Dong-Min
Lee
,
Gi-Eun
Kim
,
Jae-Hoon
Kim
* and
Chang-Jae
Yu
*
Department of Electronic Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea. E-mail: jhoon@hanyang.ac.kr; cjyu@hanyang.ac.kr
First published on 30th January 2023
Circularly polarized (CP) photoluminescence and electroluminescence (EL) generated from the emitting layer can improve the light efficiency of a commercialized organic light-emitting display. High CP emission is generated from the twisted configuration of a mesogenic conjugated polymer. However, the effect of the material and device parameters on the degree of CP emission, defined by the dissymmetry g factor, has not yet been fully studied. Through carefully investigating various parameters under various experimental conditions, it was found that the degree of CP emission can be expressed as the quadratic equation of the order parameter (S). Furthermore, a considerable degree of CP emission (g = 0.37 in the EL process) can be measured even when the order parameter was zero. Such results imply that the twisted stacks even at a random orientation are predominant rather than the formation of a uniform orientation to generate CP light. Also, the anti-reflection condition, required in conventional organic light-emitting diodes to eliminate the reflection of ambient light from a metal cathode, is still maintained since there is no retardation in the ambient light for S = 0 while emitting CPEL, which leads to an enhancement of luminance efficiency by about 18.5%. It is expected that these results, discussed herein, pave the way toward understanding the mechanism of CP emission and enhancing the performance of the CP emission.
The enhancement of light extraction efficiency by direct CP emission is related to the degree of CP emission, defined by the dissymmetry g factor g = 2(IL − IR)/(IL + IR). Here, IL and IR represent the intensities of left-handed CP (LHCP) light and right-handed CP (RHCP) light, respectively. The twisted stacks of mesogenic luminophores induced by a chiral dopant or surface treatment of the EML lead to a high dissymmetry g factor.4,7,9–13 Here, linearly polarized (LP) light emitted from locally aligned conjugate polymers experiences birefringence passing through the twisted stacks of the mesogenic polymer and the CP light is extracted. In the twisted stacks of mesogenic luminophores, the dissymmetry g factor was governed by several parameters such as the total twisted angle, birefringence, and the location of the electron–hole recombination zone (the emission zone) within the EML.7,11–14 However, the twisted mesogenic luminophores on the well-controlled alignment layer produce the phase retardation of ambient light and consequently break the anti-reflection condition to eliminate reflection of ambient light.7 Furthermore, the relationship between the dissymmetry factor and molecular ordering, directly related to the phase retardation of an EML, has not been fully analysed yet.
In this work, we systematically analyse the dissymmetry factors with respect to molecular ordering based on the Mueller matrix analysis of photoluminescence (PL) and electroluminescence (EL) processes in a twisted EML. To produce twisted configurations with different total twisted angles, different concentrations of chiral agents were doped into a mesogenic luminophore. The mesogenic luminophore was aligned by the rubbing method15,16 and its molecular ordering on the EML surface was controlled by varying the azimuthal anchoring strength, governed by the contact distance, which is the rubbing depth of the alignment layer for the EML with a rubbing cloth. The birefringent values and molecular order parameters of the EML without the chiral dopant were evaluated according to the contact distance. The dissymmetry g factors of the PL and EL devices with various total twisted angles (controlled by concentrations of the chiral dopant) and various order parameters (controlled by the contact distance) were experimentally measured and theoretically calculated based on the Mueller matrix analysis of the continuously twisted mesogenic sublayers, where the LP light emitted from the locally aligned mesogenic sublayer experiences birefringence passing through the twisted sublayers.7,11–14 The degree of LP light emitted from a certain mesogenic sublayer is governed by the order parameter which is controlled by the contact distance. That is, the degree of LP emission increases with the increasing order parameter. From the relationship between the experimentally measured g factors, produced by the twisted stacks with partially ordered luminophores, and the theoretically calculated g factors, produced by the twisted stacks with perfectly ordered luminophores, we propose that the dissymmetry g factor is proportional to the square of the order parameter of the mesogenic luminophores in the twisted configuration. Also, we achieved a considerable g factor (gEL = 0.37 in the EL process) even at an order parameter of zero satisfying the anti-reflection condition to eliminate the reflection of ambient light. This result, discussed herein, may help enhancing the performance of applications based on circular polarization, including OLEDs.
