Chemical treatment of crystalline silicon solar cells as a method of recovering pure silicon from photovoltaic modules

Photovoltaic technology is used worldwide to provide reliable and cost-effective electricity for industrial, commercial, residential and community applications. The average lifetime of PV modules can be expected to be more than 25 years. The disposal of PV systems will become a problem in view of the continually increasing production of PV modules. These can be recycled for about the same cost as their disposal.Photovoltaic modules in crystalline silicon solar cells are made from the following elements, in order of mass: glass, aluminium frame, EVA copolymer transparent hermetising layer, photovoltaic cells, installation box, Tedlar^® protective foil and assembly bolts. From an economic point of view, taking into account the price and supply level, pure silicon, which can be recycled from PV cells, is the most valuable construction material used.Recovering pure silicon from damaged or end-of-life PV modules can lead to economic and environmental benefits. Because of the high quality requirement for the recovered silicon, chemical processing is the most important stage of the recycling process. The chemical treatment conditions need to be precisely adjusted in order to achieve the required purity level of the recovered silicon. For PV systems based on crystalline silicon, a series of etching processes was carried out as follows: etching of electric connectors, anti-reflective coating and n-p junction. The chemistry of etching solutions was individually adjusted for the different silicon cell types. Efforts were made to formulate a universal composition for the etching solution. The principal task at this point was to optimise the etching temperature, time and alkali concentration in such a way that only as much silicon was removed as necessary.     

Performance evaluation of low concentrating photovoltaic/thermal systems: A case study from Sweden

Some of the main bottlenecks for the development and commercialization of photovoltaic/thermal hybrids are the lack of an internationally recognized standard testing procedure as well as a method to compare different hybrids with each other and with conventional alternatives. A complete methodology to characterize, simulate and evaluate concentrating photovoltaic/thermal hybrids has been proposed and exemplified in a particular case study. By using the suggested testing method, the hybrid parameters were experimentally determined. These were used in a validated simulation model that estimates the hybrid outputs in different geographic locations. Furthermore, the method includes a comparison of the hybrid performance with conventional collectors and photovoltaic modules working side-by-side. The measurements show that the hybrid electrical efficiency is 6.4% while the optical efficiency is 0.45 and the U-value 1.9 W/m² °C. These values are poor when compared with the parameters of standard PV modules and flat plate collectors. Also, the beam irradiation incident on a north-south axis tracking surface is 20-40% lower than the global irradiation incident on a fixed surface at optimal tilt. There is margin of improvement for the studied hybrid but this combination makes it difficult for concentrating hybrids to compete with conventional PV modules and flat plate collectors.     

Effect of concentration on the performance of flat plate photovoltaic modules

The effect of low concentration ratios on the performance of passively cooled conventional photovoltaic modules has been investigated. Peak power outputs of up to 140 W per square metre of module area have been obtained with single crystal modules of high cell packing factor using a 2.2X plane mirror concentrator. Both cell temperature and series resistance losses are found to be important in limiting module efficiency. Performance simulations indicate that the use of a 4X concentrator with polar axis tracking will increase annual peak output by a factor of 3.2 over that of a fixed flat plate module.     

A model of silicon solar cells for concentrator photovoltaic and photovoltaic/thermal system design

The design of photovoltaic (PV) and photovoltaic/thermal (PV/T) solar energy conversion systems employing optical concentration requires simple models of solar cells which nevertheless have sufficient accuracy to be employed in design decisions. Semi-empirical expressions are presented for open-circuit voltage, short-circuit current, fill factor, and conversion efficiency of silicon solar cells as explicit functions of optical concentration (C) and temperature (T). In addition similar expressions are given for the solar cell current as a function of C and T and of the operating voltage V, to enable characterization under conditions of nonoptimal power transfer. The agreement of the model with experimental data is shown to be within ? 10% for all parameters. An example of an application of the model to system design is also presented.     

Operating temperature of photovoltaic modules: A survey of pertinent correlations

The importance of solar cell/module operating temperature for the electrical performance of silicon-based photovoltaic installations is briefly discussed. Suitable tabulations are given for most of the explicit and implicit correlations found in the literature which link this temperature with standard weather variables and material/system-dependent properties, in an effort to facilitate the modeling/design process in this very promising area of renewable energy applications.     

