How do Solar Cells work?

June 07, 2019

In the last two decades the contribution of solar energy to the world’s total energy supply has grown significantly. In this article will show how solar cell or photovoltaic cell produce electricity.

Energy from the sun is the most abundant, and absolutely freely available energy on planet earth. In order to utilize this energy we need help from the second most abundant element on earth, sand. The sand has to be converted to 99.999 % pure silicon crystals to use in solar cells. To achieve this, the sand has to go through a complex purification process as shown(Fig:1A). The raw silicon gets converted into a gaseous silicon compound form. This is then mixed with hydrogen to get highly purified poly crystalline silicon(Fig:1B).

These silicon ingots are reshaped and converted into very thin slices, called “silicon wafers”.(Fig:2A) The silicon wafer is the heart of a photovoltaic cell. When we analyze the structure of the silicon atoms, you can see they are bonded together. When you are bonded with someone, you lose your freedom. Similarly the electrons, in the silicon structure, also have no freedom of movement. To make the study easier, let’s consider a 2D structure of the silicon crystals. Assume that phosphorus atoms with 5 valence electrons are injected into it. Here one electron is free to move(2B). In this structure when the electrons get sufficient energy, they will move freely (Fig:2C).

Let’s try to make a highly simplified solar cell only using this type of material. When light strikes them, the electrons will gain photon energy and will be free to move. However, this movement of the electrons is random. It does not result in any current through the load(Fig:3).

Driving force

To make the electron flow unidirectional, a driving force is needed. An easy and practical way to produce the driving force is a P-N junction. Let’s see how a P-N junction produces the driving force. Similar to N type doping, if you inject boron with 3 valence electrons into pure silicon, there will be one hole for each atom.This is called P type doping (Fig:4A). If these two kinds of doped materials join together, some electrons from the N side will migrate to the P region and fill the holes available there. This way a depletion region is formed, where there are no free electrons and holes. Due to the electron migration, the N side boundary becomes slightly positively charged, and the P side becomes negatively charged. An electric field will definitely be formed between these charges(Fig4B). This electric field produces the necessary driving force. Let’s see it in detail.

When the light strikes the P-N junction something very interesting happens. Light strikes the N region of the PV cell and it penetrates and reaches up to the depletion region. This photon energy is sufficient to generate electron hole pairs in the depletion region(Fig:5A). The electric field in the depletion region drives the electrons and holes out of the depletion region. Here we observe that the concentration of electrons in the N region and holes in the P region become so high that a potential difference will develop between them(Fig:5B). As soon as we connect any load between these regions, electrons will start flowing through the load. The electrons will recombine with the holes in the P region, after completing their path. In this way a solar cell continuously gives direct current(Fig:5C).

Practical solar cell

In a practical solar cell, you can see that the top N layer is very thin and heavily doped; whereas the P layer is thick, and lightly doped. This is to increase the performance of the cell. Just observe the depletion region formation here. You should note that the thickness of the depletion region is much higher here(Fig:6A), compared to the previous case. This means that, due to the light striking, the electron-hole pairs are generated in a wider area compared to the previous case. This results in more current generation by the PV cell. The other advantage is that due to the thin top layer, more light energy can reach the depletion region(Fig:6B).

Structure of a solar panel

Now, let’s analyze the structure of a solar panel. You can see the solar panel has different layers as showin in(Fig:7A). One of them is a layer of cells. You will be amazed to see how these PV cells are interconnected. After passing through the fingers, the electrons get collected in busbars. The top negative side of this cell is connected to the back side of the next cell, through copper strips(Fig:7B). Here it forms a “series connection”. When you connect this seriesly connected cells, parallel to another cell series, you get the solar panel. A single PV cell produces only around 0.5 Voltage. The combination of series and parallel connection of the cells increases the current and voltage values to a usable range. The layer of EVA sheeting on both sides of the cells is to protect them from shocks, vibrations, humidity and dirt.

Polycrystalline solar panels and monocrystalline cells

Why there are two different kinds of appearances for the solar panels? This is because of the difference in the internal crystalline lattice structure(Fig:8A). In polycrystalline solar panels, multi crystals are randomly oriented. If the chemical process of silicon crystals is taken one step further, the polycrystalline cells will become mono crystalline cells(Fig:8B). Even though the principles of operation of both are the same, mono crystalline cells offer higher electrical conductivity. The unique octagonal shape in mono crystalline cells utilizes the cell area more effectively without any loss of material. However, mono crystalline cells are costlier, and thus not widely used.

Even though running costs of PV cells are negligible, the total global energy contribution of solar photovoltaic is only 1.3%. This is mainly because of the capital costs and the efficiency constraints of solar photovoltaic panels, which do not match conventional energy options.(Fig:9)

Solar panels on the roofs of homes as shwon in fig(Fig:10A) have the option to store electricity with the help of batteries and solar charge controllers. However, in the case of a solar power plant the massive amount of storage required is not possible. So, generally they are connected to the electrical grid system, in the same way that other conventional power plant outputs are connected. With the help of power inverters, DC is converted to AC and fed to the grid.((Fig:10B)

Theoretical maximum efficiency of the solar cell is almost 30.02%. There are many losses in the real solar cell. Optical and electrical losses are the major losses. In optical losses reflection, shading, and transmission losses are included and in electrical losses, ohmic losses such as contacts, semiconductor, Metal SC-interface.

Open circuit voltage and short circuit current

What happens if we remove the load? Concentration of electrons and holes on either side of the P-N junction increases. This will create the potential difference between the two ends. If we check the voltage of the solar cell by multimeter then it is known as Open circuit voltage. One solar cell can give 0.5V. According to the V-I characteristics of the solar cell when the load is connected the current starts to flow and the circuit voltage starts decreasing. The highest possible current is called” Short circuit current”.

Photoelectric effect

Direct Conversion of sunlight into electricity called photoelectric effect. The highest efficiency of practical solar panel is 15 to 20% only. Generation of electricity from the sun is technically feasible but economically not viable because of high capital cost. In all resources of US electricity generation solar energy contributes only 1.3% because of some factors like dependency on weather, Expensive manufacturing process,mass energy storage problem,large space required but researchers are doing at their level best to overcome these issues. We hope that in future there will be sustainable growth in solar technology.