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PLQY (Photoluminescence Quantum Yield or Quantum Efficiency) and it's applications.

─ Key for achieving high device efficiency and stability ─

Contents

What is PL?

  Photoluminescence (PL) refers to the process in which a substance absorbs photons and then re-radiates photons. Photoluminescence (PL) is one of the various forms of substance luminescence. Substance absorbs photons and transitions to an excited state of a higher energy level. After returning to a low-energy state, it emits photons at the same time and names Photoluminescence (PL).

Photoluminescence (PL) is a type of cold luminescence, which refers to the process in which a substance absorbs photons (or electromagnetic waves) and then re-radiates photons (or electromagnetic waves). From the theory of quantum mechanics, this process can be described as a process in which matter absorbs photons and transitions to an excited state with a higher energy level, then returns to a lower energy state, and emits photons at the same time.

Reference: https://en.wikipedia.org/wiki/Photoluminescence

  The process of photoluminescence (PL) can be divided into three stages. Firstly, when light is irradiated on the material, it will be absorbed and this is called photoexcitation. The excess energy will be transmitted by the material, and finally this excess energy is then released in a luminous way. Therefore, photoluminescence (PL) is a method to explore the electronic structure of materials without contacting and damaging the materials, which can provide information about the structure, composition and atomic arrangement of the material. Commonly used for band gap detection, impurity level classification and defect detection, recombination mechanisms, and material qualification.

PL, energy diagram, ground state, singlet state, emission, luminescence

Figure 1. The energy diagram of the photoluminescence (PL) process.

The higher the energy level in the figure represents the higher the energy. The ground state refers to the state of all electrons at the lowest energy level. Others with additional energy state are generally referred to as the “singlet state / excited state”. When the fluorescent substance is excited by incident light, the electrons originally in the ground state are excited to the excited state because of absorbing the energy of the light. Electrons in the excited state can return to the ground state through various pathways. In the figure, if the electrons release energy in the form of light emission and return to the ground state, the light emitted in this way can be broadly referred to as “fluorescence”. This process is called photoluminescence (PL).

What is PLQY?

  Photoluminescence Quantum Yield (PLQY) is an important indicator for measuring luminescent materials, and it is also a basic parameter for primary classification of materials. Photoluminescence Quantum Yield (PLQY) is defined as the ratio of the number of photons emitted to the number of photons absorbed, as shown in the following PLQY formula:

PLQY formula calculation

For example, if a material absorbs 100 photons and emits 50 photons, its quantum yield is 0.5 or 50%.

PL, PLQY, Blank, Sample, excitation light, absorption, fluorescence, PLQY calculation

Figure 2. Measurement and calculation of Photoluminescence Quantum Yield (PLQY).

When measuring Photoluminescence Quantum Yield (PLQY), a blank control device will be measured first, and the measured spectrum (black spectral curve) has only one peak of excitation light. Then measure the sample: let the excitation light hit the sample with the same light intensity, and the measured spectrum (red spectral curve) has not only the peak of excitation light, but also a fluorescence peak at the same time. Comparing the two spectral curves, it can be found that the peak intensity of the excitation light of the sample will be lower than that of the blank control device, that means the sample absorbs part of the excitation light. The additional fluorescence peaks is generated by photoluminescence (PL) of the sample.

The quantum yield (Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system.

The fluorescence quantum yield is defined as the ratio of the number of photons emitted to the number of photons absorbed.

Reference: https://en.wikipedia.org/wiki/Quantum_yield

Why PLQY is important?

  In most applications, the study of efficiency is often the most concerned key indicator. Efficiency represents the ratio between the effort put into the system and the benefit obtained from the system.

  In electroluminescent devices, such as organic, perovskite or quantum dot LEDs, how to maximize the external quantum efficiency (EQE) is usually the main research motivation to drive materials research. In addition to the device architecture design and electrical properties, efficiency is also directly dependent on the intrinsic efficiency of the luminescent material used. In other word, the ratio between the photons emitted by each molecule’s excitation, which is an important key. And this efficiency is usually quantified in photoluminescence (PL) experiments, which is the so-called Photoluminescence Quantum Yield (PLQY).

How to measure PLQY?

There are two common methods for measuring Photoluminescence Quantum Yield (PLQY):

  • The first method to measure Photoluminescence Quantum Yield (PLQY) is the comparative method, which is the most frequently used method in the past. It uses some reference devices with known Photoluminescence Quantum Yield (PLQY) values to measure the absorption rate of excitation light and the emitted fluorescence light intensity of the samples respectively, and then compare the values to obtain the Photoluminescence Quantum Yield (PLQY) value of the sample. However, using the comparative method has many disadvantages and limitations, including: 1. There are not many substances that can be used as reference devices as a standard, 2. It is also necessary to find a reference devices that is close to the excitation and absorption spectrum of the device under test. 3. Moreover, the additional reference devices preparation is required for each experiment, which greatly increases the cost and time of the experiment.
  • The second method of measuring Photoluminescence Quantum Yield (PLQY) is the absolute quantum yield measurement method, that is, directly using an integrating sphere to measure Photoluminescence Quantum Yield (PLQY). It consists of an excitation light source, which can be a laser or LED, that illuminates the luminescent material located in the integrating sphere, and then all reflected, transmitted or emitted light is collected in the sphere, and then a spectrometer is used to collect the spectrum for detection.

