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We present a detailed study of the optically and electrically pumped emission in the BSB-Cz/PVK bilayer waveguide devices. By optical pumping we demonstrate that PVK as a spacer between fluorescent BSB-Cz and ITO electrode allows the significant reduction of the threshold for amplified spontaneous emission (ASE) of BSB-Cz. The simulation provides a better understanding of how the PVK thickness affects the waveguide mode field distribution and hence the ASE threshold of BSB-Cz. On the other hand, the BSB-Cz/PVK bilayer OLED exhibits the external quantum efficiency of >1% and anisotropic electroluminescence with spectrally narrowed edge emission at the cutoff wavelength controlled by the BSB-Cz thickness. When tuning the cutoff wavelength to match the peak gain of BSB-Cz, we demonstrate an intense, particularly narrow edge emission (~5 nm) without obvious degradation of efficiency at a high current density of 1000 mA/cm2, suggesting a reliable device performance for high-power applications and further exploration of electrically-pumped ASE.
© 2015 Optical Society of America
1. Introduction
Organic lasers with great advantages of low cost, processing flexibility, high luminescence quantum yield, and spectral tunability have been attracted much interest in the past two decades. In 1996, the solid-state organic laser was demonstrated for the first time by optical excitation of conjugated polymer thin films [1–3]. Since then, the optically pumped amplified spontaneous emission (ASE) and lasers have been reported in a broad range of organic materials [4–14]. The electrically pumped organic laser, however, has not been successfully demonstrated yet. The threshold for electrically pumped lasers is expected to be much higher than for optically pumped lasers due to the additional optical losses, such as polaronic absorption, absorption loss at the electrodes, and the singlet-triplet annihilation [15,16]. Even without taking into account these negative effects, a conservative estimation of a lower limit on the threshold of lasers still requires the very high current density of 1-10 kA/cm2, which is difficult to be reached due to the low mobility of organic semiconductors. It is thus highly desirable to develop novel materials [17], device architectures [18,19], and operation mechanisms [20] to minimize the optical losses while increase the carrier mobility and optical gain for lasing at a lower threshold.
Recently, the edge emission from the cutoff mode in the planar waveguide geometry was proposed as a promising mechanism to reduce the threshold for light amplification [21,22]. Based on this mechanism, C. Adachi et al. reported the very narrow edge emission from organic light-emitting diodes (OLEDs) and a superlinear relationship between the emission intensity and the current density, which they claimed as the evidence of electrically pumped ASE [23]. Despite that the occurrence of ASE is doubtful due to the unreasonably low threshold (<100 mA/cm2) [24], their research nevertheless provides a valuable strategy in designing advanced devices to further confirm the light amplification in a higher current density regime.
In this paper, we introduce a bilayer organic waveguide device and systematically study the edge emission property with optical and electrical excitations. Compared to the single-layer organic waveguide structure used in Adachi’s work [23], the bilayer structure offers multiple advantages. For example, given the fact that to date there are still very limited ambipolar organic semiconductors with both the high carrier mobility and luminescence efficiency, the bilayer structure allows combinations of different materials for optimal design of both the optical and electrical properties, which significantly relaxes the conditions of device fabrication. Another important advantage manifested in this work is that both the guided and cutoff modes can be spatially confined near the excited region and isolated from the absorptive electrodes, and so the edge emission can be more intense. Here we construct the bilayer waveguide device based on an OLED geometry, mainly consisting of 4,4’-bis[(N-carbazole)styryl] biphenyl (BSB-Cz, from Luminescence Technology Corp.) and Poly(9-vinylcarbazole) (PVK, Mw = 1.1 × 106 g/mol, from Sigma-Aldrich) deposited on ITO glass substrate [Fig. 1(a)]. BSB-Cz is a blue fluorescent small molecule, which has been demonstrated to have a high photoluminescence (PL) efficiency and low ASE threshold [25]. Figure 1(b) shows the measured absorption, PL, and ASE spectra of a BSB-Cz film on glass. The absorption spectrum shows a peak wavelength at 373 nm. The PL spectrum exhibits two vibronic peaks at 455 nm and 488 nm, which correspond to the 0-0 and 0-1 transitions, respectively. The ASE occurs at a wavelength (~480 nm) near the PL maximum. Another unique property of BSB-Cz is the anisotropy of refractive indices. A thermally evaporated BSB-Cz film comprises linear-shaped molecular crystals lying in the plane of the substrates [26], resulting in the different refractive indices in in-plane (ordinary: no) and out-of-plane (extraordinary: ne) directions [Fig. 1(c)]. Due to the stronger molecular