P3 – Silicon photocell
Safety note: the lamp housing becomes very hot, so avoid touching it.
Introduction
A photocell (or solar cell)is a semiconductor device that generates electricity (voltage and/or current) between its two terminals when light falls upon it.A photocell delivers electricity without moving parts, without consuming fuel, and without emitting exhaust fumes, and is considered a key technology for the post- fossil fuel era.
The main characteristics of a photocell are:
- Open- circuit voltage, Voc. That is the voltage that builds up between the cell’s terminals when these are not electrically connected (‘open’): The resistance (R) between the terminals is infinitely large, no current (I) flows between them. Voc is the highest voltage the cell can deliver, and depends mainly on the material the cell is made from, and only weakly on the intensity of light falling onto the cell.
- Short- circuit current, Isc. This is the current that flows between the cell’s terminals when they are electrically shorted: The resistance (R) and the voltage (V) between the terminals is zero. Isc is the highest current the cell can deliver, and strongly depends on the intensity of light falling onto the cell.
- Current- voltage (I(V)) characteristics. That is the curve that shows the current flowing between the cell’s terminals as a function of voltage between the terminals. This implies a resistance R between the terminals with 0 < R < infinite. Isc and Voc are the two extreme limits of the I/V characteristic.
- Maximum power point, VMPP. Electric power is given by P = V*I. P of a photocell is zero at Voc (because I = 0), and zero at Isc (because V = 0). P is non- zero along the I/V characteristic though, and can be calculated for every voltage by multiplication of the I/V characteristic: P(V) = V*I(V). P(V) is called ’power profile’. VMPP is the voltage at the maximumof the power profile, PMPP = P(VMPP).
- Fill factor (FF). FF is defined as FF = PMPP / (Voc*Isc)
- Power efficiency. The ratio of PMPP to the incidentpower of solar light.
The latter of these is hard to measure without a specially calibrated light source and so you are not asked to find the power efficiency. However, you willmeasure a photocell’s I/V characteristic. From the I/V characteristic, you shall find Isc, Voc, power profile, VMPP, and FF.
Experimental setup
The standard photocell circuit (which is already wired up for you) includes the photocell with a variable resistor (R) between its terminals. Also, a lamp is installed that will illuminate the photocell. A voltmeter is connectedin parallel with the resistor and photocell enabling you to measure the voltage (V)between the photocell terminals under illumination. The circuit diagram for the set-up is shown below:
From the voltage measurements, cell current (I) can be calculated as
Notice that there is a glass block between the lamp and the photocell. This is in place to absorb infra-red radiation which would heat the photocell but would not increase the current.
Method
Set the variable resistor to a value of 1 . Adjust the position of the lamp so it is centrally above the photocell and close enough to get a voltage reading of 30 – 36 mV (once you have positioned your lamp you must not move it until you have completed all of your measurements). Make a table with columns for resistance, voltage, current and power. Record your values of resistance and voltage in the table and calculate values for current and power.
You should now increase the resistance in 1 steps until you reach 10 , recording the voltage measurement for every resistance value as you go.Repeat this procedure for the interval 10 to 20 but go up in 2 steps. Repeat again for the interval 20 to 50 in 5 steps and again for the interval 50 to 100 in 10 steps.Finally increase from 100 in 50 steps until the calculated current is less than 1mA.
Analysis
From your data, plot the I(V) characteristic. Note you use the variable resistor, R, to sweep out the I(V) characteristic, but R does not explicitly appear in the I(V) plot. Read Voc from the limit I 0, and Isc from the limit V 0. Also, plot the power profile (which is a different presentation of the same data), and find VMPP, and PMPP. Finally, calculate FF.