Photovoltaic Cells

The Sun emits radiation in all parts of the EM spectrum, but most of it's photon production occurs in the visible and surrounding regions. Semiconductors that use Silicon are good, but, like all areas of research, as time marches forward advances are made and we see that other combinations of elements in the Periodic Table sometimes provide better results. We saw in the section on semiconductors that PN junctions have electric fields associated with them and that a photon can be absorbed and a hole and electron are produced. The electric field moves the holes and electrons and an electric current is produced. How much current a photovoltaic cell produces is related to how many photons are absorbed by the cell.

Recall from the section on atoms that electrons orbit the nucleus. If they receive enough energy they can leave the valence band and be promoted to the conduction band. The amount of energy it takes to do this is called the bandgap energy. If an electron absorbs a photon that has less energy than the bandgap energy for the semiconductor, it can't escape the valence band and doesn't get into the conduction band. If it absorbs a photon with more energy than the bandgap, it uses the required energy to transfer into the conduction band and the energy that is left over is dissipated as heat in the semiconductor. The voltage that a photovoltaic cell produces (and therefore it's power) is related to the energy that conduction band electrons have.

The first photovoltaic cells were known as "single-junction" cells. That is, they had one PN junction in them. Because the EM spectrum covers a large range, designers had to determine what part of the spectrum to target and what semiconductor combinations would work best for that part of the spectrum. This area is the visible light portion of the spectrum. Because there are several frequencies in this range, and single-junction cells have only one PN junction, a decision must be made on which frequency photons to target. Again, the output power of a photovoltaic cell is dependent on the energy of the electron's that get into the conduction band. Power is equal to the voltage times the current (Power = Voltage x Current). If a semiconductor is used that has a high bandgap energy, then only those electrons that have that energy (or higher) will make it to the conduction band and produce current. If this is towards the upper end of the visible spectrum then there are not a large number of electrons produced and captured. Their voltage will be higher but their number will be lower (so the current will be lower) and therefore the power output will be low.

On the other hand, if a frequency at the lower end of the spectrum is chosen, a lot more electrons will be put into the conduction band because the minimum energy required to cross the bandgap is a lot lower. But because the bandgap energy is lower, the electron's energy is lower and the output voltage is lower. In this situation you also capture the higher energy electrons. They also get promoted to the conduction band, but their excess energy is put into heating the semiconductor.

What you have is a tug-of-war between the two extremes. By choosing higher frequency photons you get a higher voltage but lower current. Choose lower frequency photons and you get lower voltage but higher current. So you compromise by choosing a semiconductor that will capture mid-range, visible light spectrum photons that give you good performance.

Wouldn't it be nice if there was a way to produce semiconductors for multiple frequency ranges in the visible light spectrum that maximized the output voltage for those frequencies? Wouldn't it also be nice if somehow all the semiconductors could be put together so that a larger, combined power output could be obtained from all of them? Well, once again you guessed it, there is! If we call photovoltaic cells with one PN junction single-junction cells, then those with more than one are referred to as "multi-junction" cells.

There are a couple of ways that multi-junction cells could be laid out in a photovoltaic panel. Consider the figure below. There are four semiconductor junctions, each one having it's own bandgap energy (E1, E2, E3 and E4).They could be laid side by side, as in figure (A), or they could be stacked one on top of the other, as in figure (B). If laid side by side the incoming photons from the Sun could randomly strike the individual junctions and each could produce it's own current and voltage. The four junctions could be wired together and the final output obtained. However, this configuration uses a lot of real estate on a photovoltaic panel. It's a much more efficient use of space to stack them one on top of the other, as in figure (B). There's a process for doing this "monolithically" (making them all in one piece). Using this scenario, the top junction is designed to capture the higher energy photons. As you go deeper in the stack you design the junctions to capture lower energy photons.

While this method is a better use of space, it has it's own challenges. These junctions are connected in series. The current produced in the yellow layer must flow into the orange, then into the green and finally into the blue.