polarizability along the long-axis of the linear-shaped crystals, no is apparently higher than ne and can reach the value of ~2 at the peak of ASE. Moreover, BSB-Cz shows the rather balanced hole and electron mobilities in terms of electrical property [27]. In the waveguide devices BSB-Cz functions as both the luminescence/gain medium and electron-transporting layer. On the other hand, PVK is a hole-transporting polymer which has been widely used in the research of OLEDs [28]. Here we choose PVK to combine with BSB-Cz for several reasons. First, PVK with a large bandgap (3.5 eV) has little optical absorption in the spectral regime for wavelength >400 nm [Fig. 1(d)], and thus the luminescence of BSB-Cz would not be absorbed by PVK. Second, the relatively low refractive index of PVK compared to the ordinary refractive index (no) of BSB-Cz at the wavelength >400 nm [see Fig. 1(d)] provides a proper waveguide effect for the edge emission of BSB-Cz. Third, PVK with a much higher LUMO level than BSB-Cz could block the electrons at the heterointerface, which confines the electron-hole recombination in BSB-Cz and enhances the electroluminescence quantum efficiency. Accordingly, in the waveguide devices PVK functions as both the low-refractive-index optical spacer and hole-transporting/ electron-blocking layer.
Fig. 1 (a) Chemical structures of PVK and BSB-Cz molecules for bilayer waveguide device. (b) Normalized absorption, PL, and ASE spectra of BSB-Cz film on glass substrate. (c,d) Refractive index and extinction coefficient of BSB-Cz and PVK films on glass substrate, which were measured by variable angle spectroscopic ellipsometry.
In the following studies, the edge emission of bilayer organic waveguide devices is investigated from two aspects. In Sec. 2, we perform the optically pumped ASE measurements to compare a series of BSB-Cz single-layer and BSB-Cz/PVK bilayer waveguides. This will allow us to verify the pronounced effect of the PVK layer on the reduction of optical loss from the bottom electrode. From simulation of the guided mode field distribution in waveguides we can further interpret the dependence of the ASE threshold of BSB-Cz on the PVK thickness. In Sec. 3, we characterize the electroluminescence of the bilayer OLEDs, and show that the spectrally narrowed edge emission satisfies the cutoff condition. The dependence of the cutoff wavelength and spectral linewidth on the BSB-Cz thickness is analyzed in detail. Finally, we investigate the dependence of the edge emission spectrum on current density, and discuss the device performance at a high current density of 1000 mA/cm2.
2. Optically pumped amplified spontaneous emission
For the optically pumped measurements, we prepared the samples as illustrated in Fig. 1(a). The samples were excited with the XeF laser (355 nm) at a repetition rate of 20 Hz and a pulse duration of 10 ns. The laser beam was focused and shaped into a strip (3 × 0.25 mm2). The pumping energy was adjusted by using neutral density filters. The edge emission was detected using an optical fiber connected with an Ocean Optics HR4000 spectrometer. We start with the examination of a 200 nm thick BSB-Cz layer deposited on the ITO substrate without applying a PVK layer. The absorption in ITO is not sufficiently low (extinction coefficient of 0.01-0.1 in the visible range) and causes severe loss for the emission of BSB-Cz. As shown in Fig. 2(a), we didn’t observe apparent spectral narrowing or rapid growth of emission intensity at a particular wavelength even with the highest pump energy of our laser setup (350 μJ/pulse). The two main peaks at 455 nm and 488 nm coincide with the two vibronic peaks of the normal PL spectrum measured at low excitation energy [Fig. 1(b)], indicating the same origin of unamplified spontaneous emission. However, by inserting a PVK layer between BSB-Cz layer and ITO electrode, the ASE phenomenon of BSB-Cz can be observed at a much lower threshold. Figures 2(b)-2(d) show the normalized PL spectra of BSB-Cz varied with the pump energy for 100 nm, 200 nm, and 300 nm thick PVK layers. Once above the threshold energy, the PL spectra in all the cases become substantially narrowed with a FWHM of <10 nm at the ASE peak wavelength close to 480 nm, while the peak intensity increases dramatically [Fig. 2(f)]. Moreover, the ASE threshold energy of BSB-Cz is significantly reduced as the PVK thickness increases. Compared to the extremely high ASE threshold for the BSB-Cz deposited on ITO (>350 μJ/pulse), the threshold energy significantly drops to 7.6 μJ/pulse with 100 nm PVK, and further decreases down to 1.84 μJ/pulse and 0.78 μJ/pulse with 200 nm and 300 nm PVK [Table 1]. We also performed the ASE measurement of a 200 nm thick BSB-Cz on glass substrate as a control sample [Fig. 2(e)], which is expected to have the lowest ASE threshold due to the negligible absorption of glass. We obtained the ASE threshold of 0.68 μJ/pulse for the BSB-Cz on glass substrate, very close to that on 300 nm PVK. These results clearly show that insertion of a sufficiently thick PVK between BSB-Cz and ITO can effectively reduce the ASE threshold of BSB-Cz by more than two orders of magnitude, even down to a similarly low level as that of BSB-Cz on glass.
