Enhancing Optical Gain Stability for a Deep-Blue Emitter Enabled by a

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Enhancing Optical Gain Stability for a Deep-Blue Emitter Enabled by a Low-Loss Transparent Matrix Jin-Qiang Pan, Jian-Peng Yi, Guohua Xie, Wen-Yong Lai, and Wei Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05273 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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The Journal of Physical Chemistry

Enhancing Optical Gain Stability for a Deep-Blue Emitter Enabled by a Low-Loss Transparent Matrix Jin-Qiang Pan,†§ Jian-Peng Yi,†§ Guohua Xie,* †,ζ Wen-Yong Lai,*, †, ‡ Wei Huang†, ‡ †

Key Laboratory for Organic Electronics and Information Displays (KLOEID) & Institute of

Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China ‡

Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU),

127 West Youyi Road, Xi'an 710072, Shaanxi, China ζ

Hubei Key Lab on Organic and Polymeric Optoelectronic Materials Department of Chemistry,

Wuhan University, Wuhan 430072, China *E-mail: [email protected]; [email protected]

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ABSTRACT: Efficient organic gain media with high optical gain and good photostability are highly desirable for organic semiconductor lasers. To shed light on the rational molecular design, in this work, a novel deep-blue organic emitter consisting of a carbazole core substituted by a diphenylamine-functionalized triazine unit and two arms of bifluorene with flexible alky chains (denoted as C2) was designed and synthesized. Various basic photophysics and amplified spontaneous emission (ASE) characterizations were conducted with a particular focus to unravel the impact of the molecular structure on the photostability. Relatively low ASE threshold of 7.0 μJ/cm2, high net gain coefficient of 79.9 cm-1 (under 40-fold threshold pump fluence) and low optical loss coefficient of 7.0 cm-1 were recorded for C2 in neat films. For comparison, C2 was also dispersed into polystyrene (PS), a low-loss transparent matrix, with various ratios to form PS-C2 blend films. Among them, 50 wt%-PS-C2 blend film exhibited a high gain coefficient of 61.8 cm-1 and a low loss coefficient of 5.4 cm-1, manifesting enhanced gain properties in comparison with that of pure C2 sample (52.2 cm-1 and 7.0 cm-1) under identical pump conditions (10-fold threshold pump fluence). With simple high-ratio blending, the photostability and the film morphology stability under annealing conditions were found to be significantly enhanced, giving rise to highly stable optical gain properties. The optical gain properties of the PS-C2 blend films remained almost unchanged even upon harsh annealing treatment in ambient atmosphere. The results suggest an efficient strategy to enhance the photostability of organic gain media without sacrificing the optical gain properties by combining the merits of robust gain media and a low-loss transparent matrix for achieving efficient and stable organic semiconductor lasers.

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INTRODUCTION Organic semiconductor lasers (OSLs) have emerged as promising coherent sources and attracted much interest due to their fascinating features: low-cost fabrication, broadly tunable emissions, potential flexibility, and easy integration with other optoelectronic devices.1-5 Although much progress has been achieved since the first demonstration of optically-pumped organic solid-state laser,6-11 it still remains a serious challenge to realize electrically-pumped OSLs.1, 3, 5 To attempt electrically-pumped OSLs, several challenging issues must be addressed. Sufficient exciton density generated within the gain layer in a fast timescale is one of the most important prerequisites for lasing actions under electrically-pumped conditions.12-14 The low gain properties of organic emitters seriously impact the lasing actions. The material stabilities including the thermal stability, the film morphological stability, and the photophysical stability are another fundamental and crucial aspects for attempting electrically-pumped OSLs.15-18 Unfavorable film morphologies as well as the poor thermal and photophysical stability will lead to high trap density and result in large optical waveguide loss, which would severely increase the lasing thresholds or even quench the lasing actions.19-22 It is therefore highly desirable to develop efficient organic emitters with high optical gain coefficients, low lasing thresholds, as well as excellent thermal, morphological, and photophysical stability simultaneously. The issue of low gain properties could be ascribed to the exciton quenching such as exciton-exciton annihilation and photo-induced absorption.23-24 The quenching effects mainly result from strong intermolecular interactions with high chromophore density, giving rise to significantly reduced exciton concentration for stimulated emission. However, a low chromophore density will otherwise restrict exciton density which is unfavorable for population inversion.25-26 In addition, 3 ACS Paragon Plus Environment


