Infrared absorption in Li-intercalated tungsten oxide

Thin films of amorphous and polycrystalline tungsten oxide were produced by reactive dc magnetron sputtering and nanocrystalline films were deposited by advanced gas evaporation. The films were submitted to electrochemical intercalation of Li ions before infrared reflectance measurements were carried out. For crystalline films, the reflectance in the wavelength region 10–30 μm increases upon intercalation, indicating an increasing free-electron contribution. On the other hand, all the films display an increased absorption at wavelengths less than 10 μm when intercalated. The thermal emittance could be varied from about 0.5 to 0.7–0.75 by intercalation in films with thicknesses in excess of 1 μm. Both absorption and interference contribute to the emittance contrast.     

Commercial white paint as back surface reflector for thin-film solar cells

In this work, commercially available white paint is applied as a pigmented diffuse reflector (PDR) on the rear surface of thin-film crystalline silicon (c-Si) solar cells with a silicon thickness in the 1–2 μm range. We show that white paint increases the short-circuit current density of the solar cells enormously, with a boost of 41% observed for very thin planar solar cells illuminated with the global AM1.5 solar spectrum. We also show that white paint is a better back surface reflector (BSR) than aluminium, air, a transparent conductive oxide (TCO)/aluminium stack, and even a detached aluminium mirror. While previous studies have investigated the influence of PDRs on silicon solar cells with thicknesses of over 27 μm, this work closes the gap that has existed for much thinner cells.     

Low-temperature growth of thick intrinsic and ultrathin phosphorous or boron-doped microcrystalline silicon films: Optimum crystalline fractions for solar cell applications

The growth kinetics and optoelectronic properties of intrinsic and doped microcrystalline silicon (μc-Si:H) films deposited at low temperature have been studied combining in situ and ex situ techniques. High deposition rates and preferential crystallographic orientation for undoped films are obtained at high pressure. X-ray and Raman measurements indicate that for fixed plasma conditions the size of the crystallites decreases with the deposition temperature. Kinetic ellipsometry measurements performed during the growth of p-(μc-Si:H) on transparent conducting oxide substrates display a remarkable stability of zinc oxide, while tin oxide is reduced at 200°C but stable at 150°C. In situ ellipsometry, conductivity and Kelvin probe measurements show that there is an optimum crystalline fraction for both phosphorous- and boron-doped layers. Moreover, the incorporation of p-(μc-Si:H) layers produced at 150°C in μc-Si:H solar cells shows that the higher the crystalline fraction of the p-layer the better the performance of the solar cell. On the contrary, the optimum crystalline fraction of the p-layer is around 30% when hydrogenated amorphous silicon (a-Si:H) is used as the intrinsic layer of p–i–n solar cells. This is supported by in situ Kelvin probe measurements which show a saturation in the contact potential of the doped layers just above the percolation threshold. In situ Kelvin probe measurements also reveal that the screening length in μc-Si:H is much higher than in a-Si:H, in good agreement with the good collection of microcrystalline solar cells     

Solar cells from thin silicon layers on Al2O3

The surface of the applied Al₂O₃ ceramic substrate consists of small crystals with a maximum grain size of 5 μm. A 40 μm thick layer deposited on this surface shows polycrystalline quality and grain sizes in the order of 10 μm. Silicon layers of different thickness and doping have been deposited on Al₂O₃ substrates to make thin film silicon solar cells. These structures have been processed to solar cells in a two mask process. From measurements of the spectral response a diffusion length of 8 μm can be extracted.     

Plasma-enhanced CVD silicon nitride antireflection coatings for solar cells

Multilayer plasma-enhanced chemical vapor deposition (PECVD) silicon nitride antireflection coatings were deposited on space quality silicon solar cells. Preliminary experiments indicated that multilayer coatings decreased the total reflectance of polished silicon from 35 per cent to less than 3 per cent over the spectral range 0.4–1.0 μm. The solar cell energy conversion efficiency was increased from an average of 8.84 per cent to an average of 12.63 per cent.     

