All rights reserved.

Numerical simulation of nc-Si : H/c-Si heterojunction solar cells

AMPS simulator, which was developed by Pennsylvania State University, has been used to simulate photovoltaic performances of nc-Si:H/c-Si solar cells. It is shown that interface states are essential factors prominently influencing open circuit voltages (V-OC) and fill factors (FF) of these structured solar cells. Short circuit current density (J(SC)) or spectral response seems more sensitive to the thickness of intrinsic a-Si:H buffer layers inserted into n(+)-nc-Si:H layer and p-c-Si substrates. Impacts of bandgap offset on solar cell performances have also been analyzed. As DeltaE(C) increases, degradation of VOC and FF owing to interface states are dramatically recovered. This implies that the interface state cannot merely be regarded as carrier recombination centres, and impacts of interfacial layer on devices need further investigation. Theoretical maximum efficiency of up to 31.17% (AM1.5,100mW/cm(2), 0.40-1.1mum) has been obtained with BSF structure, idealized light-trapping effect(R-F=0, R-B=1) and no interface states.

Fabrication and characterization of metal-semiconductor-metal (MSM) ultraviolet photodetectors on undoped GaN/sapphire grown by MBE

Interdigital metal-semiconductor-metal (MSM) ultraviolet photoconductive detectors have been fabricated on undoped GaN films grown by molecular beam epitaxy (MBE), Response dependence on wavelength, applied current, excitation powers and chopper frequency has been extensively investigated. It is shown that the photodetector's spectral response remained nearly constant for wavelengths above the band gap and dropped sharply by almost three orders of magnitude for wavelengths longer than the band gap. It increases linearly with the applied constant current, but very nonlinearly with illuminating power. The photodetectors showed high photoconductor gains resulting from trapping of minority carriers (holes) at acceptor impurities or defects. The results demonstrated the high quality of the GaN crystal used to fabricate these devices. (C) 2000 Elsevier Science B.V. All rights reserved.