For polycrystalline silicon thin films on glass, E-beam evaporation capable of high-rate deposition of amorphous silicon (a-Si) film precursor up to 1 μm/minute is a potentially low-cost solution to replace the main stream a-Si deposition method—plasma enhanced chemical vapour deposition (PECVD). Due to weak absorption of near infrared light and a target of 2 μm Si absorber thickness, glass substrate texturing as a general way of light trapping is vital to make E-beam evaporation commercially viable. As a result, the compatibility of e-beam evaporation with glass textures becomes essential. In this paper, glass textures with feature size ranging from ∼200 nm to ∼1.5 micron and root-mean-square roughness (Rms) ranging from ∼10 nm to 200 nm are prepared and their compatibility with e-beam evaporation is investigated. This work indicates that e-beam evaporation is only compatible with small smooth submicron sized textures, which enhances J sc by 21 % without degrading V oc of the cells. Such textures improve absorption-based J sc up to 45 % with only 90 nm SiN x as the antireflection and barrier layer; however, the enhancement degrades to ∼10 % with 100 nm SiO x +90 nm SiN x as the barrier layer. The absorption-based J sc is abbreviated by J sc(A), which is deduced by integrating the multiplication product of the measured absorption and the AM1.5G spectrum in the wavelength range 300–1050 nm assuming unity internal quantum efficiency at each wavelength.

The back surface reflector of a solar cell not only enhances back reflection, but also may contribute to further randomising light. In this paper, three different types of back surface reflectors (dielectric/metal bi-layer, pigmented diffuse reflector and Ag nanoparticles) are investigated and compared with one another to determine the optimal for polycrystalline Si (poly-Si) thin film solar cells grown on textured glass superstrates. The optimal dielectric layer material and thickness of an interposed dielectric layer between Si and metal contact is optimised via WVASE simulation for ∼2μm random textured poly-Si thin film solar cells on glass. Experimentally, the Si thin films are deposited on Aluminium Induced Textured (AIT) glass and those with optical absorption close to Lambertian limited absorption are selected for investigation. The experiment confirms the simulation results and finds the best back reflector configuration— 500 nm MgF2+Ag. Theoretically Ag nanoparticles on the rear surface of a cell should induce surface plasmon effects, scattering light obliquely into the Si film. However, this work indicates that Ag nanoparticles degrade Jsc of textured poly-Si thin film solar cells. The reason why Ag nanoparticles do not work on textured poly-Si thin film solar cells needs further investigation.

To maximise the degree of light incoupling, therefore improving the short circuit current and efficiency of solar cells, a novel subwavelength (SW) structured 'moth-eye' antireflection coating (ARC) is formed on the sun-facing side of the glass superstrate. This moth-eye structure in theory can suppress reflection entirely in the 300-1200 nm wavelength range. It is produced by fabricating a mask, followed by reactive ion etch. In this paper, we present a novel etch mask, a random Ag nanoparticle array on glass with feature size ranging from similar to 50 nm to similar to 350 nm. The subwavelength features can be adjusted by varying the size, shape and distribution of the Ag nanoparticle array, which in turn is tuned by varying as-deposited Ag thickness. The optimal structures reduce reflection loss by a current equivalent of 1.25 mA/cm(2) for a 3.3 mm borosilicate glass (BSG) superstrate, by 1.39 mA/cm(2) for 1.1 mm BSG and enhances J(sc) by 3.4% on average for 2.4 mu m poly-Si thin-film solar cells on 3.3 mm BSG.On Si film side of the glass surface, a preliminary modulated texture investigation, consisting of growing SW structure on micron sized aluminium induced textured (AIT) glass reduced reflection loss by similar to 7% compared to original AIT samples.(C) 2012 Elsevier B.V. All rights reserved.

