红外与激光工程

Research progress of high-performance PeLEDs based on organic light-emitting materials (invited)

Gao Chunhong, Wang Linqiang, Zhou Kewen, Yang Wei, Zhou Li, Yin Xiaojun, Ban Xinxin, Pan Shusheng

2023, 52(12): 20230630. doi: 10.3788/IRLA20230630

[Abstract](159) [FullText HTML] (71) [PDF 4185KB](57)

Objective In recent years, metal halide perovskite light-emitting materials have attracted great attention for their application in metal halide perovskite light-emitting diodes (PeLEDs) due to their outstanding optoelectronic properties, and are considered as the next generation of light-emitting sources in the field of display and lighting. Exciton utilization is one of the key factors affecting the efficiency of PeLEDs. Various methods have been employed to confine excitons in the perovskite light-emitting layer and recycle the energy of excitons to improve the utilization rate of excitons. This article will review the attempts made to improve the optoelectronic properties of green and blue PeLEDs by utilizing traditional fluorescent materials, phosphorescent materials, and thermally activated delayed fluorescent materials. It will also briefly introduce the principle of exciton confinement, as well as the energy transfer mechanism of different types of light-emitting materials introduced into green and blue PeLEDs and the physical mechanism of improving the optoelectronic properties. Methods Films are fabricated using the methods of spincoating and vacuum thermal evaporation deposition. All the perovskites films are obtained by spincoating method. Various types of organic luminescent materials are introduced into the perovskite emissive layer as additives or inserted between the perovskite emissive layer/transport layer as sensitizers for exciton recycling, or through multiple coating to create multi-quantum well structures. These materials are brought into the PeLEDs through additive-assisted methods, device interface engineering, and structural optimization methods. Results and Discussions It has been demonstrated that the introduction of traditional fluorescent emitters with a larger bandgap than that of the perovskite can better recycle singlet excitons. The incorporation of organic phosphorescent materials and different types of TADF materials, which have internal quantum efficiencies near 100% and energies of both singlet and triplet excitons that are significantly higher than the bandgap of the perovskites, can better recycle and utilize both singlet and triplet excitons of the perovskites. This leads to a potential internal quantum efficiency value of 100% for PeLEDs. Compared to traditional TADF materials, new TADF materials with a "core"-"shell" structure (such as Cz-3CzCN and Cz-4CzCN) and semiconductor TADF polymers with a "TADF core"-"shell" structure (such as P-Cz5CzCN) can not only passivate the defects in the perovskites film but also effectively suppress "exiton"-"exiton" quenching due to direct contact between the TADF emission cores. This further improves the utilization of excitons, greatly enhancing the efficiency and stability of green- and blue-emitting PeLEDs. Conclusions This article reviews the work made by the groups of Gao Chunhong, Ban Xinxin and Wang Zhaokui in the fields of exciton confinement and exciton recycling in the past five years. These approaches mentioned above have been demonstrated in PeLEDs based on 3D perovskite emissive films (CsPbBr₃) and quasi-2D perovskite emissive films (PEA₂Cs_n-1Pb_nBr_3n+1, p-F-PEA₂Cs_n-1Pb_nBr_3n+1). These methods can also be extended to various types of light-emitting devices to achieve efficient and stable PeLEDs, providing a feasible strategy for the commercialization of PeLEDs.

Research on precise measurement of phonon-polariton interference fringe period

Yin Zhijun, Wang Zhenxing, Li Quan, Song Renkang, Deng Xiao, Lei Lihua

2023, 52(12): 20230414. doi: 10.3788/IRLA20230414

[Abstract](120) [FullText HTML] (35) [PDF 5106KB](47)

