Polysilicon Solar Cells Research Articles

Effect of laser-induced conversion of silicon nitride to silicon oxy-nitride on antireflective properties of passivation layer in polysilicon solar cells

Silicon solar cell technology is dominating in the photovoltaic industry, however, a further increase in cost-effectiveness is needed. This can be achieved by increasing the light coupling in the active layer of the cell. Passivation layer of the commercial polycrystalline silicon solar cells was textured using the laser beam interference patterning technique to obtain enhanced optical properties due to the formation of periodic refractive index grating. As a consequence of laser irradiation, passivation layer of silicon nitride was partially oxidized at the intensity peaks of the interference distribution. Periodic distribution of oxidized areas was found by EDS. Investigation of optical and electrical properties of the laser treated solar cells shows increased light coupling and better photo-electrical performance. Simulations were carried out to evaluate the influence of refractive index grating formed in silicon oxy-nitride to optical properties of the patterned solar cells. Periodic oxide grating was found to be more efficient for light coupling in thinner film structures. The results of this work may find applications in other fields as it allows alteration of material composition in the well-defined periodic pattern.

Improvement in the light conversion efficiency of silicon solar cells by pure hydrogen annealing

In this report, the effects of pure hydrogen gas annealing on series resistance (Rs), shunt resistance (Rsh), open circuit voltage (Voc), short circuit current (Isc), fill factor, and efficiency were investigated systematically using standard, commercially available polysilicon solar cells. Improvements on the electrical characteristics, fill factors, and efficiency of the solar cells were observed after annealing by pure hydrogen gas at 350°C for 15min. In the best case, the conversion efficiency was raised by nearly 1% point. Judging from our experimental evidences, the improvement on cell performance could be mostly attributed to the reduction of Rs and improvement in Ag grid/emitter contact resistance in the cells during the annealing process.

Analytical model and photovoltaic parameters improvement of polysilicon solar cells with porous silicon emitter

The present study aimed to develop a simple analytical model that simulates the performance of thin polysilicon solar cells with porous silicon (PS) contact on the front surface. It provides an analytical solution to the complete set of equations needed for determining the effect of this material on the performance of the cell when acting as an antireflective coating agent. The simple analytical expressions of the emitter reverse saturation current density and light-generated current density were also obtained. The PS layer was noted to induce a decrease in emitter reverse saturation current density and an increase in solar cell photovoltaic parameters. Overall, the findings revealed that the emitter region should not be treated as a ‘dead layer’ because contact with the thin PS layer front surface was noted to improve the open-circuit voltage, photocurrent, and cell efficiency values by about 20mV, 4.5mA/cm2, and 3.3%, respectively.

Laser terahertz emission microscopy studies of a polysilicon solar cell under the illumination of continuous laser light

Terahertz wave-emission properties of a polysilicon solar cell excited by femtosecond laser pulses were visualized by laser terahertz emission microscopy (LTEM). The effects on the terahertz emissions of continuous-wave (cw) laser illumination of the solar cell at two wavelengths (808 and 365 nm) were examined under conditions of reverse and zero bias. The amplitude of the terahertz pulses in the presence of illumination was lower than that in its absence, and it decreased with increasing illumination power. The photovoltaic effect was smaller for cw laser illumination at the shorter wavelength. These phenomena can be explained in terms of screening of the electric field in the depletion layer of the p−n junction, as a result of the presence of photoexcited carriers. In addition, the LTEM images permitted the visualization of the crystalline grain structure of the solar cell and the dynamics of photocarriers, and they reflected the local electric field distribution in the cell. LTEM was therefore shown to be a promising technique for the evaluation and inspection of solar cells.

Simulation and On-Site Performance of a Novel 3D Concentrator for Photovoltaic Application

We presented a family of new 3D concentrators. Simulations showed they could significantly increase the illumination on objective plane compared with 2D trough concentrators. A 3D concentrator prototype with a nominal 35° half acceptance angle was made. Its performance was tested under an indoor solar simulator and by on-site experiment. Under solar simulator, a low cost poly-silicon solar cell coupled with a 3D concentrator achieved a 2.25 times of maximum output power compared with a similar bare solar cell. In the on-site experiment, poly-silicon solar cell with a 3D compound parabolic based reflective concentrator gained an average of 1.4 times maximum output power when the incidence sunlight within the critical angle.

