Multicrystalline Si Solar Cells Research Articles

Design Analysis of Crystalline Silicon Solar Cell Using1-Dimensional Modelling

The simulation program for solar cells, PC1D, was briefly reviewed and the device modeling of a multicrystalline Si solar cell using the program was carried out to understand the internal operating principles. The effects of design parameters on the light absorption and the quantum efficiency were investigated and strategies to reduce carrier recombination, such as back surface field and surface passivation, were also characterized with the numerical simulation. In every step of the process, efficiency improvements for the key performance characteristics of the model device were determined and compared with the properties of the solar cell, whose efficiency (20.3%) has been confirmed as the highest in multicrystalline Si devices. In this simulation work, it was found that the conversion efficiency of the prototype model (13.6%) can be increased up to 20.7% after the optimization of design parameters.

New modes of FFT impedance spectroscopy applied to semiconductor pore etching and materials characterization

AbstractA systematic approach for the application of fast Fourier transform (FFT) impedance spectroscopy to semiconductor photo‐electrochemistry is given. In particular, photo‐impedance spectroscopy in two novel modes is used in conjunction with conventional current–voltage impedance spectroscopy, and applied to some aspects of pore etching in InP and Si as well as to the characterization of multicrystalline Si or other solar cell material. The technique, applicable in situ with high time resolution, employs optimized hardware and software, extensive mathematical modeling of carrier transport, and the numerical implementation of fast parameter extraction algorithms. The complete system with a fully integrated impedance spectrometer delivers a wealth of new data including key parameters of pore etching. Examples given include the in situ tracking of the fast switch‐over between pore growth modes in InP as well as following a pore etching process over long time scales in Si. FFT impedance spectroscopy in the novel back side illumination mode, in combination with the conventional mode, allows not only some active control of macropore etching in n‐type Si by supplying crucial real‐time data like the valence of the dissolution process and the pore depth, but also provides new fundamental insights into the electrochemical processes occurring at the (porous) electrode. FFT photo‐impedance spectroscopy in the front side illumination mode used in conjunction with a scanned laser beam allows fast local measurements of all semiconductor parameters relevant for solar cell applications, like doping concentration, minority carrier lifetime, diffusion constant, or surface recombination velocity with high spatial resolution; this is demonstrated for large multicrystalline Si wafers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photovoltaic nanocrystal scintillators hybridized on Si solar cells for enhanced conversion efficiency in UV

We propose and demonstrate semiconductor nanocrystal based photovoltaic scintillators integrated on solar cells to enhance photovoltaic device parameters including spectral responsivity, open circuit voltage, short circuit current, fill factor, and solar conversion efficiency in the ultraviolet. Hybridizing (CdSe)ZnS core-shell quantum dots of 2.4 nm in diameter on multi-crystalline Si solar cells for the first time, we show that the solar conversion efficiency is enhanced 2 folds under white light illumination similar to the solar spectrum. Such nanocrystal scintillators provide the ability to extend the photovoltaic activity towards UV.

SIMS Study of C, O and N Impurity Contamination for Multi-Crystalline Si Solar Cells

AbstractThis paper is a case study of using SIMS to quantitatively measure C, O and N impurity contamination at two sequential commercial process steps: (1) Si feedstock: 7N (modified Siemens) and 5N feedstock (UMG-Si); and (2) multi-crystalline (mc-Si) solar wafers: cut and etched, from directional solidification bricks grown from 7N and 7N/5N (80:20) feedstock. The conclusion of this study is twofold: (a) the primary opportunity to reduce C, O and N contamination in mc-Si solar cells is at the directional solidification process, and (b) the costly specification of highly pure Si feedstock is unnecessary from a C, O and N perspective if a directional solidification process is used.

Carrier trap passivation in multicrystalline Si solar cells by hydrogen from SiNx:H layers

Hydrogenation by high temperature rapid annealing of SiNx:H is found to be very effective on the defects responsible for the carrier trapping effect in multicrystalline silicon. The passivation effect is reversible and is annihilated by a long thermal annealing. As for the passivation of deep, lifetime killing defects, the efficiency of “trap” removal by the short thermal treatment depends on the density of the SiNx:H layer. This effect is, in fact, well correlated with performance improvement observed in solar cells. The parallelism between the trap and recombination center passivation effects suggests that they originate from the same defect.

Silicon nitride at high deposition rate by Hot Wire Chemical Vapor Deposition as passivating and antireflection layer on multicrystalline silicon solar cells

A new regime of high rate deposition of near-stoichiometric silicon nitride by Hot Wire Chemical Vapor Deposition was investigated. The layers were tested on multicrystalline silicon solar cells in order to assess their antireflection and passivation properties. The present design of the filament arrangement and the showerhead gas supply system allows for virtually unlimited scale-up and deposition rates of around 3 nm/s were obtained. By varying the SiH 4 flow, the refractive index at 630 nm ( n) could be controlled from 1.90 ± 0.05 to 2.12 ± 0.05, and the extinction coefficient at 400 nm ( k) was < 0.007 for all films of interest. The cells had state-of-the-art values for all photovoltaic parameters, similar to cells with a conventional SiN x antireflection coating. An efficiency of 14.3% was reached using HWCVD SiN x for multicrystalline Si solar cells with an industrial emitter.

