I-V Curves: A Guide to Solar Cell, Diode and Resistor Measurement
An I-V curve (short for 'current-voltage characteristic curve'), is a graphical representation of the relationship between the voltage applied across an electrical device and the current flowing through it. It is one of the most common methods of determining how an electrical device functions in a circuit. Key properties of electronic devices can also be extracted from the shape and details of the curve, enabling greater insight into their operation.
There are as many different types of I-V curve as there are different types of electronic devices, and their shapes can be very different.
Measuring and Analysing an I-V Curve
An I-V curve measurement is performed by applying a series of voltages to the device. At each voltage, the current flowing through the device is measured. The supplied voltage is measured by a voltmeter connected in parallel to the device, and the current is measured by an ammeter connected in series. An example of this set up is shown in the diagram below.
An easier way of doing this measurement is using a source measure unit, a device capable of simultaneously supplying voltage and measuring current with high accuracy. Individual points or pixels on the device can be tested by making electrical contact using a pair of micromanipulators. Alternatively, individual pixels on a standard device architecture can be tested in one of our compatible electrical test boards.
The voltages used in an I-V measurement generally depend upon the specific device being tested. For example, a solar cell may be tested between -1 V and 1 V, whilst an LED may use a higher range of 0 V to 10 V.
Sometimes applying a voltage can alter the electronic properties of a device. This can cause the current to change over time, even when the voltage is kept constant. As such, sometimes we require a pause between setting a voltage and measuring the current.
A basic aspect of the operation of an electronic device can be deduced from the position of the curve on the I-V graph. An I-V graph can be split into quadrants around the axes, as shown in the diagram on the right. Which quadrant the devices curve passes through can reveal whether it is a passive or active device.
A device with an I-V curve that is only in Quadrants I and III is a passive device. Both current and voltage have the same polarity, i.e. current and voltage are both positive or both negative. These devices use the electrical power of the circuit, and here source measure units act as a power source. Examples of devices with I-V curves in these regions include diodes and resistors.
A device with a curve in Quadrants II and IV is an active device. Here, current and voltage have opposite polarities. An active device creates electrical power whilst in these quadrants, so here source measure units act as a power sink. Examples of devices with I-V curves in these regions include batteries and solar cells.
Examples of I-V Curves
Resistor
A resistor is one of the simplest electronic devices, and thus has one of the simplest I-V curves. It is a straight line which intercepts the origin and passes through Quadrants I and III - making a resistor a passive device. The current at each voltage is proportional to the resistance following Ohm’s law: I = V / R. Therefore, the gradient of the line is equal to 1 / R, enabling the resistance to be extracted from the I-V curve.
Diode
A diode is a semiconducting device which only allows current to flow through it in one direction. This can be seen in the I-V curve. At positive voltages, the curve rises exponentially, indicating that current is free to flow through the device. At negative voltages, the current remains nearly at zero. However, a sufficiently large negative voltage (known as the 'breakdown voltage') will cause the diode to become conductive to negative current. Similar to a resistor, a standard diode is a passive device, operating only within Quadrants I and III.
From this curve you can tell the forward current, the reverse leakage current and the reverse breakdown voltage.
Solar Cell
A solar cell is a device that uses sunlight to produce electricity. In the dark, its behaviour is identical to that of a diode. However, when illuminated, the I-V curve shifts downwards into quadrant IV. This makes a solar cell an active device, producing usable power. For this measurement, the Source Measure Unit is acting as a load in the circuit. Several key properties of a solar cell can be extracted from its I-V curve, including it’s open circuit voltage (VOC), short-circuit current (JSC) and fill factor (FF), all of which can be used to find the solar cell efficiency.
For example, the open-circuit voltage and short-circuit current are the values at which the I-V curve intercepts the x and y axes respectively. Furthermore, the gradient of the curve at each intercept can be used to estimate the series and shunt resistances, and the overall shape of the graph will give you the fill factor. You can use JSC, VOC and FF to calculate the solar cell efficiency.
Source measure units make measuring Solar Cell I-V curves quick, easy and consistent. Our Source Measure Unit is included with the Ossila Solar Cell I-V Test System and can be used with our free Solar Cell I-V testing software. Coupled with the Ossila Solar Simulator we can provide everything you need to fully test your solar cells.
For more information on the measurement and analysis of solar cells, see our solar cell guide.
I-V Measurements with the Ossila Source Measure Unit
To make it easier to perform I-V measurements with our Source Measure Unit, Ossila has developed the I-V Curve PC software, which enables you to get started with your Ossila Source Measure Unit more quickly. You can download this software for free on our Software & Drivers page.
When used with the Source Measure Unit, the I-V Curve PC software will allow you to:
- Perform I-V measurements between -10 V and 10 V, with voltage step sizes as low as 333 µV.
- Measure low currents with an accuracy of ±10 nA, or high currents up to ±200 mA.
- Customise your measurements by altering the time between applying a voltage and measuring current (settle time).
- Take more advanced measurements using the hysteresis I-V option, which will perform measurements in both forward and reverse directions.
- The software can perform an I-V measurement using one SMU channel, whilst simultaneously supplying a voltage through the other SMU channel - enabling a wider variety of experiments to be performed.
- Measurement data and settings can be saved to .csv files for easy analysis and record keeping. Settings profiles can be saved in the software, making it simpler to repeat measurements.
For more thorough solar cell characterization, the Solar Cell I-V Software can be used with the Source Measure Unit offers the following capabilities:
- It enables you to measure I-V curve (or measuring J-V curves) for your solar cell, allowing you to record curves for multiple pixels.
- The software provides all the measurements available in the Ossila I-V software.
- After performing a J-V curve, it automatically measures and calculates key metrics such as JSC, VOC, FF, and PCE, as well as shunt and series resistance.
- Both J-V curve data and calculated solar cell metrics are saved in .csv files, making it easy to analyze and plot your data.
- You can assess the long-term stability of your solar cells using the Lifetime and Stabilized Current features. The Lifetime feature continuously measures J-V curves over a defined period and at specified intervals, recording device metrics. The Stabilized Current feature represents the operational stability of your devices by allowing you to define the voltage at which your solar cell achieves maximum power output, and then tracks the current density at that voltage over time.
Source Measure Unit
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