Current in an electrical circuit is analagous to water flowing in a pipe. The image below represents three pieces of water pipe, each a different diameter, connected in series (one after the other). Water enters the pipe on the left side, flows through the pipe and exits on the right side. What is the maximum amount of water that will flow through the pipe? The pieces are all about the same length, but have different diameters. The piece in the middle has the smallest diameter, therefore, it's going to allow the least amount of water through it. Individually, more water can flow through the end pieces (if they're separated) than the middle piece, but because they're all connected in series the middle one allows the least water through the pipe. The water flow is dictated by the smallest unit in the series.

 

Armed with the information from the water pipe analogy, we return back to the multi-junction cell. Because the junctions are connected in series, we want to try and optimize each junction to produce the same amount of current. Also, remember that each junction is designed to work with photons of a specific energy, so each will produce a different voltage. For junctions that are connected in series in a photovoltaic cell, the voltages are added from each junction. So, for example, if three junctions produced voltages of 1.0 volts, 1.1 volts and 1.4 volts, the photovoltaic cell output would be the sum of these: 1.0 + 1.1 + 1.4 = 3.5 volts.

 

LET'S REVIEW:
  

Photovoltaic cells that have one PN junction are called "single-junction" cells. Designers working with these must balance the parameters of voltage and current to get the best power output. Photons with energies at the low end of the visible light spectrum produce more electrons (therefore a higher current) at a lower voltage. Photons at the higher end of the visible light spectrum produce fewer electrons (and a lower current) at a higher voltage.

Photovoltaic cells with more than one PN junction are called "multi-junction" cells. These are designed such that each junction captures photons of a certain frequency (energy) and produces a voltage proportional to that frequency. While each junction's voltage may be different, the current produced by each is the same. These junctions are connected in series and the voltages from each are added.

 

So, there you have it in a nutshell. You now have the basic idea and understanding of how photovoltaic cells work. It's not that difficult, is it? Let's take a quick moment and put all of the reviews together and have a look at the big picture.

FINAL REVIEW:
  

Atoms
Atoms have a nucleus made of protons (positively charged) and neutrons (no charge). Electrons (negatively charged) orbit the nucleus in different energy levels (orbitals). There are equal numbers of protons and electrons in electrically neutral atoms. Outer electrons can receive energy from outside the atom and rise to higher energy levels in the atom. If enough energy is received, the electron can leave the atom. Atoms like to have their outer electron orbitals full of electrons.

Semiconductors
Atoms in a solid can form a lattice or crystal by sharing electrons. "Impurity" elements inserted in a crystal can leave it (the crystal) with a surplus of electrons or holes. A surplus of electrons on one side of the crystal and holes on the other side form a PN junction. The holes and electrons near the junction can diffuse across the junction, combine with atoms on the other side and form positive and negative ions. This region is called the depletion region and has an electric field associated with it due to the positive and negative ions. If a photon of light impacts the P or N type region, an electron and hole are formed. If this happens in the P-type region, the electric field accelerates the electron into the N-type region. If this happens in the N-type region, the electric field accelerates the hole into the P-type region.

The Nature of Light
Photons are massless, chargeless particles that travel at the speed of light in wavelike fashion and carry radiation (travelling energy). Radiation is broken down into several groups, one of which is visible light. Collectively these groups are referred to as the electromagnetic spectrum. Photons, like water waves, have energy, a frequency and a wavelength. However, photons have very defined (discrete) quantites of energy. A photon's energy is directly related to it's frequency: the higher the frequency, the higher the energy. The lower the frequency, the lower the energy.

Photovoltaic Cells
Photovoltaic cells that have one PN junction are called "single-junction" cells. Designers working with these must balance the parameters of voltage and current to get the best power output. Photons with energies at the low end of the visible light spectrum produce more electrons (therefore a higher current) at a lower voltage. Photons at the higher end of the visible light spectrum produce fewer electrons (and a lower current) at a higher voltage.

Photovoltaic cells with more than one PN junction are called "multi-junction" cells. These are designed such that each junction captures photons of a certain frequency (energy) and produces a voltage proportional to that frequency. While each junction's voltage may be different, the current produced by each is the same. These junctions are connected in series and the voltages from each are added.