Solar Energy Materials and Solar Cells

 - Overview

Photovoltaics is the process of converting sunlight directly into electricity using solar cells. Today it is a rapidly growing and increasingly important renewable alternative to conventional fossil fuel electricity generation,

Photovoltaics is a most elegant energy source. Light shines on a crystal and produces electricity. Its as simple as that. There are no moving parts. The fuel source (sunlight) is free, abundant and widely distributed, available to every country and person in the world. At over 165,000 TW the solar resource dwarfs the world’s current power usage of 16 TW or even our projected future usage of 60 TW. The many advantages of photovoltaics lend itself to being the ultimate energy source. However, it required the semiconductor revolution and advances in manufacturing before photovoltaics could begin to reveal its full potential. 

 - Photovoltaic Cells – Generating Electricity

When light shines on a photovoltaic (PV) cell - also called a solar cell - that light may be reflected, absorbed, or pass right through the cell. The PV cell is composed of semiconductor material; the “semi” means that it can conduct electricity better than an insulator but not as well as a good conductor like a metal. There are several different semiconductor materials used in PV cells. 

When the semiconductor is exposed to light, it absorbs the light’s energy and transfers it to negatively charged particles in the material called electrons. This extra energy allows the electrons to flow through the material as an electrical current. This current is extracted through conductive metal contacts – the grid-like lines on a solar cells – and can then be used to power your home and the rest of the electric grid. 

The efficiency of a PV cell is simply the amount of electrical power coming out of the cell compared to the energy from the light shining on it, which indicates how effective the cell is at converting energy from one form to the other. The amount of electricity produced from PV cells depends on the characteristics (such as intensity and wavelengths) of the light available and multiple performance attributes of the cell.  

[Mohammed bin Rashid Al Maktoum Solar Park is a solar park spread over a total area of 77 km2 in Seih Al-Dahal, about 50 kilometers south of the city of Dubai]

- Solar Cells

A solar cell, or photovoltaic cell, is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. It is a form of photoelectric cell, defined as a device whose electrical characteristics, such as current, voltage, or resistance, vary when exposed to light. Individual solar cell devices can be combined to form modules, otherwise known as solar panels.  

In general, a solar cell that includes both solar and nonsolar sources of light (such as photons from incandescent bulbs) is termed a photovoltaic cell. Fundamentally, the device needs to fulfill only two functions: photogeneration of charge carriers (electrons and holes) in a light-absorbing material, and separation of the charge carriers to a conductive contact that will transmit the electricity. Solar cells are wired in series and placed into a frame. The size of the frame can vary with manufacturers as a result of the technology used. A protective coating on the top covers and protects (and sometimes increases the output) of the solar cells. 

- Three Generations of Solar Cells

[The Technical University of Denmark]: Solar cell technologies are traditionally divided into three generations. 

• First generation solar cells are mainly based on silicon wafers and typically demonstrate a performance about 15-20 %. These types of solar cells dominate the market and are mainly those seen on rooftops. The benefits of this solar cell technology lie in their good performance, as well as their high stability. However, they are rigid and require a lot of energy in production. 
• The second generation solar cells are based on amorphous silicon, CIGS and CdTe, where the typical performance is 10 - 15%. Since the second generation solar cells avoid use of silicon wafers and have a lower material consumption it has been possible to reduce production costs of these types of solar cells compared to the first generation. The second generation solar cells can also be produced so they are flexible to some degree. However, as the production of second generation solar cells still include vacuum processes and high temperature treatments, there is still a large energy consumption associated with the production of these solar cells. Further, the second generation solar cells are based on scarce elements and this is a limiting factor in the price. 
• Third generation solar cells uses organic materials such as small molecules or polymers. Thus, polymer solar cells are a sub category of organic solar cells. The third generation also covers expensive high performance experimental multi-junction solar cells which hold the world record in solar cell performance. This type has only to some extent a commercial application because of the very high production price. A new class of thin film solar cells currently under investigation are perovskite solar cells and show huge potential with record efficiencies beyond 20% on very small area. Polymer solar cells or plastic solar cells, on the other hand, offer several advantages such as a simple, quick and inexpensive large-scale production and use of materials that are readily available and potentially inexpensive. Polymer solar cells can be fabricated with well-known industrial roll-to-roll (R2R) technologies that can be compared to the printing of newspapers. Although the performance and stability of third generation solar cells is still limited compared to first and second generation solar cells, they have great potential and are already commercialized, e.g. by infinityPV.com. Research interest in polymer solar cells has increased significantly in recent years and it is now possible to produce them at a price that enables projects such as the freeOPV initiative.

[More to come ...]