What is PL?
Photoluminescence (PL) refers to the process where a material absorbs photons and re-emits photons. Photoluminescence is one of several forms of material luminescence where a substance absorbs photons, transitions to a higher energy excited state, and returns to a lower energy state while emitting photons – hence the term “photo” (light) induced “luminescence” (light emission).
Photoluminescence (PL) is a type of cold luminescence that occurs when a material absorbs photons (or electromagnetic waves) and re-emits photons (or electromagnetic waves). From a quantum mechanical perspective, this process can be described as a material absorbing photons to transition to a higher energy excited state, then returning to a lower energy state while simultaneously emitting photons. Source: https://en.wikipedia.org/wiki/Photoluminescence
The photoluminescence (PL) process can be divided into three stages: First, when light is incident on a material, it is absorbed in a process called photoexcitation. Then, the excess energy is transferred through the material, and finally, this excess energy is released through light emission. Therefore, photoluminescence (PL) serves as a non-destructive, contactless method for probing electronic structure of materials, providing information about material structure, composition, and atomic environment. It is commonly used for bandgap detection, impurity level and defect detection, recombination mechanisms, and material quality assessment.
Figure 1: Energy diagram of the photoluminescence (PL) process.
In the diagram, higher positions represent higher energy levels. The ground state refers to the state where all electrons are at their lowest energy level, while states with additional energy are broadly termed “excited states” (singlet state / excited state). When a fluorescent material is exposed to excitation light, electrons originally in the ground state absorb the light energy and become excited to higher energy states. Electrons in excited states can return to the ground state through various pathways. When electrons release energy through light emission to return to the ground state, the emitted light is broadly termed “fluorescence.” This process is called photoluminescence (PL).
What is PLQY?
Photoluminescence Quantum Yield (PLQY) is a crucial metric for evaluating luminescent materials and serves as a fundamental parameter for preliminary material classification. PLQY is defined as the ratio of emitted photons to absorbed photons, as shown in the following formula:
For example, if a material absorbs 100 photons and emits 50 photons, its quantum yield would be 0.5 or 50%.
Figure 2: Measurement and calculation of Photoluminescence Quantum Yield (PLQY).
When measuring PLQY, we first examine a blank reference sample, which produces a spectrum (black spectral curve) showing only an excitation peak. Subsequently, we introduce the sample for PLQY measurement, exposing it to excitation light of the same intensity. The resulting spectrum (red spectral curve) shows both the original excitation peak and an additional fluorescence peak. Comparing these two spectral curves, we observe that the sample’s excitation peak intensity is lower than that of the blank reference, indicating partial absorption of the excitation light by the sample. The additional fluorescence peak in the sample’s spectrum represents the fluorescence generated through photoluminescence (PL).
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 is PLQY Measurement Important?
In most applications, efficiency studies are often the most crucial parameter, as efficiency represents the ratio between system input and output benefits.
In electroluminescent devices, such as organic, perovskite, or quantum dot LEDs, maximizing external quantum efficiency (EQE) is typically the primary motivation driving materials research. However, beyond careful device architecture and electrical performance design, efficiency directly depends on the inherent efficiency of the luminescent material used – specifically, the ratio between photon emission and molecular excitation, which is a critical factor. This efficiency is typically quantified in photoluminescence (PL) experiments, known as Photoluminescence Quantum Yield (PLQY).
How to Measure PLQY?
There are two common methods for measuring Photoluminescence Quantum Yield (PLQY):
- The first method for measuring PLQY is the comparative method:
This traditionally popular method uses reference standards with known PLQY values. It involves measuring both the reference standards and the research materials for their excitation light absorption rates and fluorescence intensity, then comparing them to determine the PLQY value of the research material. However, the comparative method has several limitations and drawbacks, including the limited availability of suitable reference standards and the need to find references with similar excitation and absorption characteristics to the research material. Additionally, each experiment requires preparing additional reference standards, significantly increasing experimental costs and time. - The second method for measuring PLQY is the absolute quantum yield measurement method:
This method directly measures PLQY using an integrating sphere. It includes an excitation light source (either laser or LED) that illuminates the luminescent material placed inside the integrating sphere. All reflected, transmitted, or emitted light is collected within the sphere and subsequently measured using a spectrometer for spectrum collection.
