Solar cells (PV) are electrical devices that can convert light into electricity. These cells are made of semiconductors like silicon, germanium, or gallium arsenide. They have electrical conductivity [1]properties that fall between a conductor (metal) and an insulator (glass).
But in reality, how do Solar cells work, and how are these semiconductor properties altered by introducing impurities Into the crystalline structure? The result of all this is a semiconductor junction is created. This introducing impurity is known as doping, and the resulting semiconductors are doped semiconductors. Read on to learn more about how solar cells work.
What is a Solar Cell
A photovoltaic cell is a p-n diode, which allows the current to flow in one direction. When you combine individual Solar cells, they form Solar panels or Solar Array. Mono or Polycrystalline Solar panels have 60 Solar Photovoltaic cells combined in series to form a Solar panel. The individual Photovoltaic Solar cells produce around 0.5 to 0.6V of electricity flow. Still, when you combine Solar cells to construct Solar panels, they create a significant amount of flow to generate solar electricity.
Basic Composition of Solar Cells
When photons in the Sunlight strike the Solar panels or Solar cells, the Solar cells absorb the photon while the rest of the photons are reflected [1]. When the PN junction diode (Solar (PV) cell) absorbs sufficient photons, the electrons are knocked loose from the atoms within the Solar cell or solar panels. Because the two layers are chemically treated, it causes the electrons to move in a single direction, and this causes an electric current to be generated.
Solar cells are made of the following impurities: the upper and lower portion of the silicon cell is doped with donor (n-type) and acceptor (p-type) impurities, and the n-type is thicker than the p-type. As a result, the donor has a higher concentration of free electrons (n-type). On the other side, the semiconductor has a higher concentration of holes.
Working Principle behind How do Solar Cells work?
As the electrons move from the N region, they recombine with the holes in the P region, and a negative sign represents them. The holes move from the P region to the N region and recombine with the electrons in the N region; a positive sign represents them. The effect is a high concentration gradient of charge carriers across the junction within the doped semiconductor.
The recombination causes the n-type region to be positively charged while the p-type region to be negatively charged. [2] This charge separation between the N-type and P-type region results in a potential difference of about 0.6V. This electric field further stops any diffusion of electrons or holes from recombining. As a result, the section of the area with both negative and positive charges is called the depletion region. This region contains no mobile charge carriers and is called the space charge region. In normal thermal equilibrium conditions, the thickness of the depletion region stays fixed.
When Solar photovoltaic cells are exposed to Sunlight or Solar Radiation, some photons dissipate as heat in solar cells. At the same time, other photons that have higher energy are absorbed by the silicon photovoltaic solar cell. As a result, the photons will have enough energy to knock off the electrons or holes from the depletion region, thereby reducing the depletion region.
In the Solar photovoltaic cells designed, the electrons or holes will not get enough chances to recombine with the majority carriers in the p or n region. As a result, free electrons in the depletion region move toward the n-type side of the junction. The holes move towards the p-type side of the junction due to the electrostatic force of the junction field.
Once the newly formed electrons move toward the n-type side and the holes to the p-type side, the barrier potential prevents the electrons or the holes from moving back into the junction. The silicon cell will behave like a small battery cell as the buildup of electrons and holes happens on either side.
The voltage is called photovoltage. If we connect a small load, like a resistor, across the junction, we can see a tiny current flowing through it. This is the basic principle of how a Solar (PV) cell works.
Characteristics of Solar (PV) Cell
The figure below is the typical Voltage (V) vs. Current (I) characteristics, typically known as the I/V curve of a solar (PV) cell. The Solar (PV) cell IV characteristics curve has four terms highlighted; Short Circuit Current (Isc), Open circuit voltage (Voc), Maximum Power Point (Mpp), and Voltage Maximum (Vm). This is important because different applications require different usages. These parameters help us understand how efficiently a solar cell can convert light to conduct solar electricity that flows through the cells.
Current – Voltage (IV) Curve Parameters
Isc – Short Circuit Current – This is the maximum current produced by solar cells. Isc is measured in Amps or milliamps. As you can see from the figure, Isc is maximum when the voltage is zero. Short Circuit Current or Isc depends on cell area, solar radiation falling on the cell, and cell technology.
Voc – Open Circuit Voltage – This is the maximum voltage produced by solar cells in open circuit conditions. Voc is measured in volts or millivolts. When the cell has maximum voltage, the current produced is zero.
