1. Introduction

Light-matter interaction in organic microcavities has attracted enormous research interest over the last two decades [1–3]. The strongly-bound Frenkel excitons in organic semiconductors offer great promise for achieving strong and even ultrastrong coupling at room temperature. In the strongly coupled microcavities, the exciton-photon coupling rate becomes faster than the decoherence or dissipation of two modes, resulting in Rabi oscillation of hybrid exciton-photon states and generation of different polariton mode branches in the cavity dispersion with a minimum energy separation at resonance, known as Rabi-splitting [4,5]. Since the first discovery of organic polaritons by Lidzey et al. [1], many fascinating phenomena have been demonstrated in the strong coupling regime, such as high-temperature polariton condensation and superfluid transition [6–9], stimulated scattering [10], and optically pumped polariton lasing [11–13]. More recently, organic microcavities in the ultrastrong coupling regime (Rabi-splitting exceeds 20% of the exciton transition energy [14]) have been further achieved, bringing the prospect of non-classical physics and innovative device applications [15–19]. It is shown that strongly coupled materials are not specifically limited to the narrow-band absorbing molecules such as J-aggregate dyes [20,21]. Other organic materials with broad transition linewidths and large oscillator strengths can be more favorable for ultrastrong coupling, thus opening up a wider range of material options for polariton research [22]. Furthermore, the use of metallic mirrors in organic microcavities has great benefits for reducing the mode volume to increase coupling strength, and can also serve as electrodes to develop optoelectronic devices. With this simple metal-clad microcavity structure, ultrastrongly coupled organic light-emitting diodes (OLEDs) have been demonstrated based on several classes of fluorescent molecules and polymers, such as fluorene-based oligomer [23], squaraine dye [24], and coumarin dye [25]. On the other hand, a microcavity-integrated light-emitting transistor (LET) operating in the ultrastrong coupling regime has also been reported with a high-mobility diketopyrrolopyrole copolymer [26]. These ultrastrongly coupled devices exhibit unique electroluminescence (EL) features in terms of very low angular dispersion and narrow linewidth, making them attractive for applications requiring color-pure emission and for the study of electrically-pumped polariton lasers.

Despite these advances in polariton light-emitting devices, however, their performance is still far below that of conventional OLEDs. Due to a slow exciton-phonon scattering in organic materials, efficient polariton emission relies on radiative pumping of the bottom of lower polariton (LP) branch by emitting photons [27]. This essentially requires the strongly coupled emitters with not only good EL capabilities, but also high absorbance and moderate Stokes shift for polariton mode generation and radiative pumping. Device design is also related to multiple factors such as charge recombination balance and cavity detuning. These material requirements and design factors make it challenging to achieve high EL efficiencies, especially for planar LETs involving higher ambipolar mobility and more complex device geometry. Thus far, the highest external quantum efficiency (EQE) reported for ultrastrongly coupled OLED and OLET are 0.2% and ∼10⁻⁴%, repectively [25,26]. To accelerate the industrial applications of organic polariton devices and the realization of electrically-pumped lasers, there is an urgent need to explore more organic materials and simple device designs that enable higher EL efficiency and greater coupling strength.

In this paper, we present a systematic study of the polariton characteristics and resulting light emission properties based on the poly(1,4-phenylenevinylene) copolymer, Super Yellow (SY), in metal-clad microcavity devices, and discuss the factors influencing the device performance. SY is a well-known luminescent polymer used for LEDs and lasers [28,29], but has not yet been explored in polaritonic research. A bulk SY film features broadband high absorbance, proper Stokes shift, and high luminescence quantum yield, making it well suited as a strongly coupled emitter. The film thickness can also be controlled by simple solution processing to tune the coupling strength and optimize the electro-optical properties. Here, we demonstrate the occurrence of ultrastrong coupling and polariton emission in a series of SY-based optical microcavity and optoelectronic devices, including LED and LET. In particular, we realize a novel inorganic/organic hybrid polariton LET based on the vertical integration of a high-mobility ZnO transistor and a SY LED in a microcavity. Compared with conventional planar LETs, vertical LETs [(V)LETs] with source electrode located between outer gate and drain electrodes have much reduced channel lengths, and offer great advantages of low driving voltage, high switching frequency, and surface emission with large aperture ratios, which benefit commercial display applications [30,31]. More importantly, this hybrid VLET geometry allows flexible combination and independent optimization of organic emitters and inorganic transistors, thereby enabling to relax the high mobility requirement of strongly coupled emitters and boost EL performance to a level close to that of corresponding OLEDs.