Durability of Fresnel lenses: A review specific to the concentrating photovoltaic application

The durability of Fresnel lenses used in the concentrating photovoltaic (CPV) application is reviewed from the literature. The examination here primarily concerns monolithic lenses constructed of poly(methyl methacrylate) (PMMA), with supplemental examination of silicone-on-glass (SOG) composite lenses. For PMMA, the review includes the topics of: optical durability (loss of transmittance with age); discoloration (the wavelength-specific loss of transmittance); microcrazing and hazing; fracture and mechanical fatigue; physical aging, creep, shape change, buckling, and warping; and solid erosion. Soiling, or the accumulation of particulate matter, is examined in the following contexts: its magnitude of reduction in transmittance; variation with time, module tilt, and wavelength; the processes of adhesion and accumulation; particle size, distribution, composition, and morphology; and its prevention. Photodegradation and thermal decomposition, mechanisms enabling aging, are examined relative to the CPV-specific environment. Aspects specific to SOG lenses include: solarization of the glass superstrate; corrosion of glass; delamination of the silicone/glass interface; change in focus due to thermal misfit between the laminate layers; and the chemical stability of poly(dimethylsiloxane) (PDMS). Recommendations for future research are provided, based on the most important and the least explored topics.     

Effects of spectral variation on the device performance of copper indium diselenide and multi-crystalline silicon photovoltaic modules

We present results of an experimental investigation of the effects of the daily spectral variation on the device performance of copper indium diselenide and multi-crystalline silicon photovoltaic modules. Such investigations are of importance in characterization of photovoltaic devices. The investigation centres on the analysis of outdoor solar spectral measurements carried out at 10 min intervals on clear-sky days. We have shown that the shift in the solar spectrum towards infrared has a negative impact on the device performance of both modules. The spectral bands in the visible region contribute more to the short circuit current than the bands in the infrared region while the ultraviolet region contributes least. The quantitative effects of the spectral variation on the performance of the two photovoltaic modules are reflected on their respective device performance parameters. The decrease in the visible and the increase in infrared of the late afternoon spectra in each case account for the decreased current collection and hence power and efficiency of both modules.     

A new protocol for characterization of dislocations in photovoltaic polycrystalline silicon solar cells

Main photovoltaic properties of polycrystalline silicon solar cells are often affected by dislocation effects. Dislocations degrade functional photocurrent and considerably alter relevant parameters such as short-circuit current density, dark current intensity and open-circuit voltage. In this study, we have developed an enhanced photothermal technical protocol for diagnosing dislocation spatial distribution inside photovoltaic polycrystalline silicon solar cells. We tried to establish a qualitative and quantitative correlation between the local thermal properties alteration and dislocation spatial range. Experimental imaging profiles, yielded by this technique are compared to other diagnostic techniques results.     

A one-unit photovoltaic electrolysis system based on a triple stack of amorphous silicon (pin) cells

An arrangement is suggested in one-unit photovoltaic electrolysis whereby the land area needed can be halved.     

Interface effects on the carrier transport and photovoltaic properties of hydrogenated amorphous silicon/crystalline silicon solar cells

Current-voltage-temperature (I-V-T) characteristics evaluated near 150K and 300K were used to study the photovoltaic property variations in hydrogenated amorphous silicon (a-Si:H)/crystalline silicon (c-Si) solar cells. The possible carrier transport mechanisms in such devices were examined from the I-V-T data which indicated a significant influence of the amorphous /crystalline interface on the short-circuit current density (J_sc) and open-circuit voltage (V_oc) of the solar cells. Carrier transport near 300K for forward biases was by a multi-tunneling mechanism and became space charge limited with increasing bias. For devices having low J_sc and V_oc an additional region was seen in both forward and reverse biases, at low temperatures, where the current simply varied linearly with the applied bias. This characteristic manifested in both high and low temperatures region for devices with still lower photovoltaic properties, which has been reasoned to be due to a higher interface density. Passivating the c-Si surface with HF just prior to the amorphous layer deposition resulted in a large improvement in the properties. The most significant effect was on the J_sc which improved by an order of magnitude. The treatment also affected the lower temperature I-V-T data in that the current fell to very low levels. The spectral response of the treated solar cells showed enhanced blue/violet response compared with the unpassivated devices. The interface passivation plus reducing a-Si thickness has improved the solar cell efficiency from 0.39% to 9.5%.     