Pmp – Maximum Power Point – This represents the maximum power solar cells can produce at STC. Pmp is measured in Wpeak or Wp. The solar cells can have the maximum power Pmp at a particular voltage and current combination.
Pm = Im x Vm
Im – Current at maximum power point – This represents the maximum current produced in solar cells when operating at the maximum PowerPoint. Im is measured in ampere (A) or milli-ampere (mA).
Vm – The voltage at the maximum power point represents the maximum voltage produced in solar cells when operating at the maximum PowerPoint. Im is measured in volts (V) or mili-Volt (mv).
Fill Factor – ff – This represents the area covered by the Im-Vm rectangle with the Ioc – Voc rectangle area.
FF = PM / (ISC ×VOC)
Solar Efficiency – (ƞ) – Solar Cell efficiency (ƞ) is defined as the ratio of the maximum power output Pm to the input power (Pin). The Efficiency is measured in percentage (%). Pin is the full Sunlight on the cell and will be 1000A watts.
ƞ = PM / (PIN × Area)
Solar Manufacturers test these cell parameters under standard test conditions (STC). The STC conditions involve the following: solar radiation equals 1000 W/m2. The cell operating temperature is equivalent to 25 degrees centigrade.
Photovoltaic Technologies – IV curve characteristics
Different kinds of solar (PV) cell technologies exist in the market, like Monocrystalline, Polycrystalline, Thin film, etc. Each of them has parameters like Short Circuit Current (Isc), Open circuit voltage (Voc), Maximum Power Point (Mpp), and Voltage Maximum (Vm). The table below shows the list of commercially available cells and their parameter value range.
| Cell Type | Efficiency (%) | Open Circuit Voltage (V) | Current Density (mA/cm2) | Cell Area (cm2) | Fill Factor (FF) |
| Monocrystalline silicon | 18-23 | 0.55 – 0.68 | 30 – 38 | 5 – 156 | 70 – 78 |
| Polycrystalline silicon | 15-18 | 0.55 – 0.65 | 30 – 35 | 5 – 156 | 70 – 76 |
| Amorphous Silicon | 6 – 9 | 0.70 – 1.1 | 8 – 15 | 5 – 200 | 60 – 70 |
| Cadmium telluride (thin film) | 8 – 11 | 0.80 – 1.0 | 15 – 25 | 5 – 200 | 60 – 70 |
| Copper-indium-gallium-selenide (thin film) | 8 – 11 | 0.50 – 0.7 | 20 – 30 | 5 – 200 | 60 – 70 |
| Gallium arsenide | 30 – 35 | 1.0 – 2.5 | 15 – 35 | 1 – 4 | 70 – 85 |
Factors that affect Solar Cell power
Conversion Efficiency
Solar cell efficiency is one of the most significant factors in the solar panel’s efficiency output. Once a solar panel with lower Efficiency is chosen, nothing can change the production because the Efficiency is fixed. Conversion Efficiency is the ratio of an electric field or electrical power generated to the input solar energy system. Nearly ¾ of Sunlight on solar panels is not converted. Monocrystalline Silicon Solar cells or Mono Crystalline Solar Panels have the highest Efficiency in the market. They will produce the maximum output compared to any solar cells except Gallium Arsenide. Gallium arsenide is only used in specialized applications like space power and defense.
Input Light
The amount of incident light hitting the solar cell keeps changing throughout the day. Based on the cloud cover and angle of light hitting the surface, the current and voltage changes of the cell change accordingly. The incident light falling on the cell surface increases from the early morning to the afternoon. As a result, the output power generated increases. From the afternoon till the early evening hours, the incident light decreases, and as a result, the output power decreases. Input light is significantly reduced during cloudy days.
Cell Area
The Isc for a solar cell depends on the area of the cell. Isc is proportional to the cell area. The greater the cell area, the greater the output.
Angle of Light
The maximum output power is produced when the angle of light hitting the cell is perpendicular (90 degrees) to each other. When the angle of incident light is anything but 90 degrees, then the output power will be less than the maximum power produced for that cell.
Operating Temperature
The solar cell temperature varies due to the ambient temperature; as the cells are enclosed in glass, the temperature in the solar cell increases. The increase in temperature above the STC affects the output voltage; hence, the output power is decreased.
Final Thoughts
Based on the following, we can see that current-voltage (I-V) Curves for solar cells (diodes) are nonlinear and very different from other diodes. These characteristics are essential and can be pretty helpful in determining how different combinations of current and voltage can help us understand what is happening within a circuit and if there are additional possibilities to make it work for our needs.