The paper is constructed as follows. In Sec. 2 we describe the experiment methods. In Sec. 3 we investigate the exciton-photon coupling and photoluminescence (PL) of optical microcavities with different SY film thicknesses. In Sec. 4 we study polariton SY LEDs and compare with uncoupled devices. The effect of electron injection layer and cavity design on emission efficiency is discussed. In Sec. 5 we demonstrate a high-performance polariton VLET. A conclusion is given in Sec. 6.

2. Experiment methods

SY (M_w= 1,950,000 g/mol, purchased from Merck) was spin-coated from toluene at a controlled concentration to vary the film thickness. Reflectivity spectra at an incident angle of 5° were measured using a Hitachi UH4150 UV-VIS/NIR spectrophotometer. Angle-resolved reflectivity spectra over 5° were measured using a goniometric spectroscopy system (MFS-6300, Hong-Ming Technology). Simulated reflectivity spectra were obtained with Essential Macleod software. Angle-resolved PL spectra were measured using a k-space imaging spectroscopy in the ambient atmosphere [32]. This spectroscopy employs a microscope objective to focus UV excitation (365 nm LED) on the sample and collect PL containing parallel light at different angles, and the Fourier plane of the objective is imaged onto a 2D CMOS camera that allows snapshot capture of spectral information over a wavelength range of 450-640 nm and an angular range of 0-60°. On the other hand, the angle-resolved EL spectra were measured using a goniometric spectroscopy system in a nitrogen-filled glove box. The device emission at a specific angle was collected by a collimating lens, which was mounted on a motorized rotation stage with a 7 cm radius of gyration and coupled to an optical fiber connected with a spectrometer (HR4000, Ocean Optics). The optoelectronic characterizations of the LED and LET devices were performed using an Agilent B1500A semiconductor parameter analyzer connected with a Si photodiode (S1227-1010BQ, Hamamatsu) in the glove box. The EQE was extracted from the measured photocurrent, EL spectrum, and photodiode’s sensitivity.

3. Exciton-photon coupling and photoluminescence in SY microcavity

To understand the phenomena of exciton-photon coupling in the SY microcavity and the optimal conditions for radiative pumping, we first investigate the reflectivity and PL emission of the microcavities filled with different SY film thicknesses. Figure 1(a) shows the studied all-metal SY microcavity with Ag mirrors, where the SY film thickness (x nm) is controlled in the range of 70-120 nm, and the bottom and top Ag mirrors are 150 nm and 30 nm, respectively. Figure 1(b) shows the reflectivity of different SY microcavities measured at an incident angle of 5°, which approximates the reflectivity at normal incidence 0°. It can be observed that all the microcavities exhibit two resonant dips above and below the absorption peak energy of exciton (EX = 2.786 eV), indicating the formation of upper and lower polariton (UP/LP) modes generated from strong exciton-photon coupling. As the film thickness increases, the UP mode moves towards the exciton mode with a decreased resonant dip, while the LP mode moves away from the exciton mode with an increasing resonant dip relative to the UP mode. This measurement is further corroborated by optical simulation. We substitute the (n,k) values of SY film measured by the ellipsometry in the transfer matrix method and use the film thickness as a fitting parameter to calculate the zero-degree reflectivity (see Supplement 1, Figs. S1 and S2). The simulated reflectivity spectra are highly consistent with the measured spectra, with only a 1-2 nm deviation in the dip energy for the UP and LP modes. This confirms the high accuracy of the (n,k) values and allows determination of all the SY film thicknesses to be x = 75, 87, 106, and 119 nm. From the peak k value of 0.58 at λ=445 nm (EX = 2.786 eV), we derive the peak absorption coefficient α=4πk/λ∼1.6 × 10⁵cm^-1 of the SY film. Figure 1(c) summarizes the UP and LP mode energies versus film thickness, i.e. cavity length, extracted from the measured and simulated reflectivity spectra at 0°, as well as the bare cavity mode (EC) calculated from the optical thickness of the SY microcavity. The exciton-photon hybridization in the UP and LP modes is clearly observed for different microcavities, with the cavity detuning (defined as the difference between EC and EX at 0°) varying from 114 meV to -486 meV as the cavity length increases from 75 nm to 119 nm. As will be verified later, the microcavity with x = 106 nm has zero-degree LP mode energy close to the PL emission peak of the bare SY film [see spectrum in Fig. 1(c)], which favors efficient LP mode emission.

