Effect of porous silicon stain etched on large area alkaline textured crystalline silicon solar cells

This work shows the effects of porous silicon stain etched on alkaline textured antireflection coatings of large area monocrystalline silicon solar cells. The texturization process has been produced by immersion of the silicon wafers in different carbonate-based solutions. The porous silicon layers were formed by stain etching in a HNO₃/HF aqueous solution before or after the texturization process. We study the effects of different alkaline and acidic solutions and the etching times on the solar cell parameters and the surface reflectance of the device. We have found that the average reflectance of the surface is lowered when the porous etching is combined with the texturization in the alkaline solution. However, the solar cell characteristics are not improved.     

Ion beam deposition of photoactive nanolayers for silicon solar cells

The ion beam deposition of photoactive silicon nanolayers through the bombardment of a single-crystal silicon target with an Ar⁺ ion beam has been analyzed using computer simulation. The model thus derived is consistent with experimental data on the growth of silicon nanolayers on 100-mm-diameter c-Si(p) substrates. The process conditions have been optimized experimentally: pressure, 10⁻⁴ Pa; substrate temperature, 550 ± 50°C; target-substrate distance, 240 ± 5 mm; target-beam angle, 45° ± 2°; accelerating voltage, 450–600 V. Under these conditions, the radial asymmetry of 300-nm-thick c-Si(n) layers is within 10 nm, which reduces the efficiency of c-Si(n ⁺)/c-Si(p)/c-Si(p ⁺) solar cells by no more than 0.3%.     

Non-intrusive plasma diagnostics for the deposition of large area thin film silicon

Plasma diagnostics for large area, industrial RF parallel-plate reactors can be useful for process optimization and monitoring, provided that their implementation is practical and non-intrusive. For instance, Fourier transform infrared (FTIR) absorption spectroscopy and/or time-resolved optical emission spectroscopy (OES) can easily be retro-fitted into the pumping line of a reactor. Both techniques were used to measure the fractional depletion of silane in silane/hydrogen plasmas. By means of a simple analytical plasma chemistry model, it is shown that the silane depletion is related to the silicon thin film properties such as microcrystallinity. Uses of the diagnostics are demonstrated by two examples: (i) the optimal plasma parameters for high deposition rate of microcrystalline silicon, along with efficient gas utilization, are shown to be high input concentration and strong depletion of silane; and (ii) the optimal reactor design, in terms of fast equilibration of the plasma chemistry, is shown to be a closed, directly-pumped showerhead reactor containing a uniform plasma.     

Plasma enhanced chemical vapor deposition of silicon thin films for large area electronics

In the past 2 years major advances have been made in the understanding of silane–hydrogen plasmas. In particular, the control of the formation of clusters and even crystallites at room temperature has lead researchers to change their approach and to look for plasma conditions where clusters and crystallites contribute to the growth. In addition, hydrogen has become a key element for the growth of amorphous and microcrystalline silicon films as it can easily diffuse through the growing layers and induce their crystallization below the surface.     

Low temperature silicon epitaxy from trichlorosilane via mesoplasma chemical vapor deposition

Epitaxial silicon thick films have been deposited at around 400 °C by mesoplasma chemical vapor deposition with trichlorosilane (TCS) as source gas. The deposition rate of the Si films increases linearly with the TCS flow rate and reaches 30 nm/s at 15 sccm of TCS. These films have exhibited relatively high hall mobility (~ 200 cm²/V-s) independently of the deposition rate.     

Controlable diffusion parameters with chemical vapour deposition for p-n junction silicon solar cells

The chemical vapour deposition technique for the fabrication of p-n junction silicon solar cells is reported. This technique involves the use of native oxide on silicon to limit the diffusion flux and yields lower surface concentrations of impurities and shallow p-n junctions. Photolithography is used for cell fabrication. Data are given to demonstrate the effects of technological parameters on solar cell performance and the controllability of the diffusion parameters obtained by this technique.     

