Solar Cell: Photovoltaic Diode Technology

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The solar cell or photovoltaic diode has become the centre of solar panels used for electricity generation as well as for powering many smaller items of electronic equipment.

In recent years the importance of the solar cell has gone from a technology that is of comparatively small use to one that is now generating huge amounts of electricity and a key element of the renewable energy industry.

Along with this increased use, there has been a huge investment in solar cell technology, increasing the performance and in particular the energy conversion percentage as well as reducing production costs.

Development of the solar cell / photovoltaic diode

The photovoltaic effect has been observed in materials for many years - it is by no means as new as one might believe.

The first observations were made by Becquerel in 1839 when he observed the effect in a junction formed between an electrolyte and an electrode.

Then in 1876, similar effects were noted in selenium by a pair of researchers named Adams and Day. Then later in 1930 Shockley noticed that copper oxide produced the same effects. This observation was backed up by Grondahl in 1933.

As semiconductor research started to move forward during and after the Second World War, photovoltaic effects were noticed with germanium.

However it was not util around 1954 that the solar cell as such started to gain more scientific interest with discoveries that the photovoltaic effect was seen on single crystal silicon cells.

As semiconductor research increased the solar cell was investigated more. They were initially used for powering small devices as efficiency levels were very low. Around the late 1990s for example small calculators were often powered by a solar cell.

As interest in renewable energy grew so ddid the research into solar cells and efficiency levels as well as overall power handling grew..

The need for solar cells

Solar cells are an ideal way of capturing solar energy and putting it to good use.

There is a great need to use renewable energy like the Sun to met our needs for electrical energy.

Averaged out over the surface of the Earth, each square metre receives about 164 watts of solar energy. This is a colossal amount of energy and it mans that even if a small amount of this energy could be converted into electrical energy then it would be able to meet our needs with no need for burning fossil fuels.

Even the requirement for portable energy could be met - solar powered smartphones and the like might be possible, even if they were not practicable as they would not be exposed to light when this was needed. However it does go to demonstrate the need for solar cels.

Solar cell / photovoltaic diode technology basics

In essence the solar cel is a form of PN junction diode that has been optimised to convert light energy into electrical energy.

In fact either standard PN junctions of Schottky diodes can be used although a standard PN junction is more commonly used - either P on N or N on P. The standard PN junction offers a higher open circuit voltage along with with a better level of reliability.

The solar cell operates because when solar energy, or any light for that matter hits the PN junction electricity is generated.

Slightly more specifically, the light photons penetrate the semiconductor surface and travel down. The photons give up their energy to electrons in the P-type region causing them to have sufficient energy to travers the junction into the N-type region, thereby causing a current to flow around the overall circuit.

Basic PN junction solar cell structure

There are obviously a number of structures for photovoltaic diodes or solar cells. However they tend to have many similarities and they generally comprise a PN junction, often with an N-type substrate, a thin P-type layer.

Above the P region there is a top metallisation layer for the top connection. This consists of a metal connection region along one side and then thin fingers that extend out. This is to minimise the surface area covered by the metallisation as this would inhibit the light collection.

There is a top antireflection coating to ensure that light is absorbed rather than collected, and a bottom metallisation layer for the other contact.

Normally the top region that is normally the P region is kept thin so that the junction is between 0,5 to 1µm deep.

There is a trade off for the junction depth. If the junction is too deep then it is found that it is not as efficient in collecting carriers that are excited near the surface. This is a greater issue for light that has a shorter wavelength.

If it is too shallow then the resistivity of the top P region is increased and again efficiency levels are lower as resistive losses increase.

Schottky diode structure

It is also possible to fabricate a Schottky diode version of a solar cell. The additional layer of metallisation is about 100Angstroms thick and enables a Schottky barrier diode junction to be formed.

The Schottky barrier version of the solar cell offers a number of advantages in terms of the fabrication and the structure itself. In this first instance no high temperature processing is required for the device. This avoids enhanced diffusion along the grain boundaries for poly-crystalline materials. There is no requirement for the P-type region in the diode structure and this removes the need for a high temperature diffusion process. Instead the diode junction is created by the presence of the layer of metallisation on the surface.

Another advantage is that the diode junction is right at the surface of the material and this gives a much better short wavelength response. The disadvantage is that there is a higher level of dark current.

Series resistance of the solar cell

In view of the fact that solar cells are used to generate electricity, the series resistance is a key issue. If the series resistance of the solar cell itself is high, then a significant proportion of the power will be dissipated as heat within the device. This will naturally reduce the conversion efficiency of the device.

The series resistance of the solar cell is very dependent upon the contact grid structure used to make electrical contact near the diode junction at the top of the device, i.e. where the junction is exposed to light.

There is a compromise to be made between increasing the surface area of the grid which will improve the contact to the solar cell and also reduce the ohmic losses in the grid itself. However increasing the grid area will also reduce the area of the diode that is exposed to light.

Normally there is an optimum value that can be calculated during the design of the fabrication process for the particular solar cell.

To help in the level of light transmission into the active diode area, non-reflective coatings are often used. Additionally some transparent metallisation materials for the grid have been tried.

How a solar cell works

Solar cells work by enabling the solar energy to strike the solar cell and create electrical charge which can flow in the external circuit.

The photon beam striking the solar cell will have energy. If this energy exceeds the energy gap then it will create an electron-hole pair.

The PN junction, whether it is a standard PN junction of a Schottky barrier will then separate the electrons and holes and this results in an external current flowing.

In other words the photovoltaic process consists of light photons being absorbed by the photovoltaic cell to produce excess carriers. These are then separated by the junction to cause a current to flow in the external circuit.

Solar cell applications

Solar cells are used in a huge number of different applications. Over time they have been used in very many areas:

Solar cell power generation:   Solar cells have gained widespread use as a form of renewable energy source. Large solar panels are used to provide the collection of a sufficient amount of solar energy to be used to feed into the power grid. A number of solar panels are used in a series / parallel arrangement. These are used to drive an inverter which converts the DC supplied by the panels to AC with a waveform that synchronises in frequency and phase with the power line waveform. Either small systems, sometimes on private houses, or larger solar "farms" are seen.

Note on Solar Electricity Generation:

Solar panels are now widely used for generating electricity not only for small self contained items needing power, but more importantly for mains power systems. There are solar farms as well as local generation where panels are placed on the roofs of houses, etc.

Read more about Solar Electricity Generation.

Power for remote electrical items:   Solar cells provide the ideal source for power for relatively small electrical items in remote areas. Road signs are often seen with solar panels. They will require rechargeable batteries so that the system remains powered at night or when the level of light is low. But these systems are being seen increasingly.

Small home gadgets:   There is a host of small gadgets and the like that are used at home that are being solar powered. Electronic calculators were one of the first, but the list of solar powered items is growing. One example is a solar powered home garden watering system that can be easily installed and includes a pump to deliver water that has been collected from a water butt reducing the need for mains water.

Solar powered power banks:   There is an increasing number of lithium ion power banks that have a solar panel. Although these would take a long time to charge the power bank, the power bank can be charged normally and sunlight be used to keep it topped up. Alternatively larger arrays can be sued to charge smartphones and other gadgets.

Solar cells of photovoltaic diodes are being widely used for converting solar energy into electrical energy. As the need for remote power capabilities as well as renewable energy generation, solar cells are becoming increasingly important.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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