 

Fig. 1. (a) All-metal SY microcavity and chemical structure of SY polymer. (b) 5-degree reflectivity spectra of the SY microcavities filled with different film thicknesses, all showing the appearance of UP and LP modes above and below the exciton mode EX. The reflectivity of a pure silver mirror (x = 0) is also compared and reveals a cut-off edge at wavelengths of <350 nm. (c) The UP and LP mode energies extracted from the reflectivity dips in (b) and simulated using the transfer matrix method (solid lines) for different SY film thicknesses, along with the EX mode corresponding to the SY absorption peak and the uncoupled cavity mode (EC) dispersion simulated using the transfer matrix method (dashed lines). The absorption and PL spectra of a bare SY film are also shown for comparison with the polariton mode energies.

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We then measure the angle-resolved reflectivity and PL spectra for a series of SY microcavities. By fitting the dip energy of angle-resolved reflectivity with the Hopfield Hamiltonian and dispersionless exciton mode [17], LP/UP mode and uncoupled cavity mode (EC) dispersions can be deduced (for detailed description and results, see Supplement 1, Figs. S3-S6). To obtain an accurate fit, we also simulate the angle-resolved reflectivity to compare with the measured dip energy and to help resolve the UP mode beyond the polarizer cutoff wavelength at ∼ 375 nm. Figure 2(a) shows the dispersion curves superimposed with the angle-resolved PL intensity maps for TE- and TM-polarizations. All the fitting parameters (cavity energy at normal incidence EC₀, effective refractive index n_eff, and Rabi splitting energy $\hbar \mathrm{\Omega}$) are listed in Table 1. Regardless of TE- and TM-polarizations, thicker SY microcavities tend to have lower EC₀ and n_eff due to longer cavity lengths, as well as larger $\hbar \mathrm{\Omega}$ indicative of stronger coupling. When the SY film thickness is varied between 75-119 nm, the extracted values of $\hbar \mathrm{\Omega}$ for both TE- and TM-polarizations are similar and range between 0.86-1.1 eV, corresponding to 31-39.5% of the bare exciton energy (coupling ratio, defined as $\hbar \mathrm{\Omega}/\textrm{EX}$). This demonstrates the occurrence of ultrastrong coupling ($\hbar \mathrm{\Omega}/\textrm{EX}$>20%) in a sufficiently thick SY microcavity. The excitonic and photonic fractions of the LP/UP modes calculated from the Hopfield Hamiltonian also reveal a significant hybridization over a wide angle in various microcavities [see Supplement 1, Figs. S3(d)-S6(d)]. Essentially, TE- and TM-polarized dispersions have similar EC₀ and LP/UP mode energies at small angles, but higher n_eff of TM-polarization leads to lower EC at large angles and flatter dispersion compared with TE-polarization [15].

Table 1. Fitting parameters for the UP/LP dispersions in Fig. 2(a)

View Table

 

Fig. 2. (a) TE- and TM-polarized angle-resolved PL intensity maps for the microcavities filled with different SY film thicknesses. Also shown are the nondispersive exciton mode (EX = 2.786 eV) and the LP, UP, and cavity mode (EC) dispersion curves extracted from a Hopfield Hamiltonian fit to the dip of the angle-resolved reflectivity. (b) Normalized zero-degree TE-polarized PL spectra in various microcavities. (c) Relative intensity of LP mode emission to bare film emission over the range of 0-60° and comparison with a typical PL spectrum of SY film.

