Photoluminescence from photochemically etched silicon

The photoluminescence (PL) of photochemically etched silicon is studied. In the photochemical etching process, an n-type silicon wafer is immersed in an etchant solution of hydrofluoric acid (HF) and H₂O₂. A low-power visible laser (typically He–Ne) is used to illuminate the samples. The etching process occurs through the photogeneration of carriers. Although no electrodes are used in this etching method, the final samples show PL similar to electrochemically etched porous silicon. The samples were prepared using (1 0 0) n-type silicon with a resistivity of 1.0–5.5 Ω cm. An He–Ne laser with 20 mW of maximum power output was used and the spot radius (on the samples) was varied from 1 to 4 mm. A strong emission in the red-yellow optical region can be present in the final samples depending on the HF:H₂O₂ concentration ratio, etching time and laser intensity of the etching process. The PL spectra excited with the monochromated output of an Xe light source as excitation is studied. The peak wavelength of the PL intensity shifts to the blue region of the spectrum when increasing the laser intensity. Quantum confinement can explain this blue shifting if smaller silicon nanocrystallites are formed with higher laser intensity. The peak PL intensity also decreases when increasing the laser intensity, although a “threshold” condition must be reached to have measurable PL. Each sample also exhibits a shifting in peak PL wavelength when varying the PL excitation wavelength. The corresponding dependence and the variations of the PL intensity are studied. Other experimental conditions are discussed.     

Crystalline silicon surface passivation by amorphous silicon carbide films

This article reviews the surface passivation of n- and p-type crystalline silicon by hydrogenated amorphous silicon carbide films, which provide surface recombination velocities in the range of 10 cm s⁻¹. Films are deposited by plasma-enhanced chemical vapor deposition from a silane/methane plasma. We determine the passivation quality measuring the injection level (Δn)-dependent lifetime (τ_eff(Δn)) by the quasi-steady-state photoconductance technique. We analyze the experimental τ_eff(Δn)-curves using a physical model based on an insulator/semiconductor structure and an automatic fitting routine to calculate physical parameters like the fundamental recombination velocities of electrons and holes and the fixed charge created in the film. In this way, we get a deeper insight into the effect of the deposition temperature, the gas flow ratio, the doping density of the substrate and the film thickness on surface passivation quality.     

Low temperature silicon oxide and nitride for surface passivation of silicon solar cells

Surface passivation at low processing temperature becomes an important topic for crystalline and multicrystalline silicon solar cells. In this work, silicon oxide (250°C) and silicon nitride (300°C) have been developed by Photo-CVD and PECVD technique respectively. Effects of deposition parameters on the optoelectronic and structural properties of the films have been investigated. Interface-trap density (D_it) and fixed charge density (Qf) have been estimated by high frequency (1 MHz) capacitance-voltage measurement on Metal–Insulator–Silicon structure (CV-MIS). The effect of silicon oxide and silicon nitride on the performance of c-Si solar cells have been studied.     

Nanocrystalline silicon film grown by LEPECVD for photovoltaic applications

This work deals with the characterization of nanocrystalline (nc) silicon films, grown using the plasma enhanced chemical vapour deposition (PECVD) process based on a low-voltage–high-current arc discharge plasma named LEPECVD (low-energy PECVD).The structural, electrical and chemical properties of the LEPECVD grown films have been studied as a function of the deposition parameters (substrate temperature, growth rate, silane dilution). The results show that the films consist of elongated nanocrystals along the 1 1 1direction, embedded in an amorphous matrix. The crystallite size along the 1 1 1 direction is in the range of 9-20 nm. The volume fraction of crystallinity (χ_c) varies between 51% and 78%, depending on preparation conditions. Conductivity values of the order of 10⁻⁶ Ω⁻¹ cm⁻¹ for the layers were measured, making the material suitable for the p–i–n junction application.     