Direct Laser Doping of Poly-Silicon Thin Films Via Laser Chemical Processing

Laser chemical processing (LCP) is an attractive doping technique for thin films due to its process simplicity, high achievable doping concentrations, and relatively shallow doping depths. During LCP processing, an infinite supply of dopants is available from the pressurized doping medium. In this paper, LCP is employed for n-type doping of poly-silicon thin films on glass. We achieved a peak doping concentration of 6 × 1018 to 1 × 1019 cm-3 and a junction depth up to 350 nm, as determined by electrochemical capacitance-voltage profiling and secondary ion mass spectrometry. We evaluate the structural quality of the LCP-doped layers with cross-sectional transmission electron microscopy (XTEM), as well as ultraviolet reflectance measurements. The LCP-doped regions are of suitable material quality for device fabrication. The resulting sheet resistance and doping levels are promising for a back surface field for poly-silicon thin-film solar cells on glass (e.g., an n+/ n-/ p+/glass layer structure in superstrate configuration).

The Chemical Stain Inspection of Polysilicon Solar Cell Wafer by the Fuzzy Theory Method

This study proposed an automatic optical inspection (AOI) technique to improve the inspection of chemical stains on solar wafers. Poly-silicon solar cell wafers were inspected for chemical stains, and the inspection was rapid and stable. The system used a laser-reflection-point-based AOI method for solar wafer chemical stain inspection. Based on the fuzzy theory, the image binarization algorithm could efficiently filter irrelevant image information, and the back-propagation method was also utilized to determine if the image was stained. The inspection algorithm integrated fuzzy theory and the back-propagation method in order to shorten the comparison time and quickly find the target. The experiment proved that the validity of the proposed method could achieve a recognition rate of 98% from among 1000 images.

Laser Chemical Processing (LCP) of Poly-Silicon Thin Film on Glass Substrates

Laser chemical processing (LCP), based on the patented LaserMicroJet technology by Synova® S.A, was introduced by Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE), as a novel approach for selective doping for high efficiency (> 20%) silicon wafer solar cells. The technique consists of coupling a laser beam into a highly pressurised thin liquid jet. Total reflection inside the liquid jet enables laser light to be wave-guided towards the sample of interest. If the liquid contains a dopant source, selective doping is possible via the laser-induced, physical and chemical interactions of the substrate and doping medium. To date, this process was primarily investigated for silicon wafer solar cells. In this work, we report on a novel application of LCP for n-type doping of poly-silicon thin films on glass substrates. By using phosphoric acid as the doping medium, we have successfully realised n-type doping of poly-silicon thin films through LCP. Proof-of-principle experimental results are promising in terms of sheet resistance (< 5 kΩ/□) and active dopant concentration of 5x1018 to 1x1019 cm-3 at a doping depth of less than 250nm as measured by electrochemical capacitance-voltage (ECV) profiling. The obtained sheet resistance and doping concentration levels of LCP doped areas opens a new frontier for LCP processing. In the future, the LCP technique will be applied to fabricate back surface fields (BSF) for poly-silicon thin film solar cells.

Laser annealing of thin film polycrystalline silicon solar cell

Performances of thin film polycrystalline silicon solar cell grown on glass substrate, using solid phase crystallization of amorphous silicon can be limited by low dopant activation and high density of defects. Here, we investigate line shaped laser induced thermal annealing to passivate some of these defects in the sub-melt regime. Effect of laser power and scan speed on the open circuit voltage of the polysilicon solar cells is reported. The processing temperature was measured by thermal imaging camera. Enhancement of the open circuit voltage as high as 210% is achieved using this method. The results are discussed.

Internal quantum efficiency improvement of polysilicon solar cells with porous silicon emitter

The present study aimed to develop a simple analytical model to investigate the potential use and implications of porous silicon (PS) as an antireflective coating in thin polysilicon solar cells. It analytically solved the complete set of equations necessary to determine the contribution that this material has on the internal quantum efficiency (IQE) of the cell when acting as an antireflective coating agent. The increase in the IQE, the contribution of the different regions of the cell, and the effects of the physical parameters of each region were derived and investigated in comparison with conventional solar cells. The findings revealed that the internal quantum efficiency of the solar cell with PS emitter is higher than that of the conventional one particularly for short-wavelengths ( λ < 0.6 μm). Furthermore, for photons with higher energy, the emitter contribution in the IQE is more significant than the base and depletion regions. For photons with smaller energy, on the other hand, the absorption coefficients are also smaller, which leads to a higher generation rate in the base region and, hence, to a more pronounced contribution from this region to IQE. Last but not least, the improvement of IQE is observed to increase with decreased PS thickness and with heavily doped PS emitter ( N d ++ = 10 20 cm −3 ).