Front-side metal electrode optimization using fine line double screen printing and nickel plating for large area crystalline silicon solar cells

Industrial applicable fine-line double printing and nickel plating method was applied to single crystalline silicon (c-Si) solar cells. As the finger widths decreased, the efficiency and short circuit current density (J_SC) linearly increased. Although the increase of the J_SC was caused by the reduction of shadowing loss due to the decrease of finger width, the fill factor (FF) was slowly decreased due to increase of contact resistance. The FF of the cells using the fine line was enhanced by using a double printing and nickel plating. c-Si solar cells with the dimensions of 12.5 cm × 12.5 cm, double printed finger width of 50 μm due to spreadability of paste, a finger spacing of 2.4 μm, and aluminum back surface field were fabricated, achieving an increase of J_SC and efficiencies of up to about 0.62 mA/cm² and 0.38% compared to a reference cell at 79.8% of the FF, respectively.     

Roll-to-roll plasma deposition machine for the production of tandem amorphous silicon alloy solar cells

A roll-to-roll plasma deposition machine for depositing multilayered amorphous alloys has been developed. The plasma deposition machine has multiple deposition areas and processes a stainless steel substrate 16 in wide continuously. Amorphous photovoltaic thin films (less than 1 μm thick) with a six-layer structure (p-i-n-p-i-n) are deposited continuously in a single pass onto a roll of stainless steel substrate 16 in wide and 1000 ft long. Mass production of low cost tandem solar cells utilizing roll-to-roll processes is now possible. A commercial plant utilizing this plasma deposition machine for manufacturing tandem amorphous silicon alloy solar cells is now in operation.     

Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells

Microcrystalline silicon (μc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature T_S and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at T_S below 250 °C, μc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm⁻¹ IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages V_oc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high V_oc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high V_oc. Raman spectra show a continuously increasing amorphous peak with increasing V_oc. Crystalline fractions X_C ranging from 50% for the highest V_oc to 95% for the lowest V_oc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest X_C, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency.     

大面积碳纳米管薄膜的低温制备与表征

采用研制的大体积射频等离子体红外加热化学气相沉积设备,在600℃的低温下,在5cm×5cm大的Ni片上生长出碳纳米管薄膜。扫描电镜(SEM)和透射电镜(TEM)观察显示,碳纳米管薄膜具有很好的均匀性,管径大约为70～90nm。随机地对样品的3个不同区域进行了场发射特性的测试,结果表明这种薄膜具有良好的场发射特性及一致性。开启电场约为2.4V/μm,在电场为6.6V/μm时的发射电流密度达到1635μA/cm2。实验结果表明在低温条件下,大面积生长场发射用碳纳米管薄膜是可行的。     

Hydrogenated microcrystalline silicon for solar cells

We report a detailed study of the deposition, composition, structure, and photoelectric properties of low-temperature microcrystalline silicon layers produced by a novel method, which takes advantage of the activation of gas mixtures in an electron-beam plasma and the transport of the activated particles to the deposition zone at a supersonic speed. Under optimal conditions, we have reached deposition rates above 5 nm/s on substrates 150 × 150 mm in dimensions. The method under development is potentially attractive for the fabrication of thin-film solar cells through roll-to-roll processing on cheap substrates.     

Hot wire CVD deposition of nanocrystalline silicon solar cells on rough substrates

In silicon thin film solar cell technology, frequently rough or textured substrates are used to scatter the light and enhance its absorption. The important issue of the influence of substrate roughness on silicon nanocrystal growth has been investigated through a series of nc-Si:H single junction p-i-n solar cells containing i-layers deposited with Hot-wire CVD. It is shown that silicon grown on the surface of an unoptimized rough substrate contains structural defects, which deteriorate solar cell performance. By introducing parameter v, voids/substrate area ratio, we could define a criterion for the morphology of light trapping substrates for thin film silicon solar cells: a preferred substrate should have a v value of less than around 1 × 10^- 6, correlated to a substrate surface rms value of lower than around 50 nm. Our Ag/ZnO substrates with rms roughness less than this value typically do not contain microvalleys with opening angles smaller than ~ 110°, resulting in solar cells with improved output performance. We suggest a void-formation model based on selective etching of strained Si-Si atoms due to the collision of growing silicon film surface near the valleys of the substrate.     

Studies of key technologies for large area CdTe thin film solar cells

The structure and main manufacturing technologies of CdTe film solar cells of large area are reviewed. Among the technologies, some have been developed for application in a pilot manufacturing line. The high resistant SnO₂ (HRT) thin films have been fabricated by PECVD. The effects of annealing on the structure and properties have been studied. A surface etching process of CdTe in low temperature and lower concentration of nitric acid has been developed. The Cd_1 − xZn_xTe ternary compound films have been studied. In order to improve the back contact layer, Cd_0.4Zn_0.6Te layer with 1.8 eV band gap as a substitute for ZnTe layer is introduced in CdTe cells. The effects of the technologies on performance of CdTe cells and feasibility of application in the modules are discussed.     

