3.1 General considerations

Up to now, the PV cells used in products have been mainly limited to cells with flat surfaces. The main reason for this is found in the early PV cells made from thick Silicon wavers which were rigid and fragile. Also only flat laminations were feasible. Nowadays however there are several new emerging technologies enabling curved PV surfaces resulting in new product design opportunities for integrating PV cells into or on products. Also PV cells can have various colours. It is rather strange that this freedom in shape and colour in photovoltaic cells is not yet widespread in use in industrially designed products, while in architectural design it has already been recognised and is in fact identified as one of the major factors that pushed the growing use of photovoltaic on buildings [Pellegrino et. al., 2002].

3.2 Curved PV surfaces

In general the PV technologies with potentials for curved surface can be divided into:

1. Thin wafer mono and multi crystalline Silicon PV cells

2. Thin Film amorphous Silicon PV cells on flexible substrates

3. Thin Film PV cells deposited on or between flexible substrates

4. Dye Sensitised PV cells on or between flexible substrates

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Other emerging technologies Ad. 1.

In the same category one could place thin micro-crystalline and multi-crystalline PV wavers.

Figure 11:

Curved PV panel by using multi-crystalline wafers pressed between curved glass plates

For safety and stability the thin wafers could be pressed and sandwiched in between two curved glass plates as can be seen in Figure 11 [Beers, 2003]. Colour if functional can be
added either by colouring the individual PV cells or by colouring the thick glass plates. The prototype panel in Figure 11 is made consisting of two 5 mm thick pre-bend glass plates laminated with resin. The back panel is colored blue. This prototype panel has been made with aid of ‘Tetterode Curved Glass BV, in Voorthuizen and Energy research Centre of the Netherlands (ECN) in Petten.

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Thin amorphous Silicon PV cells can be manufactured for instance with Chemical Vapour

Deposition (CVD) techniques. Curved PV surfaces can be obtained by using for example ridged curved glass plates as substrate for these CVD techniques [Curvet, 2004]. These modules are mmanufactured in co-operation between Curvet Spa from Pesaro and Bekaert ECD Solar Systems LLC ( modules with brand name Uni-Solar®) An example of such a multi-junction amorphous Silicon PV can be seen in figure 12.

Figure 12: Amorphous Silicon PV module on curved glass substrate

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Thin film PV cells deposited on flexible substrates such as metal foils. For example Flexible CIS (copper, indium and selenium) PV cells [Solarion, 2004]

Figure 13: Example of a bendable thin CIS PV module

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Dye Sensitised PV cells can be manufactured as laminates between plastic sheets [Boschloo and Hagfeldt, 2003]. These PV devices can even be made as flexible ribbons. Still performing in indoor conditions such as for example: at 250 lux yielding 3 V and 8 uA with an active area of 10,8 cm2 The core of the dye-sensitized technology consists of nanometer-scale crystals of TiO2 semiconductor coated with a monolayer of light-absorbing dye and embedded in an electrolyte between the front and back electrical contacts.

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Other possible solutions for curved PV cells are provided by the so-called ‘Semiconductor on Insulator by Enamel Techniques’ [Poelman and Kan, 2000] and the ‘Concrete Solar Cell’ [Arthur et. al., 1997]. In the first one, doped Silicon granules are imbedded in Indium doped Emails. The Indium has two functions as doping reservoir and as conducting interconnection, resulting on large surface PV cells. In the second solution doped Silicon granules was embedded in concrete resulting in Building Integrated PV walls.