Hydrogenation of Si from SiNx(H) films: Characterization of H introduced into the Si

A promising method to introduce H into multicrystalline Si solar cells in order to passivate bulk defects is by the postdeposition annealing of a H-rich, SiNx surface layer. It has previously been difficult to characterize the small concentration of H that is introduced by this method. Infrared spectroscopy has been used together with marker impurities in the Si to determine the concentration and depth of H introduced into Si from an annealed SiNx film.

Porous silicon antireflection coating by electrochemical and chemical etching for silicon solar cell manufacturing

The formation of porous Si (PSi) antireflection coating (ARC) on n + -p multicrystalline Si solar cells has been investigated in view of its implementation at the last stage of the solar cell processing. PSi layers were formed either by electrochemical (EC) etching or by chemical stain (CS) etching and led to effective reflectivity around 13% (AM1.5). Solar cells with solar energy conversion efficiency of 13.3% and 12.6% were obtained with PSi ARC formed by EC- and CS-etching, respectively, the difference originating from a 2-3% larger short-circuit current for the EC-etched cells. Additionally, the FF was higher by 1-2% (a.v.) after PSi formation, which demonstrates that both methods are non-deleterious for the front contact quality and hence suitable for a standard solar cell process.

Mass production technology for multicrystalline Si solar cells

AbstractWorldwide shipments of PV modules in the last 10 years show an increase of about 20% per year; shipments were 277 MW in 2000. About 80% of modules are made from crystalline silicon. Shipments of multicrystalline silicon (mc‐Si) modules have shown substantial increase. The PV market may be expected to increase by the development of low‐cost, and high‐technology multicrystalline silicon solar cells. This paper reviews recent progress in mass production technologies of mc‐Si solar cells. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Influence of wafer thickness on the performance of multicrystalline Si solar cells: an experimental study

AbstractThe influence of the thickness of silicon solar cells has been investigated using neighbouring multicrystalline silicon wafers with thickness ranging from 150 to 325 μm. For silicon solar cell structures with a high minority‐carrier diffusion length one expects that Jsc would decrease as the wafer becomes thinner due to a shorter optical path length. It was found experimentally that Jsc is nearly independent of the thickness of the solar cell, even when the minority‐carrier diffusion length is about 300 μm. This indicates that the Al rear metallisation acts as a good back surface reflector. A decrease in Jsc is observed only if the wafer thickness becomes less than about 200 μm. The observed trend in Voc as a function of the wafer thickness has been explained with PC1D modelling by a minority‐carrier diffusion length in the Al‐oped BSF which is small in relation to the thickness of the BSF. This effectively increases the recombination velocity at the rear of the cell. We have shown that the efficiency of solar cells made with standard industrial processing is hardly reduced by reducing the wafer thickness. Solar cell efficiencies might be increased by better rear surface passivation. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Microwave PECVD of Micro-Crystalline Silicon

AbstractThe deposition of micro-crystalline silicon by means of PECVD with a new linear microwave plasma source is investigated. This plasma source has successfully been introduced in the large scale production of multi-crystalline Si solar cells for the deposition of passivating silicon nitride layers. Advantages of this linear plasma source are the high deposition rates and the large area (up to 80 cm width, no length limitations) on which a homogeneous deposition can be achieved. Since this source has not been applied for deposition of micro-crystalline silicon before, we explored a large parameter space (substrate temperature, pressure, MW-power, gas flow rates), in order to find optimum growth conditions. It is observed that with this microwave source it is possible to grow micro-crystalline layers at significantly higher silane/hydrogen ratios and higher deposition rates than for conventional RF PECVD. In this paper, structural properties of the silicon layers, as investigated by Raman and FTIR spectroscopy, XRD and SEM measurements are discussed.

Mass production technology for multicrystalline Si solar cells

The worldwide shipments of the PV modules in the last 10 years show an increase of about 20% per year. The worldwide PV modules shipments were 201 MW in 1999. About 85% modules have made from crystalline silicon. The multicrystalline silicon (mc-Si) modules shipments have showed substantial increase. PV market would be expected to increase by the development of low cost and high technologies of mc-Si solar cells. This paper reviews recent progress in mass production technologies of mc-Si solar cells.