Aeolian dust deposition on photovoltaic solar cells: the effects of wind velocity and airborne dust concentration on cell performance

Wind tunnel experiments were conducted to investigate the effect of wind velocity and airborne dust concentration on the drop of photovoltaic (PV) cell performance caused by dust accumulation on such cells. Performance drop was investigated at four wind velocities and four dust concentrations. I–V characteristics were determined for various intensities of cell pollution. The evolutions of the short circuit current, the open circuit voltage, the maximum power, the reduction of solar intensity received by the cells, and the fill factor variation with increasing cell pollution were examined. The deposition (and accumulation) of fine aeolian dust on PV cells significantly affects the performance of such cells. Wind velocity has an important impact on cell performance drop, since the drop is larger in high winds than in low winds. However, the wind also affects the sedimentological structure of the dust coating on the cell, resulting in a higher transmittance (of light) for coatings created during high winds. The wind tunnel experiments indicate that the former effect is more important than the latter, which means that, in general, the drop in PV cell performance due to dust accumulation is larger as wind speed increases. Airborne dust concentration also affects the drop in PV cell performance, since high dust concentrations lead to a higher accumulation on the cell. Contrary to wind speed, airborne dust concentration does not seem to affect the sedimentological structure of dust coatings (with respect to light transmittance) on PV cells.     

Calculations of efficiencies achievable using gallium arsenide and silicon cells mounted together in cavities illuminated by single-stage concentration

Detailed calculations have been performed to predict the efficiencies achievable using gallium arsenide and silicon cells mounted in cavities with concentrated light entering. Prototype devices with power output of the order of 2 W are proposed and analysed. The concept of the elliptical cavity with restriction of the angle of the escaping rays is exploited in order to avoid having to use a secondary concentrator. The light first falls on the gallium arsenide cell, with a concentration of 720 suns, before being reflected onto a silicon cell. Arrangements using both one and two silicon cells are studied. Depending on the type of cells used, and whether a dichroic filter is included, the possible efficiencies (relative to the light entering the cavity) are in the range 30.4–36%. The effect of the cavity is to increase thepower output by about 5% when compared to a similar bandsplitting arrangement not using a cavity. It is concluded that the cavity effect should be of interest for practical photovoltaic converters.     

Formation of porous silicon for large-area silicon solar cells: A new method

Luminescent porous silicon (PS) was prepared for the first time using a spraying set-up, which can diffuse in a homogeneous manner HF solutions, on textured or untextured (1 0 0) oriented monocrystalline silicon substrate. This new method allows us to apply PS onto the front-side surface of silicon solar cells, by supplying very fine HF drops. The front side of N⁺/P monocrystalline silicon solar cells may be treated for long periods without altering the front grid metallic contact. The monocrystalline silicon solar cells (N⁺/P, 78.5 cm²) which has undergone the HF-spraying were made with a very simple and low-cost method, allowing front-side Al contamination. A poor but expected 7.5% conversion efficiency was obtained under AM1 illumination. It was shown that under optimised HF concentration, HF-spraying time and flow HF-spraying rate, Al contamination favours the formation of a thin and homogeneous hydrogen-rich PS layer. It was found that under optimised HF-spraying conditions, the hydrogen-rich PS layer decreases the surface reflectivity up to 3% (i.e., increase light absorption), improves the short circuit current (I_sc), and the fill factor (FF) (i.e., decreases the series resistance), allowing to reach a 12.5% conversion efficiency. The dramatic improvement of the latter is discussed throughout the influence of HF concentration and spraying time on the I–V characteristics and on solar cells parameters. Despite the fact that the thin surfae PS layer acts as a good anti-reflection coating (ARC), it improves the spectral response of the cells, especially in the blue-side of the solar spectrum, where absorption becomes greater, owing to surface band gap widening and conversion of a part of UV and blue light into longer wavelengths (that are more suitable for conversion in a Si cell) throughout quantum confinement into the PS layer.     

