The heart of a PV array – the solar cell

As described in Chapter 2, solar thermal energy converts solar rays into heat. Photovoltaics is another way of using solar energy. This technology is based on a well known effect in physics: some semiconduc­tors convert light directly into electrical current. This chapter focuses on the main applications and the future of photovoltaics as seen from today.

Just as the absorber is the heart – the part that converts sunlight into heat – of a solar thermal array (see 2.1), the solar cell is the heart of a photovoltaic array. Spread out thinly over the largest possible area, this semiconductor converts incident light directly into electrical current.

The first solar cell was made using silicon in 1954, and 90 per cent of all solar cells currently made worldwide are still manufac­tured using this basic semiconductor material.

To reach voltage levels commonly used (such as 12V of direct current), multiple solar cells are ‘switched in parallel’ (connected in rows). When the cells are laminated into a sandwich between a glass pane on the top and a plastic foil on the back, you have a finished solar panel.

The efficiency of crystalline solar cells commonly sold on the market is generally around 15 per cent under standard test conditions. As temperatures rise, the power yield drops; ensuring that panels have some air underneath them to cool them off (natural ventilation) is therefore a good idea.

In addition to the silicon wafers described here, thin film has grown to cover roughly 10 per cent of the market. Thin-film cells generally consist of either amorphous silicon or some combination of copper, indium, gallium and selenium (CIS and CIGS), which will be discussed further in 3.5. Thin-film cells are much thinner than traditional crys­talline silicon solar cells, and the production methods are also very different.

How it works

In a process called ‘doping’, impurities (generally boron and phosphor) are intro­duced to a wafer generally thinner than 0.2mm consisting of highly pure silicon to create two layers with different electric prop­erties. When light reaches the solar cell, the charge carriers (electrons) from one layer flow to the other layer, creating a voltage of 0.5V at the contacts. This voltage within the solar cell remains relatively constant, but the current that comes out of the cell varies depending on the size of the cell and the intensity of incident light.

A number of other cell types are also being developed or in pilot production. The goal of this research and development is to make photovoltaics, which is currently relatively expensive, cheaper by using less material or less expensive material.1

Negatively doped silicon

 

Negative

electrode

 

Border layer

 

Power

consumer

 

Positively doped silicon

 

Positive

electrode

 

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The heart of a PV array - the solar cell

Figure 3.1 The heart of a photovoltaic array: The silicon solar cell

Source: Leuchtner and Preiser, Photovoltaik-Marktubersicht, 1994

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