Objective Two-dimensional materials have garnered widespread attention due to their unique photoelectric properties at the nanoscale, showcasing distinctive application advantages in the fields of nano-electronic devices, optics, and energy. Notably, the phonon polaritons generated by the coupled excitation of phonons and photons in two-dimensional materials exhibit highly localized at the nanometer scale, presenting substantial application potential in cutting-edge research fields such as optical manipulation and energy transmission of on-chip photonics. Infrared imaging of the sample revealed that at the sample's edge, the mutual interference of phonon polaritons resulted in the fomation of polarization standing wave fringes parallel to the edge. The fringe period effectively reflects the coupling characteristics of phonon polaritons. Therefore, the study of modulated phonon polariton coupling primarily relies on the precise measurement of the interference fringe period. The current measurement method depends on the linear fitting calculation of image analysis software, and its accuracy is constrained by the image resolution. Additionally, the displacement errors occur in the sample loading stage of SNOM. To enhance measurement accuracy and minimize the impact of the these errors on the measured value, this paper proposes the use a self-traceable chromium grating for the precise measurement of the period of the polariton interference fringe in hBN. Methods In this paper, we present a self-traceable grating-hBN composite structure. The construction involves depositing a chromium grating on the silicon substrate using atomic lithography. The gaps between adjacent grating structures consist of air, and the two-dimensional polar material hBN is placed on the chromium grating. The dispersion of phonon polaritons generated by the recombination of hBN and different media is analyzed using Fabry-Perot quantization conditions. The study further investigates the phonon polariton coupling enhancement and modulation characteristics of two-dimensional polar materials resulting from periodic changes in metal grating structures. Scanning near-field optical microscopy (SNOM) was employed to image the phonon polaritons of the composite structure in near-field space during sample processing. Imaging of different sizes is conducted at the same point, and two methods are employed for image processing for comparison. The first method involves traditional linear fitting based on Gwyddion for calculating the interference fringe period, while the second one utilizes the use of image analysis program to perform self-traceable grating comparison measurement on the fringe period. Results and Discussions When using the traditional Gwyddion-based linear fitting for calculating the interference fringe period, the measured fringe period in the 1 µm×1 µm near-field optical imaging is 0.264 µm, in the 5 µm×5 µm image is 0.254 µm, and in the 10 µm×10 µm image is 0.257 µm. It is observed that when the size of the measurement image is small, the error offset is large. After a large-scale scanning, the measurement value tends to stabilize, yet the overall measurement result remains unstable. Considering the 10 nm resolution of SNOM, the standard deviation value is calculated as 4 nm. Simultaneously, the image analysis program is employed to perform self-traceable grating comparison measurement on the fringe period, The grating period serves as the scale for measuring the interference fringe period. The fringe periods measured in 1 µm×1 µm, 5 µm×5 µm and 10 µm×10 µm images are 261.28 nm, 260.35 nm, 261.41 nm respectively, with a standard deviation of the measured values at 0.34 nm. Compared with the standard deviation of 4 nm from the traditional measurement method, this presents an order of magnitude reduction, achieving higher precision. When the self-traceable grating is employed, it not only measure the period size, but also calibrate the measurement point of the detection equipment. Taking SNOM in this study as an example, the calibration size ΔU of the equipment displacement device is approximately −0.18 nm/pt, with uncertainty related to the grating uncertainty, achieving sub-nanometer accuracy. However, the actual measurement still needs to consider the impact of the device resolution on measurement accuracy. Conclusions By constructing a composite structure comprising a self-traceable chromium grating and a two-dimensional polar material hBN, this study leverages the periodic enhancement principle of phonon polariton intensity generated by the structural changes in the substrate grating material. Employing a scanning near-field optical microscope for imaging, the resulting composite structure exhibits coupling-enhancing fringes consistent with the self-tracing grating pitch distribution. Through image analysis and measurement, the self-traceable grating period of 212.782 nm is utilized as the scale. The interference fringe period is measured to be (261.01±0.34) nm, achieving a measurement with higher accuracy than the uncertainty of the traditional fitting method. Simultaneously, nanoscale calibration of the device can be realized based on the measurement results. In this paper, the imaging calibration of the SNOM device is determined to be −0.18 nm/pt, with uncertainty linked to the grating uncertainty. This grating metrology method offers a measurement approach with superior accuracy and reliability for precisely measuring the excitation wavelength of phonon polaritons and controlling the coupling of photon and phonon. Additionally, it provides an experimental basis for the design and regulation of two-dimensional materials applied to nanoscale devices.

