Thin Films For Solar Cell Applications Research Articles

Fabrication of low defect density nanocrystalline silicon absorber layer and its application in thin-film solar cell

Effect of plasma excitation frequency and total gas flow rate on the structural properties as well as defect density of nanocrystalline silicon films have been investigated using Raman analysis, Fourier transform infrared spectroscopy, electron spin resonance. It has been found that defect density and microstructural defect in the films are low (1) at higher plasma excitation frequency of 54.24 MHz and (2) at higher total gas flow rate. Defect density and microstructural defect increase as crystallinity of the films increases. Performance of solar cells, prepared with different absorber layers confirm the fact that fill factor and short circuit current increases as defect density decreases.

Recent development on surface-textured ZnO:Al films prepared by sputtering for thin-film solar cell application

This study addresses the optimization of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) thin films as front contact in silicon thin-film solar cells. The front contact has to be highly conductive and highly transparent for the visible as well as for the infrared spectrum. Furthermore, it has to scatter the incident light efficiently leading to an effective light trapping inside the silicon layers. To materialize the scattering phenomenon, the surface of the magnetron-sputtered ZnO:Al thin films is textured by wet-chemical etching. In this contribution we focus on an optimized balance between electrical and optical needs maintaining a surface topography well suited for light trapping. In a first step we study the influence of vacuum annealing on ZnO:Al films on glass substrates. An increase in transmission is observed while the carrier concentration is gradually decreased. Application of vacuum-annealed ZnO:Al films in silicon thin-film solar cells allows the determination of the relationship between the front-contact carrier concentration and the short-circuit current density. Also, an optimized carrier concentration for the solar module application has been estimated. In a second step we apply this knowledge for direct fabrication of ZnO:Al layers with optimized carrier concentration by varying the target doping concentration (TDC). Nevertheless, changing the TDC alters the ZnO:Al properties and especially the texturing behavior of the thin films as well. Thus, we present a parameter study of TDC and substrate temperature during sputtering to prepare front contacts with surface topography enabling efficient light trapping and ideally balancing optical and electrical properties for solar module applications.

RF magnetron sputtered ZnO:Al thin films on glass substrates: A study of damp heat stability on their optical and electrical properties

ZnO:Al thin films were deposited on glass substrates by RF magnetron sputtering from a powder compacted ceramic target. Structural, electrical and optical properties of the films with different thickness were characterized. The damp heat stability of ZnO:Al thin film was investigated for its application in thin-film solar cells. After the 1000 h damp heat treatment in harsh conditions of 85% relative humidity at 85 °C for all samples, a degradation of electrical properties was observed, while the transmissions of the films were almost unchanged. Thick films with a relative large grain size could form compact structure to resist the corrosion by oxygen and water molecules.

Growth of Cu(In<sub>1-x</sub>Al<sub>x</sub>)Se<sub>2</sub> Thin Films by Atmospheric Pressure Selenization of Sputtered Precursors

Cu(In1-xAlx)Se2 films were prepared using a two-stage process of sputtering and selenization. Stacked elemental layer precursors of Cu, In and Al were deposited onto corning glass substrates by RF magnetron sputtering. Precursors with different Cu/(In+Al) and In/Al ratio were selenized using elemental Se-vapor at atmospheric pressure in a commercial tube furnace under constant argon gas flow. Films with good adhesion to the substrate were grown successfully. All of the films show strong (112) and (220)/(204) CIS peaks. Addition of Al, at expense of In, shifts the peaks towards higher 2θ. This paper explores the possibility to use sputtering deposition and selenization process to grow Cu(InAl)Se2 thin films for solar cells applications.

Electrical and optical characteristics of ZnO:Al films prepared by rf magnetron sputtering for thin film solar cells application

ZnO:Al(AZO) films prepared by rf magnetron sputtering on glass substrate and textured by post-deposition chemical etching were applied as front contact and back reflectors for <TEX>${\mu}c$</TEX>-Si:H thin film solar cells. For the front transparent electrode contact, AZO films were prepared at various working pressures and substrate temperature and then were chemically etched in diluted HCl(1%). The front AZO films deposited at low working pressure(1 mTorr) and low temperature (<TEX>$240^{\circ}C$</TEX>) exhibited uniform and high transmittance (<TEX>$\geq$</TEX>80%) and excellent electrical properties. The solar cells were optimized in terms of optical and electrical properties to demonstrate a high short-circuit current.

