Solar Energy to Photo Voltaic Electricity

Photo Voltaic Electricity is produced by direct conversion of light into electricity at the atomic level.

Some materials (semi-conductors) exhibit a property called “the photoelectric effect” which results in release of electrons when impinged upon by light (photons). These free electrons are captured to produce an electric current.

The photoelectric effect was discovered by a French physicist, Edmund Bequerel, in 1839. He noted that certain materials would produce small amounts of electric current when exposed to light. In 1905, Albert Einstein scientifically attempted to quantify the photoelectric effect in relation to light intensity. Photovoltaic technology is based,on his work for which he later won a Nobel prize in physics.

Some of the first photovoltaic modules were constructed by Bell Laboratories around 1954. In the 1960s, the space industry initiated using the technology to power the spaceships. The advanced as a direct consequence of the requirements of the space programs for a source of energy, far away in space. As a result of dedicated research, the technology advanced and  its reliability was established thereby reducing the costs. During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source of power for non-space applications.

The illustration above describes the operation of a photovoltaic cell/ solar cell. Solar cells are made of semiconductors e.g. silicon. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, with positive on one side and negative on the other. When photons/ light energy impinges the solar cell, electrons are knocked loose from the atoms in silicon. The electrons can be captured in the form of an electric current, which constitutes DC electricity.

A series/number of solar cells connected to each other and mounted in an aluminum support structured frame is called a photovoltaic module. Modules supply electricity at a certain voltage. The current produced is directly dependent on how much light strikes the module.

Multiple modules are wired/connected  to form an array. Generally speaking, the area of the module is proportionate  to the charge/electricity produced. Photovoltaic modules and arrays produce direct-current (dc) electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.

Most common PV devices utilize a single junction/interface, to create an electric field within a semiconductor. In a single-junctioned PV cell, photons with energy equal to or greater than the band gap of the cell material can initiate an electric current. Thus, the photovoltaic response of single-junction cells is limited to the portion of the sun’s spectrum whose energy is above the band gap of the absorbing material.

To get around this limitation, cells/modules/panels were constructed with two (or more) different cells, with more than one band gap and more than one junction, to generate a voltage. These are referred to as “multijunction” cells (also called “cascade” or “tandem” cells). Multijunction devices can achieve a higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.

As depicted below, a multijunction device is a stack of individual single-junction cells in descending order of band gap (Eg). The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-band-gap cells.

Much of today’s research in multijunction cells focuses on gallium arsenide as one (or all) of the component cells. Such cells have reached efficiencies of around 35% under concentrated sunlight. Other materials studied for multijunction devices have been amorphous silicon and copper indium diselenide.

As an example, the multijunction device below uses a top cell of gallium indium phosphide, “a tunnel junction,” to aid the flow of electrons between the cells, and a bottom cell of gallium arsenide.

 

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