## Solar cell efficiency

You can print the teacher guide without teacher hints or with teacher hints.

## Calculate the solar cell efficiency

*How do you calculate a solar cell's efficiency?*

### What you need

- 1 solar cell (e.g.: OPV School Set)
- Decade resistor / potentiometer
- Wires with banana plugs and crocodile clamps
- Voltmeter
- Ampmeter
- Light source (projector lamp or the sun)

### Background

To describe the efficiency of a solar cell, it is not enough to measure what voltage and current it produces without load. When measuring the voltage with a voltmeter directly over the solar cell contacts, you measure V_{OC}, which is the voltage of the unloaded circuit. In other words, this voltage is the maximum voltage the solar cell is capable of delivering. At a higher load, the voltage will drop until the load is infinitely high. The situation reached by infinite load is a short circuit of the system and is precisely the state you measure when you connect an ammeter to the solar cell to measure the current. The short circuit current measured in this context is called I_{SC} and is the maximum current that the solar cell can deliver. Since P = IV (the power is equal to current times voltage) we can calculate the power in both situations. At V_{OC} we have a voltage, but the current is zero, so the power is also zero. If you know I_{SC} there is a current, but the voltage is now 0, so the power is again 0.

To find the point where the solar cell produces the maximum power, we therefore need to expose the solar cell to many different loads and measure current and voltage. We can then find the maximum power as the product of power and voltage. The experiment below is about finding the maximum power by connecting a variable resistor to the solar cell, recording current and voltage and repeating this for many resistor settings.

### Experiment 1

Arrange the solar cells and components as shown in the diagram in Figure 2. Start by setting the resistance to the minimum value. Then record current and voltage. Change the resistor value a little bit and note again current and voltage. Repeat until the output voltage is very small or 0. A plot of current and voltage is called an IV curve and is a widely used way of characterizing a solar cell.

Multiply current and voltage for all values and find the maximum power.

### Experiment 2 - Calculate the efficiency of the solar cell

To calculate the efficiency of the solar cell, it is necessary to know how much power the solar cell receives and how much power it produces. From Experiment 1 we know the power the solar cell produces. If the light source used was the sun on a cloudless day, the incoming power is approx. 1000 W / m^{2}. To calculate the efficiency of the solar cell, you must use: Efficiency = P_{out} / P_{in}. To calculate P_{in} (the input power) use the area of the solar cell. You can measure the module area with a ruler. Then calculate the efficiency.

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