Capturing Solar Energy with Photovoltaic Cells
Did you know that the amount of power from a solar panel depends on what it's connected to? Explore how photovoltaic panels work, converting light into electrical energy, and learn how you can find the maximum power output from a solar panel.
- Photovoltaic panels work on the basis of the material properties of semiconductors.
- Electric field and electron concentration gradient dictate the movement of electrons within the solar cell.
- Light can excite and free a bound electron, allowing for electron movement and electrical current given the proper connections.
- The amount of power generated from a solar cell depends on the resistance of its load.
The purpose of this experiment is to explore how light energy can be transformed into electricity by measuring the power output of solar panels. Students can investigate how the power output of photovoltaic cells varies with resistance of its load.
Solar energy is the fastest growing among the renewable energy sources used today, and advances in the field of photovoltaics continue to improve the efficiency and lower the cost of solar panels.
Solar cells rely on the photovoltaic effect, or the use of light to directly create voltage. The secret behind this lies in the semiconductor material they are made of, whose properties can easily change by mixing in small amounts of relatively positive and negative atoms. In solar cells, the action takes place on the boundary between two types of semiconductor material, called the P-N junction. One side, the N-type semiconductor, has extra negatively-charged moving electrons. In contrast, the other side, the P-type semiconductor, lacks freely-moving electrons and instead has positively-charged vacancies, or "holes". These moving charges (free electrons and holes) are often referred to as charge carriers.
After the two parts come into contact, the free electrons on the N side of the junction naturally move (diffuse) to the P side where the electron concentration is lower. They fill the holes on the P side, creating a zone without any charge carriers called a "depletion zone", or "space charge region". This makes the N side acquire a slight positive charge, and the P side a slight negative charge.1 Because of this, an electric field2 is created, pointing from the N side to the P side- from the positively charged to the negatively charged semiconductor. The electric field gets stronger and stronger until the net movement of electrons slows, then stops. The result is a slightly positive N side and a slightly negative P side, and a zone between the two depleted of any charge carriers, as illustrated below. 3
The reason a P-type semiconductor lacks freely-moving electrons is that virtually all its electrons are tightly bound to its atoms. However, when light hits, or "excites", an electron, sometimes the electron gains enough energy to go free. When an electron is freed from the P side, the electric field created in the junction pulls the electron toward the N side. The N side, however, already has a surplus of free electrons, making the electron want to go back to the P side (where there is a lower concentration). Unfortunately for the electron, the force from the electric field prevents it from going back the way it came.
There is a solution to this, however! By connecting the back end of the N-type region back around to the P-side, the electron can go back to where it came from while bypassing the junction. This movement of electrons (and a corresponding movement of holes in the opposite direction) is what creates an electric current.
This connection is made via a load, something that can use the electricity generated from the solar cell to do work. The power generated by a solar cell depends both on its electric current and on its electric potential, or voltage (how much the electrons and holes "want" and are pushing to move). And these, in turn, depend on the resistance of the load.
When the load resists all movement of electrons, the voltage of the solar cell will be high, but no current will flow- so no power is generated. This is as if the two sides of the cell were not connected at all, i.e. an "open circuit". On the other hand, when the load has no resistance, current flows but with zero voltage4- so again, no power is generated. This must mean that there is an optimal resistance at which the solar panel produces the most power!
In this experiment, students will investigate how the power output of a photovoltaic cell varies with resistance of its load. They can determine the cell's highest power output, as well as its optimal load resistance.
- Two multimeters (voltmeter, ammeter)
- Connecting wires, including those with alligator clips
- Variable resistor/ potentiometer (We used a 50 kOhm potentiometer with indoor lighting)
- Photovoltaic solar panel (We used a 19 cm x 12 cm solar panel, purchased from DX
- Build the following circuit, connecting the solar panel to the load (variable resistor), and adding the multimeters as shown:
The connections should resemble the following photograph:
One of the multimeters (set to DCA - i.e. ammeter) is connected in series with the resistor and measures the electric current through the circuit (wires labelled 'A'). The second multimeter (set to DCV - i.e. voltmeter) is connected in parallel with the resistor and measures the electrical potential difference, or voltage, across the resistor (wires labelled 'V'). Start by setting the ammeter and voltmeter to 200u and 20, respectively, and adjusting as necessary.
- Measure (roughly) the power generated by recording the readings on the two multimeters and using the relationship
which states electrical power as a product of electric current and voltage. Remember to take into account the units of the multimeter display; forexample, a reading of 50.7 on a multimeter set to 200m DCA is a measurement of 50.7 milliamperes (mA).
- Turn the knob on the variable resistor to change its resistance, and repeat the power measurement. Does the solar panel produce less or more power this time? Compare the two resistances used, using the Ohm's law relationship between voltage, resistance, and current. (Ohm's Law states that voltage, in volts, is a product of current (in amperes) and resistance (in ohms). Rearranged, the equation becomes R = V/I .)
- Find the optimal load resistance- that is, the resistance of the variable resistor at which the solar cell yields the highest power output. What is that output? Hint: measure and record values of voltage and current over a wide range of resistances (turn the knob a little bit at a time). From this, calculate and plot the power outputs (on the y-axis) and corresponding resistances (on the x-axis as the independent variable).
- Do this experiment both indoors and outdoors, under stable lighting conditions (i.e. if outside, in direct sunlight on a clear day). By how much do you expect the power obtained outdoors to be bigger than that obtained indoors? What do you find? In each case, where does the energy come from?
- 1. Andrews, J., & Jelley, N. (2013). Solar Energy. In Energy science: Principles, technologies, and impacts. Oxford: Oxford University Press.
- 2. An electric field, by definition, points in the direction of the electric force on a positive test charge, and opposite to the direction of the force on a negative charge.
- 3. Image adapted from Semiconductor Technology from A to Z: Fundamentals - The p-n junction, Philipp Laube. http://www.halbleiter.org/en/fundamentals/p-n/
- 4. Refer to Ohm's Law, which describes the relationship between voltage, current, and resistance.
© Physics and Astronomy Outreach Program at the University of British Columbia (Patricia Angkiriwang 2015-12-14)