Efficient amplified spontaneous emission from ... - OSA Publishing

Efficient amplified spontaneous emission from oligofluorene-pyrene starbursts with improved electron affinity property Qi Zhang,1 Yan Zhang,1 Weidong Xu,1 Xiangchun Li,1 Jingguan Liu,1 Xiangru Guo,1 Ruidong Xia,1,* and Wei Huang2,3 1 Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China 2 Institute of Advanced Materials (IAM), Jiangsu-Singapore Joint Research Center for Organic/Bio-Electronics & Information Displays,Nanjing Tech University,30 South Puzhu Road, Nanjing 211816, China 3 [email protected] * [email protected]

Abstract: A series of monodisperse starburst molecules as optical gain media have been investigated in detail. The starburst molecules were composed of a pyrene core with four short oligofluorene arms capped by cyanophenyl moieties. The compounds exhibited low amplified spontaneous emission (ASE) thresholds of 30 nJ pulse−1 and high maximum net gain coefficient of 55 cm−1 under optically pump. Our study demonstrates that the introducing of electron-withdrawing cyanophenyl end-capper onto pyrene centered starburst molecules does not disadvantage their optical gain properties while leading to reduced LUMO level, therefore, improved electron affinity. This study suggested a promising approach for organic gain material synthesis to address the challenge of electrically pumped organic laser. ©2015 Optical Society of America OCIS codes: (140.5960) Semiconductor lasers; (160.3380) Laser materials; (250.5230) Photoluminescence; (310.6870) Thin films, other properties.

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Received 29 Jan 2015; revised 23 Mar 2015; accepted 24 Mar 2015; published 9 Apr 2015 1 Jun 2015 | Vol. 23, No. 11 | DOI:10.1364/OE.23.00A465 | OPTICS EXPRESS A465

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1. Introduction Organic semiconductors via solution processing have been extensively investigated for the promising applications such as organic light-emitting diodes (OLEDs) and organic lasers, because of their low-cost production, lightweight physical characteristics and tunable optoelectronic properties. Although the solution-processed emitters demonstrate good performance in OLEDs, some cannot fulfill the requirement of efficient optical gain at low pump thresholds. So far, great efforts have been focused on developing organic materials with efficient electroluminescence and optical gain, which are of particular significance in the commercialization of OLEDs and in pursuit of electrically pumped organic lasers. However, it remains a challenge to realize organic optical gain media with high power-conversion efficiency, low lasing threshold and high carrier mobility. Recently, star-shaped conjugated molecules, usually comprised of a central core and multiple conjugated arms, have attracted broad interesting [1] in the development of organic optoelectronics devices such as OLEDs [2, 3] and lasers [4–7], owing to their extensible molecular architectures, high photoluminescence quantum yield (PLQY), good thermal stability, film-forming property and repeatability. Thus, they could be attractive alternatives to small molecules and polymers in organic electronic devices. In our previous work, we have designed a series of star-shaped molecules, such as monodisperse six-armed triazatruxenes [8] and 2,3,7,8,12,13-hexaaryl truxenes [9]. Among them, pyrene-centered starburst oligofluorenes are found to serve as a series of state-of-the-art organic luminescent materials, which have relatively high hole-affinity and hole-injection ability (low HOMO levels), high PLQY, net gain values (~70 cm−1), low lasing thresholds, and relatively high thermal and environmental stability [10, 11]. However, similar to most of the p-type molecules, pyrene tends to possess lower electron affinity [12], which would depress current density in potential electrically pumped laser. Introducing electron-withdrawing components into the molecule to enhance electron injection and transport is an effective approach to balance carrier injection. It has been reported that the electron-withdrawing fluorine substitution in anthracene-based molecular glasses have reduced HOMO and LUMO level, therefore leading to lower turn on voltage, while having no significant disadvantage to optical gain properties [4]. On the other hand, cyano-group has been widely used for improving electron injection and transport for