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strong intermolecular interactions will bring about enhanced non-radiative losses induced by dense molecular packing, which is negative for gain properties. Therefore, deliberate control of intermolecular interactions is critical for achieving efficient gain properties. As to the molecular design, generally, there are two common strategies to simultaneously achieve depressed intermolecular interactions and highly efficient lasing properties. Firstly, introducing large steric hindrances such as bulky substitutes to construct sterically-demanding structures which can effectively separate the polymer chains and suppress the intermolecular interactions, is a representative process that can yield high lasing performance.27 For example, with the incorporation of spirofluorene units, a great number of spiro-configuration molecules with spatial conformation have been demonstrated as highly efficient emitters for organic optoelectronic applications.28 With introducing the steric hindrance groups, significant depression of π-π stacking can be obtained, which effectively prevents the intermolecular interactions.29 In our previous contributions, we have explored a novel series of starburst molecules typically composed of truxene or triazatruxene cores with varying oligofluorene arms, exhibiting superior lasing properties and thus manifesting great potentials for OSLs.24,

28, 30-35

On the other hand, physical doping into inert matrix such as

polystyrene (PS) and polymethyl methacrylate (PMMA) is another efficient strategy for regulating the intermolecular interactions.36-39 Physical doping plays an important role to improve the morphological and photophysical stability.40-41 For instance, it is widely reported that aromatic dyes are generally dispersed in PS or doped into inert polymers to achieve high photostability.42 However, the doping ratios of dyes are normally quite low (0.5~5 wt% in PS) to avoid concentration quenching of excitons.43-44 Such a low doping strategy generally results in limited optical gain properties due to the low chromophore density that restricts the exciton density for population inversion. 4 ACS Paragon Plus Environment

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In this contribution, we present our recent effort to explore robust organic gain media to address the challenge for attempting electrically-pumped OSLs. A novel deep-blue organic emitter consisting of a carbazole core substituted by a diphenylamine-functionalized triazine unit and two arms of bifluorene with flexible alky chains (denoted as C2) was designed and synthesized. The chemical structure of C2 is depicted in Figure 1(a). The thermal, morphological, photophysical and optical gain properties of the resulting compound has been systematically investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), morphology measurements, photoluminescence (PL) and amplified spontaneous emission (ASE) with a particular focus to unravel the impact of the molecular structure on the photostability. Relatively low ASE threshold of 7.0 μJ/cm2, high net gain coefficient of 79.9 cm-1 (under 40-fold threshold pump fluence) and low optical loss coefficient of 7.0 cm-1 were recorded for C2 in neat films. For comparison, C2 was also dispersed into PS matrix (a low-loss transparent matrix, see Figure 1(b)) with various ratios to form PS-C2 blend films, manifesting enhanced gain properties in comparison with that of pure C2 sample under identical pump conditions. Moreover, the photostability and the film morphology stability under annealing conditions was found to be significantly enhanced with simple high-ratio blending, giving rise to highly stable optical gain properties. In contrast to the fast decay of the neat film samples, the optical gain properties of the PS-C2 blend films maintained almost constant even at high annealing temperature up to 260 oC in ambient atmosphere. The results suggest an efficient strategy to enhance the photostability of organic gain media without sacrificing the optical gain properties by combining the merits of efficient gain media and a low-loss transparent matrix for achieving efficient and stable OSLs. EXPERIMENTAL DETAILS 5 ACS Paragon Plus Environment


