In the context of progress towards the organic laser diode, we experimentally investigate the optical and electrical optimization of an OLED in a vertical λ/2 microcavity. The microcavity consists of a quarter-wavelength TiO2/SiO2 multilayer mirror, a half-wavelength-thick OLED and a semitransparent aluminum cathode. The Alq3/DCM2 guest-host system is used as the emitting layer. This study focuses on the design and the fabrication of a half-wavelength thick organic hetero-structure exhibiting a high current density despite both the thickness increase and the cathode thickness reduction. The emission wavelength, the line-width narrowing and the current-density are studied as a function of two key parameters: the hetero-structure optical thickness and the aluminum cathode thickness. The experimental results show that a 125 nm thick cavity OLED ended by a 20 nm thick aluminum cathode exhibits a resonance at 606 nm with a full width at half maximum of 11 nm, and with current-densities exceeding 0.5 A/cm2. We show that even without a top high-quality-mirror the incomplete microcavity λ/2 OLED hetero-structure exhibits a clear modification of the spontaneous emission at normal incidence.
©2012 Optical Society of America
Although optically-pumped organic solid-state lasers have been reported as early as 1996 [1,2], the electrically pumped organic laser diode has not been demonstrated so far. An abundant literature has identified the electrical and optical issues that prevent the electrically driven laser operation [3,4]. The main limitation remains the low carrier mobility of the organic films resulting in OLED current-densities in the range 0.1 A/cm2 to 1 A/cm2 under DC excitation [5,6], and in the range of 1 A/cm2 to 1 kA/cm2 under pulsed excitation [7–9]. So far, early attempts to demonstrate electrically pumped organic laser diodes in the late 90's investigating different type of waveguide laser structures [10,11], as well as recent  but eventually controversial results  with planar cavity remain unsuccessful. Very recent works do not try to demonstrate directly lasing action in organic laser diode, but instead, in a more step-by step approach, address some of the issues of the organic laser diode [14,15].
Because of these difficulties, many efforts have been devoted to the optically-pumped experiments. Among the optically pumped experiments, one of the lowest laser threshold has been reported by Koschorreck et al.  with a quality factor laser cavity Q = 4500. The current-densities required to achieve the same excitation level is, in this case estimated at least at 36 kA/cm2. With even more efficient laser cavities, lower laser threshold could be expected. The challenge is then to realize a laser cavity with both a high quality factor and allowing a high current-density electrical pumping of the organic hetero-structure. These objectives are difficult to fulfill at the same time, because the electrical pumping of the organic hetero-structures is hardly compatible with a high quality factor micro-cavity for two reasons: Firstly, an organic hetero-structure filling a half-wavelength micro-cavity is thicker than a standard OLED, which strongly reduces the current density due to the low carrier mobility of the organic films expressed by the exponential Poole-Frenkel law . Secondly, the metallic cathode required for the current injection into the organic layers induces absorption losses that limit the quality factor and therefore needs to be minimized. However, the thickness reduction decreases the maximal current density that an organic hetero-structure can stand before destruction. This antagonism requires a trade-off on the cathode thickness between the quality factor and the current density because the optical requirements are just opposite to the electrical ones.
On the basis of several experimental results reported in the literature, we, recently, suggested , that the laser threshold can be lowered to the level of the state of the art OLED current-density (i.e. ~1 A/cm2) with a micro-cavity quality factor higher than Q > 10 000. We, then, numerically demonstrated that such a high quality (Q > 10000) factor micro-cavity OLED can be obtained with highly reflective multilayer mirror based microcavity incorporating a λ/2 thick organic hetero-structure ended with an ultra-thin (15 nm) aluminum cathode.
In this paper, we report an experimental study of the optimization of the optical and electrical properties of half-wavelength thick organic hetero-structure compatible with a multilayer mirrors micro-cavity. We aim at experimentally demonstrating that such an organic hetero-structure emitting at 600 nm ended with ultra-thin metallic cathode can still stand an important current-density. The most significant contributions of our work are firstly, the proposition of a new half-wavelength thick organic hetero-structure optimized from the optical point of view for a multilayer mirror based micro-cavity; secondly, the demonstration that the so called thick-organic hetero-structure can stand current-densities at the order of magnitude of the A/cm2, even with a cathode thickness down to few tens of nanometers. Finally, the cavity effects are observed even with the incomplete cavity; says a micro-cavity ended with the simple semitransparent metallic cathode.
The scope of this study is, thus, limited to the investigation of the incomplete cavity (i.e. without the top mirror since the latter does not impact the current density) as shown in Fig. 1(a) . It consists of glass substrate successively covered with a multilayer mirror, an organic hetero-structure and ended solely with an aluminum cathode. The bottom mirror is a multilayer mirror made of 10 pairs of alternated quarter-wavelength optically thick high-index layer made of titanium oxide (TiO2) and low-index layer made of silicon oxide (SiO2). It is capped with an Indium Tin Oxide (ITO) transparent and conductive layer playing both the role of a high-index layer and a transparent anode. The mirror reflectance is centered at 630 nm with a maximum reflectivity better than 99.5%, and with a stop-band of 100 nm.