Study on preparation and infrared properties of CoS QDs/PDMS nanocomposite films

Hu Kun, Zhang Taiwei, Li Guobin, Li Xueming, Tang Libin, Yang Peizhi

2023, 52(12): 20230393. doi: 10.3788/IRLA20230393

[Abstract](119) [FullText HTML] (28) [PDF 2857KB](30)

Objective TMCs have been widely used in photocatalysis, solar cells, lasers and other fields because of their excellent optical, electrical and photoelectric properties. As a typical TMCs material, CoS QDs have excellent near-infrared absorption properties due to their narrow band gap and are expected to be used in infrared technology. CoS QDs are expected to be an important material for infrared detector preparation. In order to improve the optical properties and processing properties of CoS QDs, CoS QDs were further prepared into nanocomposite films to expand their application range. At present, the research work on CoS mainly focuses on CoS NPs, and there are few reports on quantum dot composite films. Therefore, the CoS QDs prepared by liquid phase ultrasonic exfoliation method are blended with PDMS, and the infrared properties of CoS QDs /PDMS nanocomposite films are studied. In order to expand the application of CoS QDs in infrared optics. Methods CoS QDs solution was prepared by liquid phase ultrasonic exfoliation method. The preparation steps were as follows: 0.15 g CoS powder (purity ≥99.5%) was weighed and put into a mortar and fully ground for2 h; The ground CoS powder was evenly mixed with 50 mL of anhydrous ethanol (purity ≥99.7%) dispersant, and placed in the ultrasonic instrument at 90 W power for 2 h; The ultrasonic solution was centrifuged at a rotational speed of 500 r/min for 5 min. Taking out the supernatant, CoS QDs solution was obtained. The CoS QDs solution is dried for later use. CoS QDs/PDMS nanocomposite films were prepared by blending method. 5 mLof the basic component A of PDMS and 0.5 mL of the curing agent B were transferred to the beaker, and appropriate amount of the dried CoS QDs powder was added, stirred with a magnetic stirrer for 5 min, and then transferred to the petri dish and heated at 30 ℃ until film formation. The size, morphology, structure and elemental components of CoS QDs were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and energy dispersive spectrometry (EDS). The phase composition and bonding properties of CoS QDs were analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy (Raman); the optical properties of CoS QDs and CoS QDs/PDMS composite films were tested by UV-Vis spectrophotometer and fluorescence spectrometer. Results and Discussions Both CoS QDs and CoS QDs/PDMS nanocomposite films have obvious absorption and luminescence characteristics in infrared band, and the infrared absorption characteristics of the composite films are better than that of CoS QDs films (Fig.4(a)-(c)). With the increase of excitation wavelength, the PL peak of CoS QDs/PDMS nanocomposite films shows a redshift, which shows obvious Stokes shift effect and excitation wavelength dependence (Fig.4(k)). Conclusions The spherical CoS QDs with good dispersion, uniform particle size and average particle size of about 5 nm were successfully prepared by liquid phase ultrasonic stripping method, and the CoS QDs/PDMS nanocomposite films were prepared by blending CoS QDs and PDMS. After UV-Vis test, it was found that CoS QDs solution and CoS QDs/PDMS nanocomposite films have absorption from ultraviolet to infrared band(200-2200 nm). Compared with CoS QDs films, the infrared absorption characteristics of CoS QDs/PDMS nanocomposite films are effectively enhanced. Moreover, the absorption strength of the film samples hardly changed after six months. The PL test shows that CoS QDs and CoS QDs/PDMS nanocomposite films have PL phenomenon in infrared band, PL peak has obvious redshift phenomenon, Stokes shift effect, and both have wavelength dependence. On the other hand, CoS QDs/PDMS nanocomposite films have excellent infrared optical properties, especially the absorption and luminescence characteristics in the infrared band, and the optical properties are very stable, indicating that the composite material has important potential application value in the fields of infrared detectors, nano-photonic devices, flexible displays, infrared lasers and so on.

