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Photoluminescence quantum yield
What is a photoluminescence quantum yield measurement?
Photoluminescence (PL) quantum yield is the ratio of luminescence photons emitted to the number of excitation photons absorbed by a light-emitting material.
PL quantum yield is an important criterion in the field of light-emitting device development, where improving the luminescence efficiency is essential to reduce device power consumption. For example, in developing OLED displays, PL quantum yield measurements are used to evaluate materials, and accurate calculation of the PL quantum yield is crucial for development.
Methods and principles of measuring PL quantum yield
Relative method and absolute method
There are two main methods for measuring luminescence quantum yield: the relative method and the absolute method.
In the relative method, the PL quantum yield of a sample solution is compared to that of a standard solution with a known PL quantum yield, measured under the same conditions. Although this method requires minimal corrections, such as spectral sensitivity and solvent refractive index corrections, it lacks the accuracy of determining a precise quantum yield and is time-consuming to perform. Additionally, it cannot characterize solid samples like thin films and powders.
The absolute method determines the PL quantum yield without needing a standard sample. The primary technique for this method is the integrating sphere method, which can measure solid samples, including thin films and powders, as well as solutions. This method provides very accurate quantum yield determination and is easier and less time-consuming to perform.
Hamamatsu Photonics offers a compact, state-of-the-art PL quantum yield measurement system based on the integrating sphere method.
| Measurement method | Need for standard samples | Measurable Objects |
|---|---|---|
| Relative method | Necessary | Solution |
| Absolute method (Integrating sphere method) | Unnecessary | Solution, Solid (thin film, powder, etc.) |
What is an integrating sphere?
An integrating sphere is an optical component consisting of a hollow sphere with an inner surface coated with a highly reflective and diffusive material. Light-emitting materials and devices often exhibit unique angular light-distribution characteristics, leading to variations in measured brightness depending on the measurement location. When light enters the integrating sphere or is emitted by a sample placed inside it, the light undergoes multiple diffuse reflections off the inner wall. This process homogenizes the light distribution within the sphere, resulting in a nearly uniform radiance regardless of the measurement direction. The uniform light distribution inside the integrating sphere is directly proportional to the total radiant flux emitted by the light-emitting material or device.
By using the integrating sphere method, it is possible to accurately measure the PL quantum yield without being influenced by the anisotropic light distribution characteristics of the luminescent material. This method ensures that the measurement reflects the intrinsic emission properties of the sample, providing reliable and reproducible results.
Figure 1: Image of light emitted from light-emitting materials
Principle of PL quantum yield measurement
Figure 2 shows an example of an absolute PL quantum yield measurement system using an integrating sphere. The system generally consists of an excitation light source, integrating sphere, spectrometer, and data analyzer. The sample to be measured is placed in the sample holder inside the integrating sphere. Excitation light output from the excitation light source is introduced into the integrating sphere by a light guide and irradiated onto the sample. The excitation light and the luminescence from the sample are diffusely reflected inside the integrating sphere, resulting in uniform brightness. A portion of this light is measured with a spectrometer via a fiber probe.
Figure 3 illustrates the principle of PL quantum yield measurement. When measuring PL quantum yield, the excitation and emission spectra of both the reference and sample are measured. The order of measurement is from reference to sample. When measuring the reference at the beginning, the measurement is performed in the sample holder only, which does not contain the sample. In Figure 3, the sample spectrum shows a decrease in excitation light intensity compared to the reference spectrum, while luminescence is observed at longer wavelengths. This indicates that the sample absorbs some of the excitation light and emits light. The PL quantum yield can be calculated using the difference between the integrated intensity of the excitation light for the reference and the sample and the integrated intensity of the luminescence at longer wavelengths for the sample.
Figure 2: Example of absolute PL quantum yield measurement system with integrating sphere
Figure 3: Principle of measuring PL quantum yield
Example of PL quantum yield measurement
Quantum yield measurement of thermally activated delayed fluorescent materials with high luminous efficiency
Thermally activated delayed fluorescence (TADF) materials are well known as the third generation OLED materials. TADF is the fluorescence generated by reversed intersystem crossing process (RISC) from the lowest triplet to the singlet states. The RISC process is promoted by the small energy gap between the lowest excited states (ΔEST). A novel TADF material (4CzIPN) was successfully developed by precise molecule design. The material has small ΔEST and it shows high PL quantum yield as 0.94 +/- 0.02.
Data courtesy of Prof. Chihaya Adachi, Hajime Nakanotani
Center for Organic Photonics and Electronics Research, Kyushu Univ.