2- Experimental results
The OLED hetero-structure layers were deposited in high vacuum (10−7 mbar) system. It consists of a 4,4’,4”,tris-(3-methylphenylphenylamino) triphenylamine (m-MTDATA) as hole injection layer (HIL) with a thickness varying from 10 nm to 70 nm, a 30 nm layer of N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4-diami (NPD) as a hole transport layer (HTL), a 30 nm 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) doped at 2 wt% into the Tris(8-hydroxyquinolinato) aluminum (Alq3) host material as the emitting layer (EL) with a peak emission centered at 600 nm (Fig. 1). The emitting layer is followed by a 10 nm Bathocupuroine (BCP) as a hole blocking layer (HBL) and a 25 nm layer of Alq3 as an electron transport layer (ETL). The organic layers were followed by a 1.4 nm of lithium fluoride (LiF) and an aluminum (Al) layer as a cathode that was vapor-deposited at the same background pressure. A thin Lif layer followed by an aluminum layer provides a 2.9 eV work function . The band diagram with the details of the highest occupied molecular orbital (HOMO) and Lowest unoccupied Molecular Orbital (LUMO) level is shown on Fig. 1(b).
In order to optimize the micro-cavity OLED hetero-structure two design rules have been applied. The first one is related to the resonance condition and implies that the OLED hetero-structure optical thickness should be e = λ/2n. For an emission at λ = 598 nm with an average organic refractive index n = 1.7, the required geometrical thickness is e = 178 nm whereas an optimized OLED thickness is about 80 nm thick. Such an increase of the hetero-structure thickness is expected to dramatically decrease the current-density. In order to prevent this reduction, we choose the m-MTDATA as a HIL material. Despite a hole motility of 3.5.10−5 cm2.V−1.S−1  lower than the CuPc mobility (9.04 10−4 cm2.V−1.S−1 ) the m-MTDATA present a 5.0 eV HOMO level intermediate to the ITO (4.7 eV) and to the NPD HOMO levels (5.4 eV) which leads to an easier hole injection at the anode/m-MTDATA interface and at the m-MTDATA/HTL interface. Keeping the rest of the layers unchanged, and by increasing only the HIL thickness, the whole organic hetero-structure can be increased up to λ/2 without decreasing significantly the maximum current density.
A second design rule consists of locating the emitting layer at an antinode in order to maximize the coupling of the organic emitting dipoles with the field so as to increase the light emission. For this reason the EL, HBL and ETL thicknesses are fixed such that the HTL/Active Layer interface is located at a distance of λ/4 from the aluminum cathode.
Following these rules, several OLED hetero-structure thicknesses are studied: 105 nm, 125 nm, 145 nm and 165 nm when the m-MTDATA layer thickness is 10 nm, 30 nm, 50 nm and 70 nm, respectively. For this part of the study, we used a 20 nm thick aluminum cathode. Note that we also used a reference OLED fabricated under usual conditions in order to compare our results.
The electroluminescent measurements show a clear single peak with a Full Width at Half Maximum (FWHM) of 11 nm (Q = 55 nm) centered at 582 nm to 606 nm, 633 nm and 662 nm for 10 nm, 30 nm, 50 nm, 70 nm m-MTDATA layer thickness, respectively. By comparison, the reference OLED has a FWHM = 70 nm and Q = 8.5. For the rest of the study, we choose the 125 nm thick organic hetero-structure that exhibits a resonance (606 nm) in the vicinity of the maximum of the reference OLED electro-luminescence spectra. Note that the hetero-structure thickness difference (53 nm) between the experimental value (125 nm) and the previously calculated value (178 nm) can be explained by the penetration of the optical field into the mirrors. This value of the penetration depth of the field in the dielectric and metallic mirrors is consistent with the 40 nm already reported by Ma et al. , and should be carefully taken into account in the design of a half-wavelength thick organic hetero-structure.
From the measurements indicated above, the investigation of the current-density for different values of the aluminum layer thickness is conducted with the 125 nm thick organic hetero-structure. Figure 2 , reports the voltage-current (V-I) characteristics for different organic hetero-structures with different cathode thickness (5 nm, 10 nm 20 nm and 100 nm). The 100 nm thick aluminum layer allows a maximum current density of 583 mA/cm2 at 17.4 V and a threshold voltage of 7.2 V measured at 1 mA/cm2, whereas the OLED with the 20 nm (respectively 10 nm and 5 nm) thick aluminum layer is limited to a current-density of 524 mA/cm2 at 19 V (respectively 120 mA/cm2 at 26.8 V and 1,47 mA/cm2 at 40 V) and a threshold voltage of 7.2 V at 1 mA/cm2 (respectively 15.5 V and 21.7 V at 1 mA/cm2).