Steps for measuring absolute quantum yield.

Step 1. Setting up the excitation light source (this article takes a 405 nm laser as an example): The excitation light source is connected to the optical fiber by optical fiber coupling and connected to the integrating sphere.

PL, PLQY, Enlitech, Laser Light Source, Optical Fiber

Figure 3: The picture on the left shows a 405 nm laser light source with a fiber coupling kit, and the excitation light can be output through the fiber.

The picture on the right is the integrating sphere used for the measurement (Photoluminescence Quantum Yield, PLQY). The optical module installed on the side can be connected to the optical fiber, and the excitation light can be introduced into the integrating sphere.

Step 2. Prepare the sample: Prepare the test sample and the blank control device for measuring the Photoluminescence Quantum Yield (PLQY). For example, the blank control device of the sample with coating is the glass without coating.

PL, PLQY, Enlitech, Blank, Sample

Figure 4: The Sample on the right is a thin film sample to be measured for Photoluminescence Quantum Yield (PLQY), and the Blank on the left is a glass substrate without a coating film compared to the sample on the right.

Step 3. Put the blank control and the sample to be measured for Photoluminescence Quantum Yield (PLQY) into the integrating sphere separately. The sample need to be placed vertically to prevent falling out, and the orientation of the sample holder also needs to be noticed. Do keep the direction of the reflection mirror toward the excitation light source.

PLQY (Photoluminescence Quantum Yield or Quantum Efficiency) and it's applications. The key for achieving high device efficiency and stability. PLQY PL sample holder from integrating sphere LQ

Figure 5: The PL sample holder is placed from top of the integrating sphere.

The sample is fixed in the groove of the sample holder, and the reflection mirror towards the left to match the incident direction of the excitation light source.

Step 4. Adjust the measurement conditions: First of all, the excitation light intensity can be adjusted according to the test requirements. You can use the mouse to move the output adjustment lever, or directly input the required power. 100% means full output. Second, adjust the measurement time of the spectrometer. It needs to be adjusted according to the conditions of the excitation light intensity in the previous step. Increasing the integration time can make the spectral signal have a high signal-to-noise ratio (more than 100:1 is better), and it cannot be set too long to avoid signal saturation.

PLQY Software of Power adjustment for excitation light source LQ

Figure 6: The interface on the measurement software used to adjust the excitation light power output, read the signal, and perform preliminary tests.

The “Power” controls the output power of the excitation light, which can be manually input or adjusted with a lever. The integral measurement time of the spectrometer can be adjusted through “Int_Time”, and finally click the Pre Test button to trigger spectrum measurement to check if the settings are suitable or not.

Step 5. Spectral measurement: measure the fluorescence spectra of the blank and the sample respectively. As shown in Figure 7, the blue color is the blank spectrum, and the green color is the sample spectrum. Due to photoluminescence (PL), it can be seen that in the wavelength range of excitation light, the spectrum of the sample is lower than that of the blank, indicating that part of the excitation light has been absorbed by the sample. In the wavelength range of fluorescence, it can be seen the fluorescence spectrum of the sample appears, while the original blank does not.

PL, PLQY, Enlitech, Software, PQLY Calculation

Figure 7: Screen shot of the Photoluminescence Quantum Yield (PLQY) measurement software.

The corresponding functions on the left are (A) Blank measurement, (B) Sample measurement, (C) Calculated photoluminescence Quantum Yield (PLQY). In the spectral display in the center, the blue color is the blank spectrum, and the green color is the sample spectrum to be measured for Photoluminescence Quantum Yield (PLQY). The black dotted line is the selected excitation light calculation range, and the orange dotted line is the selected fluorescence calculation range.

Step 6. Select the calculation wavelength range: After selecting the excitation light wavelength range and the fluorescence wavelength range to be calculated, press the “calculate QE” botton to calculate the Photoluminescence Quantum Yield (PLQY).

The pain point of PLQY measurement

  Photoluminescence (PL) and Photoluminescence Quantum Yield (PLQY) are important tools for studying material characterization. Currently, there are three challenges in material testing:

(1) PLQY cannot be tested in the glove box.

(2) PLQY cannot perform in situ time spectral analysis.

(3) PLQY measuring range is not easy to expand to infrared band.