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On the other hand, PL intensities in various microcavities strongly depends on the low-energy state of LP mode relative to the exciton emission peak energy [∼2.18 eV, see PL spectrum of bare SY film in Fig. 1(c)]. In the shortest microcavity with x = 75 nm, where the LP mode is located at the edge of the high-energy tail of exciton emission, the PL intensity is relatively weak, mainly from exciton emission leakage rather than LP mode. In the thicker microcavities with the LP mode closer to the exciton emission peak, the PL distribution tends to follow the LP mode dispersion with a peak intensity at ∼0°, indicative of effective radiative pumping by exciton emission without a relaxation bottleneck. This spectral change with LP mode energy is similar in TE- and TM-polarized emission. For more clarity, in Fig. 2(b) we show the TE-polarized PL spectra at 0° for various microcavities. The polariton mode emission in the thicker microcavities (x = 87-119 nm) has a FWHM of 50-75 meV, much narrower than the bare exciton emission (345 meV). Among them, the PL intensity is highest in the microcavity with x = 106 nm, which has the best match between LP low-energy state and exciton emission peak. To further exclude the effect of different film thicknesses on the PL intensity, we normalize the total emission intensity along the LP mode dispersion to the bare film emission of the corresponding thickness over the range of 0-60° under the same excitation (defined as relative intensity). Here, we prepared the bare SY films on a 150 nm Ag mirror for normalization, and their angle-resolved PL intensity maps are shown in Supplement 1, Fig. S7. Since the k-space imaging spectroscopy only measured emission below 640 nm which accounts for approximately 90% of the spectral integration of the bare SY film, the total PL intensity of the bare film was corrected by a factor of 1.1. As the result shown in Fig. 2(c), the relative intensity clearly follows the energy distribution of bare exciton emission, demonstrating the importance of matching the LP low-energy state and exciton emission peak for effective radiative pumping. Note that in the optimal microcavity with x = 106 nm, the relative intensity can reach a maximum value of ∼80%, suggesting that most of the emissive excitons can be converted to polariton emission. In addition to good spectral overlap, the photon-like nature of LP mode with very low resonant dips at low-energy states may also facilitate efficient polariton emission. Furthermore, no significant difference between TE- and TM-polarized emission intensities is observed in various microcavities, although TM-polarized emission appears to extend to larger angles due to the flatter dispersion. This suggests that polariton emission is not particularly polarized, probably related to the amorphous conformation of the SY film [28].

4. Polariton SY LED

Following the optical study in Sec. 3, we further introduce charge injection layers in the SY microcavity to fabricate polariton LEDs. As shown in Fig. 3(a), the polariton SY LED is constructed with an inverted configuration in a λ/2-thick microcavity. Bottom 150 nm and top 30 nm Ag mirrors serve as the cathode and anode, respectively. The 80 nm SY film is designed to ensure a large coupling strength. A 15-35 nm nanocrystalline ZnO and polyethyleneimine (ZnO:PEI) blend film is used as the electron injection layer [33], while a 10 nm MoO₃ film is used as the hole injection layer. Since the addition of the injection layers in a SY microcavity has mixed effects on cavity detuning and EL performance, for simplicity, we fix the SY and MoO₃ films here and only modify the ZnO:PEI blend film for device optimization. The ZnO:PEI blend film was prepared by dissolving zinc acetylacetonate hydrate and PEI in ethanol at a controlled concentration, followed by spin-coating and annealing at 120°C for 90 min in the ambient environment. As a preliminary study, we investigated a series of ZnO:PEI blend films in uncoupled SY LEDs (with ITO cathodes) to obtain the optimal parameters for electron injection and EL efficiency without involving cavity modulation. We found that increasing the PEI weight ratio from 0.25 to 0.5 in ZnO at a fixed film thickness of 15 nm leads to a monotonic decrease in current density and luminance, but an intermediate PEI ratio of 0.33 yields the highest EQE of 4% [see Supplement 1, Figs. S8(a) and S8(b)]. This trend can be interpreted by the dual effect of blending PEI, which not only lowers the electron injection barrier from ZnO to SY, but also increases the insulating property of the blend film, both affecting the balance of electron and hole currents [34]. An appropriate PEI ratio is thus required to optimize the EL efficiency. We further found that increasing the blend film thickness from 15 nm to 35 nm at a fixed PEI ratio of 0.33 results in a slight decrease in current density and luminance; however, the maximum EQE is weakly dependent on the film thickness and remains at around 4% [see Supplement 1, Figs. S8(c) and S8(d)]. Therefore, by using the blend film with a PEI ratio of 0.33 in this thickness range, we can flexibly adjust the cavity length of polariton LEDs while ensuring bulk EL performance at optimal efficiency. Figure 3(a) illustrates the structure of the polariton SY LED with a 15 nm ZnO:PEI blend film and the electric field distribution at the exciton emission peak wavelength [λ=566 nm, see the EL spectrum of uncoupled SY LED in Fig. 3(c)] calculated by the finite-difference time-domain (FDTD) method. The antinode of the cavity field is located in the SY film bulk to maximize the exciton-photon coupling.