Fig. 2 Normalized PL spectra of a 200 nm thick BSB-Cz film deposited on (a)-(d) ITO substrates spin-coated with different PVK thicknesses and (e) glass substrate, with various pump energy in unit of μJ/pulse. (f) PL intensity versus pump energy of a 200 nm thick BSB-Cz film deposited on various substrates.
Table 1. ASE threshold pump energy of a 200 nm thick BSB-Cz film deposited on glass and ITO substrates with various PVK thicknesses.
To further clarify the dependence of the ASE threshold on the PVK thickness, we simulate the electromagnetic field distribution of the guided TE0 mode at the wavelength of 480 nm in the waveguide device (glass/ITO/PVK/BSB-Cz/air) for different PVK thicknesses [Fig. 3(a)]. From the mode profile we can calculate the ratio of the field intensity in each layer (x position from a to b) to the overall intensity of the mode by
which is defined as a confinement factor. Figure 3(c) shows the confinement factors calculated for the BSB-Cz, PVK, and ITO layers as a function of the PVK thickness. In the absence of the PVK layer, the waveguide mode is distributed widely over the BSB-Cz and ITO with the confinement factors ΓBSB-Cz = 72.5% and ΓITO = 24.4%. The value of ΓBSB-Cz is found similar to that in the control sample with the lowest ASE threshold energy (glass/BSB-Cz/air, where ΓBSB-Cz = 73.4%, see Fig. 3(b)). Therefore, the extremely high ASE threshold of the BSB-Cz on ITO is weakly related to the confinement factor in BSB-Cz, but mainly attributed to the considerable fraction of the mode field in ITO that experiences significant absorption loss. When the PVK thickness increases, the mode field tends to be more confined in BSB-Cz and less distributed in ITO, resulting in the reduced ITO absorption and hence the lowered ASE threshold of BSB-Cz. As shown in Fig. 3(c), ΓBSB-Cz increases from 72.5% up to 85.8% while ΓITO significantly decreases from 24.4% down to 0.05% for the PVK thickness from 0 to 300 nm. On the other hand, ΓPVK reaches the maximum value of 12.6% for 100 nm PVK, and then slightly decreases to 11.6% for 300 nm PVK. The simulation confirms that the guided mode field is nearly isolated from the ITO substrate with 300 nm PVK and exhibits very little absorption loss. This may well explain why the BSB-Cz on 300 nm PVK has a similarly low ASE threshold as that on glass substrate.Fig. 3 (a,b) Simulation of TE0 waveguide mode profiles at the wavelength of 480 nm in the multilayer waveguide structure with 200 nm thick BSB-Cz deposited on (a) 120 nm thick ITO substrates spin-coated with various PVK thicknesses and on (b) glass substrate. (c) Calculated confinement factors as a function of the PVK thickness. The inset shows the log-scale plot of the confinement factor in the ITO electrode.