  The glove box is a laboratory equipment that filled with high-purity inert gas, and filters out active substances such as moisture, oxygen and other organic gases through circulation. Many light-emitting components are produced in the glove box. For example, the spin coater used to coat the luminescent material on the glass will be placed in the glove box to avoid the volatilization of the organic gas used to dissolve the material when the film is thrown off, which will affect the health and safety of the personnel. The environment in the glove box is relatively clean, which can avoid the interference of many external pollutions.

  Therefore, after the material is produced, the best condition is to measure its photoluminescence (PL) and Photoluminescence Quantum Yield (PLQY) in the glove box directly. However, due to the space limitation of the glove box (common inner size is only about 1800 mm (L) x 750 mm (W) x 900 mm (H)), if the spin coater and some other equipment have been placed inside, the remaining space is obviously not enough to put another large PLQY test equipment. Enlitech’s LQ-100X-PL has a compact design with 502.4mm(L) x 322.5mm(W) x 352mm(H) dimension. Equipped a 4-inch outer diameter PTFE integrating sphere and integrated NIST traceable calibration makes it possible to integrate PL and PLQY in the glove box.

PL, PLQY, Enlitech, LQ-100X-PL, Glove box

Figure 8: The actual shot of Enlithch’s LQ-100X device in the glove box.

The LQ-100X adopts a compact design, and also considers the convenience of the operation inside the glove box, so as to make the optimal space utilization.

PL, PLQY, Enlitech, LQ-100X-PL, Glove box

Figure 9: Actual shot of the space planning and configuration of the glove box.

The glove box in the photo is a basic two-glove configuration (the front panel is temporarily removed). It shows that Enlithch’s LQ-100X-PL system with the integrating sphere and excitation light source, occupies only 50% of the space. The space on the left allows users to set up other equipment according to their requirements. The left side of the picture is Enlithch’s solar simulator measurement stage. The solar simulator is installed underneath the glove box and it illuminates the sample from bottom up.

PL, PLQY, Glove box, Space

Figure 10: PLQY measuring equipment of other brands.

The PLQY measuring equipment of other brands needs to occupy a relatively large space, so it cannot be put into common glove box. (Picture taken from the Internet.)

  In addition, as described above, since the manufacturing process of many luminescent materials is mostly carried out in the glove box, the testing of many material characterization techniques needs to be directly measured as soon as possible after the manufacturing is completed, such as in-situ time PL spectral analysis. Enlitech’s LQ-100X-PL equipped an advanced instrument control software to perform in situ time PL spectral analysis. It can generate 2D and 3D graphs that allow users to more quickly characterize material in situ time changes.

PL, PLQY, In-situ time-resolved photoluminescence, Enlitech, Software,

Figure 11: In situ time PL spectral resolution.

The LQ-100X-PL can provide a measurement function that records spectra over time and present the results through different ways: (A) Top left: 3D spectrum change graph, (B) Top right: 2D spectrum overlay, (C) Bottom left, all spectral data over time, (D) Bottom right: 2D intensity gradient map.

  In addition, Enlitech’s LQ-100X-PL system optical design can be easily extended to infrared wavelengths from 1000 nm to 1700 nm. Powder, solution and film samples are all compatible for testing.

PLQY application and practical case

PL, PLQY, perovskite solar cell, PSC, Enlitech

Figure 12: PLQY application and practical case 1.

In this paper, a dielectric annealing technique (LMA) is used to control the growth of the whole hybrid perovskite thin film crystal, which improves the stability of the power output of perovskite solar cells (PSCs). The figure shows the PLQY measurement results of the film using LMA technology and the film using the reference technology. It can be seen that the PLQY measured by the LMA technology process is higher than that of the reference process.

PL, PLQY, Enlitech, Perovskite LED (PeLED)

Figure 13: PLQY application and practical case 2.

In this thesis, researchers investigate the effect of alkali metal ions on the nucleation and growth of Quasi-2D (Q-2D) perovskites, and the results demonstrate a novel approach to optimize the performance of Q-2D perovskite LEDs. The figure shows that the higher the KBr concentration added to the Q-2D perovskite, the higher the measured PLQY. This phenomenon is positively correlated with the luminous intensity of the LED element.

PL, PLQY, Perovskite LED (Pe LED), non-radiative recombination defects, Luminous efficiency, Enlitech

Figure 14: PLQY application and practical case 3.

In this paper, ethoxylated trimethylolpropane triacrylate (ETPTA) was used as a functional additive dissolved in an anti-solvent to passivate the surface and bulk defects in the spinning process. defects). ETPTA can effectively reduce the charge trapping state to suppress defects through passivation, reduce the non-radiative recombination loss and improve the luminous efficiency. The figure below shows the comparison of PLQY measured with and without ETPTA. The group with ETPTA has higher PLQY and relatively higher luminous efficiency.

PLQY Photoluminescence Quantum Yield

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