The effect of hybrid photovoltaic thermal device operating conditions on intrinsic layer thickness optimization of hydrogenated amorphous silicon solar cells

Historically, the design of hybrid solar photovoltaic thermal (PVT) systems has focused on cooling crystalline silicon (c-Si)-based photovoltaic (PV) devices to avoid temperature-related losses. This approach neglects the associated performance losses in the thermal system and leads to a decrease in the overall exergy of the system. Consequently, this paper explores the use of hydrogenated amorphous silicon (a-Si:H) as an absorber material for PVT in an effort to maintain higher and more favorable operating temperatures for the thermal system. Amorphous silicon not only has a smaller temperature coefficient than c-Si, but also can display improved PV performance over extended periods of higher temperatures by annealing out defect states from the Staebler–Wronski effect. In order to determine the potential improvements in a-Si:H PV performance associated with increased thicknesses of the i-layers made possible by higher operating temperatures, a-Si:H PV cells were tested under 1 sun illumination (AM1.5) at temperatures of 25 °C (STC), 50 °C (representative PV operating conditions), and 90 °C (representative PVT operating conditions). PV cells with an i-layer thicknesses of 420, 630 and 840 nm were evaluated at each temperature. Results show that operating a-Si:H-based PV at 90 °C, with thicker i-layers than the cells currently used in commercial production, provided a greater power output compared to the thinner cells operating at either PV or PVT operating temperatures. These results indicate that incorporating a-Si:H as the absorber material in a PVT system can improve the thermal performance, while simultaneously improving the electrical performance of a-Si:H-based PV.     

Identification of PV solar cells and modules parameters using the genetic algorithms: Application to maximum power extraction

In this paper, we propose to perform a numerical technique based on genetic algorithms (GAs) to identify the electrical parameters (I_s, I_ph, R_s, R_sh, and n) of photovoltaic (PV) solar cells and modules. These parameters were used to determine the corresponding maximum power point (MPP) from the illuminated current-voltage (I-V) characteristic. The one diode type approach is used to model the AM1.5 I-V characteristic of the solar cell. To extract electrical parameters, the approach is formulated as a non convex optimization problem. The GAs approach was used as a numerical technique in order to overcome problems involved in the local minima in the case of non convex optimization criteria. Compared to other methods, we find that the GAs is a very efficient technique to estimate the electrical parameters of PV solar cells and modules. Indeed, the race of the algorithm stopped after five generations in the case of PV solar cells and seven generations in the case of PV modules. The identified parameters are then used to extract the maximum power working points for both cell and module.     

Investigation of multi-donor bulk-heterojunction photovoltaic cells based on P3HT:PCBM system

(2,7-bis[5′-(9,9-dioctylfluorene-2-yl)-2,2′-dithienyl-5-yl]-9,9-dioctylfluorene) (F3Th4) was used as a secondary electron donor material in the poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk-heterojunction photovoltaic cell. It is shown that the combination of F3Th4 with P3HT allows strong light absorption. The mechanism of charge transfer in the multi-donor PV cell was investigated; it shows that efficient energy transfer takes place from F3Th4 to P3HT. However, the short-circuit current (J_SC) of the multi-donor bulk-heterojunction photovoltaic (PV) cell still decreased. The possible reason for the smaller photocurrent is worsening of transport property after addition of F3Th4.     