Hydrogenated nanocrystalline silicon p-layer in amorphous silicon n–i–p solar cells

This paper describes an investigation into the impacts of hydrogenated nanocrystalline silicon (nc-Si:H) p-layer on the photovoltaic parameters, especially on the open-circuit voltage (V_oc) of n–i–p type hydrogenated amorphous silicon (a-Si:H) solar cells. Raman spectroscopy and transmission electron microscopy (TEM) analyses indicate that this p-layer is a diphasic material that contains nanocrystalline grains with size around 3–5 nm embedded in an amorphous silicon matrix. Optical transmission measurements show that the nc-Si:H p-layer has a wide band gap of 1.9 eV. Using this nanocrystalline p-layer in n–i–p a-Si:H solar cells, the cell performances were improved with a V_oc of 1.042 V, whereas the solar cells deposited under similar conditions but incorporating a hydrogenated microcrystalline silicon (μc-Si:H) p-layer exhibit a V_oc of 0.526 V.     

Multicrystalline silicon wafers prepared from upgraded metallurgical feedstock

A solution to the problem of the shortage of silicon feedstock used to grow multicrystalline ingots can be the production of a feedstock obtained by the direct purification of upgraded metallurgical silicon by means of a plasma torch. It is found that the dopant concentrations in the material manufactured following this metallurgical route are in the 10¹⁷ cm⁻³ range. Minority carrier diffusion lengths L_n are close to 35 μm in the raw wafers and increases up to 120 μm after the wafers go through the standard processing steps needed to make solar cells: phosphorus diffusion, aluminium–silicon alloying and hydrogenation by deposition of a hydrogen-rich silicon nitride layer followed by an annealing. L_n values are limited by the presence of residual metallic impurities, mainly slow diffusers like aluminium, and also by the high doping level.     

High-pressure condition of SiH4+Ar+H2 plasma for deposition of hydrogenated nanocrystalline silicon film

The characteristics of 13.56-MHz discharged SiH₄+Ar+H₂ plasma at high pressure (2–8 Torr), used for the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films in a capacitively coupled symmetric PECVD system, has been investigated. Plasma parameters such as average electron density, sheath field and bulk field are extracted from equivalent circuit model of the plasma using outputs (current, voltage and phase) of RF V–I probe under different pressure conditions. The conditions of growth in terms of plasma parameters are correlated with properties of the hydrogenated nanocrystalline silicon films characterized by Raman, AFM and dc conductivity. The film deposited at 4 Torr of pressure, where relatively low sheath/bulk field ratio is observed, exhibits high crystallinity and conductivity. The crystalline volume fraction of the films estimated from the Raman spectra is found to vary from 23% to 79%, and the trend of variation is similar to the RF real plasma impedance data.     

Influence of hydrogen passivation on majority and minority charge carrier mobilities in ribbon silicon

Multicrystalline silicon materials and ribbons in particular contain a higher amount of defects as compared to monocrystalline silicon, which have to be passivated during solar cell processing in order to reach satisfactory cell efficiencies. Within the solar cell process, this is usually carried out via the deposition of a hydrogen-rich SiN_x layer and a following firing step. During passivation, the electronic properties of the materials (conductivity, mobility) can change which might have an influence on the optimised parameters like emitter sheet resistance and grid geometry. This paper deals with the impact of hydrogen passivation on the electronic properties of majority and minority charge carriers in ribbon silicon materials. Majority charge carrier mobilities resulting from Hall measurements are strongly increasing after hydrogenation especially at temperatures below 300 K. Even at room temperature, changes in mobility up to a factor of 2 have been observed. For the determination of minority charge carrier mobilities in processed solar cells, a new method is presented based on spatially resolved internal quantum efficiency and lifetime measurements. It allows the calculation of mapped mobilities especially in materials showing small diffusion lengths. The same reductions in mobility of a factor 2–3 as compared to monocrystalline silicon for both majority and minority charge carriers could be detected in RGS silicon.     