Property control of expanding thermal plasma deposited textured zinc oxide with focus on thin film solar cell applications

Property control of expanding thermal plasma deposited textured zinc oxide is demonstrated considering intrinsic, i.e. bulk, and extrinsic transparent conducting oxide quality relevant for application in thin film amorphous silicon pin solar cells. Particularly the interdependence of electrical conductivity, film composition and film morphology, i.e. structure, feature shape and roughness of the surface, is addressed. Control of film composition is mainly governed by plasma production and gas phase chemistry inherently inducing a significant contribution to film morphology, whereas control of film morphology solely is governed by near-substrate conditions. Especially the ratio of zinc to oxygen and the reactor chamber pressure appear to be determinative in obtaining zinc oxide exhibiting the appropriate intrinsic and extrinsic quality, i.e. a high electrical conductivity, a high transmittance, a textured rough surface morphology and a strong hydrogen plasma resistance. The solar cell performance of appropriate undoped and aluminium doped textured zinc oxide inherently obtained during deposition is comparable with respect to Asahi U-type fluorine-doped tin oxide.

Low Temperature Thin-film Silicon Diodes for Consumer Electronics

AbstractWe have developed high current density thin-film silicon n-i-p diodes for low cost and low temperature two-dimensional diode-logic memory array applications. The diodes are fabricated at temperatures below 250°C on glass, stainless steel, and plastic substrates using hot-wire chemical vapor deposition (CVD). The 0.01-mm2 standalone diodes have a forward current-density (J) of near 10 kA/cm2 and a rectification ratio over 107 at ±2 V. The 25 μm2 array diodes have J &gt; 104 A/cm2 and rectification of 105 at ±2V. On plastic substrates, we have also used plasma-enhanced CVD to deposit 10-μm diameter diodes with J ˜ 5 x 104 A/cm2. We found that the use of microcrystalline silicon (μc-Si) i- and nlayers results in higher current-density diodes than with amorphous silicon. Reducing the diode area increases the forward current density by lowering the voltage drop across the external series resistances. A prototype diode array memory based on 10-micron devices was successfully demonstrated by monolithically integrating diodes with a-Si:H switching elements. High current density diodes have potential applications in a variety of large area, thin-film electronic devices, in addition to a-Si:H-based memory. This could widen the application of thin-film silicon beyond its present industrial applications in thin-film transistors, solar cells, bolometers and photo-detectors.

Nano-structured CdTe, CdS and TiO2 for thin film solar cell applications

Abstract Nano-crystalline CdTe and nano-structured CdS films in crystalline, porous and fiber forms were deposited on ITO-coated glass substrates. CdTe films exhibited a typical particle size of 10 nm and a blue shift in the absorption with an effective band gap of 2.8 eV . CdS films exhibited a typical particle size of 15 nm and an effective band gap of 2.98 eV . Also, porous CdS and porous TiO2 films were deposited on plastic substrates by a self-assembly method. Typical pore sizes were 80 and 70 nm , respectively. These can be used in nano-structured solar cell configuration where the pores are filled with a suitable absorber material. Additionally, CdS fibers and nano-crystalline films of TiO2 on plastic substrates were fabricated. Typical particle size in TiO2 was 10 nm and CdS fibers were about 80 nm wide and 1.5 μm long.

ZnO transparent conducting films deposited by pulsed laser deposition for solar cell applications

Low resistivity and highly transparent ZnO conducting films for thin film solar cell applications were fabricated at low temperature by pulsed laser deposition. Al-, B- and Ga-doped ZnO films were deposited on Corning 7059 glass substrate at a substrate temperature of 200 °C. The Al-doped ZnO films were found to have the lowest resistivity of 2.5×10 −4 Ω cm and an average optical transmission of 91% for wavelengths between 400 and 1100 nm. The values of the Ga-doped film were 2.5×10 −4 Ω cm and 81%, respectively. Owing to the higher optical transmission in the near infrared region, the photovoltaic cell performance of a Cu(In,Ga)Se 2 thin film solar cell with an Al-doped ZnO window outperformed a cell fabricated with a Ga-doped ZnO window.

Fabrication of a‐SiGe structure by rf sputtering for solar cell purposes

The deposition and electronic properties of a-SiGe and a-Si layers for thin-film solar cell applications are discussed. Technological parameters were developed for good layer quality of amorphous material. An inverse relation was discovered between the resistivity and the flow of hydrogen without annealing after deposition. Antimony was found to be a good doping material.