Note that the lowest ASE threshold achieved in our bilayer waveguide (0.7-0.8 μJ/pulse, corresponding to ~100 μJ/cm2) is three to four orders of magnitude higher than the state of art lowest threshold of organic lasers (36 nJ/cm2, Ref [14].), but only one to two orders of magnitude higher than the typical thresholds of organic DFB lasers (1-10 μJ/cm2). Despite the comparatively high ASE threshold, our organic bilayer structure is particularly useful for electrically pumped applications and shows the much reduced optical losses from electrode absorption. Based on this structure, it is possible to reach a lower laser threshold by incorporating a properly designed resonator in the PVK layer and employing the doped luminescence layer as the gain medium (e.g., the BSB-Cz doped into the CBP film has shown the very low ASE threshold of 0.7 μJ/cm2 [25], which is two orders of magnitude lower than the pure BSB-Cz film used in this work and approaches the typical thresholds of DFB lasers).
Overall, from the optically pumped ASE study we demonstrate the effectiveness of applying PVK as a spacer to eliminate the absorption loss from the bottom electrode. Moreover, PVK is shown to be essentially non-absorptive to the emission of BSB-Cz (similar to glass). This suggests that applying a PVK layer in the waveguide device enables the reduction of absorption loss not only for the guided mode, but also for the cutoff mode confined at the BSB-Cz/PVK interface, as the case of electroluminescence to be described in Sec. 3.
3. Electroluminescence of PVK/BSB-Cz OLEDs
Now we move forward to investigate the electroluminescence (EL) properties of PVK/BSB-Cz bilayer structure. Figure 4(a) shows the device configuration and energy diagram of a BSB-Cz/PVK OLED. Since the PVK has a similar HOMO level but a much higher LUMO level (~0.8 eV) with respect to the BSB-Cz, holes injected from the PVK side can cross the heterojunction into the BSB-Cz whereas electrons injected from the BSB-Cz would be confined on the BSB-Cz side of the heterojunction. This ensures the occurrence of radiative recombination within BSB-Cz layer. Moreover, due to the much higher BSB-Cz electron mobility (1.2 × 10−8 cm2/Vs) than the PVK hole mobility (2.2 × 10−10 cm2/Vs), we combine the thicker BSB-Cz and thinner PVK to balance the electron and hole current. The BSB-Cz thickness is specifically designed in the range of 200-300 nm to produce the edge emission at a cutoff wavelength close to the peak gain wavelength (~480 nm), while the PVK thickness is fixed at 100 nm. We expect that the designed thicknesses of BSB-Cz and PVK would allow the electrons and holes to recombine in the vicinity of the heterojunction. Besides, we deposit a 10 nm thick MoO3 on ITO as the hole injection layer, and a 10 nm thick Cs2CO3 and a 80 nm thick Ag on top of BSB-Cz as the electron injection layer and cathode electrode, respectively [Fig. 4(a)]. This bilayer OLED therefore comprises an asymmetric waveguide structure, in which the luminescent BSB-Cz with the refractive index nB is sandwiched between PVK and Cs2CO3 with the refractive indices nP and nC. As analyzed below, the cutoff wavelength of the TE mode is confined to the BSB-Cz/PVK interface, and is not influenced by absorption of ITO. The spatial overlap between the cutoff mode and the recombination zone offers a great opportunity to minimize losses and to produce intense, or even amplified emission [21].
Fig. 4 (a) Device configuration and energy diagram of BSB-Cz/PVK bilayer OLED. (b) J-V-L characteristics of an OLED with 250 nm thick BSB-Cz and 100 nm thick PVK. The device area is 2 mm × 5 mm. The inset shows the extracted EQE values versus current density. (c) Illustration of device preparation for measurement of side emission from the cleave edge of OLEDs (left). Also shown are the optical images of an OLED under EL measurement (right). The OLED is placed on the probe station with the Ag electrode side up, so that the back emission is mostly blocked but only appears near the uncleaved edge of the Ag electrode. The edge and back emission of the OLED clearly show different emissive colors, corresponding to the EL spectra in (d).