Theory of amorphous silicon solar cell (b): a five layer analytical model

A theoretical analysis of recombination kinetics and space charge distribution in amorphous silicon is carried out with a view to bring out the underlying physics. A uniform excitation with a flat quasi-Fermi level and a constant np product has been used as a probe to estimate the relative importance of various parameters. Recombination rates have been calculated for various ratios of capture rates for Coulomb attractive and neutral traps. In practice a large ratio of capture rates exists and for this case two peaks of recombination maxima are found to lie in the space charge regions corresponding to transitions at the energy level E₁ (for D⁺–D⁰ transition) at the p–i edge and for E₂ energy level (corresponding to D⁰–D⁻ transition) at the i–n interface. A two independent level model therefore holds to a good approximation. The dangling bond density is found to determine both the space charge distribution and the recombination rate. Based on space charge density distribution i-layer can be divided in the five parts. The two recombination rate peaks are found to exist at the p–i and i–n space charge transitions respectively. This enables us to develop a simple model for the i-layer of the p–i–n diode.     

Defect passivation of industrial multicrystalline solar cells based on PECVD silicon nitride

Low surface recombination velocity and significant improvements in bulk quality are key issues for efficiency improvements of solar cells based on a large variety of multicrystalline silicon materials. It has been proven that PECVD silicon nitride layers provide excellent surface and bulk passivation and their deposition processes can be executed with a high throughput as required by the PV industry. The paper discusses the various deposition techniques of PECVD silicon nitride layers and also gives results on material and device properties characterisation. Furthermore the paper focuses on the benefits achieved from the passivation properties of PECVD SiN_x layers on the multi-Si solar cells performance. This paper takes a closer look at the interaction between bulk passivation of multi-Si by PECVD SiN_x and the alloying process when forming an Al-BSF layer. Experiments on state-of-the-art multicrystalline silicon solar cells have shown an enhanced passivation effect if the creation of the alloy and the sintering of a silicon nitride layer (to free hydrogen from its bonds) happen simultaneously. The enhanced passivation is very beneficial for multicrystalline silicon, especially if the defect density is high, but it poses processing problems when considering thin (<200 μm) cells.     

Stability of thin film solar cells having less-hydrogenated amorphous silicon i-layers

In this study, highly stabilized hydrogenated amorphous silicon films and their solar cells were developed. The films were fabricated using the triode deposition system, where a mesh was installed between the cathode and the anode (substrate) in a plasma-enhanced chemical vapor deposition system. At a substrate temperature of 250 °C, the hydrogen concentration of the resulting film (Si–H=4.0 at%, Si–H₂<1×10²⁰ cm⁻³) was significantly less than that of conventionally prepared films. The films were used to develop the i-layers of solar cells that exhibited a significantly low degradation ratio of 7.96%.     

Comparative study of different approaches of multicrystalline silicon texturing for solar cell fabrication

Alkali etchant cannot produce uniformly textured surface to generate satisfactory open circuit voltage as well as the efficiency of the multi-crystalline silicon (mc-Si) solar cell due to the unavoidable grain boundary delineation with higher steps formed between successive grains of different orientations during alkali etching of mc-Si. Acid textured surface formed by using chemicals with HNO₃–HF–CH₃COOH combination generally helps to improve the open circuit voltage but always gives lower short circuit current due to high reflectivity. Texturing mc-Si surface without grain boundary delineation is the present key issue of mc-Si research. We report the isotropic texturing with HF–HNO₃–H₂O solution as an easy and reliable process for mc-Si texturing. Isotropic etching with acidic solution includes the formation of meso- and macro-porous structures on mc-Si that helps to minimize the grain-boundary delineation and also lowers the reflectivity of etched surface. The study of surface morphology and reflectivity of different mc-Si etched surfaces has been discussed in this paper. Using our best chemical recipe, we are able to fabricate mc-Si solar cell of 14% conversion efficiency with PECVD AR coating of silicon nitride film. The isotropic texturing approach can be instrumental to achieve high efficiency in mass production using relatively low-cost silicon wafers as starting material with the proper optimization of the fabrication steps.     