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Fig. 1 (a) Schematic diagrams of PL and EL emissions in the twisted stacking of F8BT. (b) Chemical structures of R5011 and F8BT. |
Molecular ordering in the mesogenic luminophore can be easily controlled by a surface treatment such as the rubbing process.22,23 In general, the orientational ordering of the conjugated polymer is induced parallel to the rubbing direction by mechanically rubbing the PI surface at the interface with the conjugated polymer. The degree of molecular ordering was controlled by adjusting the contact distance between the rubbing roller and the PI layer from 0 to 40 μm. It should be noted that the large contact distance gives rise to large anchoring strength and thus high molecular ordering since the rubbing cloth strongly contacts the PI surface. Fig. 2a and b show the AFM images and the corresponding Fourier transformation images (insets) of the PI surfaces rubbed by 0 and 40 μm contact distances, respectively. The Fourier transformation images clearly confirm that the rubbing process with a contact distance of 40 μm forms an anisotropic morphology along the rubbing direction but the zero-contact rubbing process still retains the isotropic morphology without any preference in the azimuthal direction. With increasing contact distance, the azimuthal symmetry is broken, and the directional preference gives rise to the azimuthal anchoring energy. Fig. 2c and d show the polarizing microscopy textures of the F8BT films coated on the PI surfaces rubbed with contact distances of 0 and 40 μm, respectively. On the zero-contact rubbed PI layer, the F8BT film presented a dark state under crossed polarizers even though the rubbing direction was rotated by 45° with respect to one of the crossed polarisers (Fig. 2c). On the other hand, on the 40 μm-contact rubbed PI layer, the F8BT film exhibited a bright state under crossed polarisers as shown in Fig. 2d. That is, the F8BT polymers were aligned along the rubbing direction.
The degree of LP emission, corresponding to the order parameter, was investigated by means of the LPPL spectral measurement of the F8BT films prepared with different contact distances since the aligned F8BT emitted LP light along the rubbing direction. Fig. 3a and b show the LPPL spectra and the phase retardation of the 100 nm-thick F8BT film without the chiral dopant on the 40 μm-contact rubbed PI layer. The LPPL spectra were recorded under polarisers parallel (I‖) and perpendicular (I⊥) to the rubbing direction. The polarization ratio (RP) of the LP light is expressed as an intensity ratio of the parallel component to the perpendicular one of LP light (RP = I‖/I⊥) and the corresponding order parameter is written as S = (RP − 1)/(RP + 2).24 In Fig. 3a, the polarization ratio was evaluated to be 12.1 at 546 nm. It should be noted that the polarization ratio depends on the thickness of the EML since the order parameter and birefringence are associated with a penetration distance of an anchoring strength from the alignment layer (AL22636) to the EML.21 By the measurement of the phase retardation as a function of the rotation angle based on the PEM method,17 the birefringence (Δn) of the aligned F8BT on the 40 μm-contact rubbed PI layer was evaluated to be 0.62 at 546 nm as shown Fig. 3b. Fig. 3c and d show the order parameter and Δn as a function of contact distance, respectively (see Fig. S3 in the ESI†). In the case of the zero-contact rubbed PI layer, both the order parameter and Δn approach zero since there is no preference in the azimuthal direction on the PI surface. With increasing contact distance, the anisotropic morphology along the rubbing direction was formed on the PI layer and mesogenic polymers were aligned along the rubbing direction. Both the order parameter and Δn increased gradually with the increasing contact distance as shown in Fig. 3c and d. Consequently, the order parameter becomes linearly proportional to Δn as shown in Fig. 3e.
