Thin Single Crystalline Elongate Silicon Solar Cells

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Energy Procedia 00 (2011) 000–000 Energy Procedia 15 (2012) 58 – 66

Energy Procedia www.elsevier.com/locate/procedia International Conference on Materials for Advanced Technologies 2011, Symposium O

Thin Single Crystalline Elongate Silicon Solar Cells
Andrew Blakers*, Vernie Everett, Jelena Muric-Nesic and Elizabeth Thomsen
Centre for Sustainable Energy Systems, Australian National University, Canberra, Australia

Abstract Efficient singlecrystalline silicon solar cells with an elongated geometry (much longer than wide) have numerous applications in both mass markets and niche markets. In this paper a commercially promising elongate technology is described (SLIVER cells), together with two specialised applications for elongate cells: micro-modules for personal mobile power, and receivers for photovoltaic-thermal linear concentrators.
© 2011 Published by by Elsevier Ltd. Selection and/or peer-review under responsibility of Solar Energyof © 2011 Published Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee International Conference on Materials for Advanced Technologies. Research Institute of Singapore (SERIS) – National University of Singapore (NUS).
Keywords: Photovoltaics; elongate crystalline silicon solar cells; concentration modules

1. Introduction Elongate solar cells are cells that are long (typically cm) and narrow (typically mm). Such cells are useful in non-standard photovoltaic (PV) modules such as linear concentrator receivers and mobile flexible panels. Elongate cells can be formed by dicing larger solar cells into strips. Alternatively, they can be fabricated directly, such as SLIVER solar cells. All of the cells in a series-connected cell assembly must operate under the same illumination intensity for optimum performance. However, non-standard modules typically have non-uniform illumination. Elongate cells allow voltage to be rapidly built by series interconnection at a rate of 1-5 V per linear cm, depending on cell width. Current can be selected by suitable series/parallel connection between groups of cells. The ability of small groups of cells to reach the required system voltage reduces requirements for

* Corresponding author. Tel.: +61 2 6125 5905; fax: +61 2 6125 8873 E-mail address: andrew.blakers@anu.edu.au

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of International Conference on Materials for Advanced Technologies. doi:10.1016/j.egypro.2012.02.007

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broad illumination uniformity to merely local illumination uniformity. A feature of some types of elongate cells is low reverse breakdown voltage, which facilities the design of non-standard modules. This paper describes recent progress in the design and fabrication of elongate cell modules utilising crystalline silicon solar cells for use in linear concentrator receivers and non-concentrator flexible modules for personal mobile power. 2. SLIVER elongate silicon solar cells Sliver solar cells are thin, monocrystalline silicon solar cells fabricated using a combination of micromachining techniques and standard silicon device fabrication technologies. SLIVER technology [1-5] was developed at the Centre for Sustainable Energy Systems at the Australian National University (ANU), and is now being commercialised by Transform Solar (a joint venture between the large Australian energy utility Origin Energy and the American semiconductor company Micron Technology [6]) in Boise, Idaho. Rather than fabricating a single solar cell on the surface of a wafer, several thousand individual Sliver solar cells are fabricated within a single wafer. The dimensions of a Sliver cell depend upon wafer size, wafer thickness, and the micro-machining method employed. Cells typically have a length of 5-10 cm, a width of 0.5-2 mm, and a thickness of 20-60 µm. There are several ways to fabricate SLIVER cells. A representative process sequence using standard process technology is to diffuse phosphorus and boron into the opposite faces of a p-type 1 mm thick starting wafer, and then to form a protective dielectric on both surfaces. An array of deep grooves are then formed extending from one surface to the other on a pitch of 50-100 µm. Grooving can be done by a variety of means, including anisotropic etching of (110) wafers to yield (111) oriented groove sidewalls. The exposed sidewalls of each groove can then be textured by non-standard means [7, 8], followed by a phosphorus diffusion into each sidewall and deposition of a passivating dielectric and antireflection coating onto each sidewall. The protective dielectric coatings can then be removed from the two wafer surface and replaced by metal contacts. Finally, each SLIVER cell can be removed from its host wafer, rotated by 90º about its long axis and encapsulated into a module. The groove sidewalls now form the front and rear solar cell surfaces. The metallised surfaces of the cells are on the cell edges (Fig. 1(a)). The area of silicon that can be harvested from a single SLIVER wafer is much larger than the face area of the host wafer. For example, a 1.5 mm thick, 200 mm diameter wafer, when processed using the SLIVER technique to produce 40-50 µm thick SLIVER cells on a pitch of 75 µm with 80% wafer utilisation, yields an area of about 0.5 m2 of silicon solar cell. Spacing each cell apart in a module by a distance equal to the width of each cell allows a module of about 1 m2 to be populated (Fig. 1(b)). Light that passes between the cells can be largely trapped within the module through use of a scattering rear module reflector [9]. A yield of 130 watts per wafer and silicon usage of less than 1 gram/W is calculated for a module efficiency of 13% (allowing 20% for ingot tips & tails and wafering kerf loss - the wafers are 1-2 mm thick which reduces kerf loss). 20% efficient Sliver solar cells using standard cell processing methods and a robust processing sequence have been fabricated at ANU [10]. Current research efforts are primarily directed towards developing and establishing fabrication techniques to further simplify the fabrication sequence and to improve cell efficiency, in preparation for commercialisation for mass power applications. However, niche applications as described below are also interesting.

