How a photovoltaic cell works: How Solar Power is Converted to Electrical Energy

So how exactly does a photovoltaic cell convert radiant sunlight into electrical power? This article breaks the process down in layman terms.

Conversion Efficiency: From 1% to 17%

The efficiency of a photovoltaic power cell is proportionate to the ratio of sunlight energy which can be absorbed and converted into an electrical charge. The more efficient a PV device is, the more energy it will generate based on the same amount of sunlight. Making Solar Power devices more efficient will make them more viable to other fuel alternatives, such as gasoline, natural gas, coal, or ethanol. More efficiency also means, less solar panels will be required to power a home, substantially cutting down the installation costs and the amount of space the system would consume.

Great strides have been made since the earliest PV devices were built. Early devices were only converting sunlight at a rate of 1% to 2% efficiency, while current devices are now able to convert as much as 7% to 17% of light energy into electrical power! Modern PV systems can now generate electrical power at a fraction of the resource cost of early systems. Both the old and new devices are based on the same principles.

The Photovoltaic Effect: Converting Sunlight To Energy

The photovoltaic effect is the process through which a photovoltaic cell converts radiant heat from the sun into electrical power. Some photons pass through, others are deflected, while a few are absorbed by the atoms in the semiconductor material. Those when enough photons from the sun are absorbed by PV cell, an electron is dislodged from the atom.

Specialized PV Material

The surface of a PV cell is designed either with an extra electron, or missing an electron to make the photocell more effective (when applied to silicon the process is referred to as doping). Crystalline silicon is the most commonly used PV conductor example because it has always been the most commonly used semiconductor in photovoltaic equipment.

How an Electrical Current Is Generated

When negative charge carrying electrons are freed, holes form, eventually creating an electrical imbalance. This imbalance between the front and back of the PV cell creates a negative and positive charge similar to a battery. The electrical current flows as DC power or as AC power (through an inverter), and may then supply an external power source. The process is displayed in the diagram below.

This image shows the process of converting sunlight into power through photovoltaic cell based solar power generating unit:


The illustration below show conceptually how each PV cells contain both a P (positive) and N (negative) layer of semiconductors. The layer carrying the extra electrons has a negative charge (due to the increased number of electrons), while the layer which contains atoms with an electron deficit contains a positive charge (due to 'holes' where electrons are missing).

The opposing positive and negative forces of the two layers combines to create an electrical field. The flow of the electrons between the two layers to correct the electrical imbalance (by moving from negative to positive to fill the holes), an electrical current is generated on the surface where the electrons meet. This meeting point, referred to as the junction, is where electrons flow towards the negative surface, making them thereby available for the electrical circuit. At the same time, holes move in an opposing direction, where they await incoming electrons.

This image displays a more conceptual explanation of the photocell process:

Photovoltaic Challenges

There are three challenges to making a photovoltaic cell extremely efficient. The goal is to maximize absorption, and minimize reflection and re-absorption.

Minimize Reflection:

The surface of a PV cell must be designed to absorb as much of the suns rays as possible, and minimize reflection. Minimizing reflected sunlight means more electrons will be knocked free, getting more electrical energy with less required solar power. The next challenge is to maximize the electron flow into the electrical circuit.

Maximizing Absorbtion:

As radiant energy from the sun is absorbed into the positive "P" layer the challenge is to free as many electrons as possible with the least amount of required sunlight or incoming photons. At the same time it is critical to prevent electrons from moving the wrong direction, back towards the holes & recombining before they can escape the cell and join the electrical current.

The materials in the PV cell were engineered to send the maximum possible amount of electrons through the conduction layer and out into the electrical circuit. The more electrons that are freed to flow into the electrical current, and the less electrons flow backwards to fill the 'holes', the more 'conversion efficient' a photovoltaic cell becomes.

Summary

Photovoltaic cells are built to absorb energy from the sun to push electrons into an electrical current, with the greatest efficiency possible. During the last few decades, great strides have been made at improving the efficiency of Solar Devices. If similar strides can be made in the future, solar power may eventually become more widespread. This would be great news for our environment, as solar power is one of the worlds greatest potential clean energy sources. Perhaps one day our cities will be powered by solar plants, in addition to traditional power plants.

The concept of advantageously using Sunlight for power is by no means a new concept. For example, In ancient Rome water containers were installed on roofs to heat water. However, our ability to capture and use sunlight energy has improved drastically since the first solar panels were developed in the mid 1800s. Early photovoltaic cells were no more than about 1% efficient. In 2008, a 40.8% efficient photovoltaic cell was developed. The same size solar panel today can generate over forty times more power than the earliest solar panels. We can only wonder what the efficiency limits of solar electric power generation may be...