Accelerated Light-Induced Defect Transformation Study of Elkem Solar Grade Silicon

Solar cells made of silicon feedstock from a metallurgical route must qualify not only the initial efficiency, but must also be comparable to the solar cells made from reference polysilicon on the spectral response after light induced degradation. A detailed comparative study of light induced defects and its impact on cell performance is necessary for both materials. We have studied accelerated light induced degradation (ALID) defect transformation for Elkem Solar Silicon and polysilicon solar cells by selecting wafers from different positions from respective silicon bricks. Active boron-oxygen complexes and iron ions in multicrystalline silicon solar cells have been analyzed, and their impacts on the current voltage characteristics of the solar cells have been studied in detail.

Effect of Temperature on Performance Parameters of Metallurgical Grade Polysilicon Solar Cells

By the stable operation of the experiments in actual outdoor conditions nearly a year, we find that the property of metallurgical silicon solar cells proves to be stable. The results show that the relationship between S-P could be expressed as a quartic equation. As the higher surface temperature of solar cells at noon on sunny day, both the open circuit voltage and the operating voltage are lower, the output power is reduced. It is concluded that the water cooling on the surface of the solar cells when the temperature T&gt;50°C could improve power generation efficiency.

Simulation of high conversion efficiency and open-circuit voltages of α-si/poly-silicon solar cell

The P+ {\alpha}-Si /N+ polycrystalline solar cell is molded using the AMPS-1D device simulator to explore the new high efficiency thin film poly-silicon solar cell. In order to analyze the characteristics of this device and the thickness of N+ poly-silicon, we consider the impurity concentration in the N+ poly-silicon layer and the work function of transparent conductive oxide (TCO) in front contact in the calculation. The thickness of N+ poly-silicon has little impact on the device when the thickness varies from 20 {\mu}m to 300 {\mu}m. The effects of impurity concentration in polycrystalline are analyzed. The conclusion is drawn that the open-circuit voltage (Voc) of P+ {\alpha}-Si /N+ polycrystalline solar cell is very high, reaching 752 mV, and the conversion efficiency reaches 9.44%. Therefore, based on the above optimum parameters the study on the device formed by P+ {\alpha}-Si/N+ poly-silicon is significant in exploring the high efficiency poly-silicon solar cell.

The Purification of Solar Grade Silicon by Chloridizing Roasting

In recent years, photovoltaic industry has developed rapidly[1]. Facing the situation of traditional energy resources exhaustion and continuing oil price rise, solar energy as a renewable clean energy has been the focus of world attention. In all kinds of solar battery, polysilicon solar cell is the most important. However, the high cost of polysilicon, which accounts for 50% of the total cost, hinders the development of photovoltaic industry. In this paper, for the purpose of reducing cost of polysilicon production, metallurgy methods, for example, chloridizing roasting has been applied for purification. A feasible technological route for polysilicon purifying was developed.

Simple analytical model and efficiency improvement of polysilicon solar cells with porous silicon at the backside

The present study developed a simple analytical model to simulate the performance of polysilicon solar cells with porous silicon (PS) layer at the backside. It analytically solved the complete set of equations necessary for the determination of the photocurrent generated under the effect of the reflected light. It also investigated the contribution of the light absorbed by the PS layer and explored the effect that the latter’s number of double porosities and high porosity have had on photovoltaic parameters. The findings suggest that the photovoltaic parameters increase with the number of double porosities that the layer might have in a given structure. When the PS layer is formed by three-double porosity layers 20%/80% and for a 5μm-thick film c-Si, the backside reflector gives a total improvement of about 2.65mA/cm2 in photocurrent density and 1.4% in cell efficiency. This improvement can even be of much more important for well passivated grain boundaries and back contact of solar cells.

Defects in poly-Silicon and amorphous Silicon solar cells

The paper introduces the issue of the typical defects present in PV-cell. Typical defects are introduced for two different technologies of photovoltaic devices: the multicrystal Silicon and the amorphous Silicon one. The defects have been implemented on a tridimensional model of a well operating cell implemented in Comsol Multiphysics environment. The aim of the paper is to simulate the behaviour of possible defect as they are pointed out by means of thermography. Defects have been modelled as they are described in literature and implemented into the model of photovoltaic cells. A multiphysical analysis, needed to study the effects of the defects on the cell behaviour, has been conducted using utilities available in Comsol environment.