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OLEDs due to its strong electron-withdrawing property and excellent luminescence efficiency [13]. For example, Taranekar [14] developed a series of alternated polymers with cyanofluorene groups emitting green to blue light with low turn-on voltages. Chen et al. reported a series of high efficient blue-emitting materials consisting of oligofluorenyl blocks and electron-withdrawing cyanophenyl groups, which would facilitate the electron injection and transport [15, 16]. However, the effect of cyanophenyl group on the gain property of the materials has not been widely explored yet. In this context, we present our recent study on laser gain property of a novel series of pyrene-centered starbursts with four radial oligofluorene arms capped by cyanophenyl moieties. These starbursts have lower LUMO levels while their HOMO levels remain almost the same compared to their non-cyanophenyl counterpart. This could be beneficial to the electron injection and transport, without harming the hole-affinity and hole-injection abilities of these pyrene-cored starbursts. The planar waveguide devices using these starbursts as the optical gain active layers were fabricated and tested. 2. Sample preparation and experimental setup Film samples for optical and morphology investigations (150 - 200 nm thickness for absorption and photoluminescence (PL) spectra, PL decay transient and ASE measurements; 100 nm for Atomic force microscopy (AFM) measurements) were prepared by spin coating 20 - 35 mg/ml solutions of T1, T2 and T3 in mixed solvents (chloroform: chlorobenzene = 3:1) onto pre-cleaned Spectrosil B substrates. UV-Vis absorption and fluorescence spectra were recorded on Shimadzu UV-3600 RF5301PC spectrophotometer, respectively. PL decay was measured using an Edinburgh FLSP920 lifetime spectrometer with a 375 nm laser (typical pulse width: 55 ps; pulse repetition frequencies: 20 MHz). AFM measurements of surface morphology were conducted on an Asylum Research MFP-3D AFM in AC mode with a NSC15/AIBS Si cantilever (resonant frequency ~325 kHz from µ-macsh). For ASE measurements, samples were optically pumped at wavelength of 375 nm with Qswitched, neodymium ion doped yttrium aluminium garnate [Nd3 + : YAG] laser (Continum Surelite II-10) pumped, type-II β-BaB2O4 [BBO], optical parametric oscillator (Panther EX) that delivered 3 ns pulses at a repetition rate of 10 Hz. Output signals are collected with a fiber-coupled grating spectrometer (Andor Co.) equipped with a CCD detector (Newton Co.). The size of narrow excitation strip is 550 μm x 4 mm. 3. Results and discussion The chemical structures of the pyrene-cored 9,9-dihexylfluorene cyanophenyl-capped starbursts studied here are shown in Fig. 1(a). The pyrene core is coupled to four equal length oligo (dihexylfluorene) arms capped by cyanophenyl, yielding molecules T1, T2 and T3 with, respectively, n = 1, 2, and 3 dihexylfluorene moieties per arm. We have undertaken density functional theory (DFT) calculations of T1, T2, T3 using Gaussian 03 (B3LYP nonlocal density functional with a 6-31G(d) basis set) and report the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital, electronic wavefunction densities for geometry-optimized structures in Fig. 1(b). The band gap does not change much as fluorine arm increased. The HOMO and the LUMO are almost concentrated on the pyrene backbone and change minorly as fluorene arm increased, which implies that its emission and absorption are mostly controlled by the pyrene unit. The measured HOMO levels of T1, T2, T3 are determined to be −5.62, −5.67, and −5.74 eV using cyclic voltammetry, which are similar to the non-cyanophenyl counterparts [11]. The LUMO levels of these starbursts are estimated according to the HOMO energy level values in combination with the band gaps derived from the absorption band edges [17]. The LUMO levels of T1, T2, T3 are −2.98, −3.03, −3.10 eV, respectively, which are lower than the non-cyanophenyl counterparts (−2.84, −2.88, −2.94 eV) [11]. This shows that introducing #233482 - $15.00 USD (C) 2015 OSA


the electron-withdrawing group cyanophenyl onto pyrene-cored starbursts can significantly improve the electron injection/transport without affecting their hole-affinity and holeinjection abilities.