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(a) Fig. 1. (a) A wafer containing Sliver cells, together with (b) a submodule.

(b)

3. Flexible micro-modules using elongate SLIVER cells Solar cells fabricated on thin monocrystalline silicon substrates share the advantages of cells made on other thin film materials in terms of the fabrication of light weight, flexible solar cells with low material consumption. They also share the advantages of conventional singlecrystalline silicon solar cells in terms of high and stable cell efficiency, durability, unlimited raw material availability, non-toxicity and market acceptance. We utilise elongate SLIVER solar cells to fabricate flexible micro-modules for personal portable power [11-13]. SLIVER cells have the following notable advantages:  Since the processing treatment of both sidewalls of a sliver is identical, SLIVER cells are perfectly bifacial. This means that modules can be illuminated from either side, and that modules can be vertically mounted in the field.  Since the cell contacts are on the edges, cells can be laid side by side and the edges connected electrically to create series-connected submodules. This allows high voltages to be obtained from small areas.  Since the cells are thin, they are highly flexible – and can be bent around a 20 mm radius. For a PV module to be flexible not only must the cells be flexible but all components must be flexible.  Since the cells are both thin and efficient, they have a high power to weight ratio. Almost all the weight of the module is in the transparent protective packaging. The first step in module fabrication is the creation of arrays of cells. All cells in one block are connected in series, and blocks are arranged in the required structure for eventual connections in series or parallel. The electrical interconnections between cells are formed using syringe-dispensed flexible conductive paste. Solder is not used for electrical interconnections due to its inability to flex without failure. Connections are simply made by drawing a conductive path between the edge of one cell and the edge of an adjacent cell. To increase the robustness of the modules, each cell has multiple connections to adjacent cells. To allow parallel connections, to increase the robustness of the modules, and to connect to external connections, tinned copper busbars are used. These busbars have similar thickness, flexibility and width to the cells, allowing them to be easily incorporated into the design. Connections between cell blocks and the busbars are made using conductive paste, as shown in Fig. 2(a).

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(a)

(b)

Fig. 2. (a) Electrical connections in a module, and (b) flexible module stored in a retractable housing.

After all the electrical connections have been established, the module array is encapsulated. For unifacial modules a white backsheet and a clear frontsheet are used, while for bifacial modules both the front and back sheets are clear. A selection of fluoropolymer materials with thicknesses from 50 µm to 150 µm has been tested for transparency, resistance to abrasion, peel strength with silicone, water vapour transmission, and dimensional stability. Optically clear two-part silicone is used as the encapsulant material between the cover sheets because of its excellent light transmission, high UV stability, weather resistance, flexibility, and ease of use [2]. Silicone in un-cured liquid form can be easily applied in very thin layers, whereas ethylene-vinyl acetate (EVA) is not easily thinned much below the original sheet thickness. EVA also requires a more complex application and curing technique. Rollable modules can be stored in a cask with a spring loaded mechanism similar to that of a blind. The module is stored in the cask when not in use, and pulled out when required (Fig. 2(b)). All the additional electronics can be stored within the cask. A kW module could be stored in a 2 litre cask. Figure 3(a) shows the performance of a 0.1 m2 module when tested outdoors with an illumination of 1.1 suns in bright sunshine. The module has a short-circuit current of 1.42 A, an open-circuit voltage of 15.2 V, a fill factor of 67%, and a power output of 14.5 W. From these results the expected power output at 1 sun is 13 W (13% efficiency). No current matching of cells was performed when constructing this module, and it is likely that if cells were matched, higher performances could be achieved. Module flexibility can be assessed in several ways. One method is to mount the module on a curved surface for a prolonged period and to measure its electrical performance before and after flexing. A second method is to repeatedly flex the module to a given radius. Figure 3(b) shows a purpose-built jig to carry out repeated flexing. The radius of curvature can be easily changed by choosing the appropriate PVC pipe for mounting the module. The jig uses a stepper motor to repeatedly rotate the PVC pipe, and hence flex the attached module. The number of rotations can easily be varied, with 1000 flexures used as a standard test. Generally, provided that the cell does not break on its first flexure, we have found that it can withstand thousands or even hundreds of thousands of flexures without degradation [14].