A comparative study of the optical properties of nickel pigmented alumina films of different thicknesses exposed to elevated temperature and humidity

Commercial solar absorbers of nickel pigmented anodized aluminium are composed of an inner nickel pigmented sublayer of about 0.3 μm thickness and a 0.4–0.5 μm thick top layer of plain alumina. Thermal emittance can be reduced from 0.17 to 0.12 if the top layer is made thinner, to be about 0.1 μm. The solar absorptance is 0.96 as for the thicker coating. In this study degradation is analysed for samples with thin or thick alumina top layer after exposure to elevated temperature, 300–500°C, or humidity. The results from these tests show that a thinner aluminium oxide top layer has the same durability as a thicker top layer. The implication of making commercial nickel pigmented anodized aluminium with an oxide half as thick as today is a reduction of the anodization time to about half the time and lower manufacturing costs.     

Influence of sputtering conditions on the solar and luminous optical properties of amorphous LixWoy thin films

Thin films of amorphous tungsten oxide were deposited by sputtering onto glass substrates coated by conductive indium–tin oxide. The films were sputtered at different oxygen-to-argon flow ratios with different pressure and power. Elastic recoil detection analysis determined the density and the stoichiometry. X-ray diffraction measurements showed that the films were amorphous. The films were electrochemically intercalated with lithium ions. At several intercalation levels of each film, the optical reflectance and transmittance were measured in the wavelength range 0.3–2.5 μm. We study the effect of various sputtering conditions on the coloration efficiency of the films and on the luminous and solar optical properties. The O₂/Ar ratio and the sputter pressure determine to a large extent the optical absorption. As-deposited sputtered tungsten oxide with sufficiently little oxygen exhibits an absorption peak similar to the case of lithium intercalation.     

CdSe thin films as solar control coatings

CdSe thin films deposited by a physical vapour deposition method were investigated as solar control coatings on architectural glazings. The optical transmittance and the near-normal specular reflectance in the range 0.40−2.40 μm and spectral distribution of reflected and transmitted intesities in the same range showed that CdSe thin films have solar control characteristics comparable to commercially available metallic coatings and other materials such as PbS and Cu_ξS films. The solar control characteristics of CdSe films were found to be dependent on film parameters, including deposition rate and deposition temperature.     

High-efficiency drift-field thin-film silicon solar cells grown on electronically inactive substrates

A drift-field in the base region of a solar cell can enhance the effective minority-carrier diffusion length, thus increasing the long-wavelength spectral response and energy-conversion efficiency. Silicon thin-films of 20–32 μm thickness as a cell base layer were grown by liquid-phase epitaxy (LPE) on electronically inactive heavily doped p⁺⁺-type CZ silicon substrates. Growth was performed from In/Ga solutions, and in a purified Ar/4%H₂ forming gas ambient, rather than pure H₂. The Ga dopant concentration was tailored throughout the p-type film to create a drift-field in the base layer of the solar cell. An independently confirmed efficiency of 16.4% was achieved on such an LPE drift-field thin-film silicon solar cell with a total cell area of 4.11 cm². Substrate thinning, combined with light trapping which is encouraged by the textured front surface and a highly reflective aluminium rear surface, is demonstrated to improve the long-wavelength response and thus, increase cell efficiency by a factor of up to 23.7% when thinned to a total cell thickness of 30 μm.     

The influence of porous silicon coating on silicon solar cells with different emitter thicknesses

The influence of the emitter thickness on the photovoltaic properties of monocrystalline silicon solar cells with porous silicon was investigated. The measurements were carried out on n⁺p silicon junction whose emitter depth was varied between 0.5 and 2.2 μm. A thin porous silicon layer (PSL), less than 100 nm, was formed on the n⁺ emitter. The electrical properties of the samples with PS were improved with decrease of the n⁺p junction depth. Our results demonstrate short-circuit current values of about 35–37 mA/cm² using n⁺ region with 0.5 μm depth. The observed increase of the short-circuit current for samples with PS and thin emitter could be explained not only by the reduction of the reflection loss and surface recombination but also by the additional photogenerated carriers within the PSL. This assumption was confirmed by numerical modeling. The spectral response measurements were performed at a wavelength range of 0.4–1.1 μm. The relative spectral response showed a significant increase in the quantum efficiency of shorter wavelengths of 400–500 nm as a result of the PS coating. The obtained results point out that it would be possible to prepare a solar cell with 19–20% efficiency by the proposed simple technology.     