Nickel silicide contact for silicon solar cells

Electroless nickel metallization on textured front surface is carried out to fabricate large area (13%) efficient silicon solar cells. It is established through XPS analysis that NiSi is formed at the front grid contact on the texturized surface at relatively low temperature leading to a low value of series resistance of the solar cells.     

Atmospheric pressure chemical vapour deposition of 3C-SiC for silicon thin-film solar cells on various substrates

The production of crystalline silicon thin-film solar cells on cost effective ceramic substrates depends on a highly reliable diffusion barrier to separate the light absorbing layers from the substrate. Ideally this intermediate layer should be deposited with cost effective techniques, be conductive and should feature optical confinement. Furthermore the intermediate layer should withstand high temperatures and harsh chemical environments like they occur during solar cell processing. Especially stability against oxidizing solvents like HNO3 or inactivity during e.g., oxide removing steps with HF is required. Crystalline silicon carbide (c-SiC) deposited by atmospheric pressure chemical vapour deposition (APCVD) can match all those requirements and additionally fits the thermal properties of crystalline silicon. The c-SiC intermediate layer is deposited from methyltrichlorosilane (MTS) and H2 at 1100 degrees C. Under these conditions, growth of solely cubic 3C-SiC could be observed by X-ray diffraction measurements. Use of such intermediate layers during high temperature steps prevents diffusion of transition metals, originating from the substrates, into active silicon layers. Doping of these 3C-SiC layers with nitrogen results in specific resistivity of less than 100 ohms cm. The different potentially cost-effective substrates are made from graphite, crystalline silicon, sintered silicon carbide and sintered zircon (ZrSiO4). Surface properties of the coated substrates were investigated, explaining changes in surface roughness and influences on the solar cell processing.     

Fabrication of selective-emitter silicon heterojunction solar cells using hot-wire chemical vapor deposition and laser doping

The Si heterojunction (HJ) solar cells were fabricated on the textured p-type mono-crystalline Si (c-Si) substrates using hot-wire chemical vapor deposition (HWCVD). In view of the potential for the bottom cell in a hybrid junction structure, the microcrystalline Si (μc-Si) film was used as the emitter with various PH₃ dilution ratios. Prior to the n-μc-Si emitter deposition, a 5 nm-thick intrinsic amorphous Si layer (i-a-Si) was grown to passivate the c-Si surface. In order to improve the indium-tin oxide (ITO)/emitter front contact without using the higher PH₃ doping concentration, a laser doping technique was employed to improve the ITO/n-μc-Si contact via the formation of the selective emitter structure. For a cell structure of Ag grid/ITO/n-μc-Si emitter/i-a-Si/textured p-c-Si/Al-electrode, the conversion efficiency (AM1.5) can be improved from 13.25% to 14.31% (cell area: 2 cm × 2 cm) via a suitable selective laser doping process.     

Fabrication of large area high density, ultra-low reflection silicon nanowire arrays for efficient solar cell applications

High density vertically aligned and high aspect ratio silicon nanowire (SiNW) arrays have been fabricated on a Si substrate using a template and a catalytic etching process. The template was formed from polystyrene (PS) nanospheres with diameter 30–50 nm and density 10¹⁰/cm², produced by nanophase separation of PS-containing block-copolymers. The length of the SiNWs was controlled by varying the etching time with an etching rate of 12.5 nm/s. The SiNWs have a biomimetic structure with a high aspect ratio (∼100), high density, and exhibit ultra-low reflectance. An ultra-low reflectance of approximately 0.1% was achieved for SiNWs longer than 750 nm. Well-aligned SiNW/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) heterojunction solar cells were fabricated. The n-type silicon nanowire surfaces adhered to PEDOT:PSS to form a core-sheath heterojunction structure through a simple and efficient solution process. The large surface area of the SiNWs ensured efficient collection of photogenerated carriers. Compared to planar cells without the nanowire structure, the SiNW/PEDOT:PSS heterojunction solar cell exhibited an increase in short-circuit current density from 2.35 mA/cm² to 21.1 mA/cm² and improvement in power conversion efficiency from 0.4% to 5.7%.