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(a)

(b)

Fig. 3. (a) Current-voltage characteristics of 0.1 m2 module; (b) Flexibility jig with module undergoing repetitive flexing.

The components occupying the largest volume within the flexible modules are the cells, the silicone encapsulant and the cover sheets. Silicon has a density of 2.3 g/cm3, silicone has a density of 1 g/cm3, and fluoropolymer cover sheets have densities of around 1.7 g/cm 3. Our current module fabrication techniques can produce total thickness of approximately 0.3 mm, allowing the power to weight ratio to comfortably exceed 300 W/kg. 4. Hybrid concentrator PV-thermal (CPV-T) receivers based upon elongate cells Hybrid concentrator PV-thermal (CPV-T) systems allow capture of what would otherwise be waste heat. Essentially, the cooling fluid for a concentrator PV receiver is utilised as an energy source in its own right. By coupling CPV with thermal energy generation, net direct normal irradiance (DNI) conversion efficiency can exceed 70%. Roof mounting of concentrator systems allows thermal and electrical energy to be delivered to the retail side of the energy meters, where energy prices are triple wholesale energy prices. Additionally, since the system is roof mounted and does not need any large plant and power distribution infrastructure (and the associated planning), projects can be realised quickly. The requirement for point-focus concentrators to track in two dimensions means that single axis tracking linear concentrators are generally preferred for building roof applications. Most linear concentration systems use large (diameter > 1 m) parabolic mirrors, Fresnel refractors or Fresnel reflectors. In these systems the large reflector areas are exposed to hail, snow, rain, dust, and other soiling sources. The optical system also generally requires significant structural support, is subject to large wind loading, and must be manufactured to high tolerances to ensure flux uniformity on the receiver in the presence of distortion by gravitational loading, wind loading, and differential thermal expansion. These requirements mean that the systems are generally significant structural installations. Hence, one of the challenges for CPV systems in the urban rooftop market is to reduce the structural requirements of the systems. Point focus concentrator systems use commercially available, efficient, but expensive, specialist solar cells placed under high concentration ratios of up to 1000 suns. On the other hand, linear concentration systems operating at 10-30 suns can use silicon solar cells, which, while less efficient than triple junction

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cells, have much lower cost. However, low-concentration CPV system development is hampered by a lack of reasonably-priced, commercially-available concentrator solar cells at industrial volumes. A solution to this problem is to adapt 1-sun monocrystalline silicon solar cells from large production runs. A joint research venture between the Australian National University and Chromasun, a San Jose-based company, has developed a hybrid, linear CPV-T system designed specifically for urban and industrial rooftop applications [15, 16]. This system is known as a micro-concentrator (MCT) system (Fig. 4). The development was motivated by the benefit of on-site generation of both thermal and electrical energy with a larger combined efficiency and reduced footprint than independent PV and solar hot water systems. To reduce the structural requirements, and hence the weight of the CPV system, to make it suitable for rooftop applications, the MCT system incorporates the entire functional system into a low profile, glazed and sealed enclosure. The MCT system can be treated as a ‘black box’, requiring only electrical and plumbing connections similar to those of PV or domestic solar hot water, to simplify installation and maintenance. This enclosure is 3.0 m long, 1.2 m wide, and 0.3 m deep and isolates all the functional components of the system from external environmental influences such as wind loading, humidity, and soiling. Because of its low weight it can be mounted on a standard flat-plate PV mounting frame. The removal of the effect of wind loading on the optics allows the use of a Fresnel array of ultra-lightweight reflectors which require no structural support other than tensioning at mounting points at each end of the enclosure. The individual tensioned mounts are also used for tracking the mirrors.

