Figure 2(a) displays a multilayer OLED device's energy level alignment diagram, including the LUMO and HOMO energies. A dc bias voltage 7 V applied across the OLED device's contacts, which causes an electron's injection from the cathode and hole from the anode, see fig. 1(a). As a result, electrons and holes' movement occurs via the hooping mechanism (because organic molecules exhibit weak van der wall force). Therefore, the accumulation of electrons and holes are observed at both sides of the active layer interface because of the difference in the organic molecule's energy barrier [13, 21]. These electrons and holes are recombined to form excitons, a coulombically bound form of electron-hole pair, and their recombination profile, see fig. 2(b). It has been observed that the three radiative recombination peaks are located at the interface and within the active layer interface. The HTL/EML1 interface, because of the low HOMO energy barrier (0.3 eV), can still effectively move the holes from the TAPC to the EML1. Besides, the LUMO barrier (0.45 eV) inhibits fewer electrons. As a result, a small peak was observed at the HTL/EML interface. But there is also energy level misalignment within the EML with HOMO energy barrier (0.22 eV), leading to a second peak within two active layer interfaces. At the EML/ETL interface, the non-reacted holes can be blocked and accrued by the 0.25 eV HOMO energy gap and encounter the electrons injected from the cathode and ETL. Hence, at EML2 / ETL interface exhibits the second-highest recombination peak. However, these formed excitons within the active layer are decay via either singlet/triplet transition and release energy in the form of photons. An average 1023 cm-3s-1 recombination of electron and hole observed in the active layer. Which directly affects device performance parameters EQE [3].
The integrated luminance as a function of array height is illustrated in fig. 3(a). The result showed that integrated luminance increases monotonically with the array's height, see fig. 3(a). Incomparable to rectangular arrays, the hexagonal shape array exhibits the highest luminance due to well-controlled geometry. As aspect ratio proportional to the height and width of the array. Thus, a higher aspect ratio has a higher intensity of light source [22]. Therefore, we are in the following results. Firstly, luminescence's high value is mainly due to using the array as an external scattering structure. Secondary, micropatterns help extract the higher light to improve the light outcoupling efficiency of OLED devices that would otherwise remain trapped inside the substrate. Tertiary, packing, and arrays are also the monitoring factors for outcoupling efficiency [23]. Fig. 3(b) shows the angular dependent radiant intensity profile of the OLED device attached with either hexagonal or rectangular shape microlens array measured at wavelength 550 nm. The angular-dependent radiance was measured to determine the exact fraction of horizontal and vertical dipoles. In the viewing angle range from ±250 for hexagonal while ±450 for rectangular, the maximum emission was observed [19]. The optical properties, therefore, vary significantly in terms of the wavelength of the emitted light. Fig. 3(c) shows the angular-dependent luminance enhancement (luminance with MLA- luminance without MLA) of the OLED device (see Table II). This angular-dependent luminance suggests that it is a Lambertian-like light source. The device luminance enhancement also decreased with an increase in the viewing angles when the hexagonal or the rectangular microlens array was positioned on the same side. The hexagonal microlenses array significantly enhances the outcoupling efficiency below the critical angle observation concerning the substrate surface normal [20, 24, 25].
TABLE II. Luminance comparison with/without a hexagonal or rectangular microlens array at different viewing angles.
Viewing angle
(degree)
|
Luminance
(Cd/m2)
|
Luminance
Enhancement (Cd/m2)
|
(w/o array)
|
Rectangular
array
|
Hexagonal
array
|
Rectangular
array
|
Hexagonal
array
|
300 |
2064
|
12913
|
16470
|
10849
|
14406
|
600 |
1536
|
9717
|
9262
|
8181
|
7726
|
It is notably from table II; a low value of luminance is observed without any micro pattern onto the glass substrate (the chosen thickness configuration, ITO (120 nm)/TAPC (40 nm)/EML1(200 nm)/EML2 (50 nm)/TPBi (50 nm)/Al (150 nm)). In contrast, a significant enhancement in luminance was observed in the hexagonal microlens array.