Study on the longitudinal mode characteristic of idler wave in MgO:PPLN infrared optical parametric oscillator

Zheng Hao, Zhao Chen, Zhang Fei, Li Pengfei, Yan Bingzheng, Wang Yulei, Bai Zhenxu, Lv Zhiwei

2023, 52(12): 20230378. doi: 10.3788/IRLA20230378

[Abstract](103) [FullText HTML] (27) [PDF 1544KB](40)

Objective Narrow linewidth solid-state lasers are characterized by their excellent coherence and beam quality. Narrow linewidth lasers of certain wavelengths are necessary to meet the absorption or transmission requirements of specific ions, molecules, and materials. Therefore, it is of great significant to investigate the longitudinal mode characteristics of lasers at different wavelengths and operating modes. The 3-5 μm spectral range falls within the atmospheric window. Mid-infrared lasers in this band have been widely used for environmental gas monitoring, spectral analysis and optoelectronic countermeasures. Currently, MgO:PPLN crystals are typically employed in optical parametric oscillators (OPOs) to generate mid-infrared lasers within the 3-5 μm spectrum. This is attributed to their high second-order nonlinearity coefficient, large damage threshold, and widely tunable wavelength range. In addition to the tunability of the output wavelength, the optical parametric oscillation process also possesses the ability to suppress multi-longitudinal-mode operation within the cavity. While previous experiments have demonstrated that multi-longitudinal-mode operation can be suppressed by placing optical parametric crystals inside the cavity, However, more specific studies are limited. In this study, a comparative analysis of the variation of longitudinal mode properties at fundamental and idler frequencies was performed using MgO:PPLN crystals. Methods The output wavelength at different temperatures is simulated based on the phase matching equation and the dispersion equation, as depicted in Fig.1. The experimental setup is illustrated in Fig.2. An fiber coupled 808 nm laser diode continuous-wave laser was used as the pump source, with a core diameter of 200 μm and numerical aperture of 0.22. A 1:2 focusing lens result in a spot radius of 400 μm at the Nd:YVO₄ crystal. The crystal has a dimension of 3 mm×3 mm×18 mm and a doping concentration of 0.3%. Plane mirrors M1 and M2 form a 1064 nm fundamental frequency optical resonator with a cavity length of 95 mm. A Q-switched pulse output of the fundamental frequency was obtained using an acousto-optic modulator. The fundamental frequency wave was directly coupled into the MgO:PPLN crystal via a 50 mm focusing lens, resulting in a beam radius of 400 μm for the fundamental frequency wave. The MgO:PPLN crystal, with dimensions of 10.5 mm ×1 mm×20 mm, a doping concentration of 5%, and a poling period of 31.0 μm was used. The OPO consist of plane mirrors M3 and M4 with a cavity length of 56 mm. The coating parameters of the lenses used in the experiments are presented in Tab.1. Results and Discussions Figure 3 depicts the output power variation of the 1064 nm fundamental frequency wave with the pump wave. A Q-switched laser output with a maximum power of 7.03 W is obtained at a repetition frequency of 120 kHz. Figure 4 illustrates the variation of output idle frequency optical power with the fundamental frequency. At a room temperature of 20 ℃ and a fundamental frequency optical power of 7.03 W, an idle frequency light with an output power of 0.702 W and a wavelength of 3.196 μm is obtained, corresponding to a conversion efficiency of 9.95%. Figure 5(a) shows the time-domain waveforms of the measured fundamental and idle frequencies. The pulse width of the 3 μm idle frequency laser is 4.7 ns, which is slightly narrower compared to the fundamental frequency laser and has a smoother waveform. Fourier transforms are performed on the waveforms, as shown in Fig.5(b). It can be seen that the multiple longitudinal modes are significantly suppressed after the OPO process, consistent with the results observed in the time domain. Conclusions By pumping the Nd:YVO₄ crystal with an 808 nm laser diode, multiple longitudinal mode output of the fundamental frequency wave with a repetition rate of 120 kHz and a pulse width of 8.1 ns was achieved, resulting in a maximum output power of 7.03 W. Based on this fundamental frequency pump source, an MgO:PPLN-OPO was developed, yielding a pulse width of 4.7 ns and an output power of 0.7 W for the 3 μm idler wave, with a fundamental-to-idler wave conversion efficiency of 9.95%. Comparing the Fourier-transformed temporal waveforms of the fundamental frequency and idler waves, we can clearly observe the suppression of higher-order longitudinal modes of the idler wave during the OPO process. This study has significant reference value for regulating the longitudinal mode characteristics in OPO and achieving low noise parametric optical output.

