We have proposed what we believe is a novel organic device pumped by an organic light-emitting diode to avoid a strong charge-induced absorption and nonradiative loss in an electrically pumped organic laser. An organic light-emitting diode was fabricated on the transparent anode substrate with a microcavity structure and driven under intense pulse voltages, and an organic film with a low threshold of amplified spontaneous emission was spin-coated on the opposite side of the substrate. Edge emission, spectral from the film, has been measured under the pulse voltages. The optical characteristics of organic films under an Nd3+: YAG laser have also been studied.
© 2006 Optical Society of America
The organic solid laser diode (OSLD) is a promising research topic in the field of organic semiconductor devices. Optically pumped organic thin films or single crystals have been demonstrated to produce stimulated emission [1–4]; however, electrical pumping of the organic light-emitting diode (OLED) has not, to date, proved successful in achieving gain or lasing. According to reports on the lasing threshold of optically pumped organic dye-doped films, the current density of ~KA/cm2 is necessary for realizing electrical pumping [5–7].
To avoid strong charge-induced absorption and device degradation resulting from heat accumulation [8–12], a pulse-driven scheme on a small-size OLED is necessary. High luminance of ~Mcd/m2 has been achieved through the use of pulsed OLED, as reported in Refs. [5–7]. In this paper we have developed a novel application of the high-luminance OLED as an excitation source to an organic film with a low amplified spontaneous emission (ASE) threshold, which was spin-coated on the opposite side of the substrate. In comparison with noncavity OLEDs, the microcavity OLED (MOLED) has been demonstrated to be spectrally narrowing and enhancing in forward-direction electroluminescence (EL) intensity [13–15]. Moreover, an EL emission with strong spectral narrowing can improve the power density by a factor of approximately 10% over that of a noncavity OLED with the same emission intensity, as estimated from Eq. (1):
where W and L are the power density (W/cm2) and the luminance (cd/m2); Km is maximum luminous efficiency at 555 nm, 683 lm/W; k(λ) and V(λ) are emission spectrum and photonic spectral luminous efficiency as a function of wavelength λ, respectively.
The structure of the OSLD is illustrated in Fig. 1(a). Before the device was fabricated, the indium-tin-oxide (ITO) substrate with a microcavity structure was cleaned, and the organic film was spin-coated on the opposite side of the glass. The organic films used in this study were DCM2, Pyrromethen 580, and Pyrromethen 597 in a polymer binder (polystyrene); their molecular structures are shown in Fig. 1(b). The solvents for these molecules are methylene chloride (MDC) for DCM2, and tetrahydrofuran for Pyrromethen 580 and Pyrromethen 597. We also fabricated a mixed film of DCM2 and Pyrromethen 580 using the solvent MDC. The concentration and thickness of organic films are listed in Table 1.
We have utilized a vacuum evaporation deposition system to deposit 4,4’-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl as a hole transport layer, coumarin6 [see Fig. 1(b)] in tris(8-hydroxyquinoline)aluminum (0.5-wt%) as a light-emitting layer, and tris(8-hydroxyquinoline)aluminum as an electron transport layer, subsequently, upon ITO-coated glass substrate with alternating high and low refractive index dielectric mirrors (Ta2O5/SiO2) under a base pressure lower than 5.5×10-6 Torr. The electron injection layer of 0.4 nm LiF and the cathode of 150 nm Mg:Ag (9:1 mass ratio) were evaporated from the top organic layer owing to its enhanced electron injection-to-electron transport layer . A shadow mask was used to form the LiF and Mg:Ag electrode pattern, which resulted in devices with an active area of 1.00 mm2. In addition, a 150 nm Al layer was deposited on the ITO as a path electrode before the evaporation of the organic layer to help reduce the series resistance with the device and avoid the breakdown of the device due to heat accumulation at the application of bias. The thickness of organic layers has been optimized for making the peak wavelength of microcavity OLED consistent with a noncavity device.
The OSLD was driven under intense pulse voltages. The pulse width was set to be 10 ms, and the pulse interval was fixed to be 100 ms by use of a pulse function generator (Agilent 81110A, rise time is 0.5 ns). In addition, we have employed the spectrophotometer USB 2000 (OceanOptics Company) to record the EL emission intensity from the pulsed OLEDs. The EL spectrum from the fabricated microcavity OLED and absorbance of organic films used were illustrated in Fig. 2. Figure 2 suggests that the wavelength of excited light is nearly equal to the peak position of absorbance for different films. Full Wavelength at Half Maximum (FWHM) changes with the pairs of dielectric mirrors, such as from 40 nm for one pair to 10 nm for six pairs. Therefore, a MOLED can be used as an excitation light source to be absorbed by organic film and to investigate optical properties of pumped films.
The absorption and PL spectrums were measured by UV-VIS-NIR spectrophotometer (Shimadzu Corporation) and spectrofluorometer FP-750 (JASCO Corporation), respectively. In addition, the second harmonic at 532 nm of the pulsed Nd3+:YAG laser (LOTIS TII, Ltd) (pulse duration is 11 ns) was also used to investigate the ASE characteristics of organic films.
