Improvement of multicrystalline silicon wafer solar cells by post-fabrication wet-chemical etching in phosphoric acid

In this study, we have improved electrical characteristics such as the efficiency (η) and the fill factor (FF) of finished multicrystalline silicon (mc-Si) solar cells by using a new chemical treatment with a hot phosphoric (H₃PO₄) acidic solution. These mc-Si solar cells were made by a standard industrial process with screen-printed contacts and a silicon nitride (SiN) antireflection coating. We have deposited SiN thin layer (80 nm) on p-type mc-Si substrate by the mean of plasma enhanced chemical vapour deposition (PECVD) technique. The reactive gases used as precursors inside PECVD chamber are a mixture of silane (SiH₄) and ammonia (NH₃) at a temperature of 380°C. The developed H₃PO₄ chemical surface treatment has improved η from 5·4 to 7·7% and FF from 50·4 to 70·8%, this means a relative increase of up to 40% from the initial values of η and FF. In order to explain these improvements, physical (AFM, EDX), chemical (FTIR) and optical (spectrophotometer) analyses were done.     

Influence of stain etching on low minority carrier lifetime areas of multicrystalline silicon for solar cells

In this work the use of HF/HNO₃ solutions for texturing silicon-based solar cell substrates by stain etching and the influence of texturing on minority carrier lifetimes are studied. Stain etching is currently used to decrease the reflectance and, subsequently improve the photogenerated current of the cells, but also produces nanostructures on the silicon surface. In the textured samples it has been observed that an improvement on the minority carrier lifetime with respect to the samples treated with a conventional saw damage etching process is produced on grain boundaries and defects, and the origin of this effect has been discussed.     

Properties of plasma enhanced chemical vapor deposited silicon nitride for the application in multicrystalline silicon solar cells

Hydrogenated films of silicon nitride (SiNx:H) were investigated by varying the deposition condition in plasma enhanced chemical vapor deposition (PECVD) reactor and annealing condition in infrared (IR) heated belt furnace to find the optimized condition for the application in multicrystalline silicon solar cells. By varying the gas ratio (ammonia to silane), the silicon nitride films of refractive indices 1.85-2.45 were obtained. Despite the poor deposition rate, silicon wafer with the film deposited at 450 °C showed the best minority carrier lifetime. The film deposited with the gases ratio of 0.57 showed the best peak of carrier lifetime at the annealing temperature of 800 °C. The performance parameters of cells fabricated by varying co-firing peak temperature also showed the best values at 800 °C. The multicrystalline silicon (mc-Si) solar cells fabricated in conventional industrial production line applying the optimized film deposition and annealing conditions on large area substrate (125 mm × 125 mm) was found to have the conversion efficiency of 15%.     

Optical properties of black silicon prepared by wet etching

In this paper, the optical properties of black silicon have been studied. The black silicon samples were fabricated by alkaline etching and metal assisted etching. The micro-columns and nanopores on the silicon surface were obtained in KOH and Au-induced HF/H₂O₂ solution, respectively. The height and diameter of micro-columns prepared by KOH etching is about 470?nm and 2?μm. In the Au-induced HF/H₂O₂ etching, the metallic nuclei behave as a cathode and their surrounding area acts as an anode, resulting in nanopores with diameters ranging from 80 to 120?nm. These microstructures formed in the etching process directly affect the optical properties of black silicon such as reflectance, transmittance and absorptance. According to the measurement of integrating sphere detector, the absorptance of the black silicon produced by wet etching remains roughly 90% from 250 to 1,000?nm wavelength, which is almost 150% of the absorptance of conventional silicon. However, the reflectance of black silicon is less than 13% and the transmittance is less than 4%.     

Inverted organic solar cells with ZnO nanowalls prepared using wet chemical etching in a KOH solution

We report on the photovoltaic (PV) performances of inverted organic solar cells (IOSCs) that were fabricated from PCBM:P3HT polymer with a ZnO thin film and ZnO nanowalls as electron transport and hole block layers. ZnO thin film on ITO/glass substrate was deposited using a simply aqueous solution route. ZnO nanowall structures were obtained via wet chemical etching of ZnO thin films in a KOH solution. The power conversion efficiency (PCE) of the IOSC with ZnO nanowalls was significantly improved by 44% from 1.254% to 1.811% compared to that of the IOSC with ZnO thin film. The short circuit current in IOSCs fabricated with the ZnO nanowalls was increased mainly due to the increase in the charge transport interface area, as a result of enhancement in the PCE. This work suggests a method for fabricating efficient PV devices with a larger charge transport area for future prospects.     

