Multicrystalline Silicon Cells Research Articles

Study of the Electron Lifetime in Crystalline and Multicrystalline Si Solar Cells

Crystalline (including multicrystalline) silicon based devices dominate the present solar cell industry due to their durability, relatively low cost, and the vast silicon knowledge base developed by the microelectronics industry. Multicrystalline silicon is the becoming dominating substrate material of the photovoltaic cells. This is obviously due to the significant cost savings compared with the single crystalline silicon wafers. Consequently, the conversion efficiency of the mc-Si solar cell is suffering because of the low cost requirement of the starting materials. One contributor to the lower performance of the mc-Si cells is the grain boundaries which reduce the charge carrier collection properties. Thus, reducing solar cell fabricate is an attractive way to reduce material costs. However, the model calculations in the presented paper show that the rear surface recombination velocity (S) in multicrystalline silicon cell is greater than (mc-Si) cell and the efficiency of mc-Si cell is lower than the efficiency of a crystalline cell.

Angle of Incidence Effects on External Quantum Efficiency in Multicrystalline Silicon Photovoltaics

The accurate prediction of photovoltaic power output under a wide range of conditions observed in the field is of significant interest to the effective insertion of solar power into the electric grid. Spectrally resolved external quantum efficiency (EQE) is used to compute changes in the short-circuit current density as a function of the incidence angle, thus providing a cell-level performance metric that can be integrated into an irradiance-to-power model, allowing an alternative to empirically derived parameters. Multicrystalline silicon cells have been examined both in their as-received condition and after the addition of an EVA/top glass laminate structure. The use of a physical mask in the EQE measurement has allowed the isolation of the expected cosine law behavior from the contribution of optical effects associated with reflection and scattering that are unique to cell/module materials and architecture.

Polysilicon production in India

Polysilicon is the basic semiconductor material used in the manufacture of the most popular type of solar photovoltaic (PV) cells as well as the ubiquitous integrated circuit (IC) chips. With increased importance of renewable energy, the production of polysilicon globally has increased from hundreds to thousands of tonnes. Comparing the various available technologies, it is seen that single crystal and multicrystalline silicon cells comprise 87% of the market. This note provides comparative data of the production of polysilicon and PV modules in various countries. In spite of the importance of solar PV in the Jawaharlal Nehru National Solar Mission, India does not produce any polysilicon.

An Alternative One-diode Model for Illuminated Solar Cells

A novel one-diode model is proposed for illuminated solar cells, which contains an additional variable resistance describing minority carrier diffusion from the bulk to the p-n junction. This model naturally describes the differences between photo- and electroluminescence imaging without the need for correcting the photoluminescence images by short circuit images. It was successfully applied to the quantitative interpretation of photoluminescence images of an industrial multicrystalline silicon cell, where it provides a realistic prediction of the local short circuit current density. Moreover, the novel model explains a known departure from the superposition principle.

Failure Assessments for Outside-Exposed Photovoltaic Modules

Photovoltaic modules with single-crystalline silicon cells and multi-crystalline silicon cells, which were exposed outside for over nine years from 1992, have been evaluated for their failures. Current–voltage characteristic, electroluminescence, and thermography (dark mode) measurement of the modules were carried out as well as those of each solar cell in the modules. Application of these measurements in combination has been shown to be beneficial for investing the failure positions and failure factors in the modules. A new method of analyzing the positions of interconnection failures in modules was also adopted in the present study that is by using electroluminescence measurement and by connecting wires to the interconnector of each individual solar cell.