Fig. 4. Chromasun micro-concentrator.

Elongate silicon solar cells lend themselves very well to linear concentrators. Flux uniformity across the width of the receiver is generally poor – a pronounced peak in irradiance is observed that oscillates across the receiver while tracking the sun. The requirement for uniform current from each cell means that the cells should span the width of the receiver. On the other hand, flux uniformity along the length of the receiver is generally good. The desirability of building voltage rather than current dictates an elongate geometry for the cells. A hybrid PV-thermal MCT receiver has been developed based upon elongate silicon solar cells. We are using diced one-sun cells that come from very large production runs, and hence have low cost. SLIVER cells might also play a role in future receivers. The cells are integrated into a sub-module

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assembly using a single substrate which incorporates structural support, heat sinking, electrical interconnection, and by-pass diode protection. By surface mounting the cells using a custom reflow process, the sub-module assemblies can be produced with inter-cell spacing of as little as 100 μm. Each submodule contains 30 elongate cells approximately 27 mm long and 9 mm wide, electrically connected in series, with four by-pass diodes per sub-module. Hybrid receivers consist of a series of sub-modules thermally bonded to the base of an aluminium extrusion which incorporates a cooling fluid channel along the rear of the extrusion (Fig. 5(a)). The cooling fluid is used to cool the cells, extracting the thermal energy for applications such as domestic water heating or driving absorption chillers. Once bonded to the extrusion the sub-modules are electrically interconnected. To protect the receivers, and increase their efficiency, the receivers are encapsulated using optically clear silicone gel covered with a sheet of 1 mm thick low-iron glass. These materials are designed to provide maximum transmission of light to the solar cells by creating a refractive index gradient between the cell and air. Practical limitations on the operating temperature of the PV cells place an upper bound on the thermal output temperature of the hybrid MCT. However, the MCT is readily configurable for PV-Thermal or thermal-only operation according to consumer demands. A modular approach to system installations offers PV energy generation in conjunction with thermal energy ranging from 60 to 220 °C. The thermal performance of the CPV-T hybrid MCT is currently being characterised at ANU. Receiver sub-modules were tested under one-sun illumination using a solar simulator, and under concentrated illumination using a custom-built test rig incorporating the same mirrors and tracking system as used in a full MCT unit. The I-V curve for a receiver sub-module, measured under approximately 14.5 suns DNI concentration, is shown in Fig. 5(b). The voltage gain under concentrated illumination almost offsets the decrease in fill factor, returning sub-module efficiencies under concentrations in the range of 10 to 20 suns similar to one-sun efficiency, despite increased series resistance losses.

(a)

(b)

Fig. 5. (a) Full 3 m long receiver; (b) Measured I-V curve of a 30-cell receiver sub-module under 14 suns concentration.