New silicon architectures by gold-assisted chemical etching

Silicon nanowires (SiNWs) were produced by nanosphere lithography and metal assisted chemical etching. The combination of these methods allows the morphology and organization control of Si NWs on a large area. From the investigation of major parameters affecting the etching such as doping type, doping concentration of the substrate, we demonstrate the formation of new Si architectures consisting of organized Si NW arrays formed on a micro/mesoporous silicon layer with different thickness. These investigations will allow us to better understand the mechanism of Si etching to enable a wide range of applications such as molecular sensing, and for thermoelectric and photovoltaic devices.     

Photochemical wet etching of silicon by synchrotron white X-ray radiation

We have investigated photochemical wet etching of n-type silicon (100) using synchrotron white X-ray radiation. During electroless photochemical wet etching under high flux white X-ray beam, the surface is electropolished. However, when the photon is reduced, the silicon surface becomes porous instead. The pore formation is greatly enhanced when an external potential is applied through a Pt counter electrode. The porous silicon layer exhibits strong photoluminescence signal.     

Relationship between trapping density and open circuit voltage in multicrystalline silicon solar cells

Minority carrier trapping frequently exists in solar grade multicrystalline silicon. At low illumination levels, the effect of trapping centers on open circuit voltage of multicrystalline silicon solar cells is dependent on the trap density and illumination level. In this paper, the relation between trapping density and open circuit voltage of multicrystalline silicon solar cells at different illumination levels is studied by a series of experiments. The experimental evidence suggests that the effect of trapping on open circuit voltage of multicrystalline silicon solar cells is obvious at carrier injection levels equal to and below the trap density, the trapping effect of multicrystalline silicon can be reflected by measuring open circuit voltage at low illumination levels, instead of complicated lifetime measurements, and some multicrystalline silicon solar cells with higher trap densities have higher open-circuit voltages at weak illumination levels. The measurement and analysis of the trapping effect is a relative tool to diagnose the quality of multicrystalline silicon, so a new method is presented to analyze relative quality of multicrystalline silicon by measuring open circuit voltage at weak illumination levels.     

Construct hierarchical superhydrophobic silicon surfaces by chemical etching

We present a simple approach for preparing hydrophobic silicon surfaces by constructing silicon nanowire arrays using Ag-assisted chemical etching without low-surface-energy material modification. The static and dynamic wetting properties of the nanostructured surfaces and their dependence on etching conditions were studied. It was revealed that the surface topologies of silicon nanowire arrays and their corresponding wetting properties could be tuned by varying the etching time. Under optimized etching conditions, superhydrophobic surfaces with an apparent contact angle larger than 150 degrees and a sliding angle smaller than 10 degrees were achieved due to the formation of a hierarchical structure. The origin of hydrophobic behavior was discussed based on Wenzel and Cassie models. In addition, the effects of surface modification of Si surface nanostructures on their hydrophobic characteristics were also investigated.     

Interaction between process technology and material quality during the processing of multicrystalline silicon solar cells

Multicrystalline silicon is the most used material for the production of silicon solar cells. The quality of the as grown material depends on the quality of the feedstock and the crystallization process. Bulk impurities, crystal defects like dislocations and of course the grain boundaries determine the material quality and thus the solar cell conversion efficiency. Therefore minority carrier lifetime measurements are often done to characterize the material quality. But the measured values are from limited use because it is known that the solar cell process itself can dramatically change the minority carrier lifetime and the solar cell efficiency. In order to obtain more detailed information of the behaviour of different defect types additionally high-resolution LBIC (light beam induced current)-measurements have been done. Since LBIC needs a pn-junction for photocurrent generation the LBIC technique has been combined with the a-Si/c-Si heterojunction cell process, which makes it possible to manufacture solar cells even from as cut wafers without changing the material quality. With this combination of measurement and preparation techniques it was possible to analyze the influence of the diffusion process and the firing process on the behaviour of the three different defect types: grain boundaries, dislocation networks and bulk impurities.     

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.     

Nitride-based Schottky diodes and HFETs fabricated by photo-enhanced chemical wet etching

Photo-enhanced chemical (PEC) wet etching technology was used to etch GaN and AlGaN epitaxial layers. It was found that the maximum etch rates were 510, 1960, 300, and 0 nm/mm for GaN, Al_0.175Ga_0.825N, Al_0.23Ga_0.77N, and Al_0.4Ga_0.6N, respectively. It was also found that we could achieve a high Al_0.175Ga_0.825N to GaN etch rate ratio of 12.6. Nitride-based Schottky diodes and heterostructure field effect transistors (HFETs) were also fabricated by PEC wet etching. It was found that we could achieve a saturated I_D larger than 850 mA/mm and a maximum g_m about 163 mS/mm from PEC wet etched HFET with a 0.5 μm gate length. Compared with dry etched devices, the leakage currents observed from the PEC wet etched devices were also found to be smaller.     