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Sample preparation: C2 waveguides with varying thicknesses were fabricated from chloroform (CF) solutions (30 mg/mL) or toluene solutions (20 mg/mL) by spin-coating. Film thickness was determined by Bruker DektakXT Stylus profiling system. PS-C2 blend solutions were prepared by mixing two individual solutions (toluene as processing solvent at a concentration of 25 mg/mL) with various ratios. The solutions were stirred and heated at 60 oC for 1 h, and then mixed with various C2 ratios (5%, 10 %, 15 %, 20 %, 30%, 40%, 50%, 60%, 70%, 80%, and 90%, weight ratio). The blend solutions were stirred and heated upon 60 oC for ~3 h to obtain well-mixed solutions. The film thicknesses were regulated by modulating spinning speeds. Basic characterizations: UV-vis absorption and photoluminescence (PL) spectra were recorded by PerkinElmer UV/VIS Spectrometer Lambda 650s and PerkinElmer LS 55 Fluorescence Spectrometer, respectively. The PL quantum yields (PLQYs) and PL decay measurements were carried out by a steady state/transient fluorescence spectrometer FLS-920. The microscopic surface morphology measurements were conducted by Bruker icon Dimension with ScanAsyst atomic force microscope (AFM, Bruker). Macroscopic morphologies were recorded by SPL-50 polarized optical microscope. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were conducted on Shimadzu DTG-60 simultaneous DTA-TG apparatus and Shaimadzu DSC-60A differential scanning calorimeter, respectively. ASE characterizations: All planar waveguides were prepared by spin-coating the solutions onto pre-cleaned quartz substrates, and pumped with an optical parametric oscillator (OPO), which was excited by pulsed Nd3+:YAG nanosecond laser (Continuum Ltd, Surelite, frequency 10 Hz, pulse width 5 ns). The output beam from OPO was shaped by an adjustable circular slot and then focused with a cylindrical focal lens (focus length, 10 cm). The resulting rectangle pump stripe area is 4.1 6 ACS Paragon Plus Environment

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mm× 440 μm. A set of neutral density optical attenuators were inserted into the beam path to regulate the pump intensity. The emission signal from the edge of planar waveguide was collected by optical fiber, dispersed by spectrograph and detected by CCD detector. The waveguides were excited at their maximum absorption wavelengths (pumped at 365 nm for C2). Standard variable stripe length (VSL) method was used to determine the net gain coefficients. By monitoring the pump strip length from 0.05 cm to around 0.25 cm with an interval of 0.0025 cm, varied output ASE intensities were collected. It is noted that we have recorded the results based on different pump fluences, namely 2-, 5-, 10-, 20-, and 40-fold ASE threshold intensities, respectively. Due to the long-stripe-length optical gain saturation, we only fitted the experimental data in the small-signal region according to the equation: (1) Where I out denotes the output intensity of the edge-emission light, and l is the stripe length. Different from the net gain coefficient determination method, the optical waveguide loss coefficient was evaluated by regulating the distance of non-pumped area between the edge of waveguide and pump stripe. The distance ranged from 0 to around 0.12 cm with an interval of 0.0025 cm. The output ASE intensities were recorded and fitted to single exponential decay with the following equation: (2) Where x is the length of the region without pumping light beam, and α indicates the loss coefficient. RESULTS AND DISCUSSION 7 ACS Paragon Plus Environment


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Figure 1. The chemical structures of C2 (a) and polystyrene (b). (c) The absorption (open squares) and fluorescence (open circles) spectra of C2 film. The corresponding ASE spectrum is shown as the curve filled in blue.

Figure 2. (a) The full width at half maximum (FWHM) of the emission spectra (left, solid spheres) and output intensity (right, solid triangles) of the planar waveguide as a function of pump energy 8 ACS Paragon Plus Environment

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density. (b) The output spectra of the planar waveguide at various pump fluences. The corresponding normalized spectra shown in the inset. (c) The ASE intensity as a function of pump strip length under different pump fluences. (d) The output intensity as a function of the distance between the edge of C2 waveguide and the fiber detector. Noted that all characterizations were carried out in ambient atmosphere.

