Snapshot Angle-Resolved Spectroscopy and Its Application for Study of Highly Efficient Polariton OLEDs

Abstract

The multifunctional snapshot angle-resolved spectroscopy (ARS) system capable of electroluminescence, photoluminescence, and reflectance measurements for thin film devices is developed based on the k-space imaging technique. Compared with the conventional goniometric ARS system, this snapshot spectroscopy system offers great advantages of rapid and simple measurement, suitable for characterizing thin film devices that are unstable or degraded under long-time or high-power driving conditions, such as OLEDs. We perform a detailed calibration of the snapshot system and show that the measured results closely match with those obtained using a goniometric system. Furthermore, we show the capabilities of the system with application in studying polariton OLEDs. The result provides comprehensive information on the polariton mode dispersion and emission distribution, and shows an effective radiative pumping of the lower polariton branch for high emission efficiency.

1. Introduction

In the past decades, there has been tremendous progress in the research of organic light-emitting diodes (OLEDs) for display applications, which offer great advantages of high brightness, large contrast, and a wide viewing angle [1,2,3]. Recently, the strong light-matter coupling in the OLEDs has also become an interesting topic due to the fascinating polariton phenomena and potential for realizing low-threshold lasing [4,5,6,7]. When developing the novel materials and complex structures of OLEDs, angle-resolved spectroscopy (ARS) is an important technique for characterizing the emission profile and photonic dispersions. The conventional ARS systems are based on a goniometer with a single-arm or dual-arm rotating stage, by which the optical and mechanical components can be arranged to perform various measurements (reflectance, transmittance, emission, scattering, etc.) at controlled angles [8,9,10]. Such goniometric ARS systems are reliable, but the rotation scan is time-consuming. Since most organic devices tend to degrade under long-time or high-power excitation, especially when exposed to an ambient environment, it is not easy to maintain a constant brightness during the rotation scan in a goniometric system to obtain accurate spectral information. On the other hand, the ARS systems based on the k-space (momentum space) imaging technique have been developed for numerous types of research work [11,12,13,14,15]. In these systems, the emission or reflected/transmitted light from the sample is collected by a microscope objective and Fourier-transformed into a far-field image in the Fourier (back focal) plane of the objective, which corresponds to the spectral information at specific angles and can be imaged onto a camera spectrometer to perform ARS measurement with a snapshot, thereby minimizing the errors caused by device instability or degradation over the measuring time. Generally, the k-space imaging configuration can be adapted for spectroscopy, imaging, and time-resolved measurements with high spatial and spectral resolution, and is popularly used in the study of novel photonic phenomena and technologies [16].

Based on the k-space imaging technique, here we present a multifunctional snapshot ARS system for measuring electroluminescence (EL), photoluminescence (PL), and reflectance spectra of thin film devices such as OLEDs. In Section 2, we describe the details of system settings and calibration methods for different measurements, and compare the results of the system with those of the typical goniometric system. In Section 3, we exemplify the application of the snapshot ARS system to explore the polaritonic phenomena of a microcavity OLED, from which the polariton mode dispersion, coupling strength, and angular distribution of the emission can be quantitatively and precisely analyzed. A conclusion is given in Section 4.