For polycrystalline silicon (poly-Si) thin-film solar cells on similar to 3 mm borosilicate glass, glass thinning reduces the glass absorption and light leaking to neighbouring cells; the glass texturing of the sun-facing side suppresses reflection. In this Letter, a labour-free wet etching method is developed to texture and thin the glass at the same time in contrast to conventionally separated labour-intensive glass thinning and texturing processes. For 2 cm2 size poly-Si thin-film solar cells on glass superstrate, this wet etching successfully thins down the glass from 3 mm to 0.5 mm to check the ultimate benefit of the process and introduces a large micron texture on the sun-facing glass surface. The process enhances Jsc by 6.3% on average, with the optimal Jsc enhancement of 8%, better than the value of 4.6% found in the literature. This process also reduces the loss in external quantum efficiency (EQE loss), which is due to light leaking to neighbouring cells, dramatically. (c) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

It is desirable to be able to estimate the light trapping performance of glass in photovoltaic applications prior to depositing the active semiconductor layer onto it. However, conventional methods such as haze, root-mean-square roughness (Rms) are not very effective. Additionally, decoupling the geometries of two surfaces ("surface decoupling") is known to enhance light trapping effectively. In this investigation, clear surface decoupling is observed between the inner surface in contact with an aluminium induced textured (AIT) glass surface and the exposed Si film surface grown on such glass by using cross-sectional focused ion beam (FIB) and scanning electron microscope (SEM) imaging. Results suggest that light travelling from a texture on glass to a texture on the exposed Si film surface growing from the same glass texture gets randomised to some extent depending on the decoupling degree between the two textures. The surface decoupling degree can be characterised by a calculated correlation coefficient (CC) between decoupled surfaces. The CC calculation requires the topographies of both AIT glass and the Si film grown on such glass, with the former extracted from atomic force microscopy (AFM) and the latter calculated by a self-coding model based on the underlying AIT glass topography.This CC is found to form a stronger correlation with light trapping capability of AIT glass compared to the conventional light trapping estimation methods. (C) 2012 Elsevier B.V. All rights reserved.

This paper investigates the electrical properties of non-hydrogenated and hydrogenated germanium thin films deposited on silicon nitride coated glass in order to develop a material for the bottom cells of low cost monolithic tandem solar cells. Films were deposited by RF magnetron sputtering over a series of substrate temperatures up to 500°C. A structure-dependent conduction property of germanium films was found. As the substrate temperature increased from 255 to 400°C, both series of films first showed n-type conductivity with progressively increasing room-temperature dark resistivity that peaks around the type switch. Upon attaining p-type character the resistivity decreased rapidly with further increase in T s. Accompanying these trends, the film grain orientation evolved from predominantly (220) to (111).

Boron is an excellent dopant for forming germanium (Ge) shallow junctions because of its low diffusivity. This work investigates fabricating heavily boron-doped hydrogenated polycrystalline Ge thin films with an aim to develop a thin p(+) emitter of the bottom cell of a monolithic tandem solar cell. The films were deposited on glass by cosputtering in an argon and hydrogen mixture at 500 degrees C. Rapid thermal annealing at 600 degrees C for 60 s enhances the boron activation level from 6.24 x 10(19) to 1.21 x 10(20) atoms/cm(3). The high activation level obtained with a low thermal budget indicates that the simple fabrication approach is promising. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3473730] All rights reserved.

This paper examines the effectiveness of a range of aluminum induced textured (AIT) glass topographies at enhancing light absorption in silicon thin film diode structures deposited on the textured glass side, operating in the superstrate configuration. The aluminum layer used to produce the AIT can be deposited either by thermal evaporation or magnetron sputtering. Varying AIT process parameters produces a wide range of feature roughness and uniformity, providing scope to optimize texture effectiveness and process repeatability. We report strong correlation between the degree of absorption enhancement from these textures and both dark field microscope images of the AIT glass and reduction of the interference envelope in spectral reflectance of the deposited silicon films. Our findings corroborate earlier modeling work based on ray tracing, which predicted that the best enhancement occurs when the feature size is close to the film thickness. In this paper we investigate AIT samples in the 1-3 mu m film thickness range, some of which trap light in silicon as strongly as at the Lambertian limit. Copyright (C) 2010 John Wiley & Sons, Ltd.