Figure 1(c) shows the absorption, PL, and the corresponding ASE spectra of C2 in film states. A broad absorption band in the UV zone is observed with the maximum absorption wavelength at around 365 nm. In light of the PL spectrum of C2 film, three evident characteristic vibronic bands at around 414 nm (0-0), 436 nm (0-1), and 464 nm (0-2), respectively, are observed. The ASE spectrum of C2 film shown as the filled curve was obtained when exciting the planar waveguide by using Nd3+:YAG laser as the pumping source with the incident energy well above the ASE threshold. The ASE peak locates at 436 nm with a full-width at half maximum (FWHM) of 4.3 nm. It is noted that the ASE peak locates at 0-1 emission band for C2 waveguide, suggesting a quasi-four-level system that is beneficial for low-threshold lasing actions. Standard measurements were carried out to explore the ASE behaviors and thus the gain properties of C2 waveguides. The planar waveguide was fabricated by spin-coating their chloroform (CF) solutions onto the pre-cleaned quartz substrates. By varying the solution concentrations and spinning speeds simultaneously, various film thicknesses ranging from 34±1 nm to 230±5 nm were obtained. The optimal film thickness is 112 nm. Figure 2(a) depicts the FWHM and output intensity as a function of pump fluence. With increasing the pump energy, the FWHMs of the spectra show a rapid decrease from ~77 to ~40 nm. Further increasing the pump fluence to 7.0 μJ/cm2, a much smaller FWHM of 8.0 nm value was obtained. The FWHM remains almost 9 ACS Paragon Plus Environment


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constant at around 4.3 nm even with higher pump intensities. For the output intensity of the waveguide, at first, a slow growth is observed as the pump fluence gradually increases. However, when the pump fluence exceeds 7.0 μJ/cm2, the output intensity shows dramatical increment with a much higher increasing rate, demonstrating a slope transition occurring upon the input-output curve. This phenomenon, similar to those previously reported, refers to light amplification or amplified spontaneous emission (ASE). As previously reported, we define this transition as the ASE threshold (I th ASE).34 Figure 2(b) shows the emission spectra under various pump fluences. The inset shows the corresponding normalized spectra. When the pump fluence is

6.0 μJ/cm2, broad emission spectra

with three evident vibronic bands are observed at ~412, ~437, and ~464 nm, respectively. As the pump fluence increases, the output intensity of the central vibronic band grows much faster than those of the side oscillations. As a result, the side oscillations are effectively suppressed and the central wavelength dominates the emission profile, resulting in narrow emission spectra. Taking the notable light amplification and spectral narrowing into consideration, the ASE action is demonstrated with a threshold (I th ASE) of 7.0 μJ/cm2, a peak wavelength of 437 nm, and a FWHM of 4.3 nm. The optical gain was determined by using standard variable stripe length (VSL) method. As shown in Figure 2(c), a low value of 29.3 cm-1 is obtained for C2 film with the pump fluence of 14.0 μJ/cm2 (2-fold I th ASE). A much higher coefficient of 52.2 cm-1 was observed upon the pump fluence of 70 μJ/cm2 (10-fold I th ASE). Impressively, the maximum net gain value of 79.9 cm-1 was achieved for C2 by increasing pump intensity to ~280 μJ/cm2 (40-fold I th ASE). In this case, the resulting optical gain property of C2 is comparable to those based on well-known organic gain media, i.e. g = 74 cm-1 for polyfluorene derivatives45,46 and g = 78 cm-1 for the anisotropic 10 ACS Paragon Plus Environment

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terfluorene films.47,48 The optical waveguide loss characterizations were then carried out to determine the loss coefficient for C2 waveguide. The optical waveguide loss can be caused by absorption effects induced by intrinsic self-absorption and light scattering due to the rough surface of gain media or the interfaces between various functional layers in the multilayer devices. As shown in Figure 2(d), relatively low loss coefficient of 7.0 cm-1 was achieved for C2, indicating relatively low-loss ASE property for C2 waveguide. A combination of remarkably high net gain coefficient and relatively low optical waveguide loss coefficient provides C2 with relatively low ASE threshold. With the excellent ASE behaviors, C2 shows great potentials as highly efficient optical gain medium for various photonic devices such as organic light amplifiers and organic lasers.