2. Angle-Resolved Spectroscopy Setup and Calibration

Figure 1 shows the schematic of the snapshot ARS system, which is constructed in an ambient atmosphere. Essentially, the system consists of an objective, an aperture, a fine slit, a blazed grating, six lenses, and a 2D CMOS digital camera. We use a 0.95 NA microscope objective (Olympus Corp. UPLSAPO 40X2) that allows the maximum half-angle of the cone of light to be collected up to 71.8°. For the EL measurement (Figure 1a), the device is placed in front of the bore of the objective lens. The device emission containing parallel light at different angles will be focused on the Fourier plane of the objective lens, forming a beam in diameter of ~8 mm. The combination of the six lenses is designed to adjust the beam size to match the detective area of the CMOS (13.312 × 13.312 mm², Hamamatsu Photonics K. K.). Every two lenses are separated by the sum of their focal lengths. The objective/lens 1 (f₁ = 10 cm), lens 2 (f₂ = 6 cm)/lens 3 (f₃ = 2.54 cm), lens 4 (f₄ = 10 cm)/lens 5 (f₅ = 25 cm) serve as the first, second, and third afocal systems, repectively. An aperture is placed at the confocal plane of lens 1 and lens 2 to eliminate the stray parallel light far from the optical axis. Importantly, through lens 1 and lens 2, the momentum space information on the Fourier plane of the objective lens is imaged onto a fine slit (50 µm × 1.5 cm) at the confocal plane of lens 2 and lens 3. This fine slit filters the undesired spatial frequency and determines the angle dimension of the CMOS image in the vertical direction (y-axis). A high-quality slit is required to obtain a high angular resolution and prevent significant noise in the CMOS image. The rest of the lenses essentially work to image the spectral information at different angles on the fine slit to the CMOS camera. A blazed grating (384.6 grooves/mm, blaze wavelength of 605 nm, blaze angle of 6.7°) is placed at the confocal plane of lens 5 and lens 6 (f₆ = 18 cm) to produce wavelength dispersion in the horizontal direction (x-axis). The grating normal is oriented at 15° from the optical axis of lens 5 to meet the first-order diffraction condition, and the optical axis of lens 6 is aligned at the averaged diffraction angle (~2.7°) for the wavelengths of 450~650 nm with respect to the grating normal. Lens 6 focuses the diffracted parallel light of different angles onto the CMOS at the rear focal plane, forming a two-dimensional image.

The snapshot ARS system can also be modified for the photoluminescence (PL) and reflectance measurements, as shown in Figure 1b,c. Here we use a UV (365 nm) LED as an excitation light source to generate fluorescence of organic semiconductor thin films. The UV excitation is collimated from an off-axis direction and incident on a dichroic mirror (409 nm cut-on wavelength) placed between lens 1 and the aperture, which allows the excitation wavelength to be reflected toward the sample and the PL band to be transmitted toward the CMOS. A OD 4 longpass filter (400 nm cut-on wavelength) is inserted between the dichroic mirror and aperture to reject the stray UV excitation. On the other hand, for the reflectance measurement, we use a broadband white LED (5700 K) as the light source. The LED emission is focused by an aspheric lens from an off-axis direction and incident on a 50/50 beam splitter placed between the objective lens and lens 1. The beam splitter serves to deflect partial LED emission toward the test sample and to pass through half of the light reflected by the sample toward the CMOS. In this configuration, since the incident light on the sample cannot be directly measured by the CMOS, the reflectance of the sample is measured relative to a reference standard with high reflectivity.

Considering that most organic devices exhibit emission/transmission from the glass substrate, all the CMOS images in this work were measured with the glass side of the sample in direct contact with the objective lens. This operation is also simple and repeatable. The device was prepared with the active area larger than the bore of the objective lens (2 mm in diameter) to ensure that the parallel light of different angles could be captured from the entire bore region. The time of the measurement with the snapshot system is determined by the integration time of the CMOS camera, which was set to 1 s for all the devices.

After the system was built, we sequentially calibrated the wavelength and angle dimensions and the relative intensity of the 2D CMOS image (2048 pixels × 2048 pixels). The following mainly introduces the calibration procedure based on the EL measurement and the different parts for the PL and reflectance measurements.