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Initial testing of 3 m long, 830 cm2 full receivers was performed under approximately one-sun AM1.5G illumination, with the receiver surface normal to the sun. Output was a Voc of 197 V, Isc of 118 mA, fill factor of 68%, and efficiency of 19.0%. The high voltage and low current of the module is due to the use of elongate cells. The reduced fill factor seen in these results is due to slight misalignment of the protective Alanod strips at the end of each sub-module, which can cause partial shading of cells adjacent to the end of each sub-module. In real-world testing with production-standard tracking and optics, submodule electrical efficiencies of 18.4% have been achieved. Testing under 20 suns concentration will take place soon. 5. Conclusion The development and use of elongate solar cells is a unifying theme for several projects at ANU. SLIVER technology is a novel high-efficiency elongate solar cell technology which has the advantages of crystalline silicon solar cells while offering many of the benefits of thin-film technology in terms of flexibility and reduced material usage. Thousands of SLIVER solar cells can be manufactured in a single conventional monocrystalline wafer. A single wafer can populate a 100 W module. SLIVER cells have been fabricated with efficiencies exceeding 20%. Current Sliver technology research at ANU is directed towards simplifying the fabrication process and improving cell efficiencies through the use of state-ofthe-art photovoltaic processing techniques. Currently, Transform Solar is undertaking a major commercialisation effort, having transferred manufacturing to a former semiconductor facility in the US. Portable flexible photovoltaic micro-modules based on monocrystalline silicon have been designed, fabricated, and tested. The micro modules use elongate SLIVER solar cells, which have many advantageous properties for flexible modules. Each element of the modules (cells, electrical connections, encapsulation) is flexible, and high power to weight ratios above 300 W/kg have been demonstrated. The ANU/Chromasun hybrid CPV-T MCT is a novel implementation of linear, hybrid CPV-T technology, utilising elongate silicon solar cells. The aim of the project is to develop a hybrid MCT capable of delivering reliable, cost-effective, flexible, and adaptable solar power generation in the form of electricity and thermal output to residential, commercial, and industrial rooftops. The thermal output can be used for process heat, hot water, and solar cooling. Laboratory-based testing has demonstrated submodule efficiencies of up to 19.4% at 14X optical concentration. In real-world testing with productionstandard tracking and optics, sub-module electrical efficiencies of 18.4% have been achieved. Acknowledgements Support from the Australian Research Council, the Asia Pacific Partnership program of the Australian Government and the CTD Programme managed by the Defence Science and Technology Organisation is gratefully acknowledged. The authors would like to acknowledge the support of the project commercial partners, Transform Solar and Chromasun Inc. Transform Solar is currently undertaking commercial development of Sliver technology, and the current Sliver technology research at ANU is being conducted in partnership with the company. Chromasun Inc. is presently undertaking commercial development of the hybrid CPV-T micro-concentrator system. These projects have been supported by the Australian Government through the Australian Solar Institute (ASI), part of the Clean Energy Initiative, and the Department of Climate Change. The Australian Government, through the ASI, is supporting Australian research and development in solar photovoltaic and solar thermal technologies to help solar power

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become cost competitive with other energy sources. The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein. References
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Weber KJ, Blakers AW. Semiconductor Processing. International patent application PCT/AU01/01546, 2001. Weber KJ, Blakers AW, Stocks MJ, Babaei JH, Everett VA, Neuendorf AJ, Verlinden PJ. A novel low cost, high efficiency micromachined silicon solar cell. Electron Device Lett. 2004; 25:37. Blakers AW, Stocks MJ, Weber KJ, Everett V, Babaei J, Verlinden P, Kerr M, Stuckings M, Mackey P. Sliver® solar cells. Proc. 13th Workshop on Crystalline Si Solar Cell Materials and Processes, Vail, Colorado; 2003. Weber KJ, Blakers AW, Everett V, Franklin E. Results of a cost model for Sliver® cells. Proc. 21st European Photovoltaic Solar Energy Conf., Dresden, Germany; 2006, p. 1314-7. Franklin E, Everett V, Blakers A, Weber K. Sliver solar cells: high-efficiency, low-cost PV technology. Advances in OptoElectronics 2007; 2007, Article ID 35383, doi:10.1155/2007/35383. Transform Solar website, www.transformsolar.com [accessed 22 June 2011]; 2011. Weber K, Blakers A. Semiconductor texturing process. US patent US2005104163, USA; 2005. Weber K, Blakers A. A novel silicon texturisation method based on etching through a silicon nitride mask. Prog. Photovoltaics 2005; 13: 691-5. Weber KJ, MacDonald J, Everett VA, Deenapanray PNK, Stocks MJ, Blakers AW. Modelling of Sliver® modules incorporating a Lambertian rear reflector. Proc. 19th European Photovoltaic Solar Energy Conf., Paris; 2004. Franklin E, Blakers AW, Everett V, Weber KJ. A 20% efficient Sliver solar cell. Proc. 22nd European Photovoltaic Solar Energy Conf., Milano, Italy; 2007. Thomsen E, Everett V, Blakers A, Brauers M, Davies E, Muric-Nesic J, Zhao HH, Skryabin I. Flexible modules of elongate solar cells. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion , Valencia, Spain; 2010, p. 617-21. Thomsen E, Everett V, Blakers A, Brauers M, Davies E, Muric-Nesic J, Surve S, Zhao HH, Skryabin I. Characterisation of flexible modules. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion, Valencia, Spain; 2010, p. 4223-6. Blakers AW, Armour T. Flexible silicon solar cells. Sol. Energy Mat. Sol. Cells 2009; 93: 1440-43. Everett V, Walter D, Harvey J, Vivar M, Van Scheppingen R, Surve S, Blakers A, Le Lievre P, Greaves M, Tanner A. Enhanced longitudinal and lateral flux uniformity for linear Fresnel reflectors in concentrating photovoltaic systems. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion, Valencia, Spain; 2010, p. 1060-2. Everett V, Walter D, Harvey J, Vivar M, Van Scheppingen R, Surve S, Blakers A, Le Lievre P, Greaves M, Tanner A. A linear Fresnel hybrid PV/thermal micro-concentrator system for rooftop Integration. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion, Valencia, Spain; 2010, p. 3937-41. Everett V, Walter D, Harvey J, Vivar M, Van Scheppingen R, Surve S, Blakers A, Le Lievre P, Greaves M, Tanner A. A closed loop tracking system for a linear Fresnel hybrid PV/thermal micro-concentrator system. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion, Valencia, Spain; 2010, p. 1063-5.

