Solar Cells and Modules

Solar Cells

A solar cell or photovoltaic cell is a semiconductor device that converts light directly into electricity by the photovoltaic effect. The most common material in solar cell production is purified silicon that can be applied in different ways.

Monocrystalline Silicon PV Cells

Monocrystalline silicon PV cells are made from silicon wafers that are cut from cylindrical single-crystal silicon ingots. To form nearly quadratic cells, that can be easily integrated in one module, the rotund cells have to be cut. Thus, refined silicon is wasted in the cell production process. Monocrystalline silicon shows predictable and uniform behaviour but due to the careful and slow manufacturing processes required, it is also the most expensive type of silicon.

Modules consisting of monocrystalline silicon PV cells reach commercial efficiencies between 15 % and 18 %. So far, they are the most efficient modules and have the largest market share.

A solar cell made from a monocrystalline silicon wafer

Polycrystalline Silicon PV Cells

Polycrystalline or multicrystalline silicon PV cells are made from cast square ingots - large blocks of molten silicon, carefully cooled and solidified. They are less expensive to produce than monocrystalline silicon PV cells, but are marginally less efficient, with module conversion efficiencies between 13 and 16 %. 

PVCDROM, an educational website on photovoltaics, shows the production process of polycrystalline silicon PV cells with pictures and viedeo sequences.

Polycrystaline PV cells laminated to backing material in a PV module

Thin Film PV Cells

The various thin film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs from that of bulk materials (in the case of silicon thin films) but also tends to reduce energy conversion efficiency (an average 6 to 12 % module efficiency),

Thin film PV cells are constructed by depositing extremely thin layers of photovoltaic semi-conductor materials onto a backing material such as glass, stainless steel or plastic. Thin film materials commercially used are amorphous silicon (a-Si), cadmium telluride (CdTe), and copper-indium-(gallium)-diselenide (CI(G)S).

Commercially available thin film modules:

• Are potentially cheaper to manufacture than crystalline cells
• Have a wider customer appeal as design elements due to their homogeneous appearance
• Present disadvantages, such as low-conversion efficiencies and requiring larger areas of PV arrays and more material (cables, support structures) to produce the same amount of electricity

Partly taken from: IEA PVPS

A CdTe photovoltaic array

Solar Modules

For almost all applications, the one-half volt produced by a single cell is inadequate. Therefore, cells are connected together in series to increase the voltage. Several of these series strings of cells may be connected together in parallel to increase the current as well.

In order to use the connected cells in practical applications they must be:

• protected from mechanical damage during manufacture, transport and installation and use (in particular against hail impact, wind, sand and snow loads). This is especially important for wafer-based silicon cells which are brittle.
• protected from moisture, which corrodes metal contacts and interconnects, (and for thin-film cells the transparent conductive oxide layer) thus decreasing performance and lifetime.

Therefore, the interconnected cells and their electrical connections are sandwiched between a top layer of glass or clear plastic and a lower level of plastic or plastic and metal. An outer frame is attached to increase mechanical strength, and to provide a way to mount the unit. This package is called a "module" or "panel". Typically, a module is the basic building block of photovoltaic systems

The peak power output of a solar module depends on the number of cells connected and their size. Module performance is generally rated under Standard Test Conditions (STC) : irradiance of 1,000 W/m², solar spectrum of AM 1.5 and module temperature at 25°C. Solar modules are rated in peak watts [W_p] according to their output under STC. Thus, a 50 W_p module can be expected to supply 50 W of power under optimal conditions. The performance is reduced by high temperatures.

Like solar cells modules can be connected in series and/or in parallel to increase the voltage and/or the current depending on the system requirements.

Partly taken from: Polar Power Inc.

Comparison of Different Types of PV Modules 

Cell material	Module efficiency	Surface area needed for 1 kWp	Advantages	Disadvantages
Monocrystalline silicon	15-18 %	7-9 m²	- most efficient PV modules - easily available on the market - highly standardised	- most expensive  - waste of silicon in the production process
Polycrystalline silicon	13-16 %	8-9 m²	- less energy and time needed for production than for monocrystalline cells (= lower costs) - easily available on the market - highly standardised	- slightly less efficient than monocrystalline silicon modules
Micromorph tandem (aµ-Si)	6-9 %	9-12 m²	.	- more space for the same output needed
Thin film: Copper indium diselenide (CIS)	10-12 %	9-11 m²	- higher temperatures and shading have lower impact on performance - lower production costs	- more space for the same output needed
Thin film: Cadmium telluride (CdTe)	9-11 %	11-13 m²	- higher temperatures and shading have lower impact on performance - highest cost-cutting potential	- more space for the same output needed
Thin film: Amorphus silicon (a-Si)	6-8 %	13-20 m²	- higher temperatures and shading have lower impact on performance - less silicon needed for production	- more space for the same output needed

Partly taken from: Solarpraxis (Hrsg.): Photovoltaik für Profis. Verkauf, Planung Montage von Solarstromanlagen, 2., vollständig überabeitete Auflage, 2009, S. 131-135.

Due to their high efficiency, market penetration and standardisation mono- and polycrystalline silicon modules are the most common modules used in solar home systems. However, the market share of thin film modules is growing, especially in free-standing and building integrated PV systems.

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