Step-by-Step Process for Absolute Quantum Yield Measurement
Step 1. Setting up the Excitation Light Source (Using 405 nm laser as an example):
The excitation light source is connected to the integrating sphere via fiber optic coupling.
Figure 3: The left image shows a 405 nm laser source with fiber optic coupling accessories, allowing excitation light to be transmitted through the fiber. The right image shows the integrating sphere used for PLQY measurements, with optical modules mounted on the side for fiber connection and excitation light input into the integrating sphere.
Step 2. Sample Preparation:
Prepare both the test sample for PLQY measurement and a blank reference. For example, when testing a thin film sample, the blank reference would be an uncoated glass substrate.
Figure 4: The right image shows the Sample for PLQY measurement (thin film sample), while the left image shows the Blank (uncoated glass substrate) as a reference.
Step 3. Integrating Sphere Placement:
Place both the blank reference and the PLQY test sample into the integrating sphere vertically to prevent sample displacement. Pay attention to the sample holder orientation, aligning the reflector direction with the excitation light incidence direction.
Figure 5: The PL sample holder is inserted from the top of the integrating sphere, with the sample placed in the holder’s groove and the reflector and sample oriented toward the left (excitation light incidence direction).
Step 4. Adjusting Measurement Parameters:
First, adjust the excitation light intensity according to measurement requirements using either the mouse to move the output adjustment slider or by directly inputting the desired power, where 100% represents full power output. Second, adjust the spectrometer integration time to complement the excitation light intensity settings. Increasing integration time can improve the signal-to-noise ratio (preferably above 100:1), but avoid setting it too long as this can affect measurement time and potentially cause signal saturation and data distortion.
Figure 6: Software interface for adjusting excitation light power output, signal acquisition, and preliminary testing.
The Power control adjusts excitation light output power through manual input or slider adjustment. The SPM Int_Time allows input of spectrometer integration time. Click the Pre Test button above to examine the spectrum and verify whether the settings are appropriate.
Step 5. Spectral Measurement:
Measure the fluorescence spectra of both the blank reference and sample. The blue spectrum represents the blank reference, while the green spectrum shows the PLQY test sample. Due to photoluminescence, the sample spectrum shows lower intensity than the blank reference in the excitation wavelength range, indicating partial absorption of excitation light. In the fluorescence wavelength range, the sample spectrum shows a fluorescence peak absent in the blank reference.
Figure 7: PLQY measurement software interface.
The left side shows the corresponding functions: (A) Blank reference measurement, (B) Sample measurement, (C) PLQY calculation. In the central spectrum display, the blue spectrum represents the blank reference, while the green spectrum shows the PLQY test sample. The black dashed line indicates the selected excitation light calculation range, and the orange dashed line shows the selected fluorescence calculation range.
Step 6. Selecting Wavelength Ranges for Calculation:
Select the wavelength ranges for both excitation light and fluorescence, then activate the calculation function to determine the PLQY value.
Pain Points in PLQY Testing
Photoluminescence (PL) and Photoluminescence Quantum Yield (PLQY) are essential tools for material characterization. Currently, material testing faces three major challenges:
- PLQY cannot be measured inside a glovebox.
- PLQY lacks in-situ time-resolved spectral analysis capabilities.
- PLQY infrared range extension is challenging.