CHEMICAL MAKERS INVEST IN SOLAR

THREE NEW SOLAR technology investments signal a rare bright spot of capital spending by the chemical industry. The projects will increase capacity and advance research for chemical components of both traditional polysilicon solar cells and newer thin-film modules. The two largest outlays involve Dow Corning and will increase manufacturing capacity for polysilicon and silane (SiH 4 ); both raw materials have been in short supply in recent years. Hemlock Semiconductor—a joint venture involving Dow Corning, Shin-Etsu Handotai, and Mitsubishi Materials—will spend an initial $1.2 billion to build a polysilicon plant at a new site in Clarksville, Tenn. In addition, Hemlock plans a $1 billion expansion of its current location in Hemlock, Mich., to come on-line in 2011. Overall, the investments may total up to $3 billion and add 34,000 metric tons per year of capacity. Dow Corning’s second investment is in a silane plant to be located next to the Michigan facility. The company, which does ...

Shunting problems due to sub‐micron pinholes in evaporated solid‐phase crystallised poly‐Si thin‐film solar cells on glass

AbstractRecent progress in the metallisation of poly‐silicon thin‐film solar cells on glass, created by solid phase crystallisation (SPC) of evaporated amorphous silicon (EVA), revealed that shunting through sub‐micron holes (density 100–200 mm−2) in the films causes severe shunting problems when the air‐side metal contact is deposited onto these diodes, by creating effective shunting paths between the two highly doped layers of EVA cells. We present evidence of these pinholes by optical transmission and focussed ion beam (FIB) microscopic images and confirm the point‐like pinhole shunts using lock‐in thermographic images. The latter revealed that the Al rear electrode induces strong ohmic shunts below the grid lines and a high density of weak non‐linear shunts away from the grid lines. Two distinctly different approaches are shown to reduce the shunting problem to a negligible level: (i) to contact only a small fraction of the rear Si surface via a point contacting scheme, whereby the metal layer needs to be thin (&lt;1 µm) and the fractional area coverage small (&lt;5%), and (ii) to deposit line contacts in a bifacial interdigitated scheme, whereby a thick layer of metal is deposited followed by a wet‐chemical etching step that effectively reduces shunting by preferentially etching away the shunting paths. Test devices with an area of 1 cm2 achieve pseudo fill factors (pFF) of above 75% and diode ideality factors of below 1·3, demonstrating that the proposed methods are well suited for the metallisation of the rear surface of EVA solar cells. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Effect of quasi-monocrystalline porous silicon at the backside on the photovoltaic parameters of a polycrystalline silicon solar cell

A three-dimensional model that simulates the performance of quasimonocrystalline porous silicon (QMPS) at the backside reflector of an elementary polysilicon solar cell is developed. Analytical expression for the photocurrent generated under the effect of the reflected light is derived in the base region. An improvement effect is obtained on the photovoltaic parameters compared to conventional BSF polysilicon solar cell (without QMPS). The QMPS layer gives an improvement which overtakes 4 mA/cm2 for the photocurrent, and 2.25 % for the cell efficiency. In addition, the effect of the QMPS layer is more important for a thin solar cell with passivated grain boundaries.

Silicon Thin-Film Solar Cells

We review the field of thin-film silicon solar cells with an active layer thickness of a few micrometers. These technologies can potentially lead to low cost through lower material costs than conventional modules, but do not suffer from some critical drawbacks of other thin-film technologies, such as limited supply of basic materials or toxicity of the components. Amorphous Si technology is the oldest and best established thin-film silicon technology. Amorphous silicon is deposited at low temperature with plasma-enhanced chemical vapor deposition (PECVD). In spite of the fundamental limitation of this material due to its disorder and metastability, the technology is now gaining industrial momentum thanks to the entry of equipment manufacturers with experience with large-area PECVD. Microcrystalline Si (also called nanocrystalline Si) is a material with crystallites in the nanometer range in an amorphous matrix, and which contains less defects than amorphous silicon. Its lower bandgap makes it particularly appropriate as active material for the bottom cell in tandem and triple junction devices. The combination of an amorphous silicon top cell and a microcrystalline bottom cell has yielded promising results, but much work is needed to implement it on large-area and to limit light-induced degradation. Finally thin-film polysilicon solar cells, with grain size in the micrometer range, has recently emerged as an alternative photovoltaic technology. The layers have a grain size ranging from 1 μm to several tens of microns, and are formed at a temperature ranging from 600 to more than 1000∘C. Solid Phase Crystallization has yielded the best results so far but there has recently been fast progress with seed layer approaches, particularly those using the aluminum-induced crystallization technique.