Solar Panel Physics: Harnessing the Power of the Sun
The sun’s energy can be converted into electricity using a silicon solar cell designed to create a flow of electrons.
Silicon Solar Cells
A photovoltaic solar panel is made up of silicon. Silicon is a semiconductor and the second most abundant element on earth. Silicon crystals can be grown in a lab to make solar cells. As they are grown, very small quantities of other elements are incorporated into the crystal. Boron and phosphorous are typical elements added to silicon. This process is called “doping” silicon.
Solar Cell P/N Junction
Phosphorous (n-type) has an extra electron in its electron cloud. Boron (p-type) lacks an electron in its electron cloud. Some of the extra electrons from the silicon doped with phosphorous (n-type) move to the boron side (p-type). Because the n-type loses an electron, it becomes positive while the p-type becomes negative. As the crystal is grown, boron is replaced by phosphorous and this creates a p/n junction, or a negative end and a positive end within the silicon cell.
Lacks electrons Extra electrons
Negative (-) Positive (+)
Photons from sunlight hit the silicon cell and excite electrons due to the photoelectric effect. These excited electrons leave the n-type region because there are already extra electrons present. They are collected by metal electrodes on the n-type surface of the solar cell and flow through a circuit on their way to the p-type region. This circuit is completed by wires and can include other components like a simple light bulb. The electrons flow through another electrode into the p-type region. They then flow across the crystal to replace the electrons that left the n-type region, completing the circuit. That’s how sunlight produces electricity!
When electrons are excited they produce a direct current (DC). Your inverter will change this into an alternating current (AC), then send it to the breaker panel of your building, powering the circuits and devices in your house.
A silicon cell is 0.5 Volts. See MINI PHYSICS LESSON: I=V/R
Multiple cells make up a module. Multiple modules wired in series make a string. Multiples strings wired in parallel make an array.
Electrons are the only “moving” part of the solar system so panels can be used for decades.
MINI PHYSICS LESSONI = V / R
Current (I) is equal to the amount of voltage (V) pushing against an amount of resistance (R). Another name for current is Amperes, and another name for resistance is Ohms.
Understanding Panel Specifications Data Sheet
A solar panel has multiple characteristics describing its electrical function. These can mostly be found on the back of a solar module or on its datasheet. Here are some common specs.
Open Circuit Voltage (Voc)
Short Circuit Current (Isc)
Maximum Power Voltage (Vmp)
Maximum Power Current (Imp)
Maximum Power (Pmp)
Standard Test Conditions (STC)
Solar Irradiance (W/m2)
Darker Blue Color
More efficient (2% more power in the same place)
Better performance in heat
Better performance in shade
Square shaped molded
Crystal-like blue appearance
Less expensive (due to easier manufacturing process)
Performance affected by heat
Performance affected by shade
To understand the power of a solar panel system, it is best understand the Voltage-Current characteristics. The maximum power of any electrical system can be found by reading an I-V Curve. On the x-axis is voltage (V) and on the y-axis is current (I).
The graph above can be used to find the maximum power (Pmp) output of a solar panel. Power is current times voltage (P = I x V). Therefore, the point in the curve with the highest amount of current and highest amount of voltage will provide the most power. The highest power can be found at the start of the downward slope of each curve, the “knee” of the curve.
Each curve represents a different amount of solar irradiance. An increase in irradiance means there is a higher density of photons falling on the array. Interestingly, voltage changes very little with irradiance but irrandiance does increase current. Therefore, more solar irradiance provides more power (1000 W/m2 solar irradiance provides the highest amount of power in this graph).
Series Vs Parallel
In series, the negative end of a panel is connected to the positive end of the next panel, and so on. Panels connected in series increase the voltage. The current stays the same, and so does power (watts). It’s important to raise the voltage to closely match 120 Volts inside your home.
In parallel, the negative end of a panel is connected to the negative end of the next panel, and so on. Panels connected in parallel increase the current. The voltage stays the same, and so does power (watts). A steady, sufficient amount of current is important to power all the electrical appliances in your home.