Treatment of antireflection on tin oxide coated anodized aluminum selective absorber surface

Nickel-pigmented anodic aluminum oxide, Ni–Al₂O₃ was pyrolytically coated with tin oxide (SnO₂). The undesirable increase of reflectance in the solar spectrum due to the high refractive index of the SnO₂ film was compensated by an antireflection layer. The layer was applied by a simple dipping technique in a bath containing a commercial colloidal silica sol which forms a silica (SiO₂) layer. The infrared reflectance is nearly unaffected by the silica sol treatment process. Preliminary test results indicate that treated samples are resistant to temperatures as high as 300°C as well as to corrosion in an 8% sulfuric acid solution. In addition, the optical properties were unaffected by outdoor exposure for two months.     

Enhancement of optical absorption for below 5 μm thin-film poly-Si solar cell on glass substrate

Optical confinement effect of thin-film polycrystalline-Si (poly-Si) solar cell on glass substrate fabricated at low-temperature has been investigated as a function of cell thickness of less than 5 μm. We found that it is possible to fabricate the textured Si thin film in situ on a glass substrate and that the reflectance at long-wavelength light is reduced by surface texturing. Thin-film poly-Si solar cell and a-Si:H/(0.45 μm)/poly-Si (5 μm) tandem solar cell exhibit the efficiency of 8.6% and 12.8%, respectively. The numerical study in terms of the light trapping explains the excellent high short-circuit current density (_sc above 27 mA/cm² at the 4.7 μm thin-film poly-Si solar cell.     

Microstructured extremely thin absorber solar cells

In this paper we present the realization of extremely thin absorber (ETA) solar cells employing conductive glass substrates functionalized with TiO₂ microstructures produced by embossing. Nanocrystalline or compact TiO₂ films on Indium doped tin oxide (ITO) glass substrates were embossed by pressing a silicon stamp containing a μm size raised grid structure into the TiO₂ by use of a hydraulic press (1 ton/50 cm²). The performance of these microstructured substrates in a ETA cell sensitized by a thermally evaporated or chemical bath deposited PbS film and completed by a PEDOT:PSS hole conductor layer and a Au counter electrode is compared to that of planar substrates. Surprisingly planar films produced better performance than micro-structured films. A simple model implying photoconductive shunting paths revealed by junction breakdown at negative bias under illumination is presented.     

Electrodeposited cuprous oxide on indium tin oxide for solar applications

Semiconducting cuprous oxide films were prepared by electrodeposition onto commercial conducting glass coated with indium tin oxide deposited by spraying technique. The cuprous oxide (Cu₂O) films were deposited using a galvanostatic method from an alkaline CuSO₄ bath containing lactic acid and sodium hydroxide at a temperature of 60°C. The film's thickness was about 4–6 μm. This paper includes discussion for Cu₂O films fabrication, scanning electron microscopy and X-ray diffractometry studies, optical properties and experimental results of solar cells. The values of the open circuit voltage V_oc of 340 mV and the short circuit current density I_sc of 245 μA/cm² for ITO/Cu₂O solar cell were obtained by depositing graphite paste on the rear of the Cu₂O layer. It should be stressed that these cells exhibited photovoltaic properties after heat treatment of the films for 3 h at 130°C. An electrodeposited layer of Cu₂O offers wider possibilites for application and production of low cost cells, both in metal–semiconductor and hetero-junction cell structures, hence the need to improve the photovoltaic properties of the cells.     

Chemically deposited CdS by an ammonia-free process for solar cells window layers

Chemically deposited CdS window layers were studied on two different transparent conductive substrates, namely indium tin oxide (ITO) and fluorine doped tin oxide (FTO), to determine the influence of their properties on CdS/CdTe solar cells performance. Three types of CdS films obtained from different chemical bath deposition (CBD) processes were studied. The three CBD processes employed sodium citrate as the complexing agent in partial or full substitution of ammonia. The CdS films were studied by X-ray diffraction, optical transmission spectroscopy and atomic force microscopy. CdS/CdTe devices were completed by depositing 3 μm thick CdTe absorbent layers by means of the close-spaced vapor transport technique (CSVT). Evaporated Cu-Au was used as the back contact in all the solar cells. Dark and under illumination J-V characteristic and quantum efficiency measurements were done on the CdS/CdTe devices to determine their conversion efficiency and spectral response. The efficiency of the cells depended on the window layer and on the transparent contact with values between 5.7% and 8.7%.     

