OLED Testing: Current Efficiency, Power Efficiency and Lifetime

Good organic light emitting diodes (OLED) must offer sufficient brightness in the desired spectral range, good conversion efficiency and stable output. Therefore, OLED characterization requires several different methods of testing, including:

• Measuring OLED Efficiency
• Measuring OLED Lifetime
• Measuring OLED Light Output

Therefore, fully testing OLEDs requires a range of equipment including a current-voltage measurement system (such as a source measure unit) to test the system under different electrical conditions, a probe to measure the intensity of the light emitted over time, and a spectrometer or spectrofluorometer to measure spectral output. 

Custom systems like the Ossila LED Measurement system are designed to make measuring key OLED metrics, such as current and power efficiency, easy and straight forward.

 

OLED Efficiency

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Some key metrics used to measure OLED efficiency are the external quantum efficiency (EQE), current efficiency and power (or luminance) efficiency.

External Quantum Efficiency

One key metric for determining OLED efficiency is the external quantum efficiency, EQE or η_ex. EQE essentially measures the ratio of electrons inputted to photons outputted across a range of different wavelengths. To be more specific, this measures the number of outcoupled photons per injected charge.

This measurement encompasses several factors such as charge carrier dynamics, recombination effects and outcoupling efficiency (i.e. how efficiently generated electrons can be extracted into a circuit). This measurement requires simultaneous measurement of current density and light output inside an delicately calibrated system, often using an integrating sphere.

Current Efficiency

Current density (J) is a metric used in characterizing various electronics. Since the amount of current (I) produced by a device depends on device area, using current density allows researchers to compare OLEDs of different sizes directly. Current density is calculated using the following equation:

By measuring current density and luminance (L), you can work out current efficiency. Current efficiency can be calculated by dividing the luminance emitted in a forward direction by the OLED device to the input current and is given by the following equation.

Luminance is metric of the luminous intensity per unit area of light traveling in a given direction. In OLED measurement, it is used to quantify how bright a display appears to the human eye from a specific angle. This is different from irradiance measurements used to characterize other optical devices, which purely measure radiative power falling on a surface. Therefore, luminance has unique units of candelas per square meter (cd/m²).

Power Efficiency

Power efficiency, also known as luminous efficiency, describes the ratio of luminous flux to input power. This uses current density, luminance and bias voltage, and can be estimated using following equation.

This equation assumes the OLED emission is isotropic.

How to Measure OLED Efficiency

Current efficiency or luminance efficiency are both voltage dependent, so it is important to measure current and luminance over a range of voltages. This is known as a current voltage luminance measurement (I-V-L measurement).

The Ossila LED Measurement System has a calibrated photodiode in the lid, so can take current, voltage and luminance measurements simultaneously. You can also take I-V-L measurements using a combination of equipment including an electrical test board connected to a source measure unit, and a luminance meter that is focused onto one of the pixels to be measured. However, this would require significant calibration in order to measure efficiencies reliably.

Proper measurement of EQE requires an integrating sphere and a high levels of photon detection, which is beyond the scope the Ossila LED Measurement System.

The typical working range of an OLED is less than 100 mA. The maximum efficiency of an OLED is typically observed in the brightness range of tens to hundreds of candelas per meter squared (cd/m²).

With the LED measurement system, you simply put your device into the correct riser board and you can begin testing. This is demonstrated in the short video below.

OLED Lifetime and Stress Testing

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The most common method for measuring OLED stability involves holding a device under a driving current or constant voltage, and measuring how long the device can maintain a certain luminance. Holding the driving current constant is the most popular measurement, as maintaining a constant voltage may allow current to fluctuate according external factors, leading to accelerated deterioration. However, constant voltage measurements may still be useful for stress testing or for initially assessing operational lifetime.

OLED lifetime is often measured in L_p, defined as the amount of hours that a LED can maintain a percentage p of its initial luminance output L₀. This is a measurement of relative decay. The standards for this measurement are often LT₉₅ or LT₅₀, representing the time it takes for a device to reach 95% or 50% of its original luminosity respectively.

Taking OLED Stability Measurement (Constant Current Method)

You can use initial I-V-L measurements to decide on choose important parameters used in lifetime measurements such as the drive current (I_OLED). You should use a drive current that corresponds to your desired luminance L₀ value. Alternatively, you can choose the current which delivers the highest power efficiency.

An example of Luminance vs. Current plot of the OLED is shown below, with the drive current selected based on a desired L₀ value.

Once you have decided on the L₀ value and found the corresponding i_OLED value, the experiment can be set up.

• The chosen i_OLED current must be kept constant throughout the whole experiment, continuously adjusting the voltage.
• The photocurrent of a photodiode (i_PD) is used to follow the luminance decay.

The typical shape of the Luminance decay of an OLED, driven at constant current, is shown in the figure below.

Taking OLED Stability Measurement (Constant Voltage Method)

You can use the Ossila LED Measurement System to take constant voltage lifetime measurements, as shown in the video below.

Accelerated Aging and Stretched Exponential Decay

For particularly stable devices, different methods can reduce the testing time.

In reality, measuring an OLED with a 10,000 hour lifetime would require an experiment longer than 1 year. For this reason, it is vital to have the means to run accelerated tests and extrapolate the lifetime at lower operational conditions.

There are different ways of doing this:

• Stretched Exponential Decay (SED) - This involves taking a portion of the initial data, and using a model to fit the data and extrapolate the test results (had the experiment continued to run). One of the most-used fit models is the Stretched Exponential Decay (SED) :

  

  Where L(t) is the value of the luminance over time, L₀ the initial value of the luminance, t is the time, β and τ are fitting parameters.

  This is a trusted method to predict lifetime. However, the accuracy of the extrapolation depends heavily on the amount of actual data acquired.

• Accelerated lifetime testing - The second method is to run highly-accelerated lifetime tests at different initial L₀ values, and use the obtained lifetime data to extrapolate what the results for lower L₀ values would have been.

  In 1996, Van Slyke et al described the OLED degradation process as “coulombic”, i.e. determined only by the total charge injected into the device. Subsequently in 2002, Popovic and Aziz demonstrated that the product between the initial luminance (L₀) of the lifetime test and the lifetime obtained (LT₅₀ if considering 50% decay from L₀) obtained is a constant.

  This can be written as the following equation:

  

  Where K is a constant and n is an aging acceleration factor.

  Therefore, by plotting the logarithm of lifetime values obtained (e.g. LT₅₀) for the various different initial luminance values L₀, n is equal to the gradient of this straight line graph. From this plot, you can predict the lifetimes for lower luminance values.

  

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