Fig. 1. (a) Chemical structure and (b) calculated LUMO and HOMO electronic wavefunction distributions for T1-T3.

The absorption and PL characteristics of T1, T2 and T3 films are shown in Fig. 2. As shown in Fig. 2(a), with the increase of arm length, the main absorption peak red-shifts progressively and the relative absorption intensity in the long wavelength region decreases. There are two peaks, 350 nm and 415 nm, with a shoulder at around 300 nm in the absorption spectrum of T1 film. However, these two peaks redshift to 365 nm and 425 nm respectively in T2 film. The 425 nm peak becomes less obvious and degenerate to a shoulder. For T3 film, there is only one main absorption peak at 373 nm with two shoulders at around 430 nm and 305 nm. Nevertheless, the absorption edges of T1–T3 are very close, located at around 470 nm, which agree with previous report [11]. The PL spectra of T2 and T3 exhibited a peak at 477 nm, which blue-shifted from that of T1 (484 nm). The relative small difference in the PL spectra of T2 and T3 compared to the PL of T1 indicates that the effective conjugation saturates quickly with the arm length increases. The overlaps between the absorption edge and the onset of the PL for these starbursts are moderate, thereby limiting self-absorption losses and offering the prospect of low threshold optical gain. The PLQY of the starbursts were measured in thin-film (thickness~100 nm) using an integrating sphere with a Xenon lamp as the exciting light source. T1, T2 and T3 films show similar quantum efficiencies of 0.41, 0.48 and 0.44, respectively, suggesting that different lengths of oligofluorene arms have little influence on PLQYs of these materials. Figure 2(b) shows the PL decay spectra of T1, T2 and T3 films, which suggest single exponential decays with excited state lifetimes τ = 1.35 ns, 1.13 ns and 1.06 ns for T1, T2 and T3, respectively.

Fig. 2. (a) UV−Vis absorption (dashed lines) and PL (solid lines) spectra of T1, T2 and T3 films. (b) PL-Decay spectra of T1 (open squares), T2 (filled circles) and T3 (filled triangles).

AFM measurements (Fig. 3) show that film morphology becomes poorer with the increase of arm length. The measured root mean square roughness of film samples (thichness~100 nm) was only 0.4 nm for T1 and 0.6 for T2, but up to 1.6 nm for T3. Film morphology evolves gradually as the arm length of the starbursts increases, suggesting a size dependent texture of

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the starburst molecules. Poor film morphology may affect the performance of future electrically driven devices. Therefore, the LUMO levels lowering and the film-forming ability must be balanced by limiting the length of the arms. However, the relatively poor film morphology may not significantly disadvantage the ASE performance of T3 film as shown later.

Fig. 3. AFM topography images of oligofluorenes T1 (a), T2 (b) and T3 (c) films.

For ASE measurements, all the film samples of T1, T2 and T3 were pumped by 375 nm light, which was very close to the peaks of the absorption spectra of T1, T2 and T3 films. As the pulse energy increased, a narrow ASE peak appeared and grew to dominate the emission spectra rapidly. The ASE peaks were centered at 501, 493, and 489 nm for the T1, T2 and T3 slab waveguides, respectively. In order to quantify our observations, we define the pulse energy of exciting light at which the full width at half maximum (FWHM) intensity of the emission spectrum drops to half of its PL value as the ASE threshold, Eth. In this definition, the Eth of T1, T2 and T3 is 36 nJ pulse−1, 32 nJ pulse−1 and 38 nJ pulse−1, respectively, which is similar to that of those pyrene-cored starbursts in our previous study (~30 nJ pulse−1) [10]. The comparison of ASE peak positions, FWHMs and output intensities as a function of pump energy for T1, T2 and T3 are shown in Fig. 4(a) and 4(b). The ASE peak wavelength of the starbursts reported here is blue-shifted as the arm length increase; FHWM values remain ~9 nm for T1-T3 samples; the edge emitted output-intensity versus pump energy at the ASE wavelength of T1-T3 samples displays similar tendency, indicating that all the materials have similar ASE threshold. In Fig. 4(c), the three dimensional emission spectra of T1 under different pump energy demonstrate that ASE dominated the emission gradually as incident light intensity enhanced.