Encapsulation of PV modules using ethylene vinyl acetate copolymer as a pottant: A critical review

The primary purpose of this work is to review the literature about what is and is not known about using ethylene vinyl acetate (EVA0 copolymer as the encapsulant (or pottant) material in photovoltaic (PV) modules. Secondary purposes include elucidating the complexity of the encapsulation problem, providing an overview about encapsulation of PV cells and modules, providing a historical overview of the relevant research and development on EVA, summarizing performance losses reported for PV systems deployed since ca. 1981, and summarizing the general problems of polymer stability in a solar environment. We also provide a critical review of aspects of reported work for cases that we believe are important.Failure modes resolved in the early work to establish reliability of deployed modules and the purposes and properties of pottants, are summarized. Typical performance losses in large field-deployed, large-scale systems ranging from 1% to 10% per year are given quantitatively, and qualitative reports of EVA discoloration are summarized with respect to ultraviolet (UV), world-wide location and site dependence.The general stability of polymers and their desirable bulk properties for solar utilization are given. The stabilization formulation for EVA, its effectiveness, and changes in it during degradation are discussed. The degradation mechanisms for the base resin, e.g., unstabilized Elvax 150^TM, and stabilized EVA are indicated for literature dating to the early 1950s, and the role played by unsaturated chromophores is indicated. The limited number of studies relating discoloration and PV cell efficiency are summarized.Observed degradation of EVA or the unstabilized base resin in the laboratory and examples used to measure the degradation are summarized in sections entitled: (1) thermally-induced degradation; (2) photodegradation and photothermal degradation of EVA in different temperature regimes; (3) photobleaching and photodegradation of the UV absorber and cross-linking agent; (4) acetic acid and metal and metal-oxide catalyzed oxidative degradation; and (5) discolaration and PV cell efficiency losses.Processing effects/influences on EVA stability are discussed in sections entitled: (1) EVA raw materials and extruded, uncured films; (2) thermal encapsulation processes; (3) effects of lamination, curing, and curing peroxide on gel content and chromophores formed; and (4) incomplete shielding of curing-generated chromophores. A summary is given for the limited number of accelerated lifetime testing efforts and examples of erroneous service lifetime predictions for EVA are discussed. The known factors that effect the discoloration rate of several EVA formulations are discussed in which the reduction in rate by using UV-absorbing superstrates is a prime example. A summary is given of what is and is not known about EVA degradation mechanisms, degradation from exposures in field-deployed modeules and/or laboratory testing, and factors that contribute to EVA stability or degradation. Finally, conclusions about using Elvax 150 in EVA formulations are summarized, and future prospects for developing the next-generation pottant for encapsulating PV modules are discussed.     

Analysis of power efficiency of concentrator photovoltaic modules based on solar cells with GaInP/Ga(In)As/Ge structure in various geographic regions

The simulation of the response of the working parameters of multijunction solar cells (MJSCs) in the composition of concentrator photovoltaic module (PVM) on the change in the spectral composition of the incident solar radiation in different climatic conditions. The problems of optimizing MJSC structures to increase the annual output of electric power by PVM were studied. The simulation of results showed the possibility of the power gain for the modules based on MJSCs with optimized structures ranging from 1.2% for tropical and 3.4% for arctic zones on Earth.     

Spectrally selective silicon paint coatings: Influence of pigment volume concentration ratio on their optical properties

Spectrally selective paint coatings based on silicon resin were prepared at different pigment volume concentration (PVC) ratios. Inorganic type pigment was found to be more suitable than the organic type for preparation of spectrally selective paint coatings. The temperature stability of thickness sensitivity spectrally selective (TSSS) paintcoatings was studied. The obtained results prove the possibility to the use the prepared coatings on laboratory scale for industrial application (coil-coating line) for solar collector.     

Effect of molecular aggregation by thermal treatment on photovoltaic properties of MEH-PPV: Fullerene-based solar cells

The effect of a thermal annealing treatment at various temperatures on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and buckminsterfullerene (C₆₀) composites was investigated. With the increase in temperature, three stages were observed: from 60 to 120 °C, the photovoltaic performance of the composite devices became better with the rising temperature; from 120 to 180 °C, the photovoltaic performance decreased to a minimum; but, at about 200 °C, a sharp rise occurred, followed by a gradual decline. The obvious red-shift of the photoluminescence (PL) spectra and the new PL peak and shoulder at about 620 and 660 nm, respectively, indicated the formation of molecular aggregation and the lengthened and the ordered conjugated segments. It was suggested that the crystallization of C₆₀ after thermal annealing at 200 °C, which matched the nano-sized ordered domains of MEH-PPV led to a sharp enhancement in photovoltaic performance.