 

Fig. 3. (a) Schematic of a polariton SY LED consisting of a 150 nm Ag as the bottom mirror, 15 nm ZnO:PEI blend film, 80 nm SY, 10 nm MoO₃, and 30 nm Ag as the top mirror in a λ/2-thick microcavity. The electric field superimposed on the microcavity was calculated by the FDTD method at λ=566 nm. (b) J-V-L characteristics and extracted EQEs of the polariton SY LEDs with different ZnO:PEI blend film thicknesses. (c) Zero-degree EL spectra of different polariton SY LEDs in (b) and a typical uncoupled SY LED as a reference. The inset shows the ratio of the maximum EQE of polariton LED to that of uncoupled device. (d) TE- and TM-polarized angle-resolved reflectivity and EL intensity maps of the polariton SY LED labeled “B”, along with the LP, UP, and EC dispersion curves extracted from a Hopfield Hamiltonian fit to the reflectivity dip energy. (e) EL spectra at selected angles in (d).

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Figure 3(b) shows the J-V-L curves and extracted EQEs of the polariton SY LEDs with 15, 25, 35 nm ZnO:PEI blend films (labeled as A, B, C, respectively). The current density decreases with increasing blend film thickness as in uncoupled SY LEDs. However, the luminance first increases at the film thicknesses of 15-25 nm and then decreases at 25-35 nm. Consequently, the EQE is lowest in device A (0.96%) and reaches a maximum in device B (2.8%), and then reduces again in device C (1.94%). Since all the devices have similar bulk EL efficiencies, this EQE variation may be related to the degree of energy matching between LP mode and bare exciton emission. Indeed, as the zero-degree EL spectra shown in Fig. 3(c), all the devices exhibit narrowband LP mode emission with a FWHM of 65-75 meV, but device B has the best match of the LP mode energy to the exciton emission peak (LP₀-EMI_peak< 10 meV), thus enabling efficient radiative pumping to yield the highest EQE. In contrast, devices A and C have zero-degree LP mode energies farther away from the exciton emission peak, resulting in a lower radiative pumping efficiency and reduced EQE. By normalizing the maximum EQE with that of the corresponding uncoupled SY LED in Fig. S8(d), the relative EQE can reach a highest value of 70% in device B [see the inset of Fig. 3(c)]. This result is consistent with the relative PL intensity of ∼80% obtained in the optimal SY microcavity. Figure 3(d) summarizes the angle-resolved reflectivity and EL intensity maps of device B for TE- and TM-polarizations. The reflectivity and EL spectra at selected angles can be seen in Supplement 1, Fig. S9, and Fig. 3(e). The reflectivity spectra reveal a negatively detuned LED cavity and weak angular dispersion of LP and UP modes. From the Hopfield Hamiltonian fit to the polariton mode dispersion, we deduce that the Rabi splitting energies for TE- and TM-polarizations are 0.94 eV and 0.9 eV (33.7% and 32.3% of EX), respectively, confirming the SY LED operation in the ultrastrong coupling regime. On the other hand, the angle-resolved EL spectra indicate that the SY LED emission mainly comes from the LP low-energy state at low angles, and no UP mode emission is observed. Both TE- and TM-polarized emissions exhibit weak angular dispersion following the LP mode, with a peak energy shift of less than 50 meV from 0° to 60°. However, their intensity distributions differ slightly at large angles. For TE-polarized emission, the intensity decreases more rapidly at large angles as the LP mode is tuned to higher energies to overlap with the high energy tail of exciton emission. By contrast, TM-polarized emission exhibits higher intensity at large angles due to better overlap of the flatter LP dispersion with the exciton emission peak. Overall, these angle-resolved measurements provide clear evidence of nearly dispersionless polariton emission from the ultrastrongly coupled SY LED, and demonstrate that an optimized cavity design allows efficient radiative pumping over relatively wide angles along the LP mode, which may account for the high conversion from exciton to polariton emission and high EQE.