Concentrator silicon solar cells from silicon industry rejects

Two types of silicon (Si) substrates (40 n-type with uniform base doping and 40 n/n⁺ epitaxial wafers) from the silicon industry rejects were chosen as the starting material for low-cost concentrator solar cells. They were divided into four groups, each consisting of 20 substrates: 10 are n/n⁺ and 10 are n substrates, and the solar cells were prepared for different diffusion times (45, 60, 75 and 90 min). The fabricated solar cells on n/n⁺ substrates (prepared with a diffusion time of 75 min) showed better parameters. In order to improve their performances, particularly the fill factor, 20 new solar cells on n/n⁺ substrates were fabricated using the same procedure (the diffusion time was 75 min)—but with four new front contact patterns. Investigation of current–voltage (I–V) characteristics under AM 1.5 showed that the parameters of these 20 new solar cells have improved in comparison to previous solar cells' parameters, and were as follows: open-circuit voltage (V_OC=0.57 V); short circuit current (I_SC=910 mA), and efficiency (η=9.1%). Their fill factor has increased about 33%. The I–V characteristics of these solar cells were also investigated under different concentration ratios (X), and they exhibited the following parameters (under X=100 suns): V_OC=0.62 V and I_SC=36 A.     

SiO2 passivation layer grown by liquid phase deposition for silicon solar cell application

Surface passivation is one of the primary requirements for high efficient silicon solar cells. Though the current existed passivation techniques are effective, expensive equipments are required. In this paper, a comprehensive understanding of the SiO₂ passivation layer grown by liquid phase deposition (LPD) was presented, which was cost-effective and very simple. It was found that the post-annealing process could significantly enhance the passivation effect of the LPD SiO₂ film. Besides, it was revealed that both chemical passivation and field-effect passivation mechanisms played important roles in outstanding passivation effect of the LPD SiO₂ film through analyzing the minority carrier lifetime and the surface recombination velocity of n-type and p-type silicon wafers. Although the deposition parameters had little influence on the passivation effect, they affected the deposition rate. Therefore, appropriate deposition parameters should be carefully chosen based on the compromise of the deposition rate and fabrication cost. By utilizing the LPD SiO₂ film as surface passivation layer, a 19.5%-efficient silicon solar cell on a large-scale wafer (156 mm × 156 mm) was fabricated.     

High-rate deposition of polycrystalline silicon thin films by hot wire cell method using disilane

A new process, hot wire cell method, was developed and successfully used to grow polycrystalline silicon thin films at a low-temperature and high deposition rate. In the hot wire cell method, reactant gases are decomposed by a heated tungsten filament. Polycrystalline silicon films can be deposited at deposition rates of 1.2 nm/s for mono-silane (SiH₄) and 2.8 nm/s for disilane (Si₂H₆). By using disilane as a reactant gas, it is possible to achieve a high deposition rate without any change in the quality of the films.     

Properties of amorphous silicon solar cells fabricated from SiH2Cl2

Chlorinated intrinsic amorphous silicon films [a-Si:H(Cl)] and solar cell i-layers were fabricated using electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) and SiH₂Cl₂ source gas. n–i–p solar cells deposited on ZnO–coated SnO₂ substrates had poor photovoltaic performances despite the good electronic properties measured on the a-Si:H(Cl) films. Improved open–circuit voltage (V_oc) of 0.84 V and fill factor (FF) of 54% were observed in n–i–p solar cells by providing an n/i buffer layer and by using Ga-doped ZnO coated glass substrates. However, the FF improvement was still rather poor, which is thought to originate from high interface recombination in the ECR deposited solar cells. The V_oc and the FF showed much stable feature against light soaking.     

