Background - luminescence
Certain types of materials emit radiation when supplied with energy – these processes are broadly called luminescence. There are different ways of providing energy to the material that give us distinct forms of luminescence that help us understand that material, or may be used to form useful devices. These include:
Photoluminescence – energy is supplied by the material absorbing photons.
Cathodoluminescence- energy is supplied by the material absorbing electrons. This is used, for example in cathode ray tube (CRT) televisions.
Electroluminescence – energy is supplied to the material by applying an electric field.
Electroluminescence (EL) and LEDs
Certain types of materials will emit radiation when energy is supplied under an electric field (or an electric current). When this radiation is visible, these materials can be used to produce light for different applications – for example, display devices.
The building block of many of these types of devices are diodes – devices that allow current to flow in one direction but are highly resistive in the other. These are often measured by sweeping the voltage applied to a device and measuring the current passing through it. This can be referred to as an I-V curve, or often units of current density are used to compare different devices and are given the symbol J. This electrical measurement can be very useful in showing that a device is working properly, due to its characteristic shape:
Figure 1: An I-V curve of an LED
When these diodes are made from materials that can produce electroluminescence, we can produce light-emitting diodes, or LEDs for short. In these types of materials, light is emitted when current is passing through a device. We call the light emitted from the whole device the luminous flux of the device (measured in lumens), and often divide this by the area of the device to give the luminance (measured in cd/m2).
Figure 2: Luminance - voltage curve on an LED
We can measure the luminance of the device as we increase the voltage (and thus the current) going through it. This is a measure of the light of all wavelengths emitted from the device. However, we typically find that materials will emit light in a characteristic spectrum, which often tells us a lot about the material. In this case, the LED has a spectrum with a peak around 550 nm, giving it a yellow colour.
Figure 3: EL intensity plotted against wavelength of a yellow LED
External quantum efficiency (EQE)
In a perfect LED device, all of the current we pass through the device would contribute to the emission of light. However, in real devices, not all current passing through the device will do this. It is, therefore, useful to measure the external quantum efficiency (EQE) of a device, which is defined as the ratio of the number of photons emitted from the device to the number of electrons passing through the device.
We use a HAMAMATSU C9920-12 system to characterise LEDs. An integrating sphere is used to count for the emission in different directions. Various sample holders are available to correspond to different device layouts, allowing us to rapidly prototype and test different OLED systems.
Figure 4: An illuminated yellow LED
A source meter (Keithley 2400) provide constant current or voltage to derive the device electroluminescence. The electroluminescence is detected via two photonic multichannel analysers, PMA12 and C10028, depending on the wavelength of interest.
Our apparatus allows for measuring the electroluminescence (EL) spectrum from 300nm to 1650 nm. The light flux [lm] is then calculated from the spectrum to measure the external quantum efficiency of light-emitting devices. The PMA software, provided by HAMAMATSU, controls the measurement parameters such as exposure time and the applied current/voltage. It also allows for scanning through sequential current or voltage steps. The measurement results can be displayed in various graphic representation using the PMA software.
Figure 5: Overview of the LED characterization equipment, including the Keithley Source Measure Unit, HAMAMATSU PMA, and integrating sphere
Figure 6: PMA software under operation