H. Uoyama, K. Goushi, K. Shizu, H. Nomura, and C. Adachi, Nature. 492, 234 (2012).
Fluorescence spectra and fluorescence quantum yields of anthracene solutions
The quantum yields of fluorescence standard solutions were measured with our absolute PL quantum yield measurement system. The fluorescence standard solutions have been used for determining PL quantum yield based on a relative method. For most compounds, the quantum yield measured by our system shows excellent agreement with the values given in the literature, proving the high reliability of our system.
Collaborative research of Hamamatsu Photonics K.K.; A. Kobayashi, S. Kaneko, K. Takehira, T. Yoshihara, and S. Tobita, Faculty of Engineering, Gunma University; H.Ishida, Y.Shiina, and S.Oishi, School of Science, Kitasato University
K. Suzuki, A. Kobayashi, S. Kaneko, K. Takehira, T. Yoshihara, H. Ishida, Y. Shiina, S. Oishi, and S. Tobita, Phys. Chem. Chem. Phys., 11, 9850 (2009).
Fluorescence quantum yield and levigation effect on p-terphenyl and anthracene single crystals
We utilized high-purity single crystals of the typical organic materials p-terphenyl and anthracene to determine their respective fluorescence quantum yields. Measuring the p-terphenyl resulted in a fluorescence quantum yield of 0.67 for this high-purity single crystal (blue curve in Figure A). Levigating this single crystal to a fine powder increased the fluorescence quantum yield to 0.80 (red curve in Figure A). On the other hand, levigating the high-purity, single crystal anthracene decreased the fluorescence quantum yield from 0.64 to 0.27 (Figure B). Measuring the p-terphenyl showed a higher fluorescence quantum yield and appearance of structures on the short wavelength side of the fluorescence spectrum. So this higher fluorescence quantum yield was possibly caused by the fine powder from levigation that acts to inhibit reabsorption. Examining the anthracene revealed another luminescent component at longer wavelengths caused by levigation in addition to the usual luminescent components on shorter wavelengths of the fluorescence spectrum. This luminescent component on the lower wavelengths resembles the fluorescence spectrum of anthracene dimer and so was found to be luminescence from a dimer state. This proves that the decrease in fluorescence quantum yield of anthracene single crystal was caused by dimers induced by levigation that formed structural flaws and acted as a center for light extinction.
Collaborative research of Hamamatsu Photonics K.K.; Ryuzi Katoh, Akihiro Furube, Ph.D., Research Institute of Instrumentation Frontier, Advanced Industrial Science and Technology; Masahiro Kotani, Ph.D., Department of Chemistry, Gakushuin University; and Katsumi Tokumaru, University of Tsukuba.
R. Katoh, K. Suzuki, A. Furube, M. Kotani, and K. Tokumaru, J. Phys. Chem. C, 113, 2961(2009).
Phosphorescence quantum yield measurement of benzophenone at −196 ˚C (77K)
Phosphorescence quantum yields in benzophenone organic solution were measured at room temperature (22 ˚C (295 K)) and at a low temperature (-196 ˚C (77 K)) and both compared on the graph. Benzophenone is known to generate a triplet excitation state at a high efficiency (fISC to 1.0) after being excited by light from the ground state to the singlet state. Observing phosphorescence from general organic compounds is usually difficult because phosphorescence is a forbidden transition. In benzophenone, a phosphorescence spectrum, though weak, was definitely observed (fP to 0.01). The result also shows that phosphorescent intensity greatly increased at low temperature compared to room temperature and produced a high phosphorescence quantum yield (fP>0.8).
Collaborative research of Hamamatsu Photonics K.K.; A. Kobayashi, S. Kaneko, K. Takehira, T. Yoshihara, and S. Tobita, Faculty of Engineering, Gunma University
A. Kobayashi, K. Suzuki, T. Yoshihara, and S. Tobita, Chem. Lett., 39, 282 (2010).
Featured products and systems
Absolute PL quantum yield measurement system with easily expandable functions. By adding options, it supports near-infrared measurement, high-sensitivity (low quantum yield) measurement, and up-conversion luminescence measurement.
Compact 1-box type absolute PL quantum yield measurement system. Its straightforward operation allows for the fast measurement of luminescence quantum yield, excitation wavelength dependence, excitation spectrum and more.
This system measures photoluminescence spectra in all directions using an integrating sphere to determine the luminous efficiency of a sample. It easily and instantly measures IQE (internal quantum efficiency), which is necessary for the evaluation of GaN single crystals, perovskite crystals, etc.
Other measurement methods
- Confirmation
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