Texturing of large area multi-crystalline silicon wafers through different chemical approaches for solar cell fabrication

Surface texturing of silicon can reduce the reflectance of incident light and hence increase the conversion efficiency of solar cells. Comparatively lesser concentrated (10%) standard alkaline (NaOH/KOH) solution does not give good textured multi-crystalline silicon (mc-Si) surface, which could give satisfactory open-circuit voltage. This is due to grain-boundary delineation with steps formed between successive grains of different orientations. In this work an attempt has been made to obtain a well-textured mc-Si surface through three different approaches. The first two are with two different types of acid solutions and the third with concentrated alkaline NaOH. Solutions of HF–HNO₃–CH₃COOH/H₂O system with different concentrations of HF and HNO₃ were used for texturing. The results on the effect of texturing of these three solutions on the surface morphology of very large area (125 mm×125 mm) mc-Si wafer as well as on the performance parameters of solar cell are presented in this paper. Attempts have been made to study extensively the surface morphologies of mc-Si wafers in two effective regions of the isoetch curves of the HF:HNO₃:diluent's system. Also we studied the reflectance, uniformity, spectral response, short-circuit current, open-circuit voltage, fill factor and dark current–voltage of the cells fabricated using wafers textured with the three different methods. Short-circuit current of the solar cells fabricated using acid-textured wafers were measured to be in the range of 4.93 A. This value is 0.37 and 0.14 A higher than the short-circuit current values measured in the cells fabricated with isotextured and alkaline-textured wafers, respectively.     

Comparative analysis of embodied liabilities using an inter-industrial process model: gasoline- vs. electro-powered vehicles

We developed a model of economy-wide production systems by incorporating a material balance concept into the standard input–output framework. This inter-industrial process model represents the physical flow of materials throughout the industrial network and thus is able to address the entire process involved with the production of a target product according to its material content. The model, which is based on some physically allowable assumptions, was calibrated using the available input–output coefficients for aggregated processes (sectors). We used detailed data on environmentally hazardous emissions and labour requirements for each sector to analyse the liabilities of substitutable products in terms of different factors, origins, and stages of the inter-industrial process network. To empirically examine the model, we applied it to analyse the production of a popular gasoline-powered vehicle and an electro-powered vehicle.     

Design and simulation of antireflection coating systems for optoelectronic devices: Application to silicon solar cells

Reflectance calculation for various single-, double- and triple-layer Antireflection coatings (ARCs) on silicon substrate are presented. A calculation program is developed to determine the optimum thickness and the refractive index of each layer at a single wavelength for optoelectronic applications and through the visible spectrum for photovoltaic applications.Ta₂O₅, ZnS, Al₂O₃ single layer, MgF₂/Zns double layer and MgF₂/Al₂O₃/ZnS triple layer ARC systems are deposited on silicon substrate using electron beam and thermal evaporation as deposition techniques. The reflectance as a function of the wavelength of AR coating systems on silicon substrate is measured. All curves show good accordance between the theoretical and the experimental reflectance. As application in the photovoltaic field, a ZnS single-layer AR coating is evaporated on concentrator silicon solar cells. Spectral response and current–voltage characteristics are measured before and after ZnS ARC deposition to estimate the improvement of the cell performances. Short-circuit current and cell efficiency are increased by about 31% and 29.4%, respectively.     

Experimental evidence of very high open-circuit voltages of inversion–layer silicon solar cells

In this paper the first experimental evidence of the high V_oc-potential of inversion-layer silicon solar cells is given. Minority-carrier lifetime measurements on inversion-layer emitters have been performed and the diffused p–n contact of PN-IL silicon solar cells has been optimized for high open-circuit voltages. PN-IL silicon solar cells with open-circuit voltages of 693 mV have been fabricated on 0.2 and 0.5-Ω cm FZ p-Silicon wafers. These values are the highest ever reported V_oc's for inversion-layer silicon solar cells on p-Silicon. This demonstrates that inversion-layer silicon solar cells exhibit a similar potential for achieving high open-circuit voltages as silicon solar cells with a diffused p–n junction.