First, the wavelength dimension (x-axis in the CMOS image) is calibrated using the Hg-Ne lamp (Newport, 6034) and 473 nm, 532 nm, and 635 nm laser diodes as the light sources. The Hg-Ne lamp placed vertically in front of the objective lens exhibits three straight lines perpendicular to the x-axis in the CMOS image, from which the x-axis pixel values corresponding to the characteristic wavelengths of 546 nm, 577 nm, and 579 nm can be extracted (Supplementary Materials Figure S1a). On the other hand, the lasers are incident into the system at 0° and exhibit a beam profile in the CMOS image, where the x-axis pixel value of the most intense position corresponds to their respective wavelengths (Figure S1b). By fitting all the extracted pixel values and the corresponding wavelengths, we obtain a linear relationship between pixel value and wavelength, and determine the wavelength range of the system from 449 nm to 641 nm (Figure 2a). From the FWHM of the spectral line, the wavelength resolution of the system is estimated to be ~1.8 nm. Then, we calibrate the angle dimension (y-axis in the CMOS image) by vertically changing the incident angle of a laser beam into the system. The y-axis pixel value of the beam profile as a function of incident angle is recorded. As the result shown in Figure 2b, the relationship between the pixel value and the incident angle can be fitted to a third-order polynomial, and the angle detected by the system ranges from −60° to 60° (0° is defined as being parallel to the optical axis of the system). Moreover, 473 nm, 532 nm, and 635 nm lasers follow the same polynomial curve, suggesting that the pixel value in the angle dimension is independent of wavelength.

Next, the relative intensity in the CMOS image needs to be calibrated due to the aberration of the system and the different wavelength responses of optical components such as the objective lens, blazed grating, and CMOS sensor. Intensity calibration requires correlating the raw CMOS image with the angle-resolved spectra acquired from other standard measurements. Considering the lower response of the CMOS sensor at wavelengths of <500 nm, here we employ a blue-green OLED [ITO/m-MTDATA (30 nm)/NPB (30 nm)/DPVBi (11 nm)/Alq₃ (50 nm)/LiF (1 nm)/Al (150 nm), see Figure 2c] with a broadband EL spectrum to cover the detected wavelength range and high emission intensity and stability as a calibration light source. We use m-MTDATA [4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)-triphenylamine] as a hole-injection buffer layer [17], which facilitates hole injection from the ITO anode to the hole transporting layer, NPB [N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine)]. DPVBi [4,4′-Bis(2,2-diphenylvinyl)biphenyl] acts as a blue emitting layer, whereas Alq₃ [Tris(8-hydroxyquinoline)aluminum(III)] acts as an electron transporting and green emitting layer. LiF is used as an electron injection layer from the Al cathode. Figure S2a shows the raw CMOS image of the blue-green OLED emission. On the other hand, we measure the angle-resolved EL spectra using a goniometric spectroscopy system as a calibration reference. The detail of the goniometric system is illustrated in the Supplementary Materials. Figure 2d,e show the angle-resolved EL spectra and radiation pattern of the blue-green OLED measured with the goniometric system, revealing the peak wavelength of 489 nm and an almost perfect Lambertian distribution. To compare the results of the two systems, we convert the CMOS image and angle-resolved spectra into the same matrix form and normalize their respective maximum intensities to unity. Finally, by dividing the angle-resolved spectra with the raw CMOS image, we obtain the EL intensity correction matrix (EL-ICM), as shown in Figure S2b. For other devices under test, their raw CMOS images can be multiplied by EL-ICM and then transformed into the corresponding angle-resolved spectra.

Regarding the calibration of the PL and reflectance measurements, we find that using different light sources and additional optical components does not obviously change the wavelength and angle dimensions, but affects the intensity distribution of the CMOS image. The PL intensity is calibrated in a similar manner to the EL, based on the angle-resolved fluorescence spectra of a reference sample (PFO: 1 wt % F8BT blend film) measured by the goniometric system. This reference sample consists of a blue light-emitting host polymer, PFO [Poly(9,9-dioctylfluorene)], and a small weight ratio of a green light-emitting guest polymer, F8BT [Poly(9,9-dioctylfluorene-alt-benzothiadiazole)], which can produce a broadband PL spectrum containing a minor emission from PFO (400–500 nm) and major emission from F8BT (>500 nm) due to incomplete Förster energy transfer. Figure S3a,b show the raw CMOS image of the PFO:F8BT blend film and the angle-resolved spectra measured with the goniometric system. By dividing the two, we obtain the PL intensity correction matrix (PL-ICM) (Figure S3c). On the other hand, the reflectance is measured relative to a reference standard and does not require a careful intensity calibration. We use a silver mirror coated on glass as a reference standard, which has high reflectivity in the visible light and wide angle ranges. By dividing the CMOS image of the reflected light from the test sample with that from the silver mirror, and then correcting with the reflectance of the silver mirror measured by the goniometric system, the reflectance of the test sample can be obtained.