Stability of microcrystalline silicon for thin film solar cell applications

The development of microcrystalline silicon (μc-Si:H) for solar cells has made good progress with efficiencies better than those of amorphous silicon (a-Si:H) devices. Of particular interest is the absence of light-induced degradation in highly crystalline μc-Si:H. However, the highest efficiencies are obtained with material which may still include a-Si:H regions and light-induced changes may be expected in such material. On the other hand, material of high crystallinity is susceptible to in-diffusion of atmospheric gases which, through adsorption or oxidation, affect the electronic transport. Investigations are presented of such effects concerning the stability of μc-Si:H films and solar cells prepared by plasma-enhanced chemical vapour deposition and hot wire chemical vapour deposition.

Non-destructive 3D-characterization of Zn 2-2x Cu x In x S 2-thin films with ion beam analysis.

Thin layers of ZnS-CuInS(2) mixed crystals (called ZCIS) are promising absorber materials for thin film solar cell applications. The ZCIS-films investigated in this study were grown on (001)GaP, SiO(2) and CeO(2)/Al(2)O(3) with different elemental compositions by Pulsed Laser Deposition (PLD). In order to optimize the sample preparation process a quantitative three-dimensional (i.e. laterally and depth resolved) determination of the compositions and thicknesses of the ZCIS-films is needed. It is demonstrated how this difficult analytical task can be addressed with ion microbeam analysis. For this purpose the films have been analysed non-destructively by means of Rutherford Backscattering Spectrometry (RBS) and Particle Induced X-ray Emission (PIXE) using a 2 MeV He(+) ion microbeam at the high-energy ion nanoprobe LIPSION. A large variation in film thickness caused by particulates deposited on the film was observed. The elemental compositions of the film and the particulates have been determined and compared with the target composition. The deviations found varied substantially for the individual elements. It could be concluded from these measurements, that the quality of the sintered PLD-target is of crucial importance for the roughness of the films. Furthermore concentration-depth-profiles of the individual elements have been derived non-destructively by means of RBS.

Investigation of lateral parameter variations of Al-doped zinc oxide films prepared on glass substrates by rf magnetron sputtering

Sputter-deposited Al-doped zinc oxide (ZnO : Al) is an interesting transparent conductive oxide (TCO) material for application in electronic devices and thin-film solar cells. A phenomenon in the planar magnetron sputtering of the ZnO : Al films that is not well investigated as yet are the laterally non-uniform film properties resulting from the laterally inhomogeneous erosion of the target material, whereby the lateral distribution of the film properties depends strongly on the sputtering parameters. In this work, the lateral distributions of the electrical, optical, and surface structure properties of the ZnO : Al films prepared by the rf magnetron sputtering on glass substrates are investigated across a distance of 64 mm using the four-point probe technique, optical transmission and reflection measurements, X-ray diffraction, and scanning electron microscopy. We find that the lateral variations of the parameters of the ZnO : Al films prepared by the rf magnetron sputtering can be reduced to acceptable levels by optimising the deposition parameters. Hence, it seems that the sputter-deposited ZnO : Al is a promising TCO material for large-area thin-film solar cells.

Boron doping of silicon layers grown by liquid phase epitaxy

This paper presents the results of a study of the incorporation of boron into silicon layers grown from a tin melt by liquid phase epitaxy. Boron was added to the melt through the use of boron-doped silicon source wafers. There is a large discrepancy between the amount of boron incorporated into the epitaxial layer and that available in the source wafer. This mismatch is explained by the gradual removal of boron from our system, most likely as a result of boron precipitation in the tin melt. This situation allows for control of the boron profile by adjusting the cooling rate and adding a dwell time. In this way, we have grown an epitaxial layer with an abrupt and highly doped p-type region at the epitaxial layer/substrate interface. This is useful for thin film solar cell applications as it allows the growth of a back surface field and a lightly doped bulk in a single growth step.

