Solar Cells and Modules

Overview

A solar cell or photovoltaic (PV) 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 Photovoltaic (PV) Cells

Monocrystalline silicon PV cells are made from silicon wafers that are cut from cylindrical single-crystal silicon ingots. The rotund cells have to be cut to form nearly quadratic cells, that can be easily integrated in one module. 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, with about 85% in 2010, have the largest market share. However, other alternatives are challenging this technology.

A solar cell made from a monocrystalline silicon wafer

Polycrystalline Silicon Photovoltaic (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.org, 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 Photovoltaic (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

A CdTe photovoltaic array

Third Generation Solar Cells

Currently there are solar cells based in different new technologies in the way to market maturity, for example the high efficiency cells:

Thin film III-V solar cells:

• nion of the semiconductors from the third and the fifth group from the periodic table.
  
• Efficiency of 20-25%
  
• A variety of possible combination increasing price while increasing efficiency
  
• Most common connection: gallium arsenide (GaAs),
• Application: Power supply of satellites
  

Multi-stack thin film:

• "Stacking" III-V solar cells or silicon cells
• Efficiency up to approximately 37% - Each cell absorbs a certain wavelength, and then the stack can absorb more from the solar spectrum
• The top cell material has the highest band gap and covers the highest absorption area. Underlying cells absorbs the section of the solar spectrum with smaller wavelengths.
• Series connection of the overlying cells
• Other names for mutli-stack solar cells (depending on the number of layers) are: Tandem, triple, or multiple cascade cells.

Concentrator Photovolatics

Concentrator photovoltaics (CPV) are based on lenses or mirrors which focus direct sun light on solar cells. These cells consist of a small amount of highly efficient, but expensive, PV-material (silicon or III-V compounds, generally gallium arsenide or GaA). At present concentrating intensities vary from a factor of 2 to 100 suns (low concentration) to 1000 suns (high concentration). Commerical module efficiencies lay in the range of 20 to 25 percent, although efficiencies of 25 to 30 percent could have been achieved with gallium arsenide . An efficiency of 41,1 percent has been achieved in the laboratory by the Fraunhofer Institut für solare Energiesysteme, Germany (concentrating intensity: 450 suns) . In order the course of the sun Concentrator modules are mounted on a 2-axis tracking system. In case of low-concentration-PV there exist 1-axis tracking systens and less complex lenses.

Concentrating module

Organic Solar Cells

Hybrid dye-sensitised solar cells:

instead of semi-conductor material hybrid dye-sensitised solar cells use an organic dye for light absorption. Simple silk-screen allows cost-efficient production costs and offers different scopes for design as implementation in frontages or in advertising purposes . hybrid dye-sensitised solar cells are able to use diffuse sun light. Average efficiencies vary from 2 to 4%.

Fully organic solar cells:

Organic solar cells are still at an early stage of development. These cells consist of organic hydrocarbon compounds as well as a special electron structure and are able to generate electricity on foils and textiles. At present average cell efficiencies vary between 3 and 5 percent . Minor material consumption and more efficient production technologies lead to a high potential for cost-efficient production. However, there is still need for research activities as life spans are very short.

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.

Comparison of Different Types of PV Modules

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

To compare the price of the different modules, the next database present monthly updated values: solarserver.de

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

Further Information

• Solar Portal on energypedia
• Further information on PV modules in solar home systems see standards for the PV generator.