Figure 4(b) shows the typical optoelectronic characteristics of a BSB-Cz(250 nm)/PVK(100 nm) OLED. Due to the birefringence of BSB-Cz and the planar waveguide structure, the BSB-Cz/PVK OLED generally exhibits anisotropic EL intensity and spectrum, in particular with a strong edge emission. Note that the EL intensity in Fig. 4(b) was simply measured by using a photodiode (Hamamatsu S1133-01) to detect the photocurrent for light outcoupling through the back of ITO substrate. Such a measurement without collecting the edge emission somewhat underestimates the total EL intensity of the OLED, but allows us to extract a lower limit of the external quantum efficiency (EQE) of >1%. To further characterize the spectral property of the edge emission, we carefully cleaved the OLED from the inner edge of the ITO electrode [Fig. 4(c)]. An optical fiber connected with the spectrometer (Ocean Optics HR4000) was then placed close to the cleaved edge of the OLED and adjusted in height such that the maximum EL intensity can be collected. The EL image in Fig. 4(c) clearly shows the different color emission from the edge and back sides of the BSB-Cz/PVK OLED, corresponding to the different spectra in Fig. 4(d). In particular, the edge emission spectrum appears to be well-defined, and reflects the nature of the cutoff mode with a narrow bandwidth and the peak wavelength depending on the BSB-Cz thickness. This can be seen in a complete set of the edge emission spectra for the BSB-Cz thickness of 200-300 nm [Fig. 5(a)]. On the other hand, the back emission strongly affected by the waveguide property exhibits a rather complex spectral shape, which is also varied with the BSB-Cz thickness.
Fig. 5 (a) Edge emission spectra of BSB-Cz/PVK bilayer OLEDs with the BSB-Cz thickness in the range of 200-300 nm. (b) Comparison of the peak wavelengths extracted from the edge emission spectra for different BSB-Cz thickness and the calculated cutoff wavelengths for the TE and TM modes according to Eq. (5). (c) The TE and TM refractive indices of BSB-Cz at the cutoff wavelength. The refractive index of PVK is also shown for comparison. (d) The FWHM of the main peak in the edge emission spectrum versus the BSB-Cz thickness.
By analyzing the cutoff wavelength for different BSB-Cz thicknesses, we can clarify the spatial position and polarization characteristics of the cutoff mode for the edge emission. Because the BSB-Cz with the refractive index nB is sandwiched between PVK and Cs2CO3 with the refractive indices nP and nC (nB, nP > nC [29]), we assume that the cutoff mode is confined at the BSB-Cz/PVK interface. According to the theory of an asymmetric slab waveguide [23], the cutoff wavelength (λc) satisfies the phase-matching condition at a given BSB-Cz thickness (d)
where θc = sin−1(nP/nB) is the critical angle for internal refraction at the BSB-Cz/PVK interface, ϕBP and ϕBC are the phase shift at the BSB-Cz/PVK and BSB-Cz/Cs2CO3 interfaces, respectively, and m = 0, 1, 2, … is the mode number. Due to the birefringence of BSB-Cz, the refractive indices and phase shifts at the interfaces depend on the polarization direction, and can be represented for the TE and TM modes by, and From Eqs. (2)-(4) we can obtain the relation between the TE and TM cutoff wavelengths and the BSB-Cz thickness as follows Figure 5(b) shows the calculated cutoff wavelengths of the TE and TM modes by using Eq. (5) and the peak wavelengths extracted from the edge emission spectra in Fig. 5(a). A good agreement is obtained between the experimental result and the calculation of the TE1 mode. This confirms that the edge emission is dominated by the TE1 cutoff mode propagating along the BSB-Cz/PVK interface. The TM0 mode with a slightly shorter cutoff wavelength than the TE1 mode may also contribute to the edge emission, but the intensity is too weak to be identified in the spectrum. The very different cutoff conditions of the TE and TM modes can be understood from the contrast between (/) and nP. As shown in Fig. 5(c), is apparently higher than nP while approximates to nP in the spectral regime of BSB-Cz luminescence. Therefore, the cutoff thickness is much larger for the TM mode than the TE mode at a given mode number. Moreover, the TM mode would not be cut off at the BSB-Cz/PVK interface for the wavelength longer than ~640 nm (where < nP).From the spectra in Fig. 5(a), it can be also observed that the FWHM at the cutoff wavelength is varied with the BSB-Cz thickness [Fig. 5(d)]. Interestingly, the FWHM abruptly decreases as the cutoff wavelength approaches the ASE peak wavelength (477 nm for 100 nm PVK, see Fig. 2(b)) of the BSB-Cz with the thickness of ~250 nm, possibly due to the occurrence of a particularly strong coupling between the cutoff mode and the BSB-Cz emission. The minimum FWHM is close to 5-6 nm. Overall, by controlling the cutoff wavelength to match the peak gain wavelength we can obtain the edge emission with the highest intensity and the narrowest linewidth.
