Angular selective thin film glazing

Angular selective control of daylight, solar heat gain and visual performance is the aim of a variety of emerging technologies for windows, roof and wall glazing. Certain oblique thin metal and metal/insulator films on glass have transmittance as a unique function of direction of incidence and they are also spectrally selective. A variety of angular selective control options result according to choice of film materials covering one or a combination of (i) light and glare from high angles (ii) solar heat gain (iii) visual amenity (iv) glare from lower angles (iv) emittance control. They are thus able to be adapted by materials choice to the needs of different latitudes and window orientations. A new and simple way of categorizing performance will be presented.     

Hybrid equivalent model algorithm for the prediction of glazing angular properties

Daylighting and energy consumption evaluation for actual rooms requires knowledge of the luminous and energetic parameters of the fenestration, both at normal and off normal incidence. In the case of coated glazings, the off normal parameters can be achieved only with very complex measurements. The Hybrid Equivalent Model Algorithm allows the prediction of the fenestration characteristics solely on the basis of the transmittance and reflectance spectra, measured at normal incidence, without need of any information about the coating composition. In this paper, the algorithm effectiveness is tested on single, double and triple-pane glazings by applying it to a set of synthetic coated glasses, inspired by real specimens; the variety of the considered coatings makes this set quite representative of the industrial production. The results are very encouraging: for 19 samples, on the 22 considered here, the mean deviation of the g-factor from the true values is less than 1%; for the remaining 3 it is not larger than 3.7%, and probably it could be reduced by further refining of the hybrid model because these 3 samples belong to the same product class.     

Cermets for angular selective transmittance

Cermets have been reactively deposited at an oblique angle onto transparent glass substrates at ambient temperature by thermal and cathodic arc evaporation. They have been found to display angular selective transmittance that is in general better than that found in oblique columnar films which are predominantly metallic. The structure and optical properties of Al/Al₂O₃ and Ag/Al₂O₃ films are discussed including the angle of incidence dependence, over a 120° range, of the integrated solar and photoptic transmittance. The spectral characteristics of these films can vary continously with angle of incidence with large differences possible between spectral properties at the same angle of incidence on opposite sides of the normal. Results indicate that those surfaces can provide the basis for applications including automobile and building glazing. The cathodic arc samples have the advantage of being very durable, which is essential for such applications.     

Thin film amorphous silicon solar cells

Amorphous silicon-based materials have attracted considerable attention over the past few years mainly due to their potential as inexpensive photovoltaic devices. There has been a rapid progress in this field and conversion efficiency of up to 7.5 per cent has been reported. In this paper, we outline some of the criteria that are required for a thin-film device and consider how these materials can satisfy them.     

Thin film photodetectors grown epitaxially on silicon

Epitaxial germanium films of thickness d≈4 μm, grown on silicon (001) by a low-temperature MBE process, constituted the base material of a silicon-integrated infrared detector. Characterization of the fully relaxed films was performed mainly by ex situ techniques, such as X-ray diffraction, Rutherford backscattering spectrometry and channeling, room temperature Hall effect and defect etching. Mesa diodes, fabricated from the originally p-type Ge films after pn-junction formation by thermal diffusion of antimony (Sb), showed quantum yields above 40% at wavelengths between 1200 nm and 1500 nm without an antireflection coating. The rise time of the photodiode signal in response to a picosecond laser pulse (τ = 300 ps) at wavelength of 1300 nm was 530 ps. Forward current-voltage characteristics of the devices were described by an ideality factor n = 1.25, while excess current under reverse bias was attributed to leakage caused by threading dislocations in the active layers.     

Thin film module fabrication and reliability advances

This paper presents the Chronar experience of deploying amorphous silicon (a-Si) modules in the field and the resulting efforts to understand and correct observed degradation mechanisms. The data were obtained from several on-site observations, and evaluation and analysis of corrosion effects in a-Si modules. These data are correlated with laboratory tests, and a design for a new a-Si module is proposed.     

Thin film PV modules for low-concentrating systems

The high cost of photovoltaic (PV) energy has imposed extensive research efforts in order to provide alternatives to the conventional crystalline silicon (c-Si) PV technology. Thin film PV modules based on Cu(In,Ga)Se₂ (CIGS) is considered one of the most promising alternatives for mass production of low-cost PV. In parallel to the development of new module technologies, there is an increasing interest for using concentrating optics in PV systems in order to increase radiation onto the modules. By replacing the relatively expensive PV absorbers with low-cost concentrators there is a potential reduction of overall system costs. The reflector types considered in this study are based on the compound parabolic concentrator (CPC) and the planar reflector. These are low-concentrating devices with concentration ratios of 1–4. With the CPC as well as the planar reflector, the illumination on the PV module will be non-uniform, with local light intensities that are considerably larger than the average 4 suns. For conventional c-Si modules, this is detrimental to module performance. It is demonstrated in the present work that modules based on thin film technology are better candidates for reflector applications. The principles of design and fabrication of CIGS thin film PV modules for low-concentrating systems are discussed, and experimental results from measurements of CIGS modules under concentrated illumination are evaluated.     