Surface potential imaging of PV cells with a Kelvin probe

This paper describes a Kelvin probe based inspection technique which measures surface potentials (SP) at various illumination conditions and derives the delta surface potential (ΔVSP) and surface band bending (ϕ) of photovoltaic (PV) cells. The ΔVSP is the difference of surface potential measured in light and dark conditions respectively. The delta surface potential (ΔVSP) is related to charge injection and carrier lifetime, while the surface band bending (ϕ) is associated with the surface condition. A white light and short wavelength light illuminations are used to distinguish the bulk effects from that of the surface.A scanning vibrating Kelvin probe system was built to image PV cells with and without illumination. Experiments were performed on both single crystalline silicon (sc-Si) and multi-crystalline silicon (mc-Si) cells. It is found that the sc-Si cells possess a higher ΔVSP but a lower ϕ than that of mc-Si cells. The average ΔVSP and ϕ are 350mV and 50mV for the sc-Si cells, and 280mV and 110mV for the mc-Si cells. Process defects on a surface could affect both parameters, and it is found that the ΔVSP is not uniform and could be reduced to 170mV at a defect on the mc-Si cells. The high ΔVSP and low ϕs are expected to contribute to the high conversion efficiency on a cell.

EFFECTS OF DUST ON THE PERFORMANCE OF PV PANELS

EFFECTS OF DUST ON THE PERFORMANCE OF PV PANELS

Investigation of bulk and solar cell properties of ingots cast from compensated solar grade silicon

AbstractIn this work, the effect of the concurrent presence of B and P on bulk and solar cell properties of directionally solidified multicrystalline ingots from commercially compensated solar grade silicon (SoG‐Si) feedstock produced by Elkem Solar was investigated. The initial B and P content prior to the directional solidification experiment was 1260 and 762 ppba, respectively. Two reference ingots have been solidified in a silica crucible from 100% electronic grade silicon (EG‐Si) feedstock, with 332 ppba of boron added. All ingots have been cast under similar process parameters. The resistivity measurements by Four Point Probe (FPP) are in good agreement with the net dopant content, i.e., NA − ND for p‐type material, measured by Glow Discharge Mass Spectrometer (GDMS). Bulk lifetime measurements show a decrease in the values compared to the EG reference. Lifetime distributions show the highest values of 13 and 19 µs at approximately half ingot height, compared to 30 and 44 µs in the reference ingots. This decrease can be due to the concurrent effect of compensation and of other impurities present in the ingot. However, the content of several transition metals measured by GDMS at half ingot height was not significantly higher than that of the reference ingots. Oxygen content as measured by Fourier Transform Infra‐Red (FTIR) spectroscopy shows no significant difference compared to the references. Solar cells made from the compensated ingots and processed under standard process conditions show efficiency values up to 15.5% and fill factor values up to 78%, comparable to conventional multicrystalline silicon cells. Copyright © 2010 John Wiley & Sons, Ltd.

RIE 표면 텍스쳐링 모양에 따른 결정질 실리콘 태양전지의 영향

We fabricated a plasma texturing for multi-crystalline silicon cells using reactive ion etching (RIE). Multi-crystalline Si cells have not benefited from the cost-effective wet-chemical texturing processes that reduce front surface reflectance on single-crystal wafers. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. We will discuss reflectance, quantum efficiency and conversion efficiency for multi-crystalline Si solar cell by each RIE process conditions.

The path to 25% silicon solar cell efficiency: History of silicon cell evolution

AbstractThe first silicon solar cell was reported in 1941 and had less than 1% energy conversion efficiency compared to the 25% efficiency milestone reported in this paper. Standardisation of past measurements shows there has been a 57% improvement between confirmed results in 1983 and the present result. The features of the cell structure responsible for the most recent performance increase are described and the history of crystalline and multicrystalline silicon cell efficiency evolution is documented. Copyright © 2009 John Wiley & Sons, Ltd.