Finally, we investigate the edge emission phenomena as driving the bilayer OLEDs with high current density. The BSB-Cz thickness is fixed at 250 nm to have the cutoff wavelength around 476-477 nm. To achieve the current density as high as possible, we perform the J-V measurement by driving the device with pulsed voltage (width = 5 ms, period = 10 ms) while collecting the edge emission spectrum with an integration time of 100 ms. Figure 6(a) shows the selected spectra for different current densities. As the current density increases, the peak wavelength tends to slightly red-shift from 476 nm (62 mA/cm2) to 477 nm (1098 mA/cm2), accompanied by a little decrease of FWHM from 6.5 nm to 5.6 nm. The peak intensity is approximately linear proportional to the current density up to 1200 mA/cm2 [Fig. 6(b)]. The optical image in Fig. 6(c) shows the intense edge emission at the high current density of 1000 mA/cm2. However, before the device breakdown at an even higher current density we didn’t observe a further narrowing of the spectrum and superlinear increase of the peak intensity that signify the light amplification. We speculate that the maximum current and the emission intensity of the device are mainly limited by the low conductivity of PVK, since there might be large amounts of holes trapped in PVK as well as electrons accumulated at BSB-Cz/PVK heterojunction without effective recombination. Nevertheless, at this stage we demonstrate that this bilayer OLED can be reliably operated at the current density of at least 1000 mA/cm2 without obvious degradation of emission efficiency, and clearly, there is still room for improvement of device performance. By taking into account the lowest optically-pumped ASE threshold (~100 μJ/cm2) obtained in Sec. 2 and the PL transient lifetime of BSB-Cz (~1 ns) [25], we can roughly estimate the electrically-pumped ASE threshold of at least 105 A/cm2 for the guided modes. For the emission of cutoff modes from the bilayer OLEDs, the ASE threshold is expected to be lower than the guided modes due to the less loss in the electrodes [21], but could be still few orders of magnitude higher than the maximum current density of the present device. To further realize the electrically-pumped ASE, it is required to optimize the maximum current density and emission efficiency with more conductive materials and improved device processing and design [30].
Fig. 6 (a) The edge emission spectra of BSB-Cz(250 nm)/PVK(100 nm) bilayer OLEDs as driven with pulse voltage condition. (b) The peak intensity of the edge emission spectrum versus the current density. (c) The optical image of the edge emission at the high current density of 1000 mA/cm2.
4. Conclusions
We present a detailed study of the optically and electrically pumped emission based on BSB-Cz/PVK bilayer waveguide device. In the optical pump study, we demonstrate that insertion of a sufficiently thick PVK layer between BSB-Cz and ITO electrode can effectively reduce the ASE threshold of BSB-Cz, even down to a low level comparable to that of BSB-Cz on glass. The simulation confirms that as the PVK thickness increases, the waveguide mode becomes better confined in BSB-Cz while moves away from ITO electrode, resulting in less ITO absorption loss and hence the reduced ASE threshold of BSB-Cz. In the electrical pump study, the BSB-Cz/PVK bilayer OLED shows the EQE value of >1% and anisotropic emission, in particular with a well-defined, spectrally narrow edge emission at the cutoff wavelength. A particularly narrowed linewidth is obtained when the cutoff wavelength matches the peak gain of BSB-Cz. Moreover, the emission efficiency of the OLED is not apparently degraded at a high current density of ~1000 mA/cm2. Overall, this bilayer waveguide device shows reduced optical loss and produces intense edge emission with wavelength tunability, narrow bandwidth, and stable efficiency at high current densities, which is believed suitable for physical study of electrically-pumped ASE and high-power light source applications.
Acknowledgments
The authors are grateful for the financial support from the Ministry of Science and Technology of Taiwan (Contract No. MOST 103-2112-M-008-016-MY2).
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