The spectral extraction efficiency of the OLED device enhances significantly by employing the hexagonal or rectangular microlens array on the top of OLED, and their variation with wavelength as shown in fig. 4(a). With an average of 75%, the spectral extraction efficiency has been achieved for the hexagonal packing type. The following points to be noted. The micropattern introduced onto the glass substrate (they don't induce any alteration either in the architecture of the device or in the functioning) to outcouple more photons that were trapped mostly in conventional OLED in substate mode due to the combined effect of TIR and waveguiding at boundaries of different refractive index materials. Thus, many rays are emitted from the OLED point in any direction and have further polarization. Fig. 4(b) compares EQE as a function of array height with keeping the aspect ratio constant. The EQE increases monotonically with the microlens array's height and appears to saturate for a higher value of microlens height. It means that a "perfect" microlens array may not be necessary for practical applications. In particular, we are at the following results; firstly, the extraction efficiency depends upon the aspect ratio of an array (aspect ratio defined as the height of the considered array divided by their radius). Secondly, the light extraction efficiency can enhance by outcoupled more photons trapped inside the bottom-emitting OLEDs substrate due to the TIR at the glass/air interface. Tertiary, if all incident rays are perpendicular to the lens's surface, TIR does not occur according to simple ray optics. The light generated inside the device is efficiently coupled to the substrate. Light now traverses through the microlens array without any deflection [26, 27]. The comparison of device performance parameters such as external quantum efficiency of different packing types onto a glass substrate of OLED device as a function of luminance characteristics of three separate arrays on the top of OLED as an external scattering structure is shown in fig—4 (c). Notably, from the figure that the hexagonal array exhibits the highest EQE, 35% (32% Rectangular MLA), then another array (which is higher than 34% EQE reported in the literature [16]). While the roll-off takes place with a minimum reached EQE 30% at the brightness level of 105 lm/m2,
Fig. 5 shows the angular-dependent E.L. spectra of OLED devices attached with different microlens micropattern arrays as a wavelength function. The El spectrum has two peaks because of the singlet and the second due to triplet transition due to the attribution of two primary colors that function as a phosphorescent active layer. The E.L. spectrum shows the two peaks at wavelength 512 nm and 600 nm. The emission curve shows a slight shoulder band around 650 nm as the peak intensity corresponds to exciton dynamics. Hence, the corresponding maximum generation of an exciton exhibits in hexagonal packing type. It has been observed that the intensity of the first peak is greater than the second peak, which means that electrically generated triple excitons can be exchanged via crossing the EML1 and transferring the Dexter energy from the EML2. Firstly, the following points to note: Due to reduced electrons' reduced repulsion, a singlet exciton's energy is more significant than a triplet exciton's energy. Secondly, due to a different pair of spin properties, singlet to triplet transition involves a change in the electronic state. Due to the following reason, the singlet state lifetime is less than that of the triplet state [13, 28]
Fig. 6(a, b) displays the angular dependent CIE coordinates of the OLED device attached with/without a hexagonal or rectangular microlens array. The CIE x index of a device without microlens array decreased first and then increased, see fig. 6(a) while the CIE y index first increased then dropped, see fig. 6(b). In the studied range of viewing angle, the maximal variations of the CIE x and CIE y panel indices were 0.0621 and 0.0768, respectively. However, when the OLED device was attached with a microlens array with packing type hexagonal or rectangular, the CIE x index decreased, and the CIE y index increased initially and then decreased with increasing the viewing angle. Table III illustrates the angular dependent CIE x and CIE y indices maximum variations (i.e., the difference between maximum and minimum values). More considerable variation in the CIE x and CIE y index values are observed for OLED devices without microlenses array in the calculated range of viewing angle. This suggested that the OLED device connected with either hexagonal or rectangular microlenses is also more sensitive than the OLED device without microlenses to the viewing angle range [20, 21].
Table III. Angular dependent CIE indices maximal variation of the OLED device attached with/without hexagonal or rectangular microlens arrays
Item
|
Without microlens
|
Hexagonal microlens arrays
|
Rectangular microlens arrays
|
(CIE X)max - (CIE X)min |
0.0621
|
0.0381
|
0.0331
|
(CIE y)max - (CIE y)min |
0.0768
|
0.0190
|
0.0205
|
The Comparisons of outcoupled efficiency as a function of ETL-TPBi thickness of OLED attached with microlens arrays, in which the orientation of point dipole was set up as isotropic (fig 7(a)), T.M. (fig 7(b)), and T.E. (fig 7(c)) polarization mode concerning the interface. All three-polarization mode shows an oscillation pattern in air medium with radiative quantum efficiency value as 0.6. The governing oscillation pattern in all three-mode due to the multi-beam interference effect, i.e., it is a difference in optical path length and phase shift that occurred throughout the reflection at the metal cathode, the electron transport layer thicknesses. Therefore, the photons generated in the active layer propagate toward the HTL, ITO, glass substrate, and out of the device. Part of them propagate toward the metal cathode and reflect back [31]. Hence, the emissive dipole moment's orientation significantly impacts the outcoupling efficiency as the proportion of light trapped within the OLED in parasitic waveguide mode. As a result, the OCE strength sinusoidally decreased with ETL thickness. The orientation of point dipole was set up as isotropic concerning the interface, see fig. 7(a). The first maxima with the highest OCE observed for the hexagonal array at 70 nm and 220 nm second maxima for air medium. The strength of oscillations decreases as the distance rises due to reduced dipole radiation intensity with increasing distance from the dipole. The intensity of the outcoupling efficiency is thus reduced by increasing the thickness of ETL. However, we found a higher value of OCE (for ETL-50 nm, see fig. 7(b)) than the transverse electric mode, i.e., perpendicular polarization (ETL 180 nm, see fig. 7(c)) in point dipole orientation. Thus, the emitter's dipole orientation is essential if OLEDs' light emission characteristics are determined. The TM mode provides more performance efficiency than the T.E. in the air because OCE has the main concern with the air mode, i.e., how much light has come out of the system and is stuck in a parasite mode [13, 19, 29].