A luminescent solar concentrator with 7.1% power conversion efficiency

AbstractThe Luminescent Solar Concentrator (LSC) consists of a transparent polymer plate, containing luminescent particles. Solar cells are connected to one or more edges of the polymer plate. Incident light is absorbed by the luminescent particles and re‐emitted. Part of the light emitted by the luminescent particles is guided towards the solar cells by total internal reflection. Since the edge area is smaller than the receiving one, this allows for concentration of sunlight without the need for solar tracking. External Quantum Efficiency (EQE) and current–voltage (I –V) measurements were performed on LSC devices with multicrystalline silicon (mc‐Si) or GaAs cells attached to the sides. The best result was obtained for an LSC with four GaAs cells. The power conversion efficiency of this device, as measured at European Solar Test Installation laboratories, was 7.1% (geometrical concentration of a factor 2.5). With one GaAs cell attached to one edge only, the power efficiency was still as high as 4.6% (geometrical concentration of a factor 10). To our knowledge these efficiencies are among the highest reported for the LSC. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Imaging physical parameters of pre‐breakdown Sites by lock‐in thermography techniques

AbstractLocal pre‐breakdown sites in solar cells can be studied by lock‐in thermography (LIT). Three new LIT techniques are proposed and demonstrated here, which are TC‐DLIT for studying the local temperature coefficient of pre‐breakdown sites, Slope‐DLIT for measuring the normalized local slope of the I–V characteristics, and MF‐ILIT for imaging the local carrier multiplication factor. First results on multicrystalline silicon cells show that the pre‐breakdown mechanism cannot completely be described by the conventional impact ionization and internal field emission models. Copyright © 2008 John Wiley & Sons, Ltd.

A Comparison of Bulk Lifetime, Efficiency, and Light-Induced Degradation in Boron- and Gallium-Doped Cast mc-Si Solar Cells

High-efficiency boron- and gallium-doped multicrystalline silicon (mc-Si) cells were fabricated and compared in this paper. The quality of three different boron-doped mc-Si ingots and one gallium-doped mc-Si ingot was investigated and compared by means of lifetime measurements and solar cell efficiencies. Untextured screen printed 4-cm2 cell efficiencies in excess of 16% were achieved in this paper when the lifetime after gettering and hydrogenation exceeded 100 mus. This was true for most wafers from top, middle, and bottom regions of the boron-doped ingots. Lifetimes in excess of 300 mus were achieved from the middle region of some boron- and gallium-doped mc-Si ingots. High efficiencies in excess of 16.7% were attained from the middle region of most ingots investigated in this paper regardless of gallium or boron dopant. Light-induced degradation in efficiency (2%-3% relative) was observed in some of the boron-doped mc-Si wafers in which oxygen concentration was high (15 ppm). In contrast, gallium-doped solar cells were found to be very stable under illumination irrespective of their location in the ingot. Device characterization and modeling were performed to show that the combined effect of large variation in resistivity and lifetime along the gallium-doped mc-Si ingots results in variation in the cell efficiency from different regions of the gallium-doped ingots. Design rules were established to determine the optimum thickness of the solar cell for extracting maximum efficiency when the bulk lifetime and resistivity vary along the length of the ingot for a better utilization of the whole ingot

Requirements of PECVD SiNx:H layers for bulk passivation of mc-Si

Abstract Optimization of plasma enhanced chemical vapor deposited hydrogenated silicon nitride (SiN x :H) towards bulk passivation of multi-crystalline silicon cells has been carried out for both low and high frequency (HF) plasma deposition. Experimental results showed that bulk passivation is not caused by hydrogen incorporation in the top silicon layer during deposition and subsequent diffusion towards the bulk during firing, but that it is released from the SiN x :H film. We demonstrate that the amount of passivation depends on the SiN x :H density and its resistance against etching in HF. Optimization of the density, varying deposition temperature and using hydrogen dilution resulted in an optimized passivation.

Low cost surface passivation for p-type mc-Si based on pseudobinary alloys (Al2O3)x(TiO2)1−x

Abstract The conventional process for back side passivation with full face Al screen printing layer is not suitable for very thin multicrystalline (mc-Si) solar cells and approaches to new technological processes are searched for. More investigations have been concentrated on local aluminum contacts and passivation coatings with different layers on mc-Si wafers . The aim of this work is to prove that (Al 2 O 3 ) x (TiO 2 ) 1− x is one promising candidate to be applied as passivation layer on multicrystalline Si. Investigations were performed on dielectric films of pseudobinary alloy (PBA) (Al 2 O 3 ) x (TiO 2 ) 1− x , prepared by chemical solution deposition known initially as sol–gel method. It was determined that their optical, dielectric and electrophysical properties are suitable for applications of these layers as back side surface passivation for thin multicrystalline silicon cells.