Microstructural characterization of transparent conducting aluminium doped zinc oxide films prepared by spray pyrolysis

X-ray diffraction patterns of pyrolytically sprayed aluminium doped zinc oxide films have been recorded and X-ray line profile analysis studies have been carried out after correcting for instrumental broadening. Different microstructural parameters such as crystallite size, r.m.s. strain and dislocation density have been determined from the variance analysis of X-ray line profiles. Texture coefficient, the degree of preferred orientation of the crystallites and stacking fault have been estimated from the X-ray diffraction data. It is observed that the figure of merit as a transparent conductor depends on the dopant concentration and microstructural parameters of the films deposited under identical growth conditions.     

Highly efficient 1 μm thick CdTe solar cells with textured TCOs

Thickness reduction of CdTe absorption layer down to 1 μm has been achieved by controlling the temperature profile used during the close-spaced sublimation (CSS) growth. Transparent conducting oxides, such as indium tin oxide (ITO) and textured fluorine doped tin oxide (SnO₂:F) films have been investigated as transparent electrodes for such 1-μm-thick CdTe absorption layers to increase the incident light confinement and thus to achieve higher conversion efficiency. The contribution in solar cell performance has been found in the case of textured TCOs with optimum haze ratio (roughness). Conversion efficiencies of 10.6% (V_oc: 0.75 V, J_sc: 22.02 mA/cm², FF: 0.64, area: 1 cm²) and 11.2% (V_oc: 0.78 V, J_sc: 22.6 mA/cm², FF: 0.63) have been achieved for only 0.6-μm-thick CdTe absorption layers with SnO₂:F-TCO of 11% and 3% of haze ratios, respectively.     

Solution growth, characterization and applications of zinc sulphide thin films

Zinc sulphide (ZnS) thin films were successfully deposited on glass substrates under varying deposition conditions using the electroless or solution growth technique. The film properties investigated include their transmittance/reflectance/absorbance spectra, bandgap, optical constants, and thicknesses. Films grown under certain parametric conditions were found to exhibit high transmittance ( 64–98%), low absorbance, and low reflectance in the ultraviolet (uv)/visible/near infrared (nir) regions up to 1.00 μm. Those obtained under other conditions exhibited high transmittance ( 78–98%) and low absorbance ( 0.01–0.1) in the uv/visible regions but low transmittance ( 30–37 and high absorbance ( 0.56) in the nir region. These characteristics revealed their suitability for various solar device applications. Bandgap range E_g 3.7–3.8 eV and thickness range t 0.07 – 0.73 μm were obtained.     

Back electrode formation for poly-Si thin film solar cells on glass having AIC-grown seeding layer

Various conductive materials (Al, Mo and TiN) were deposited onto glass substrates to evaluate whether poly-Si seed layers can be formed on such substrates by means of Al-induced crystallisation (AIC) of a-Si at low temperature around 450°C. The material located between the glass and the poly-Si film serves as the back electrode of a substrate-type thin-film solar cell configuration. The outcome of the investigation is that Mo is found to be not compatible with the AIC process. In contrast, Al and TiN showed moderate to good compatibility. TiN is the only viable choice for high-temperature applications (>540°C). Al has satisfactory back electrode properties whereas TiN has a medium high resistivity (120 μΩ cm) and an estimated low back reflectance at the near-infrared wavelengths critical for light trapping.     

An assessment of solar energy availability in different regions of the solar spectrum

The use of spectrally sensitive devices such as solar cells for electricity generation and selective absorber coatings for solar heaters, require a knowledge of the spectral composition of solar energy arriving at the point of utilization. Work is in progress at the University of Bradford to measure the amounts of available energy in different regions of the solar spectrum.The solar spectral measurement system uses a circularly variable interference filter covering the wavelength range 0.3–1.06 μm.