Structural and optical properties of germanium thin films deposited on silicon nitride coated glass are investigated with the aim to develop a material for the bottom cells of low cost monolithic tandem solar cells. The films were deposited by radio-frequency magnetron sputtering at various substrate temperatures (Ts)≤450°C. X-ray diffraction spectra reveal the structural evolution from amorphous to crystalline phase with increasing Ts We find that the film sputtered at 450°C is poly-crystalline with strong (111) preferential orientation, confirmed by cross-sectional transmission electron microscopy. Optical band gaps of these films derived from Tauc plots, using absorption coefficient values derived from both reflectance/transmittance measurements and spectroscopic ellipsometry data, are in a reasonable agreement. Optical band gap values decrease from ∼0.88 to 0.68eV over the transition from the amorphous to poly-crystalline phase. The absorption coefficient of the poly-crystalline Ge film is higher than that of bulk Ge over a wide waveband and exhibits an absorption tail. The optical properties upon substrate temperature are correlated with the structural properties of Ge films.

Properties of heavily boron-doped hydrogenated polycrystalline germanium (Ge) films sputter-deposited on glass are investigated for developing p+ emitters of the bottom cells of low cost monolithic tandem solar cells. The films were deposited and in-situ doped by co-sputtering Ge with boron at various power levels (PB) in a mixture of argon and hydrogen at 500°C, and then followed by a rapid thermal anneal process at 550°C for 120s or 600°C for 60s, respectively. The dependence of the structural properties of the films on the various boron incorporations as well as annealing conditions was explored by Raman and X-ray diffraction measurements. We find that the films as deposited at 500°C are polycrystalline with strong (220) preferential orientation, which are normally of columnar structure, confirmed by cross-sectional transmission electron microscopy. Revealed by Hall measurements, a boron activation level of 6.24×1019atoms/cm3, well above reported maximum solid solubility in crystalline Ge, was obtained in the as deposited film at PB=55W. Annealing at 600°C enhanced the concentration to 1.21×1020atoms/cm3. A fact that boron hardly diffuses in Ge at temperatures below 800°C makes the resultant material an excellent candidate for thin p+ emitters of the bottom cells of monolithic tandem solar cells.

Significant photocurrent enhancement has been achieved for evaporated solid-phase-crystallized polycrystalline silicon thin-film solar cells on glass, due to light trapping provided by Ag nanoparticles located on the rear silicon surface of the cells. This configuration takes advantage of the high scattering cross-section and coupling efficiency of rear-located particles formed directly on the optically dense silicon layer. We report short-circuit current enhancement of 29% due to Ag nanoparticles, increasing to 38% when combined with a detached back surface reflector. Compared to conventional light trapping schemes for these cells, this method achieves 1/3 higher short-circuit current.

To improve the properties of polycrystalline Ge thin films, which are a candidate material for the bottom cells of low cost monolithic tandem solar cells, ∼300nm in situ hydrogenated Ge (Ge:H) thin films were deposited on silicon nitride coated glass by radio-frequency magnetron sputtering. The films were sputtered in a mixture of 15sccm argon and 10sccm hydrogen at a variety of low substrate temperatures (Ts)≤450°C. Structural and optical properties of the Ge:H thin films were measured and compared to those of non-hydrogenated Ge thin films deduced in our previous work. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increase in Ts. It is found that the introduction of hydrogen gas benefits the structural properties of the polycrystalline Ge film, sputtered at 450°C, although the onset crystallization temperature is ∼90°C higher than in those sputtered without hydrogen. Compared with non-hydrogenated Ge thin films, hydrogen incorporated in the films leads to broadened band gaps of the films sputtered at different Ts.

Poly-crystalline germanium (poly-Ge) thin films have potential for lowering the manufacturing cost of photovoltaic devices especially in tandem solar cells, but high crystalline quality would be required. This work investigates the crystallinity of sputtered Ge thin films on glass prepared by in situ growth and ex situ solid-phase crystallization (SPC). Structural properties of the films were characterized by Raman, X-ray diffraction and ultraviolet–visible reflectance measurements. The results show the transition temperature from amorphous to polycrystalline is between 255°C and 280°C for in situ grown poly-Ge films, whereas the transition temperature is between 400°C and 500°C for films produced by SPC for a 20h annealing time. The in situ growth in situ crystallized poly-Ge films at 450°C exhibit significantly better crystalline quality than those formed by solid-phase crystallization at 600°C. High crystalline quality at low substrate temperature obtained in this work suggests the poly-Ge films could be promising for use in thin film solar cells on glass.