Fig. 4. (a) ASE peak wavelengths, (b) FWHM values and edge emitted output intensity (insert), as a function of the pump energy of T1 (open squares), T2 (filled circles) and T3 (filled triangles) films. (c) ASE narrowing process, (d) gain coefficients as a function of excitation energy, and loss as a function of the displacement of pump stripe (insert) of T1 film waveguide.

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To further study the gain characteristics of these starbursts, we measured the optical gain and loss of the waveguides. The gain characteristics of the waveguides were measured with the variable stripe length method. The net gain coefficients of T1 waveguide as a function of pump energies is shown in Fig. 4(d) and the maximum value is ~55 cm−1. The optical loss of the waveguide was measured using the stripe displacement method. The inset of Fig. 4(d) shows the output light intensity at the ASE wavelength (~501nm) as a function of the displacement of a fixed pump stripe (2 mm x 550 μm) on T1 waveguide. The pump energy was ~320 nJ/pulse, 10 times higher than the ASE threshold of T1. The loss of T1 film is ~3.6 cm−1, representing a low level of loss for dendrimer films [5–7, 18]. Table 1. Summary of key electrical and optical parameters PLQE

Lifetime (ns)

λ@ASE (nm)

FWHM (nm)

Eth ASE (nJ)

Pth ASE (kW/cm2)

2.98

0.41

1.35

501

10

36

0.55

5.67

3.03

0.48

1.13

493

10

32

0.49

5.74

3.10

0.44

1.06

489

8.8

38

0.57

HOMO (eV)

LUMO (eV)

T1

5.62

T2 T3

Materials

All the optical gain related studies show that introducing the cyanophenyl end groups onto the pyrene-centered starburst molecules is harmless to the optical gain properties [10]. However, it can improve the electron injection/transport ability of these resulting materials in electrically driven devices. This discovery suggests a possible application of pyrene-centered, with cyanophenyl end-capper, starburst materials in electrically pump organic laser. Further characterization and development of this series of materials could be warranted by the encouraging initial results presented here. (See Table 1.) 4. Conclusion We have investigated the optical gain properties of a series of starburst oligofluorenes (T1T3) based on a rigid planar pyrene core combined with the cyanophenyl end groups. The LUMO energy levels of these starbursts lowered ~0.15 - ~0.2 eV, while the HOMO levels remained about the same of the HOMO levels of non-cyanophenyl counterparts. Gain property investigations indicate that the ASE threshold is as low as ~30 nJ pulse−1, which is comparable to previous reported organic gain media. The net gain of T1 film is as high as ~55 cm−1 and the loss was 3.6 cm−1. These studies suggest it is possible to improve electron affinity by introducing electron-withdrawing components such as cyanophenyl groups while keeping good optical gain properties of organic optoelectronic materials. This is a promising approach for organic gain material synthesis toward the challenge of electrically pumped organic laser and, thus it deserves further development. 5. Acknowledgments We thank the National Natural Science Foundation of China (Grants 61376023 and 61136003), the National Key Basic Research Program of China (973 Program, 2015CB932203), the Program for Changjiang Scholars and Innovative Research Teams in the Universities of Jiangsu Province (Grant IRT1148), the Priority Academic Program Development Fund of Jiangsu Higher Education Institutions (PAPD) and the Natural Science Foundation of Nanjing University of Posts and Telecommunications (NUPTSF Grants NY212013) for financial support.

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