Ion beam analysis of Zn 2−2 xCu xIn xS 2 films

Thin layers of ZnS–CuInS 2 mixed crystals (called ZCIS) may be promising absorber materials for thin film solar cell applications. The ZCIS films studied here have been grown on (0 0 1) GaP, SiO 2 and (0 1 2) CeO 2/Al 2O 3 substrates with different stoichiometries by pulsed laser deposition (PLD). In order to optimize the film deposition process the thickness, stoichiometry and crystallinity of the films were determined by means of Rutherford backscattering spectrometry (RBS), particle induced X-ray emission (PIXE) and RBS/channeling using a 2 MeV He + ion beam. Furthermore the stoichiometry of the targets used for the PLD was determined with backscattering spectrometry and PIXE using a 2 MeV H + ion beam. A large variation in film thickness was observed and the film stoichiometry did not agree with the PLD-target stoichiometry in general. The element transfer efficiencies from the PLD-target to the deposited film varied substantially for the individual elements. Furthermore, on some films deposited on (0 0 1) GaP a remarkable out-diffusion of Cu into the substrate was found with RBS and confirmed by secondary neutral mass spectroscopy and transmission electron microscopy. RBS/channeling measurements show that mono-crystalline films can be grown on (0 0 1) GaP and (0 1 2) CeO 2/Al 2O 3 substrates, however, they are characterized by a large defect density.

Aluminium BSF in silicon solar cells

The purpose of this work is to develop a back surface field (BSF) for industrial crystalline silicon solar cells and thin-film solar cells applications. Screen-printed and sputtered BSFs have been realised on structures which already have a n +p back junction due to the diffusion of the phosphorus in both faces of the wafer during solar cell emitter elaboration. Rapid thermal annealing temperatures from 700°C to 1000°C have been used. Thickness of the BSF has been measured by SIMS and confronted to the theoretical expected value and simulations. Electrical and optical measurements have been done in order to characterise the BSF. For 250 μm thick industrial solar cells, 6% relative increase in photocurrent has been reached.

Structural, Optical and Electrical Properties of Natively Textured ZnO Grown by PECVD for Thin-Film Solar Cell Applications

Natively textured ZnO layers have been deposited between 150 and 350 °C with an expanding thennal plasma created by a cascaded arc. As shown by Scanning Electron Microscopy, Atomic Force Microscopy, X-ray Diffraction, and optical measurements, the crystallite size and surface roughness of the films increase with increasing substrate temperature. At the same time, columnar textured growth becomes less pronounced, and a change to granular structure is observed.The ZnO films exhibit low sheet resistance ( 80%) and effective light scattering properties. Preliminary amorphous silicon pin solar cells deposited on ZnO show an initial efficiency approaching 10 %, close to the value obtained on Asahi U-type Sn02:F.

Intrinsic amorphous and microcrystalline silicon by hot-wire-deposition for thin film solar cell applications

The influence of various deposition parameters on the electrical and optical properties and the structure of amorphous and microcrystalline silicon films was investigated for material prepared by hot-wire (HW) CVD in a new multichamber deposition system, designed for the development of thin film solar cells. Prior to the material studies, careful measurement of the real substrate temperature under the influence of additional HW heating was performed. While good electronic quality and solar cell performance was found for a-Si:H layers, the μc-Si:H material showed very high spin densities, porosity and a characteristic structural inhomogeneity along the growth axis.

Surface textured ZnO films for thin film solar cell applications by expanding thermal plasma CVD

An expanding thermal plasma created by a cascaded arc is used to deposit surface textured ZnO films. Films have been deposited at 150–350°C at a rate of typically 0.70 nm/s. They exhibit low resistivity (<10 −3 Ωcm), high transmittance in the visible wavelength region (>80%) and a rough surface texture. The crystallite size and surface roughness increase with increasing deposition temperature and flow of argon ions. At the same time columnar textured growth gets less pronounced, a change to granular growth is observed. First p-i-n a-Si:H solar cells deposited on this material with initial efficiencies approaching 10% have been realised.

Amorphous Hydrogenated Carbon-Nitrogen Alloy ThinFilms for Solar Cell Application

Amorphous hydrogenated carbon-nitrogen alloy (a-CNx:H) thin filmshave been deposited on silicon substrates by improved dc magnetronsputtering from a graphite target in nitrogen and hydrogen gasdischarging. The films are investigated by using Ramanspectroscopy, x-ray photoelectron spectroscopy, spectralellipsometer and electron spin resonance techniques. The optimizedprocess condition for solar cell application is discussed. Thephotovoltaic property of a-CNx:H/silicon heterojunctions can beimproved by the adjustment of the pressure ratio of hydrogen to nitrogen andunbalanced magnetic field intensity. Open-circuit voltage and short-circuitcurrent reach 300 mV and 5.52 mA/cm2, respectively.