To validate the self-consistency and accuracy of the calibration, we use the snapshot ARS system to measure the EL, PL, and reflectance of a series of prototype samples (NPB/Alq₃ OLED, DCM doped PMMA film, DBR reflective mirror), and compare with the measurements using the goniometric system. The NPB/Alq₃ OLED is configured as glass/ITO (150 nm)/MoO₃ (10 nm)/NPB (50 nm)/Alq₃ (50 nm)/LiF (1 nm)/Al (100 nm). The red dye molecule, DCM [4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran], is doped in the PMMA (polymethylmethacrylate) film at a concentration of 0.5 wt %. Such a DCM doped film has been used as a gain medium for optically pumped lasers [18]. The distributed Bragg reflector (DBR) with a central wavelength of ~600 nm is deposited using electron beam evaporation. Figure 3 shows the measurement results of the two systems. All the angle-resolved spectra extracted from the calibrated CMOS images match well with the results of the goniometric system, and the root-mean-square deviations are less than 2%. In particular, a close look can be taken at the EL spectra of the NPB/Alq₃ OLED, which show no obvious angular dependence of peak wavelength and spectral shape, but only a decreased intensity at higher viewing angles. This indicates a weak microcavity effect in the designed OLED, consistent with other studies with similar device thicknesses [19,20]. In addition, the measured EL spectra are in good agreement with the simulation using the finite difference time domain (FDTD) method (see Figure S4), which also supports the high accuracy of the system calibration.

3. Characterizations of Polariton OLED

We further apply the snapshot system to investigate a highly efficient polariton OLED. Here the polariton OLED is constructed based on an intracavity pumping architecture, which has separate materials for strong coupling and radiative pumping [7,21]. This intracavity pumping scheme can resonantly populate the lower polariton (LP) branch via radiative pumping by the weakly-coupled emitter without the need for direct excitation of the strongly-coupled medium; therefore, the losses such as exciton–exciton annihilation and slow polariton relaxation [22], or perturbation of strong coupling under high current density can be circumvented. Figure 4a shows the architecture of the polariton OLED, containing an absorbing J-aggregate film as the strongly coupled medium and a thermally activated delayed fluorescence (TADF) OLED as the weakly coupled pumping emitter in a λ-thick microcavity. A 40 nm silver film deposited on glass serves as the emission side mirror, whereas a 150 nm silver cathode of the TADF OLED serves as the top mirror. The J-aggregate film is fabricated based on a cyanine dye molecule, DEDOC [5-chloro-2-(2-[(5-chloro-3-(3-sulfopropyl)-2(3H)-benzoxazolylidene)methyl]-1-butenyl)-3-(3-sulfopropyl)-benzoxazolium inner salt, sodium salt], with a thickness of 9 nm and a narrow absorption J-band at the wavelength of 546 nm [7]. The TADF OLED is configured as Au (15 nm)/MoO₃ (10 nm)/TAPC (40 nm)/mCP:2 wt % TXO-TPA (40 nm)/TmPyPb (40 nm)/LiF (1 nm)/Ag (150 nm). TXO-TPA [2-(4-(diphenylamino)phenyl)-10,10-dioxide-9H-thioxanthen-9-one] is used as the emission dopant of TADF OLED [23] due to its high emission efficiency and peak wavelength of 580 nm close to the low energy state of LP mode (λ~587 nm), favorable for radiative pumping. mCP [1,3-Bis(N-carbazolyl)benzene] is selected as the host material that enables efficient energy transfer to TXO-TPA. TAPC [1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane] and TmPyPb [1,3,5-Tris(3-pyridyl-3-phenyl)benzene] act as the hole and electron transporting layers, respectively. MoO₃ and LiF are used as the hole and electron injection layers from the Au anode and Ag cathode, respectively. Figure S5 shows the absorption and EL spectra of the DEDOC J-aggregate film and the TADF OLED, respectively. The cavity is designed to position the J-aggregate film and the OLED emissive layer at the two antinodes of the cavity field to maximize the coupling strength and emission, while placing the Au anode at the node to minimize the absorption, as in the electric field distribution shown in Figure 4a. To meet this design condition, two ~λ/4 spacer layers (sputter-deposited SiO₂ and spin-coated PMMA) are introduced below and above the J-aggregate film, and the total thickness of the TADF OLED is controlled to ~λ/2. Figure 4b shows the optoelectronic characteristics of the TADF OLED with the Au anode and the optimal polariton OLED. Compared with the TADF OLED, the polariton OLED has a similar current density but a three-times-lower EL intensity due to cavity modulation. As a result, the maximum external quantum efficiency (EQE) can reach 9–10% in the TADF OLED and about 3% in the polariton OLED.