Clearly, the OLEDs with an aluminum cathode thickness between 10 nm and 5 nm restrict the current-density to very low values and thicknesses higher than 10 nm are preferable in term of current density. From the optical point of view, the different aluminum layer thicknesses results in different line-width narrowing as shown in the inset of Fig. 2. The 5 nm, 10 nm, and 20 nm of aluminum cathode thicknesses lead to the spectra with a FWHM of 37 nm, 21 nm and 11 nm, respectively. The corresponding quality factors (Q = 16, Q = 29 and Q = 55, respectively) are increased with the aluminum layer thickness as a result of a better reflectivity despite a larger absorption from the metallic layer. If a top mirror is to be added, the reflection from the metallic cathode is parasitic and is to be minimized. Therefore, taking into account both the current density and the parasitic reflection leads to an optimal aluminum thickness of 20 nm.
Figure 3 presents both the top-side and bottom-side electro-luminescence spectra of a 125 nm thick micro-cavity OLED with a 20 nm thick aluminum cathode. The spectra have been measured under normal incidence with a 2 mA/cm2 OLED current density which corresponds to 8.3 V. Measurements are normalized to the maximum of the bottom side emission of the reference OLED. Figure 3(a) shows the bottom side emission of the reference OLED (red), the bottom emission of the micro-cavity OLED (black), the measured reflectance of the bottom mirror (blue) and the calculated transmittance (green). The transmittance of the mirror being much smaller than the transmittance through a simple glass substrate, the light emitted from the bottom of the microcavity is much lower than the light emitted by the reference OLED. Therefore, for the sake of visibility the bottom emission of the microcavity OLED is multiplied by a factor 20 before being plotted with the same scale as the reference OLED spectrum. The calculations are based on the matrix transfer model . The oxide layers have a quarter-wavelength thickness and their residual absorption is taken into account in their complex refractive index measured by ellipsometry. The thicknesses of the organic layers are set to the same values as in the experiment, and the refractive index is set to n = 1.7. The calculation of the micro-cavity transmittance is in good agreement with the experimental results as can be seen from the 606 nm, the 708 nm and the 780 nm peaks. Our simulations indicate that the existence of the 708 nm and 780 nm peaks is explained by the filtering of the DCM emission spectrum by the side transmission peaks of the micro-cavity transmittance (green). This is a strong indication that the spectral narrowing is due to the filtering properties of the microcavity and not to a light amplification.
Figure 3(b) shows the experimental and the calculated top side emission spectra. Since the top and bottom-mirrors transmittances differ, the top and bottom emission are also different. There is a good agreement between the calculated wavelength resonance and the experimental one (λres = 606 nm).
In Fig. 3(b), at the resonance wavelength (606 nm), the top-side emission of the micro-cavity OLED is 12.5 times higher in amplitude than the top-side emission of the reference OLED with the same aluminum thickness. In order to take into account the redistribution of light induced by the mirrors, we plotted on Fig. 3(c) the sum of the bottom and the top-side emissions of the micro-cavity OLED and that of the reference OLED. These sums are obtained by the numerical addition of the data of the top and bottom measurements. The ratio of their total emission gives the external efficiency enhancement (Gext) which shows that the light emitted from the micro-cavity OLED at the resonance wavelength (606 nm) is Gext = 1.37 times higher than that emitted from the reference OLED. This increase of the light emission at the resonance cannot be explained by a filtering effect. Rather, the possible enhancement of the spontaneous emission  should be carefully considered since it can play a key role in a low laser-threshold approach.
In summary, we have experimentally investigated the maximum current density of a half-wavelength thick organic hetero-structure ended with an ultra-thin metallic cathode and compatible with a high Q micro-cavity. The new half-wavelength-thick organic hetero-structure is based on a m-MTDATA/NPD/Alq3:DCM/BCP/ Alq3/LiF/Al stack deposited on a TiO2/SiO2 multilayer bottom mirror. The resonance wavelength and the current density of this hetero-structure have been experimentally studied and optimized as a function of two key construction parameters: the thickness of the organic hetero-structure and the thickness of the semitransparent aluminum cathode. A 125 nm-thick organic hetero-structure ended with a 20 nm aluminum cathode allows a 524 mA/cm2 DC excitation and a resonance at 606 nm with a spectral narrowing down to 11 nm (FWHM). Unexpected result is the enhancement of the spontaneous emission at the resonance by a factor of 1.37 compared to the reference OLED.
Although this work deals with the optical and electrical optimization of a thick organic hetero-structure and its maximum current density, we believe that it is an essential step towards the electrically pumped organic laser diode that remains to be demonstrated. Our results open the way to kA current-densities pulsed excitation of high Q micro-cavity OLED available with a highly reflective multilayer mirror added on the top of the aluminum cathode. In future studies the modification of the spontaneous emission will be investigated through fluorescence lifetime measurement.
The authors thank the Agence National de la Recherche (ANR) for their financial support in the projects OLD-TEA and RTB + . The authors are very grateful to the Région Ile de France (C'nano Ile de France), and the Fonds Européens de Développement Economique Régional (FEDER) for their financial supports.
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