A demonstration site-built solar air collector using selective surface glazing, Boston, Massachusetts

A 450 ft ‘site-built’ air collector was completed in February 1981 in Boston, Massachusetss. The collector uses the Airco/Guardian ‘Passive Solar Glass’ with a selective surface coating on the inner lite of the collector's double glazing. This paper discusses the issues and theory that led to the building of the collector and estimates performance for the system. Preliminary data taken in late winter indicates that the system will perform seasonally as predicted.     

PCM glazing systems

This paper presents a different approach for thermal effective windows, i.e. windows that reduce the energy transmitted into or out of a room. The idea is to use a double sealed glass filled with a phase change medium (PCM) whose fusion temperature is determined by solar–thermal calculations. The PCM used is polypropylene glycol. The investigation includes modelling of the heat and radiation transfer through a composite window and optical investigation of conventional and PCM filled windows, testing of the window and comparison with numerical simulations. A one-dimensional model for the composite glass window is developed to predict the thermal performance as a function of the geometrical parameters of the panel and the PCM used. Optical measurements were realized using photo-spectrometry to determine the transmittance, reflectance and absorptance. The specimens used include single glass of different thicknesses, double glass of different gap spacing and thicknesses filled with air or PCM, and finally coloured PCM. The results indicate big reductions in the energy transmitted, specially in the infra-red and ultraviolet regions, while maintaining a good visibility. © 1997 by John Wiley & Sons, Ltd.     

Durability of electrochromic glazing

Fundamental properties of all-solid-state electrochromic windows to control the solar energy have been investigated. This system comprises of multilayer represented as Glass/ITO/NiO/Inorganic Electrolyte (Ta₂O₅, etc.)/ WO₃/ITO/ Adhesive Film/ Glass. Of the various electrochromic systems examined so far, the most important features are their environmental stability and the possibility of large area applications. Our system can control the visible transmittance between 72.6% and 17.6% and has a cyclic life over 100000 cycles at 60°C. Based on the accelerated weathering tests, the stability of the system is estimated to be over five years for outdoor applications. For the problem of scaling up, some technical aspect is given and the prototype window of size 40×60 cm is exemplified. The present system could be more suitable for architectural and automobile applications in the near future by developing production technology.     

Thin film polycrystalline silicon solar cells on mullite ceramics

In this work, we present the structural quality of polycrystalline silicon films formed by high-temperature chemical vapor deposition (CVD) on mullite ceramics coated with spin-on flowable oxides (FO_x) serving as intermediate layers (ILs). The average grain size and the size distribution were investigated by optical microscopy. It is found that more than 65% of the surface of polysilicon films grown on boron-doped FO_x is covered by large grains of 5–10 μm. The intra-grain and inner-grain defects as well as the grain orientation were analyzed with the electron backscattering diffraction (EBSD) technique. Twin-type defects such as Σ3 and Σ9 are frequently present in these silicon layers, which are slightly (1 1 0) preferentially oriented. Finally, we present the photovoltaic data on test solar cells made on these CVD polysilicon films. An efficiency of about 3.3% is reported. The limiting factors, as well as possible improvements, are discussed.     

Highly insulated window glazing

The paper is introduced by a short survey of heat-transfer processes through a double-glazed window system. Network calculations show the advantage of a double-glazed window including at least one heat-reflecting filter and an IR-trans-parent foil. In the optimum system with one foil, the U_L value is slightly below 1 W/m²K. Several insulating glazing systems with air and with krypton filling are compared, and all U_L values are calculated. the application of an IR-transparent foil to a solar collector is discussed, and the measured top losses are given.     

Thin film tandem solar cells based on CuInSe2

The CuInSe₂/(CdZn)S heterojunction is the best developed low bandgap solar cell for use in a two-junction tandem device. The potential performance of large area terrestrial systems based on this junction is reviewed. A monolithic tandem cell in which the high bandgap cell is a (CdHg) Te/CdS heterojunction deposited onto a CuInSe₂/(CdZn)S cell is being developed and progress with this system is described.     