3.1. Surface emission
The PL spectrum of surface emission from the films excited by the MOLED are shown in Fig. 3. The first peak emission in Fig. 3 was produced from one part of the transmitted light of the MOLED. The second peak emission in the spectrum was the PL of the thin film that was excited by EL emission from the MOLED. Therefore, we can conclude that the organic films can be excited by the biased OLED and emit PL. Relatively, Py580 film produces stronger surface emission than the DCM2 and Py597 films. However, the PL emission from a mixed film of Py580 and DCM2 was found to be very weak, showing a strong interaction between the molecules of DCM2 and Py580.
By taking the integral of the EL intensity over the wavelength, we can directly estimate the absorption of light by organic film. This is based on the assumption that EL emission into the film can be divided into absorption and nonabsorption by the organic film. The integration shows that 13.4% and 26.9% of OLED emission has been absorbed by the organic films of DCM2 and Py580, respectively. The increasing molecular concentration of organic material in a polymer binder may enhance the absorption; however, a concentration quenching will occur.
3.2. Edge emission
We have further investigated the emission guided in the film excited by the same MOLED. Figure 4 is an emission spectrum measured from the edge of the organic films at the same driving schemes. An interesting phenomenon is that the emission for Py580 is weakest among the films, which is contrary to its surface emission. On the other hand, Py597 and DCM2 have been found to produce a strong PL at the peak wavelength of 580 nm and 620 nm, which are consistent with the PL from optical excitation, as seen in Table 1. However, we have noticed that the emission from the blend film of DCM2 and Py580 is still weak, and there is a redshift in the peak wavelength, which indicates that the molecular interaction will not only inhibit light absorption but also change the emission spectrum.
The Bragg condition that determines the lasing action of the organic film pumped by the MOLED is proposed to guide light into edge emission completely. If a distributed feedback (DFB) resonator using LiF as a transparent insulator (n=1.3) was prepared on the opposite side of the glass before spin-coating organic film, a complete film-waveguided edge emission will be obtained. For a flat DFB resonator, the lasing wavelength (λ DFB) can be calculated from the following Eqs. (2) and (3):
where λ Bragg is Bragg wavelength, n eff is the effective refractive index (n=1.8), and m and Λ are the diffraction order and period of grating, respectively. According to waveguided analysis by Kappa software, λ Bragg is 508 nm, which is almost consistent with the emission wavelength (512 nm) of the coumarin6-doped MOLED.
3.3. Optical pumping of organic films excited under YAG laser
The lasing threshold for different films under YAG laser excitation has been summarized in Table 1. DCM2 has been found to have relatively the lowest threshold of ASE among the films used in the study, which is about 3.9 KW/cm2, as shown in Fig. 5. Mixed film of two organic molecules, DCM2 and Py580 increases the lasing threshold greatly, e.g., from 3.9 KW/cm2 of single film DCM2 to 8.6 KW/cm2, which may be attributed to the multiexciton annihilation of DCM2 and Py580. Compared with Py580, Py597 has a lower lasing threshold, which is only 5.0 KW/cm2. This can be compared from an edge emission measurement (see Fig. 4) in which Py597 produces a stronger PL emission pumped by OLED.
We have developed a novel OLED-pumped organic solid laser diode and demonstrated that the MOLED can be used as an excitation source to pump organic films. Small-size OLEDs have been found to realize the luminance of ~Mcd/m2 under pulse voltages up to now. Based on our research about the coumarin-6 doped OLED with a microcavity structure, a peak luminance of 18 Mcd/m2 has been achieved. The power density can reach about 16.0 W/cm2, calculated from Eq. (1). However, this value of power density is far smaller than the ASE threshold of the organic films, ~KW/cm2, as shown in Table 1. Therefore, new approaches to achieving an OLED-pumped organic solid laser are necessary.
One such approach may be to develop the organic material with a lower ASE threshold. The organic film of 1,4-bis[2-[4-[N,N-di(p-tolyl)amino]phenyl]vinyl]benzene doped in 4,4′-bis(9-carbazolyl)biphenyl (4.0-mol%) has been realized to have a low lasing threshold of about 200 W/cm2 under a pulsed nitrogen laser . Since its peak wavelength of absorption spectrum is about 380 nm , an ultraviolet OLED can be used as an excitation source to pump this type of film. High-luminance ultraviolet OLED under pulse voltages is expected to be applied in pumping organic films to achieve lasing.
Furthermore, edge emission intensity from an OLED has been found capable of reaching the order of 1000 times stronger than the external emission, since the ITO/organic waveguided light may be restricted within the thickness of hundreds of nanometers . Therefore, it will be potential to employ the edge emission of pulsed OLED as an excitation source to pump the organic film if the peak wavelength of edge emission is consistent with the maximum absorbance of organic films.
This work was supported by the CLUSTER of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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