Now, we investigate the CP luminescence and the corresponding g factors in both PL and EL devices by introducing the chiral dopant (R5011) with high HTP to the achiral mesogenic polymer (F8BT) with different order parameters. As you already know, no CP luminescence was observed in the mesogenic luminophore without the chiral dopant even at a uniformly aligned EML since no optical phase retardation of the LP light was experienced within the birefringent EML. Furthermore, in the case of the randomly aligned EML, the emitted light is not only unpolarized but also does not experience optical phase retardation; thus, the resultant g value is zero.7 In the twisted stacks of the mesogenic luminophores, it is assumed that the F8BT film consists of uniformly twisted sublayers and the LP light emitted from an aligned sublayer within the EML experiences the phase retardation and changes to the other polarized light including CP light propagated through the twisted birefringent F8BT.7,11–14 In the PL process, the LP light is emitted from each sublayer and propagated in the twisted sublayers as shown in Fig. 1a (left image). On the other hand, in the EP process, the LP light is emitted from an emission zone and propagated toward the anode and the cathode by experiencing the phase retardation. The LP light propagated toward the cathode reflects from the metal cathode and experiences additional phase retardation passing through the entire twisted sublayers as shown in Fig. 1a (right image). Here, since the orientational ordering of F8BT was varied by the contact distance, the emitted LP light is partially polarized, and the degree of LP emission is governed by the polarization ratio and the corresponding order parameter. As a result, the measured dissymmetry g factor, representing the degree of CP emission, is governed by the order parameter. When the partially polarized LP light passes through the film, the measured g value is written as g = S·gideal, where gideal represents the ideal dissymmetry factor for fully polarized LP light.7,11
In the first step, we determined the twisted angles of the F8BT films with different concentrations of the chiral dopant by using Stokes parameters (see Fig. S4 in the ESI†). The chiral dopant plays a predominant role in the helical molecular conformation for CP emission.25 The twist angles lie within a certain range and increase consistently only by increasing the concentration of the chiral dopant regardless of molecular ordering (see Fig. S5 in the ESI†). Here, in the case of the zero-contact distance, the estimation of the twisted angle was excluded since there was no birefringence.
Next, the ideal dissymmetry g values (gideal) in the case of fully polarized LP light were calculated as a function of Δn in both PL (gPL) and EL (gEL) processes based on the Mueller matrix analysis at specific twisted angles (see Section 5 in the ESI,† for calculation details). As shown in Fig. 4a and b, the ideal dissymmetry g values were linearly proportional to Δn for various concentrations of the chiral dopant in the case of a perfectly ordered F8BT film. From the linear relationship between the order parameter and Δn as shown in Fig. 3e, the gideal value is linearly proportional to the order parameter (S). As a result, the measured g value is expressed as the following quadratic equation:
g = a·S2 + g0, |
Concentration of R5011 | Twisted angle | Fitted gPL | Fitted gEL | ||
---|---|---|---|---|---|
g 0 | a | g 0 | A | ||
6 wt% | 35.60° | 0.19 | 0.05 | 0.22 | 0.18 |
9 wt% | 51.74° | 0.24 | 0.06 | 0.29 | 0.19 |
12 wt% | 72.66° | — | — | 0.37 | 0.18 |
Fig. 4e shows all PL gPL values (symbols) measured in our experiments, the calculated PL gPL value for S = 1 (solid line), and the extrapolated PL gPL value for S = 0 (dashed line) as a function of the twisted angle. All PL gPL values lie within the boundary region between the solid and dashed lines. It should be noted that the PL gPL value for S = 1 was directly calculated from Stokes parameters after analysing the Mueller matrix without any fitting parameter. The EL gEL values (symbols) averaged over EL devices with various order parameters at the given concentration of R5011 (the given twisted angle) as a function of the twisted angle are shown in Fig. 4f. In the EL process, the EL gEL value was directly calculated by using the Mueller matrix analysis used in the PL process except for the emission zone. The ideal EL gEL values (lines) were calculated for various emission zones as a function of the twisted angle as shown in Fig. 4f. Here, the emission zone (z) is defined as a distance from the TPBi layer (an isotropic medium). Comparing the experimental results and calculations, the emission zone is located at around z = 35.
Footnote |
† Electronic supplementary information (ESI) available: Light efficiency of OLEDs with respect to circularly polarized light; evaluation of the total twist angle; and the Mueller matrix analysis of the g value in the PL and EL cases. See DOI: https://doi.org/10.1039/d2tc04814k |
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