Characterization of random reactive ion etched-textured silicon solar cells

Hemispherical reflectance and internal quantum efficiency (IQE) measurements have been employed to evaluate the response of Si nanostructured surfaces formed by using reactive ion etching (RIE) random texturing techniques. Random RIE-textured surfaces typically exhibit broadband anti-reflection behavior with solar-weighted-reflectance (SWR) of /spl ap/3% over 300-1200-nm spectral range. RIE-texturing has been demonstrated over large areas (/spl sim/180 cm/sup 2/) of both single and multicrystalline Si substrates. Due to the surface contamination and plasma-induced damage, as formed RIE-textured solar cells do not provide enhanced short-circuit current. However, improved surface cleaning combined with controlled wet-chemical damage removal etches provide a significant improvement in the short-circuit current. For such textures, the internal quantum efficiencies are comparable to the random, wet-chemically-textured solar cells. In both the UV and near-IR wavelength regions, the RIE-textured subwavelength surfaces exhibit superior performance in comparison with the wet-chemically-textured surfaces. Due to their large area, low-reflection capability, random, RIE-texturing techniques are expected to find widespread commercial applicability in low-cost, large-area multicrystalline Si solar cells.

Statistical analysis of local shunts and their relationship with minority-carrier lifetime in multi-crystalline silicon solar cells

Local shunts in multi-crystalline Si solar cells were investigated statistically using minutely meshed mesa diodes fabricated on commercially available cells. The dark reverse current density J dr in each mesa varied in a wide range of 10 −9–1 A/cm 2, which showed that local shunts over n +–p junction distributed spatially. The histogram of J dr was analyzed to clarify cell performances, and a critical value of J dr was proposed to classify the cell efficiency. Minority-carrier lifetime estimated using transient behavior of switching current was also discussed relating with the material quality.

Rapid thermal processing of next generation silicon solar cells

Rapid and potentially low-cost process techniques are analysed and successfully applied towards the fabrication of high-efficiency mono-and multicrystalline Si solar cells. First, a novel dielectric passivation scheme (formed by stacking a plasma silicon nitride film on top of a rapid thermal oxide layer) is developed that serves as antireflection coating and reduces the surface recombination velocity (S{sub eff}) of the 1.3 {omega}-cm p-Si surface to approximately 10 cm/s. The essential feature of the stack passivation scheme is its ability to withstand short 700 - 850degC anneal treatments used to fire screen printed (SP) contacts, without degradation in S{sub oeff}. The stack also lowers the emitter saturation current density (J{sub oe}) of 40 and 90 {omega}/{open_square} emitters by a factor of three and 10, respectively, compared to no passivation. Next, rapid emitter formation is accomplished by diffusion under tungsten halogen lamps in both belt line and rapid thermal processing (RTP) systems (instead of in a conventional infrared furnace). Third, a combination of SP aluminium and RTP is used to form an excellent back surface field (BSF) in 2 min to achieve an effective back surface recombination velocity (S{sub eff}) of 200 cm/s on 2.3 {omega}-cm Si. Finally, the above individual processes are integrated to achieve: (1) >19% efficient solar cells with emitter and Al-BSF formed by RTP and contacts formed by vacuum evaporation and photolithography, (2) 17% efficient manufacturable cells with emitter and Al-BSF formed in a belt line furnace and contacts formed by SP. (Author)

Interaction of Crystal Defects with p-n Junctions in Multicrystalline Si Solar Cells

Interaction of Crystal Defects with p-n Junctions in Multicrystalline Si Solar Cells

Method for the Evaluation of the Influence of Gettering and Bulk Passivation on Non-Uniform Block-Cast Multicrystalline Si Solar Cells

Method for the Evaluation of the Influence of Gettering and Bulk Passivation on Non-Uniform Block-Cast Multicrystalline Si Solar Cells

Chemically etched porous silicon as an anti-reflection coating for high efficiency solar cells

Porous silicon was formed on mono- and multicrystalline Si substrates by stain etching in aqueous HF/HNO 3 solutions. Optical properties of the resulting films were investigated by reflectance spectroscopy and related to the main etch parameters. Porous Si films have been shown able to reduce surface reflectance to under 3% in a wavelength range larger than 400–800 nm. Efficient anti-reflection coatings were developed and used in multicrystalline Si solar cells with area up to 12.8×12.8 cm 2. Photovoltaic conversion efficiency up to 12% was obtained under standard AM1.5G simulated sunlight. © 1997 Elsevier Science S.A.

Efficiency improvement by porous silicon coating of multicrystalline solar cells

Shallow junction multicrystalline Si solar cells have been processed by an anodical etching technique. More than 25% improvement in short-circuit current and photovoltaic energy conversion efficiency was demonstrated. It was shown that improved performance was caused by antireflection action of the porous silicon layer as well as by the cell surface and grain boundary passivation.

Porous-silicon coatings for photovoltaic devices

Porous silicon was formed on multicrystalline Si substrates by stain etching in aqueous HF/HNO3 solutions. In this work optical and electrical properties of the resulting films are discussed as a function of process parameters. Porous-Si films have been shown to be able to reduce surface reflectance to 3% in 350–700 nm wavelength range and their application in anti-reflection coating of photovoltaic solar cells has been demonstrated, obtaining (10×10) cm2 multicrystalline Si solar cells with efficiency approaching 12% under standard AM 1.5 simulated sunlight.