Figure 4c-g present the angle-resolved reflectance, EL, and PL spectra of the polariton OLED measured by the snapshot ARS system. A polaritonic feature is evidenced by the reflectance spectra (Figure 4c,f), which exhibit an anticrossing angle dispersion of two resonant dips moving toward and away from the peak absorption energy of the J-aggregate film (EX = 2.27 eV), corresponding to the lower and upper polariton (LP/UP) modes generated from the exciton-photon coupling. By fitting the LP and UP dispersion curves based on a two-mode coupled oscillator model with the dispersionless exciton mode (EX) and the cavity mode (EC) dispersion relation [24], a Rabi splitting energy of 190 meV can be deduced. On the other hand, the angle-resolved EL and PL spectra essentially follow the LP mode dispersion and show very similar intensity distributions (Figure 4d,e). It can be observed that the emission intensity of the polariton OLED mainly comes from the low energy states of the LP mode at low angles, suggesting an effective radiative pumping of the photon-like LP mode with the TADF OLED. The maximum intensity is distributed at 0~5°. At higher angles, the LP mode becomes more exciton-like and overlaps with the high energy tail of the TADF OLED spectrum, resulting in a significantly decreased intensity, which drops to <20% of the maximum intensity at 60°. Intriguingly, a relatively weak emission from the UP mode is also observed (Figure 4g), which can be attributed to a slight overlap between the tail of the TADF OLED spectrum and the UP mode dispersion. Compared with the emission from the LP mode, the emission intensity from the UP mode appears to decrease more slowly with angle—only by half from 0° to 60°. This intensity variation may reflect that the UP mode has a more photon-like behavior but overlaps less with the TADF OLED spectrum as the angle increases. Based on this result, we conclude that in such an intracavity pumping device, the distribution of emission intensity along the LP and UP mode dispersion strongly depends on the degree of spectral overlap with the emission states of the pump OLED. In order to achieve a high emission efficiency, there must be a good match between the low energy state of the LP mode and the peak emission of the pump OLED [7]. From the angle-resolved EL spectra we extract the radiation pattern of the polariton OLED, showing the directional emission at a limited angle (see the inset in Figure 4g and the corresponding EL image). With the aid of the snapshot ARS measurements, it is possible to study a variety of novel materials for higher EQE and coupling strength, and also explore polariton lasing via high-energy excitation.

4. Conclusions

We have developed a multifunctional snapshot ARS system that enables simple and instant measurements on the EL, PL, and reflectance of thin film devices. Based on a detailed calibration, we have shown that the measurements of the snapshot system are consistent with the results obtained using a conventional goniometric system, and can be corroborated by FDTD simulation. By applying the snapshot system for spectral characterization of a designed polariton OLED, we gained a comprehensive understanding of the polariton mode dispersions and emission distribution along the modes, and demonstrated an effective radiative pumping of the lower polariton branch for the high device efficiency. Overall, this snapshot ARS system can capture accurate spectral information over a wide-angle range in a second without a strict requirement for sample stability; therefore, it may serve as a powerful tool for investigating a versatile set of thin film devices, particularly useful for developing the photonic devices that operate in high power conditions, such as lasers.