How does a PV cell work? - Sustainable Energy - TU Delft - YouTube

This is currently the world's biggest solar farm.

It has an installed capacity of 850MWp.

This is huge, but at the base of even the biggest solar farm is the solar cell.

So let's take a closer look into the basic working principles of a solar cell.

Photovoltaic or PV cells are made by slicing a very thin layer of a silicon rod called

an ingot.

In this image you can see some monocrystalline ingots and slices called wafers.

A single silicon wafer does not act like a solar cell yet.

It misses some essential layers that are required to make it operate as a solar cell.

A typical simplified structure of a conventional solar cell looks like this:

So, how can we get electricity directly from sunlight?

The mechanism in which solar light is directly converted into voltage or current is called

the photovoltaic effect, or in short PV.

Here we see a simplified representation of a silicon based solar cell.

It consists of the c-Si absorber layer, a pn-junction to separate the light-excited

charge carriers, and a metal front and back contact.

To give you a first idea, how the PV effect works, I will show its principle using this

simple animation.

Here we see the same structure, but then in a cross-sectional view.

The light enters the solar cell from the front side - in this illustration that is the top

side.

The light is transmitted into the absorber layer where its energy is absorbed.

The energy is used to excite charge carriers in the semiconductor material, which are a

negatively charged electron, indicated by the red dot, and a positively charged hole,

indicated by the blue dot.

These charge carriers diffuse and need to be separated.

This separation occurs at the depletion region between the n- and p-type doped silicon and

the depletion region at the back of the solar cell.

Doped layers are areas where we intentionally have put impurities that make either the electron

or the holes the dominant charge carriers in these regions.

In n-type materials the electrons are the dominant charge carrier and in p-type the

holes are.

Therefore, the electrons are collected at the n-type layer and holes at the p-type layer.

Then the charge carriers have to be collected at the contacts.

In this example, the contacts are connected with a load, in this case a lamp.

The electron will move through the load and back to the solar cell.

Both charge carriers recombine at the metal/p-layer interface.

You can see that the photovoltaic process is based on three important principles:

the first is excitation of free mobile charge carriers- the hole and electron- due to light

absorption, the second is separation of the charge carriers,

seperation of the hole and electron and the third one is collection of the charge

carriers at the contacts.

The performance of a solar cell can be demonstrated by measuring its current versus the voltage

employed on the device.

Such measurement is reflected in a current-voltage, or iv curve.

The electrical bias can be a reverse voltage (negative) or a forward voltage (positive).

Letís first look at a solar cell in the dark.

A solar cell behaves much like a diode under non illumination.

It will block the current under reverse bias conditions and will produce a current under

forward bias.

However, when the solar cell is illuminated, it conducts additional current related to

the light excited charge carriers.

An ideal solar cell can be represented by an electrical circuit as shown here.

The photogenerated current is represented by a current source and the photodiode characteristic

of the solar cell in the dark is represented by the triangle shaped symbol.

Under which operation condition is the device producing electrical power?

For that we will define some so-called external parameters.

Here we can see the situation of open circuit voltage.

Nothing is connected to the cell.

The cell is only producing voltage and no current.

This is ant important characteristics of a solar cell.

On the IV-curve this is a point on the horizontal axis of the graph, at J = 0.

Another important parameter is the short circuit current.

Now the external circuit of the cell is connected in a short circuit without any external resistance.

The solar cell is only producing current and no voltage.

The current that is running now is called the short circuit current.

If we look back the at the IV-curve, we can see that the short circuit current is a point

on the vertical axis of the diagram, at V = 0.

The power of the solar cell can be defined as the voltage multiplied by the current at

any point on the iV-curve.

With the open circuit voltage and the short circuit current known, we can now plot the

entire IV-curve and determine the Maximum Power Point.

The power is equal to voltage times current, the horizontal axis times the vertical axis

in this plot.

The red curve here represents the power as a function of the voltage.

It is clearly seen that the this graph has a maximum.

This operation point is indicated by the red dot in the curved part of the IV-curve, and

is called the maximum power point.

If you move left from this point, the voltage will decrease without an increase in the current,

resulting in less power.

If you more right and down from the maximum powerpoint, you can observe that the current

will reduce, with only a small increase in the voltage, resulting again in a lower power.

The voltage and current at maximum power point are the maximum power point voltage and current.

These are two other important characteristics of a solar cell, which will in turn determine

the characteristics of the solar module they will end up in.

Now what is the significance of the Power on the P-V curve of a solar module?

Well, this is the power that is produced and delivered to the rest of the PV system, and

eventually the load.

Therefore, it is clearly advantageous that the solar module operates at maximum power

as seen in the figure as the peak of the P-V curve.

Without any external electrical manipulation, the PV module's operating point is largely

dictated by the electrical load seen by the PV module at its output.

To get maximum power delivered by the PV module, it is therefore imperative to force the module

to operate at the operating point corresponding to the maximum power.

This point corresponds to the peak of the P-V curve or the "knee" of the I-V curve.

The simplest way to do this, is to force the voltage of the PV module to be that at the

MPP, which is the maximum power point voltage, or Vmpp, or regulate the current to the right

amount as that of the MPP (called Impp), using converters.

But what if, after forcing the PV module to operate at MPP, the ambient conditions, like

irradiance or temperature change and in turn cause the I-V and P-V curve to change as well?

This would mean that the old MPP is no longer valid under these conditions.

Thus, to be continuously at the MPP at all times, we would need to track any such changes

in the I-V curve, and find out the new MPP.

This process is called maximum power point tracking or MPPT, and the devices that perform

this process are called MPP trackers.

Finally, people often use the term conversion efficiency of a solar cell.

People refer then to the solar-to-electricity conversion efficiency, or in other words,

the fraction of energy in the solar light incident on the solar module that is converted

into electrical energy.

The efficiency is therefore calculated as the maximum power point divided by the total

power of the light that is incident on the solar module.

Well, you now understand the output of an illuminated solar cell, the heart of any PV

system, and how to look at it with a critical eye.

In addition, you now know what the maximum power point is, and how that influences the

performance of a solar cell, and consequently a PV module.

Next we will quickly introduce the various PV technologies.