[15] [16]

References: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Weber KJ, Blakers AW. Semiconductor Processing. International patent application PCT/AU01/01546, 2001. Weber KJ, Blakers AW, Stocks MJ, Babaei JH, Everett VA, Neuendorf AJ, Verlinden PJ. A novel low cost, high efficiency micromachined silicon solar cell. Electron Device Lett. 2004; 25:37. Blakers AW, Stocks MJ, Weber KJ, Everett V, Babaei J, Verlinden P, Kerr M, Stuckings M, Mackey P. Sliver® solar cells. Proc. 13th Workshop on Crystalline Si Solar Cell Materials and Processes, Vail, Colorado; 2003. Weber KJ, Blakers AW, Everett V, Franklin E. Results of a cost model for Sliver® cells. Proc. 21st European Photovoltaic Solar Energy Conf., Dresden, Germany; 2006, p. 1314-7. Franklin E, Everett V, Blakers A, Weber K. Sliver solar cells: high-efficiency, low-cost PV technology. Advances in OptoElectronics 2007; 2007, Article ID 35383, doi:10.1155/2007/35383. Transform Solar website, www.transformsolar.com [accessed 22 June 2011]; 2011. Weber K, Blakers A. Semiconductor texturing process. US patent US2005104163, USA; 2005. Weber K, Blakers A. A novel silicon texturisation method based on etching through a silicon nitride mask. Prog. Photovoltaics 2005; 13: 691-5. Weber KJ, MacDonald J, Everett VA, Deenapanray PNK, Stocks MJ, Blakers AW. Modelling of Sliver® modules incorporating a Lambertian rear reflector. Proc. 19th European Photovoltaic Solar Energy Conf., Paris; 2004. Franklin E, Blakers AW, Everett V, Weber KJ. A 20% efficient Sliver solar cell. Proc. 22nd European Photovoltaic Solar Energy Conf., Milano, Italy; 2007. Thomsen E, Everett V, Blakers A, Brauers M, Davies E, Muric-Nesic J, Zhao HH, Skryabin I. Flexible modules of elongate solar cells. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion , Valencia, Spain; 2010, p. 617-21. Thomsen E, Everett V, Blakers A, Brauers M, Davies E, Muric-Nesic J, Surve S, Zhao HH, Skryabin I. Characterisation of flexible modules. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion, Valencia, Spain; 2010, p. 4223-6. Blakers AW, Armour T. Flexible silicon solar cells. Sol. Energy Mat. Sol. Cells 2009; 93: 1440-43. Everett V, Walter D, Harvey J, Vivar M, Van Scheppingen R, Surve S, Blakers A, Le Lievre P, Greaves M, Tanner A. Enhanced longitudinal and lateral flux uniformity for linear Fresnel reflectors in concentrating photovoltaic systems. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion, Valencia, Spain; 2010, p. 1060-2. Everett V, Walter D, Harvey J, Vivar M, Van Scheppingen R, Surve S, Blakers A, Le Lievre P, Greaves M, Tanner A. A linear Fresnel hybrid PV/thermal micro-concentrator system for rooftop Integration. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion, Valencia, Spain; 2010, p. 3937-41. Everett V, Walter D, Harvey J, Vivar M, Van Scheppingen R, Surve S, Blakers A, Le Lievre P, Greaves M, Tanner A. A closed loop tracking system for a linear Fresnel hybrid PV/thermal micro-concentrator system. Proc. 25th European Photovoltaic Solar Energy Conf., 5th World Conf. on Photovoltaic Energy Conversion, Valencia, Spain; 2010, p. 1063-5. [15] [16]