5. Polariton VLET

Next, we develop an inorganic/organic hybrid polariton VLET by integrating a ZnO transistor and a SY LED in a λ-thick microcavity. As the device architecture shown in Fig. 4(a), the SY LED is stacked on the ZnO transistor, where the source electrode is encapsulated with an insulator to confine the current path from the bottom source surface to the top drain. This hybrid VLET geometry is adapted from the previous studies of organic vertical transistors [35,36]. The operation of VLET relies on the interaction between vertical gate–source and drain–source fields, so that electrons injected from the source electrode can be gate-modulated and pulled toward the SY layer by the drain–source field to recombine with holes injected from the drain electrode. A 150 nm Ag film deposited on glass serves as the gate and bottom mirror. The gate dielectric (30 nm Al₂O₃/30 nm HfO₂ bilayer) and a 50 nm ZnO layer were deposited using the atomic layer deposition technique, which yields a high areal capacitance of 150 nF/cm² and a high electron mobility of ∼10 cm²/Vs for the ZnO transistor (see transfer characteristic in Supplement 1, Fig. S10). Our recent study has shown that the high transistor mobility is essential to support long-range lateral transport of electrons away from source injection and generate strong surface emission in this type of VLET [37]. We patterned the ZnO layer into a square with area A = 1 × 1 mm², which defines the maximum electron distribution range and emission area of the VLET. As a key element for VLET operation, the source electrode (40 nm Ag) encapsulated with 250 nm SiO_X was fabricated using a self-aligned photolithography process [37], which enables complete SiO_X coverage without reducing the source aperture as the channel region. The source pattern also has an impact on the current density and distribution in the VLET. To achieve strong and uniform emission over the ZnO pattern, we designed the source pattern with 9 periodic stripes on ZnO, each stripe width of 10 µm, and the bare aperture width of 90 µm between two adjacent stripes [Fig. 4(b)]. Therefore, the effective area of the VLET (A_eff= 0.91 mm²) can be defined by subtracting the total area of the source stripes (i.e., the non-channel regions) from the ZnO area, corresponding to a geometrical aperture ratio of ∼91%. The SY LED stacked on the ZnO transistor is an inverted configuration, comprising a 25 nm ZnO:PEI blend film as the electron injection layer, a 80 nm SY film, a 10 nm MoO₃ film as the hole injection layer, and a 30 nm Ag anode as the top drain electrode and emission-side mirror of the VLET. This microcavity VLET is designed to optimize the electron-hole recombination in the SY layer while matching the LP low-energy state with the EL peak energy of SY (λ=566 nm) for effective radiative pumping. The Supplement 1, Fig. S11 shows the electric field distribution at λ=566 nm calculated using the FDTD method. The total thickness of the ZnO transistor and SY LED is about λ/2, where an antinode of the cavity field is located in the SY film bulk and the node is located on the surface of the ZnO transistor.

 

Fig. 4. (a) Schematic of polariton VLET architecture. (b) Optical image of interdigitated SiO_X-encapsulated Ag source electrode on the ZnO pattern with area A = 1 × 1 mm². Also shown are the EL images of polariton VLET recorded at V_ds= 4 and 8 V during the output scan at V_gs= 4 V. (c,d) Transfer and output characteristics of polariton VLET (P-VLET). (e) EQE extracted from the output characteristics. (d,e) also show the output characteristics and EQE of reference uncoupled VLET for comparison. (f) TE- and TM-polarized angle-resolved EL intensity maps of polariton VLET, as well as the LP, UP, and EC dispersion curves extracted from a Hopfield Hamiltonian fit to the reflectivity dip energy. TE- and TM-polarized EL spectra show a FWHM of 53-55 meV at 0° and a similar intensity variation along the LP mode.