Thin film silicon solar cells by AIC on foreign substrates

This paper presents the fabrication of thin film crystalline silicon solar cells on foreign substrates like alumina, glass–ceramic (GC) and metallic foils (ferritic steel—FS) using seed layer approach, which employs aluminium induced crystallisation (AIC) of amorphous silicon. Effect of hydrogen content in a-Si:H precursor films on the AIC process has been studied and the results showed that defects in the AIC grown films increased with increase of hydrogen content. At the optimal thermal annealing conditions, the AIC grown poly-Si films showed an average grain size of 7.6, 26, and 8.1 μm for the films synthesised on alumina, GC, and FS, respectively. The grains were (1 0 0) oriented with a sharp Raman peak around 520 cm^?1. Similarly, n-type seed layers were also fabricated by over-doping of as-grown AIC layers using a highly phosphorus doped glass solution. The resistivity of as-grown films reduced from 8.4×10^?2 Ω cm (p-type) to 4.1×10^?4 Ω cm (n-type) after phosphorus diffusion. These seed layers of n-type/p-type were thickened to form an absorber layer by vapour phase epitaxy or solid phase epitaxy. The passivation step was applied before the heterojunction formation, while it was after in the case of homojunction. Open circuit voltage of the junctions showed a strong dependence on the hydrogenation temperature and microwave (μW) power of electron cyclotron resonance (ECR) plasma of hydrogen. Effective passivation was achieved at a μW power of 650 W and hydrogenation temperature of 400 °C. Higher values of solar conversion efficiencies of 5% and 2.9% were achieved for the n-type and p-type heterojunction cells, respectively fabricated on alumina substrates. The analysis of the results and limiting factors are discussed in detail.     

Thin film silicon photovoltaics: Architectural perspectives and technological issues

Thin film photovoltaics is a particularly attractive technology for building integration. In this paper, we present our analysis on architectural issues and technological developments of thin film silicon photovoltaics. In particular, we focus on our activities related to transparent and conductive oxide (TCO) and thin film amorphous and microcrystalline silicon solar cells. The research on TCO films is mainly dedicated to large-area deposition of zinc oxide (ZnO) by low pressure-metallorganic chemical vapor deposition. ZnO material, with a low sheet resistance (<8 Ω/sq) and with an excellent transmittance (>82%) in the whole wavelength range of photovoltaic interest, has been obtained. “Micromorph” tandem devices, consisting of an amorphous silicon top cell and a microcrystalline silicon bottom cell, are fabricated by using the very high frequency plasma enhanced chemical vapor deposition technique. An initial efficiency of 11.1% (>10% stabilized) has been obtained.     

Thin film solid oxide fuel cells with copper cermet anodes

Thin film solid oxide fuel cells (SOFCs), composed of thin coatings of 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ) and thick substrates of (La_0.8Sr_0.2)_0.98MnO₃ (LSM)-YSZ cathodes, are fabricated using the conventional tape casting and tape lamination techniques. Densification of YSZ electrolyte thin films is achieved at 1275 °C by adjusting the cathode tape formulation and sintering characteristics. Two types of copper cermets, CuO-YSZ-ceria and CuO-SDC (Ce_0.85Sm_0.15O_1.925)-ceria, are compared in terms of the anodic performance in hydrogen and propane. Maximum power densities for hydrogen and propane at 800 °C are 0.26 W cm⁻² and 0.17 W cm⁻² for CuO-YSZ-ceria anodes and 0.35 W cm⁻² and 0.22 W cm⁻² for CuO-SDC-ceria anodes, respectively. Electrochemical impedance analysis suggests that CuO-SDC-ceria exhibits a much lower anodic polarization resistance than CuO-YSZ-ceria, which could be explained by the intrinsic mixed oxygen ionic and electronic conductivities for SDC in the reducing atmosphere.     

Thin film PV manufacturing: Materials costs and their optimization

Thin film PV technologies face a number of hurdles as they advance towards low-cost goals that would make them competitive with traditional sources of electricity. The US Department of Energy cost goal for thin films is about $0.33/W_p, which corresponds to module efficiencies of about 15% and module manufacturing costs of about $50/m². Past papers have provided a framework for examining thin film efficiencies and manufacturing costs, especially those costs for equipment, labor, materials, utilities, and others. Although materials costs appear to be a large fraction of the total, we have not yet broken them down in enough detail to seek significant improvement. In the future, with more mature thin film production, materials costs such as those from semiconductor layers, contacts, pottants, substrates, and electrical interconnection will dominate total module cost. This paper (1) breaks down the materials costs into two broad categories (active and inactive materials) and then (2) investigates the issues associated with reducing their costs much below today's levels. Materials will likely be such an overwhelming cost-driver for mature manufacturing of thin film PV that issues associated with their optimization should be examined as soon as possible in order to meet the DOE long-term goals for PV module costs.     

Thin film CdTe-CdS heterojunction solar cells on lightweight metal substrates

An unconventional thin film CdTe-CdS solar cell device configuration, in which the substrate is molybdenum foil and CdS is deposited on top of CdTe, was developed. A conducting contact between Mo and CdTe was achieved by using thin interlayers. The CdTe–CdS junction process development led to an open circuit voltage of 824 mV. CdCl₂ treatment followed by oxygen annealing greatly improved solar cell performance. An initial study of capacitance–voltage data has shown a depletion layer width of 4.62 μm into CdTe in the dark. Cell efficiency of 5.3% was achieved. Higher cell efficiency is now being sought by improving the top contact to CdS and lowering series resistance.