Highly-efficient hybrid TADF/phosphorescent white organic light-emitting diodes based on an exciplex host

Zhang Yuanbo, Liu Yuan, Li Yanan, Bian Haodong, Li Jiarui, Zhu Lianqing

2023, 52(12): 20230222. doi: 10.3788/IRLA20230222

[Abstract](161) [FullText HTML] (55) [PDF 2283KB](36)

Objective White organic lighting-emitting diodes (WOLEDs) have attracted significant interest in the fields of flexible flat panel displays and large-area solid-state lighting due to their merits of ultrathin, large-scale and low-cost. Phosphorescent OLEDs can achieve 100% exciton utilization. However, the lack of stable blue phosphorescent materials hinders the commercial application of all phosphorescent WOLEDs. Thermally activated delayed fluorescence (TADF) materials, which can harvest triplet excitons through efficient reverse intersystem crossing (RISC) and achieve nearly 100% internal quantum efficiency (IQE) are emerging as next generation emitters for OLEDs. Therefore, hybrid TADF/phosphorescent WOLEDs have become an alternative for preparing high efficiency and stable WOLEDs. Generally, in WOLEDs, unbalanced carrier transport in light-emitting layers (EMLs) usually leads to narrow exciton recombination regions, which reduces the efficiency and color stability at a high current density. Various methods, including inserting interlayers between EMLs have been proposed to improve color stability. However, the organic-organic barriers between the interlayers and EMLs enlarge the driving voltages and exacerbate exciton accumulation. Therefore, developing WOLEDs with balanced carrier transport and broadening the exciton recombination zones are the key to simultaneously achieving high efficiency and stable white emission. Methods High efficiency hybrid TADF/phosphorescent WOLEDs are prepared in this study. An exciplex system TCAT:DPEPO is chosen as the host to improve charge balance and optimize exciton distribution. Moreover, a cascaded exciton energy transfer route is constructed to improve exciton utilization efficiency. The working mechanism of devices is illustrated by examining host effects in EMLs. Moreover, the carrier balance is further enhanced by optimizing the transport layer. Results and Discussions The bipolar exciplex host (TCTA:DPEPO) and traditional host DPEPO are comparably investigated in blue TADF devices (Fig.1). By modulating the thicknesses of light-emitting layers, high-efficiency hybrid TADF/phosphorescent WOLED_S based on exciplex host have been achieved with excellent color stability and a high color rendering index (CRI) of 88 (Fig.3). The comparison experiment shows that the outstanding performance of hybrid TADF/phosphorescent WOLEDs is attributed to the widened exciton recombination region and reasonable exciton utilization routes (Fig.4). In addition, by optimizing the electron transport layer, the power efficiency is further improved, achieving maximum values of 52.6 lm·W⁻¹ and 19.3% for power efficiency and EQE, respectively (Fig.6). Conclusions High efficiency, color stable and low efficiency roll-off TADF/phosphorescent hybrid WOLEDs based on exciplex host are achieved. In the proposed WOLEDs, an exciplex host is utilized in EMLs to broad exciton recombination region and a cascaded exciton energy transfer route is constructed to improve exciton utilization. Hybrid WOLEDs exhibit excellent color stability and low efficiency roll-off. Maximum values of PE and EQE are 36.4 lm·W⁻¹ and 17.5% (maintaining 18.2 lm·W⁻¹ and 12.3% at 1000 cd·m⁻²), respectively. With balanced white emission, the WOLED reaches a CIE of (0.451, 0.428) and a high CRI of 88. By further optimizing the transport layer of WOLEDs, the EQE is further improved to 19.3%, and a maximum power efficiency of 52.6 lm·W⁻¹ and a CRI of 90 are achieved. The design strategy proposed in this study provides a simple but feasible approach for high performance hybrid TADF/phosphorescent WOLEDs.