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Figure 4(c) shows the transfer characteristics of the polariton VLET, which exhibits n-type transistor behavior and good switching performance. In the OFF state (V_gs< 0 V), the SiO_X encapsulation can suppress the off-current density below 10⁻²mA/cm² for V_ds up to 8 V. The current is switched on at V_gs= 0∼1 V, and the emission starts to increase with the current at V_gs> 0.5 V. In the ON state at V_gs= 4 V, the maximum on-current density of 23 mA/cm² and highest luminance of 1750 cd/m² can be obtained at V_ds= 8 V. Both the current density and luminance have the largest ON/OFF ratios of >10³. From the output characteristics at V_gs= 4 V [Fig. 4(d)], it can be seen that the VLET in the ON state behaves like an OLED, with current and luminance increasing superlinearly with V_ds. The EL images of the VLET recorded during the output scan are shown in Fig. 4(b). At low V_ds (4 V), the device already exhibits uniform light emission over the entire ZnO pattern except for the nonchannel regions on the encapsulated source stripes. This demonstrates that combining the high mobility nature of ZnO with an appropriate source pattern can indeed support long-range electron transport and uniform distribution across the source aperture, and confirms the effective area of the VLET defined above. When operating at high V_ds (8 V), the device can produce a highly intense glow that even eliminates the shadow of the source stripes and appears to emit uniformly from the entire ZnO pattern. Compared to the reference uncoupled VLET using ITO bottom gate as the emission side [see transfer and output characteristics in Supplement 1, Fig. S12 and Fig. 4(d)], the polariton VLET has a similar on-current density but 1-2 times lower luminance. As a result, the maximum EQE extracted for the polariton VLET is 2.4%, approximately 0.58 times that of the uncoupled VLET [4.1%, similar to the uncoupled SY LED, see Fig. 4(e)].

To gain more insight into the emission of polariton VLET, we perform the angle-resolved reflectivity and EL measurements. The reflectivity was measured on the same VLET microcavity without the source pattern, and the result is shown in Supplement 1, Fig. S13(a). By fitting the reflectivity dip energy using the Hopfield Hamiltonian, we extract the TE- and TM-polarized polariton mode dispersions with similar Rabi splitting energies of ∼0.7 eV (∼25% of EX), and demonstrate the polariton VLET in the ultrastrong coupling regime. Since Rabi energy is inversely proportional to the optical mode volume [38], the VLET with a cavity length about twice that of an OLED may lead to a larger mode volume and a smaller Rabi splitting energy, which in turn exhibits a stronger angular dispersion of the LP mode. As the dispersion curves shown in Fig. 4(f), the LP mode energies of TE- and TM-polarizations are ∼2.2 eV at 0° close to the exciton emission peak, and blue-shifted to 2.36 eV (TE) and 2.32 eV (TM) at 60° near the high-energy tail of exciton emission. Correspondingly, the TE- and TM-polarized EL emissions exhibit similar intensity distributions along the LP mode dispersion, where the intensity is highest at ∼0° and drops to <10% above 50° [see EL intensity maps in Fig. 4(f) and selected spectra in Supplement 1, Fig. S13(b)]. It is evident that VLET emission decreases faster with angle than OLED due to stronger LP mode dispersion. This may explain a lower conversion ratio from exciton to polariton emission in the VLET than in the OLED (58% vs. 70% in optimal cavity design). Nonetheless, our studied VLET affords multiple advantages to overcome severe performance and application limitations of planar LETs. Using a high-mobility ZnO transistor as the current source and SY as the strongly coupled emitter, the polariton VLET can reach a high luminance (>10³ cd/m²) and a maximum EQE (>2%) about 4 orders of magnitude higher than the previously reported planar LET in the ultrastrong coupling regime [26]. It also enables low driving voltage and uniform emission in a defined channel area with a large aperture ratio, which is difficult to achieve in planar LETs [39,40]. To our knowledge, this is the first demonstration of polariton VLET, and can serve as a useful model for studying more strongly coupled emitters to develop efficient polariton devices and high-resolution displays.

6. Conclusion

We study in detail the exciton-photon coupling and emission in a series of SY microcavity devices. We demonstrate that the optical microcavities with sufficiently thick SY films can generate ultrastrong exciton-photon coupling with Rabi splitting energies of >1 eV and exhibit narrowband PL emission along the flat LP mode dispersion. The degree of energy matching between LP low-energy state and exciton emission peak is proven to be important for radiative pumping of polariton emission. In an optical microcavity with the best energy matching condition, the conversion efficiency from exciton to polariton emission can reach about 80%. We further introduce appropriate charge injection layers in the SY microcavity and develop an ultrastrongly coupled LED with a maximum EQE of 2.8% and weakly dispersive EL emission following LP mode. Furthermore, by integrating a SY LED with a ZnO transistor in a microcavity, we demonstrate an ultrastrongly coupled VLET that exhibits a favorable switching function, uniform emission in the patterned ZnO area, and LP mode emission with a high EQE of 2.4%. Compared with typical planar LETs, this inorganic/organic hybrid VLET not only enables superior emission performance, but also can precisely control the emission area and incorporate various solution-processed and evaporated strongly coupled emitters, which may pave the way for polariton research towards practical display and lighting applications.