Efficiency improvement of multicrystalline silicon solar cells after surface and grain boundaries passivation using vanadium oxide

The aim of this work is to investigate the effect of vanadium oxide deposition onto the front surface of multicrystalline silicon (mc-Si) substrat, without any additional cost in the fabrication process and leading to an efficient surface and grain boundaries (GBs) passivation that have not been reported before. The lowest reflectance of mc-Si coated with vanadium oxide film of 9% was achieved by annealing the deposited film at 600 °C. Vanadium pentoxide (V₂O₅) were thermally evaporated onto the surface of mc-Si substrates, followed by a short annealing duration at a temperature ranging between 600 °C and 800 °C, under O₂ atmosphere. The chemical composition of the films was analyzed by means of Fourier transform infrared spectroscopy (FTIR). Surface and cross-section morphology were determined by atomic force microscope (AFM) and a scanning electron microscope (SEM), respectively. The deposited vanadium oxide thin films make the possibility of combining in one processing step an antireflection coating deposition along with efficient surface state passivation, as compared to a reference wafer. Silicon solar cells based on untreated and treated mc-Si wafers were achieved. We showed that mc-silicon solar cells, subjected to the above treatment, have better short circuit currents and open-circuit voltages than those made from untreated wafers. Thus, the efficiency of obtained solar cells has been improved.     

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.     

Correlation between spatially resolved solar cell efficiency and carrier lifetime of multicrystalline silicon

The correlation between the spatially resolved carrier lifetime of multicrystalline silicon and the spatially resolved monochromatic solar cell efficiency is investigated by means of microwave-detected photoconductance decay (MW-PCD) measurements and illuminated lock-in thermography (ILIT). Local monochromatic solar cell efficiencies are determined from ILIT measurements under short-circuit conditions and at the maximum power point of the cell. The resulting efficiency images are compared with efficiency images obtained from MW-PCD lifetime images of unprocessed neighbouring wafers using PC1D simulations. We observe a qualitative correlation between the measured and the simulated efficiency images. Areas with reduced efficiency are found in the same locations using both methods. However, the dynamic range in the monochromatic efficiency is larger for the images obtained from ILIT measurements. Possible explanations for this difference are a change in carrier lifetime during cell processing and varying lifetimes on microscopic scales, leading to averaging faults in the lifetime images.     

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.     

Fabrication of ordered arrays of InP microstructures by wet chemical etching with Au masks

Ordered arrays of InP microstructures have been fabricated on InP(001) substrates by wet chemical etching in aqueous HCl with patterned Au masks. The masks were produced by Au deposition through copper grids or a monolayer of polystyrene microspheres. Square InP mesas (20 x 20 microns) and pillars (approximately 100 nm in both diameter and height) were both produced and characterized by scanning electron microscopy and atomic force microscopy.     

Spectroscopic ellipsometry characterization of SiNx antireflection films on textured multicrystalline and monocrystalline silicon solar cells

We present a spectroscopic ellipsometry study of silicon nitride based antireflection films deposited on chemically textured multi- and monocrystalline silicon wafers. The ellipsometric parameters were measured from the near infrared to the ultra violet spectral region. We report the effective thickness and complex index of refraction parameters of the antireflection films from all studied surfaces, regardless of their microscopic morphology. We report on a method to make ellipsometric measurements of the effective optical constants and thickness parameters of thin films deposited on alkaline etched (100)-oriented monocrystalline silicon. The effect of the texture on the complex index of refraction can be described within an effective medium approximation approach. The optical properties are consistent with those obtained from a series of reference films deposited on flat silicon surfaces.     

Three-dimensional fabrication on GaAs surfaces using electron-beam-induced carbon deposition followed by wet chemical etching

We propose a method for fabricating three-dimensional structures on GaAs surfaces using electron beam (EB) irradiation followed by wet chemical etching. An etch-resistant hydrocarbon layer forms on the GaAs surface with the EB irradiation. Structures can be fabricated after etching using the hydrocarbon layer to block the etching. The height dependence on the irradiation and etching conditions was investigated as a means of controlling the height of the structures. A higher structure was fabricated at higher doses. The etching selectivity changed with the concentration of the etchant. A three-dimensional structure was fabricated based on these results, demonstrating the possible use of this method as a novel three-dimensional fabrication method for GaAs surfaces.