Investigators: Dr. Dirk Konig – Characterisation/Modeling Strand Leader, GCEP, CoE Dr. Eun-Chel Cho – Processing Strand Leader, GCEP, CoE Dr. Tom Puzzer – Professional Officer, GCEP, CoE (part time) Ms. Yidan Huang – Professional Officer, GCEP, CoE Dr. Shujuan Huang – Postdoctoral Fellow, GCEP, CoE Dr. Dengyuan Song – Postdoctoral Fellow, GCEP, CoE Dr. Thorsten Trupke – Associate Professor, GCEP, CoE (part time) Dr. Chu-Wei Jiang – Postdoctoral Fellow, GCEP, CoE (part time) Dr. Patrick Campbell – Research Associate, GCEP, CoE (part time) Mr. Edwin Pink – Research Associate, GCEP, CoE (part time)

A simplified nanosphere lithography process has been developed which allows fast and low-waste maskings of Si surfaces for subsequent reactive ion etching (RIE) texturing. Initially, a positive surface charge is applied to a wafer surface by dipping in a solution of aluminum nitrate. Dipping the positive-coated wafer into a solution of negatively charged silica beads (nanospheres) results in the spheres becoming electrostatically attracted to the wafer surface. These nanospheres form an etch mask for RIE. After RIE texturing, the reflection of the surface is reduced as effectively as any other nanosphere lithography method, while this batch process used for masking is much faster, making it more industrially relevant.

Polycrystalline silicon (poly-Si) thin-films are made on planar and textured glass substrates by solid phase crystallization (SPC) of in situ doped amorphous silicon (a-Si) deposited by electron-beam evaporation. These materials are referred to by us as EVA materials (SPC of evaporated a-Si). The properties of EVA poly-Si films are characterised by Raman microscopy, transmission electron microscopy, and X-ray diffraction. A narrow and symmetrical Raman peak at a wave number of about 520 cm−1 is observed for all samples, showing that the films are fully crystallized. X-ray diffraction (XRD) reveals that the films are preferentially (111)-oriented. Furthermore, the full width at half maximum of the dominant (111) XRD peaks indicates that the structural quality of the films is affected by the a-Si deposition temperature and the surface morphology of the glass substrates. A-Si deposition at 200 instead of 400 °C leads to an enhanced poly-Si grain size. On textured glass, the addition of a SiN barrier layer between the glass and the Si improves the poly-Si material quality. No such effect occurs on planar glass. Mesa-type solar cells are made from these EVA films on planar and textured glass. A strong correlation between the cells' current–voltage characteristics and their crystalline material quality is observed.

This paper presents progress made in developing a method of measuring light trapping intrinsically free of uncertainties associated with reflector absorption and collection losses. These problems presently restrict analysis to uniformly absorbed wavelengths, which in thin films especially accounts for only a minor part of available light trapping benefit. We aim to extend photoconductance measurements, which presently are also similarly restricted, to nonuniformly absorbed wavelengths in order to fully characterise light trapping. Specimen preparation and measurement guidelines are given.

A glass substrate texture of pyramids formed by embossing, suitable for polycrystalline silicon solar cells 5-20 mum thick, is described. A monocrystalline silicon wafer textured with inverted pyramids was used as a die. We evaluate antireflection and light trapping properties for an undoped a-Si:H silicon film (volumetric thickness 5.6 mum; no reflector) simultaneously deposited on this texture and sandblasted glass, using total reflectance and transmittance measurements. J(sc) enhancement potential from light trapping in equally thick polycrystalline silicon films on the same substrates is estimated to be 6.7 (sandblasted), 8.7 (embossed) mA/cm(2). Light trapping characteristics obtained by spectral photoconductance measurements of the specimens are compared.