Figure 3. (a) ASE threshold of C2 film as a function of annealing temperature, the inset: the corresponding normalized emission spectra upon high pump fluences. (b) TGA and DSC (shown in 11 ACS Paragon Plus Environment


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the inset) results of C2. (c) PL spectra for C2 planar waveguides based on various annealing temperatures. Inset: the normalized spectra. Noted that all characterizations were carried out in ambient atmosphere.

For attempting electrically-pumped OSLs, the ambient stability is one of the key issues to be addressed. To evaluate the material stability, C2 films were tested under harsh ambient conditions. ASE thresholds of the planar waveguides as a function of the annealing temperature were monitored to assess the thermal effects on gain properties of C2. The samples were annealed at each temperature for 10 min under ambient conditions. As shown in Figure 3(a), C2 shows a slight increase of threshold from 11.0 to 17.5 μJ/cm2 as the temperature increases from 60 to 80oC. The threshold of 55 μJ/cm2 was recorded upon annealing at 100oC. Unexpectedly, ASE actions were not detectable with higher annealing temperature. In other words, the ASE actions were totally quenched at high annealing temperature over 100oC. According to the emission spectra shown in the inset in Figure 3(a), a slight blue-shift of the peak wavelength from 434.8 to 433.2 nm was noticed when the temperature increased to 100oC. The broad spontaneous emission spectrum was observed at 100oC, confirming the poor gain stability of C2 film. To figure out the origin of the poor gain stability under ambient conditions for C2, PL spectra measurements, TGA, DSC, surface morphology measurements including atomic force microscope (AFM) and polarized optical microscope were carried out. As shown in Figure 3(b), C2 is thermally stable with 5% weight loss temperature (T d ) of 369oC. Furthermore, the glass transition temperature (T g ) of ca. 97oC (see the inset) was recorded for C2. Taking the results into consideration, it appeared abnormal that C2 with high T d exhibited poor ambient stability of gain properties. The result indicates that T d is not the only factor that determines the optical gain properties upon thermal 12 ACS Paragon Plus Environment

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annealing. For deeper understanding of the correlation between the gain properties (the threshold increase and the ASE quenching) and the annealing treatments, PL spectra of C2 under varying annealing temperatures in air were investigated. As shown in Figure 3(c) and the inset, the spectral shape keeps almost constant although the PL intensity of C2 decreased slightly with increasing the annealing temperature, indicating undamaged intrinsic chemical characteristics. The chemical stability of the samples under thermal annealing (25oC~160oC) has been further confirmed by time-resolved PL measurements and NMR tests at various annealing temperatures (Figure S3 and Figure S4), which showed no obvious additional energy transfer and chemical degradation within the detection limit. The results suggest the robust chemical structure of C2 that exhibits excellent ambient stability towards oxygen and moisture.

Figure 4. The macroscopic surface morphologies of C2 films upon various annealing conditions, room temperature (RT, a), 100oC for 10 min (b), 120oC for 10 min (c) and 160oC for 10 min (d).

Film morphology measurements were then conducted by using polarized optical microscope. As shown in Figure 4, relatively good film quality was obtained at room temperature, while obvious cracks were observed in the regions on the surface upon thermal annealing up to 100oC. Overall, based on these results including TGA, DSC, PL spectra, time-resolved PL, NMR, and 13 ACS Paragon Plus Environment


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morphology measurements, we come to conclude that the ASE quenching of C2 waveguide upon annealing at 120oC for 10 min is close related to the poor stability of film morphology other than the chemical structure variation.

Figure 5. (a) The evolution of ASE threshold of PS-C2 blend films with different ratios of C2. Inset: the corresponding normalized ASE spectra with different C2 ratios. (b) The gain/net gain coefficient and waveguide loss coefficient as a function of C2 ratio. Noted that all samples without any annealing treatment were characterized in ambient atmosphere.