Enhancement of solar cell performance by employing planar spectral converters

The effect on solar cell performance of planar converters containing quantum dots (QDs) as wavelength-shifting entities on top of multicrystalline silicon cells was investigated by means of model studies with varying incident spectra. These included global, direct, and diffuse spectra with Air Mass (AM) values ranging from 1 to 10. In case of AM1.5, a planar converter with QDs emitting at 603nm yields a short-circuit current increase of 6.3%, 9.6%, and 28.6% for direct, global, and diffuse irradiation, respectively, as a result of the larger blue/green content of diffuse spectra with respect to direct and global ones. For other AM values, similar results are calculated, with a lower increase toward high AM values.

Rie-texturing of multicrystalline silicon solar cells

We developed a maskless plasma texturing technique for multicrystalline silicon cells using reactive ion etching that results in higher cell performance than that of standard untextured cells. Elimination of plasma damage has been achieved while keeping front reflectance to extremely low levels. Internal quantum efficiencies as high as those on planar cells have been obtained, boosting cell currents and efficiencies by up to 7% on evaporated metal and 4% on screen-printed cells.

A 19.8% efficient honeycomb multicrystalline silicon solar cell with improved light trapping

This paper reports a substantially improved efficiency for a multicrystalline silicon solar cell of 19.8%. This is the highest ever reported efficiency for a multicrystalline silicon cell. The improved multicrystalline cell performance results from enshrouding cell surfaces in thermally grown oxide to reduce their detrimental electronic activity and from isotropic etching to form a hexagonally-symmetric "honeycomb" surface texture. This texture, largely of inverted hemispheres, reduces reflection loss and improves absorption of infrared light by effectively acting as a randomizer. Results of a ray tracing model are presented, with the notable finding that up to 90% of infrared light is trapped in the substrate after the first two passes, compared with only 65% for the well known inverted pyramid structure. These optical features are considered to contribute to an exceptionally high short-circuit current density of 38.1 mA/cm/sup 2/. A further improvement is expected by using under-etched wells for these honeycomb cells.

High-efficiency Multicrystalline Silicon Solar Cells using Standard High-temperature, Float-zoned Cell Processing

This paper reports recent results of fabricating multicrystalline silicon solar cells with the standard PERL (passivated emitter, rear locally-diffused) cell high-temperature processing sequence originally developed for float-zoned wafers. One of these multicrystalline silicon cells with a planar front surface demonstrated a 645-mV open-circuit voltage and 18.2% energy conversion efficiency tested at the National Renewable Energy Laboratory and Sandia National Laboratories under the 100 mW cm−2 AM1.5 global spectrum at 25°C. This is the highest confirmed voltage and one of the highest confirmed conversion efficiencies ever reported to date for a multicrystalline silicon cell. Further optimization of the standard PERL processing and texturing of the cell surfaces is expected to improve the cell efficiency to over 19% in the near future. © 1997 John Wiley & Sons, Ltd.

Trends in industrial silicon solar cell processes

The performance and technology of industrial silicon solar cells have improved considerably in recent years. Conversion efficiencies exceeding 18% are reproducibly obtained by cost-effective technologies on large area Cz-silicon. The performance of multicrystalline silicon cells is closing-in at 17.2%. Improved material casting techniques, a refined technology, and efficient in-process material improvement techniques are found to be the major causes behind such advancement. The trend to towards thinner substrates leads to considerable material cost reduction while yielding better performance. The major processing technologies and steps are critically discussed in this article, keeping in mind the priorities of today's PV industry: cost, and environmental issues. The future trends of the technology are outlined.