A glovebox is a laboratory equipment that contains high-purity inert gas and filters out reactive substances such as moisture, oxygen, and other organic gases through circulation. Many light-emitting device fabrication processes are completed inside gloveboxes, including spin-coating luminescent materials onto glass substrates. The spin coater is placed inside the glovebox to prevent organic solvent vapors from affecting personnel health and safety during film deposition. Additionally, the controlled environment inside the glovebox minimizes interference from external conditions, making it ideal to test materials’ PL and PLQY directly inside the glovebox after film deposition.
However, typical gloveboxes have limited space dimensions of approximately 1800 mm (L) × 750 mm (W) x 900 mm (H). After installing the spin coater and other essential equipment, there’s insufficient space for large testing equipment. Enlitech’s LQ-100X-PL features a compact design measuring 502.4mm(L) x 322.5mm(W) x 352mm(H), equipped with a 4-inch outer diameter PTFE integrating sphere and integrated NIST-traceable calibration, making glovebox integration of PL and PLQY measurements possible.
Figure 8: Actual photograph of Enlitech’s LQ-100X installed in a glovebox.
The LQ-100X employs a compact design that considers operator ergonomics inside the glovebox, maximizing efficiency in the limited workspace.
Figure 9: Actual glovebox space planning and configuration.
The photo shows a basic two-glove configuration (front panel temporarily removed). The Enlitech LQ-100X-PL setup, including the integrating sphere and excitation source, occupies only about half the glovebox space, leaving the left side available for other testing equipment. The left side shows Enlitech’s solar simulator testing platform, with the solar simulator mounted beneath the glovebox, projecting light upward into the chamber.
Figure 10: PLQY measurement equipment from other manufacturers.
PLQY measurement equipment from other manufacturers requires larger installation space, making it impractical for glovebox integration. (Images sourced from the internet)
As mentioned earlier, since many luminescent material fabrication processes occur inside gloveboxes, various material characterization techniques should ideally be performed immediately after fabrication, such as in-situ time-resolved PL spectral analysis. Enlitech’s LQ-100X-PL utilizes advanced instrument control programs to perform in-situ time-resolved PL spectral analysis, generating 2D and 3D plots that enable users to more rapidly characterize material changes in real-time.
Figure 11: In-situ time-resolved PL spectral analysis.
The LQ-100X-PL offers time-resolved spectral measurement capabilities with multiple data visualization options: (A) Top left: 3D spectral evolution plot, (B) Top right: 2D spectral overlay, (C) Bottom left: Complete temporal spectral data, (D) Bottom right: 2D intensity gradient map.
Furthermore, Enlitech’s LQ-100X-PL system features optical designs that facilitate infrared range extension from 1000 nm to 1700 nm. The system is compatible with powder, solution, and thin film samples.
PLQY Applications and Case Studies
Figure 12: PLQY Application Case Study 1.
This research paper utilizes a dielectric annealing technique (LMA) to control the crystal growth of mixed perovskite thin films, enhancing the power output stability of perovskite solar cells (PSCs). The figure shows PLQY measurement results comparing films produced using the LMA technique versus reference processing methods. The results demonstrate that films processed using the LMA technique exhibit higher PLQY values compared to those produced using reference methods.
Figure 13: PLQY Application Case Study 2.
This research paper investigates the influence of alkali metal ions on the nucleation and growth of Quasi-2D (Q-2D) perovskites, validating a novel method for optimizing Q-2D perovskite LED performance. The figure demonstrates that higher KBr concentrations in Q-2D perovskites correlate with higher PLQY values, which positively correlates with LED device luminescence intensity.
Figure 14: PLQY Application Case Study 3.
This research paper employs ethoxylated trimethylolpropane triacrylate (ETPTA) as a functional additive dissolved in anti-solvent, introduced during the spin-coating process to passivate surface and bulk defects. ETPTA effectively reduces charge trap states through passivation, suppressing defects, reducing non-radiative recombination losses, and enhancing luminescence efficiency. The figure compares PLQY measurements with and without ETPTA addition, showing that samples containing ETPTA exhibit higher PLQY values and correspondingly higher luminescence efficiency.