As discussed above, although C2 possesses a variety of fascinating gain properties such as relatively low ASE threshold, high gain coefficient and low loss coefficient, the poor film morphological stability under heating conditions largely impedes its further applications in optoelectronic/photonic devices. Improving the optical gain stability is highly desirable. To this end, C2 was dispersed into the inert transparent PS matrix with various concentrations. The ratios of C2 in the matrix were regulated from 5 wt% to 90 wt%. To begin, the variation of the photophysical properties for PS-C2 blend systems was recorded. The influence of C2 ratio in PS matrix on ASE performance has been investigated by monitoring the ASE threshold as a function of the concentration. Standard ASE characterizations were conducted on the PS-C2 blend samples which 14 ACS Paragon Plus Environment

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were optically pumped by OPO with pump wavelength at 365 nm. The data are depicted in Figure 5(a) and summarized in Table 1. ASE threshold decreased from 12.6 (5 wt%) to 7.5 μJ/cm2 (20 wt%). With further increment of the concentration, the threshold maintained at around 7.0 μJ/cm2, suggesting relatively low ASE quenching effects induced by high doping ratios. The optimal concentration was determined to be 50 wt%. Gradual increase of ASE threshold was observed with increasing the concentration over 70 wt%. ASE threshold increased to ~9.8 μJ/cm2 with the C2 ratio of 90%. The ratio of 100% representing pure C2 film was used as a reference for comparison. The inset shows the normalized emission spectra of PS-C2 samples with various concentrations. The gradual red-shift of the peak wavelength from 430.8 to 435.1 nm was observed with increasing the C2 ratio, which was ascribed to the gradual increase of chromophore concentration that induced intermolecular interactions. In order to reveal the origin of the ASE threshold evolution trend with various ratios for PS-C2 blend system, a series of measurements including net gain coefficient (under 10-fold I th ASE pump fluence), waveguide loss coefficient, and AFM images were conducted. Moreover, the radiative recombination and non-radiative recombination were calculated to gain deeper understanding. Net gain and waveguide loss coefficients with various ratios have been studied in detail. It has been well established that the prerequisite of ASE/lasing action is

, where g is net gain, g’ is

gain coefficient, and α is waveguide loss coefficient.1 As shown in Figure 5(b), with increasing the concentration from 5 wt% to 20 wt%, the gain value showed a large increase from 28.1 to 58.4 cm-1, indicating its positive correlation with the ratio. Furthermore, the gain value exhibited a slight decrease to 54.7 cm-1 with the ratio of 40 wt% in contrast to the largest gain value of 67.2 cm-1 (50 wt%). The gradual decreases of gain coefficients, from 60 cm-1 (60 wt%) to 51.0 cm-1 (70 wt%), 15 ACS Paragon Plus Environment


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56.3 cm-1 (80 wt%), and 55.0 cm-1 (90 wt%), were observed by increasing the ratio consecutively. Table 1. ASE performance parameters of PS-C2 blend films with various ratios. Ratio/% 5% 10% 20% 30% 40% 50% 60% 70% 80% 90%

ASE threshold/μJ cm-2 12.6 9.1 7.5 7.4 7.4 6.8 7.5 7.3 7.9 11.9

Peak/nm FWHM/nm 431 432 433 434 434 435 435 435 435 436

7.0 6.1 5.0 5.0 3.4 3.5 3.6 3.7 3.6 3.7

Gain/cm-1

Loss/cm-1

28.1 34.9 58.4 56.5 54.7 67.2 60.0 51.0 56.3 55.0

7.7 7.2 6.6 6.2 6.2 5.4 7.6 11.7 13.0 15.0

Net gain/cm-1 20.4 27.7 51.8 50.3 48.5 61.8 52.4 38.3 43.3 40.0

As for waveguide loss, gradual decrement from 7.7 to 5.4 cm-1 was observed as the ratio increased from 5 wt% to 50 wt%. The waveguide loss value exhibited a fast increase from 5.4 to 15 cm-1 as the ratio further increased to 90 wt%, indicating the lowest loss coefficient was recorded at the ratio of 50 wt%. For the net gain coefficient, the evolution trend is similar to those of gain coefficient in the range between 5 wt% and 50 wt%, while a faster decrease is noticed with further increasing ratio to 90 wt%. This phenomenon can be explained from the perspective of gain coefficients and loss coefficients. The gain coefficient takes a dominant role in the ASE process due to their relatively large values and the relatively low loss coefficients with the ratio below 50 wt%. On one hand, the gain coefficient decreases with the increasing ratio, leading to the decrement of net gain coefficient. On the other hand, the waveguide loss value shows a large increment with a high ratio ( 50 wt%), which further gives rise to the decrease of net gain coefficient. It is worthy to mention that the result of the net gain coefficient is consistent with that of the ASE threshold, suggesting the ASE threshold evolution trend is monitored by the net gain coefficient. Therefore, with the maximum net gain coefficient, PS-C2 with the ratio of 50 wt% manifested the lowest ASE 16 ACS Paragon Plus Environment