Study on preparation and infrared properties of FeS quantum dots and their composite films

Zhang Taiwei, Hu Kun, Li Guobin, Li Xueming, Tang Libin, Yang Peizhi

2023, 52(12): 20230489. doi: 10.3788/IRLA20230489

[Abstract](136) [FullText HTML] (17) [PDF 1876KB](35)

Objective Compared with other quantum dots (QDs), infrared QDs have narrower band gaps, wider absorption ranges, and longer fluorescence wavelengths. Therefore, they show greater potential in areas such as bioimaging, tumor treatment, photodetector and solar concentrators. As transition metal chalcogenides (TMCs), FeS QDs are promising infrared detection materials due to their narrow band gap, low toxicity, and strong near-infrared absorption. Forming therm into thin films is an effective approach to enhance the stability and processability of QDs. At present, the research about FeS mainly focuses on nanofilms and nanoparticles, and there are few reports on FeS QDs and their composite films. In this paper, we studied the preparation of FeS QDs by liquid-phase ultrasonic exfoliation, and prepared FeS/PVA composite films by mixing FeS QDs with polyvinyl alcohol (PVA). We tested and analyzed the infrared characteristics of FeS QDs in order to explore their potential applications in the field of infrared, and its application in the field of infrared optics was prospected. Methods FeS QDs solution was prepared by liquid phase ultrasonic exfoliation method. The preparation steps were as follows: 0.15 g of FeS powder (purity ≥99.9%) was weighed and placed in a mortar, followed grinding for 2 h. The ground FeS powder was then mixed with 50 mL of isopropyl alcohol (IPA, purity ≥99.7%) dispersant, and placed in the ultrasonic instrument at 120 W power for 2 h. After ultrasonic, the solution was centrifuged at 500 r/min for 5 minutes, taking out the supernatant, FeS QDs solution was obtained. Collect in a reagent bottle for further use.　　FeS QDs/PVA nanocomposite films were prepared using a blending method, following the steps below: 0.4 g of PVA powder was weighed and added to a beaker containing 20 mL of deionized water. The mixture was placed on a magnetic heating stirrer and continuously stirred at elevated temperature for 45 min until the powder was completely dissolved. Then, 4 mL of the FeS QDs solution was added to the mixture, and the heating and stirring kept on an additional 15 min. Subsequently, 4 mL of the mixed solution was drop-cast onto a metal sample holder, and the film was formed by heating the sample holder on a heating plate at 40 ℃ for 4 h. 　　FeS QDs were characterized and analyze for size, morphology, structure, and elemental composition using transmission electron microscopy (TEM), atomic force microscopy (AFM), and energy spectroscopy (EDS). The phase composition and bonding properties of FeS QDs were analyzed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The optical properties of FeS QDs and FeS QDs/PVA nanocomposite films were studied using UV-Vis spectrophotometer and fluorescence spectrometer. Results and Discussions Both FeS QDs and FeS QDs/PVA nanocomposite films exhibit significant absorption and luminescence characteristics in the infrared band (Fig.4(a), (b), (c)). As the excitation wavelength increase, the PL peak of the FeS QDs/PVA nanocomposite film shows a clear redshift, which shows obvious Stokes shift and excitation wavelength dependence (Fig.5(e)). Conclusions FeS QDs with an average particle size of 8.1 nm were successfully prepared by liquid phase ultrasonic exfoliation method. FeS/PVA nanocomposite films were prepared by blending FeS QDs with PVA. UV-Vis tests show that FeS QDs and FeS/PVA nanocomposite films exhibit absorption from ultraviolet to infrared band (200-2500 nm). PL test shows that they have photoluminescence in infrared band. PL peaks show significant redshift and Stokes shift, indicating that both are wavelength dependence. In addition, FeS/PVA nanocomposite film shows excellent infrared optical properties, especially the absorption and luminescence characteristics in the infrared band. These results show that FeS QDs and its nanocomposite films have important application potential in the field of infrared optics, and provide a new idea for the development of infrared optical devices.

Special issue—Low-dimensional optoelectronic materials and devices

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