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threshold.

Figure 6. (a) The PL decay results of PS-C2 blend samples with various C2 ratios. (b) The average lifetime (left) and PLQY (right) versus the concentration of C2 in PS matrix. (c) The radiative decay rate (k r , left) and non-radiative decay rate (k nr , right) as a function of the concentration.

To further confirm the doping effects on the photophysics of C2, we have carried out PL decay and PLQY measurements for the samples. As shown in Figure 6(a), all samples show two-exponential decays with a fast component and a slow component. The fitting results are summarized in Table 2. Overall, the average lifetime of the samples exhibits gradual decrease from 0.85 to ~0.64 ns as the ratio increases, which can be ascribed to the enhanced intermolecular interactions. As shown in Figure 6(b), a remarkable increase of PLQY value, from 44% to 77 %, is observed with the ratio increasing from 5 wt% to 20 wt% followed by a decrease to 67% (30 wt%). 17 ACS Paragon Plus Environment


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The maximum PLQY is up to 77% with the ratio of 50 wt%. Over 50 wt%, the PLQY decreased with the increasing doping ratio. Table 2. Fitted results of the time-resolved PL decays for PS-C2 blend films with various ratios. Compound 5%-C2 10%-C2 20%-C2 30%-C2 40%-C2 50%-C2 60%-C2 70%-C2 80%-C2 90%-C2

τ 1 /ns 0.66 0.66 0.54 0.60 0.52 0.56 0.45 0.42 0.46 0.48

τ 2 /ns 2.95 2.80 2.24 2.94 2.53 2.90 2.42 2.67 3.08 4.66

Fraction/% 91.75 92.48 88.80 92.92 91.50 93.29 90.41 89.74 91.78 96.22

Fraction/% 8.25 7.52 11.20 7.08 8.50 6.71 9.59 10.26 8.22 3.78

Average/ns 0.85 082 0.73 0.77 0.69 0.72 0.64 0.65 0.68 0.64

Based on the lifetime and PLQY results, we have calculated the radiative recombination rate (k r ) and non-radiative recombination rate (k nr ) according to the equations of τ = 1/(k r +k nr ) and Ф = k r /(k r + k nr ). As shown in Figure 6(c), a fast increase of k r , from 5×108 to 1.1×109 s-1, is observed with the ratio increasing from 5 wt% to 20 wt%. For the ratio between 20 wt% and 70 wt%, k r values of the waveguides stay at around 1.0×109 s-1 with small fluctuation. Further increase of ratio to 80 wt%, a remarkable decrease to 5×108 s-1 is obtained. As presented, the maximum k r value is attained at 50 wt%. The k nr exhibits gradual decrease from 6.6×108 s-1 to 3.3×108 s-1 as the ratio increases from 5 wt% to 50 wt%. Dramatic increase to 9.8×108 s-1 is obtained with further increasing the ratio to 90 wt%. Further, we have studied the surface morphologies of PS-C2 thin films with various ratios. As shown in Figure 7, the films with low ratios of 5 wt% and 10 wt% exhibited a small root-mean-square (RMS) roughness of 0.3 nm indicating highly smooth surface morphology. However, a large number of randomly-distributed small-sized holes were observed over the PS-C2 18 ACS Paragon Plus Environment

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films. This phenomenon might explain the waveguide loss coefficient, and relatively higher ASE threshold in comparison with those of pure C2. As the ratio increased (

Enhancing Optical Gain Stability for a Deep-Blue Emitter Enabled by a

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