Our device architecture (Fig. 1a) borrows from the "gated" AC-OEL structure that were introduced recently in the literature31. In this case, however, the gated structure is coupled to a bilayer organic emitter consisting of [poly(N-vinylcarbazole) (PVK):bis(2-methyldibenzo[f, h]quinoxaline) (acetylacetonate)iridium(Ⅲ) (Ir(MDQ)2(acac)) and poly[(9, 9-bis(3'-((N, N-dimethyl)-N-ethylammonium)-propyl)-2, 7-fluorene)-alt-2, 7-(9, 9-dioctylfluorene)] (PFN-Br)]. Surface morphology analyses of PVK:Ir(MDQ)2(acac) and PFN-Br emissive layers are performed via atomic force microscopy (AFM); the obtained images can be found in Figure S2. As in an earlier work32, the carrier gate layer was composed of [poly(3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) that was loaded with ZnO nanoparticles (NPs)] and the electron-transporting layer was [2, 2′, 2"-(1, 3, 5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi)]. The two conductive electrodes were indium tin oxide (ITO) and aluminum (Al). The chemical structures of the emitters, namely, PFN-Br and Ir(MDQ)2(acac), are listed in Fig. 1b. With 365 nm ultraviolet (UV) excitation, PFN-Br and Ir(MDQ)2(acac) show strong fluorescent and phosphorescent luminescence peaks at 474 and 585 nm, respectively (as shown in Fig. 1c). The lifetimes of short-lived blue fluorescence and long-lived red phosphorescence are given in Fig. 1d for 0.31 ns and 1.87 μs, respectively. With ZnO NPs (~35 nm in diameter) gating the interface of ITO/PEDOT:PSS, injected charges can be efficiently manipulated under the forward and reverse biases of AC cycles32-34. The AC-OEL devices are driven by a sinusoidal voltage with a wide frequency range from 50 Hz, which causes the gate to allow bi-polar injection and the device to act like a diode, to 60, 000 Hz, at which most light is created by field-generated polarons/excitons with little diffusive transport in the active volume of the emitter. The operating mechanism of AC-OEL devices is described in detail in Supplemental Information.
PFN-Br is a high-performance ionized electron-transporting polymer35, 36 with an electron mobility of 1.41 × 10−7 cm2/V/s, and PVK is a typical p-type semiconductor with a hole mobility on the order of 1.0 × 10−6 cm2/V/s. Thus electrons and holes are transferred and interact at the PVK/PFN-Br interface under an external electric field. The time for accumulation at this interface depends on the driving frequency. However, as already noted at high frequencies, the gate of the system allows only for field-generated carrier injection into the emitting volume, while at lower frequencies the gate allows for direct injection from the contacts. Nevertheless, both conditions result in charge drift toward the interface (illustrated in Fig. 1e). The electrons and holes are transferred to the heterointerface in the positive cycle of the AC electric field and drift along the opposite direction under the reverse bias37-39. Therefore, the time-dependent electric field generates an interfacial magnetic field at the PVK:Ir(MDQ)2(acac)/PFN-Br heterointerface according to Maxwell's equations. In an ideal case, the pixel dimension is 4 × 4 mm2, which is significantly larger than its thickness (~300 nm); hence, it is reasonable to ignore fringing effects (the infinite-area parallel-plate-capacitor assumption). Via engaging a high-frequency driving (60, 000 Hz) under a strong AC electrical field (1.6 × 108 V/m), the temporal and spatial characteristics of the internal magnetic field are shown in Fig. 1f. The upper and lower half-planes represent the opposite "clock directions" of the magnetic field in the positive and negative halves of an AC cycle. The amplitude of the magnetic field is estimated to be approximately 0.85 mT. More details can be found in Supplemental Information, along with simulation results with various frequencies in Figure S3. The dynamic revolution of the magnetic field is shown in Movie S1.
The heterointerface also plays the role of an e–h pair recombination zone for hot carrier injection, as shown in the energy-level diagram in Fig. 1g. These e–h pairs not only move in the applied electric field but also experience the induced magnetic field. The device shows a blue emission (in Figure S4) due to the PFN-Br fluorescence at near-DC driving (50 Hz) since the dissociation rate of e–h pairs is negligible in the absence of an induced magnetic field (< 0.00005 mT). When this internal AC magnetic field is of the same order as the nuclear hyperfine field (~1 mT), ISC suppression should occur11 and this would naturally lead to singlet-spin e–h pair accumulation. Many secondary carriers are produced in PFN-Br through the magnetically mediated dissociation of the e–h pairs. The secondary charges diffuse to nearby Ir(MDQ)2(acac) sites, which yields decay of triplet-state excitons, as shown in Figure S4. There is no significant position shift of the recombination zone in the device, as shown in Figure S5. Thus, as illustrated in Fig. 1g, in the low-frequency driving regime (50–1000 Hz), hot carrier injection is the main mechanism for fluorescent excitons in PFN-Br. In the high-frequency regime (30, 000–70, 000 Hz), the high-intensity AC magnetic field at the F–P interface populates singlet-excited e–h pairs mostly via ISC suppression, which leads to secondary carriers. The secondary carriers exist in the form of bonded electrons in the PFN-Br polymer matrix, more specifically, with Br atoms, which are strong electron acceptors. The charged Br ions significantly improve the carrier diffusion length40, resulting in negative charges moving across the interfacial energy barrier. Consequently, the secondary carriers are transferred to Ir(MDQ)2(acac) for red phosphorescent emission. For the same reason, the charged movable Br ions greatly facilitate magnetic field current, even under very subtle magnetic intensity with non-ionized polymer, which normally requires over hundreds of mT41. More discussion is presented in supplementary information.
Figure 2a, b show the evolution of the device's EL spectrum with increasing electric field at low and high frequencies. At the frequency of 50 Hz, there are trivial spectral shifts in Fig. 2a when the applied voltage varies from 15.4 to 20.4 V; however, no phosphorescence is observed. This is attributed to the dominant hot carrier injections, as shown in Fig. 2c. Electrons and holes are injected in the positive halves of voltage cycles while no current injection occurs in the reverse bias. Hot carrier injection is strongly related to the 474-nm-wavelength fluorescence.
For 60, 000 Hz (secondary-carrier-related), as shown in Fig. 2b, the EL intensity of the fluorescent emission at 474 nm is nearly identical to that observed at 50 Hz. However, the 595 nm phosphorescent peak grows as the external electric field increases. Examining current transients in Fig. 2d, the total current density is the sum of the root-mean-square (RMS) values of the sinusoidal waveform and the direct current (DC) offset (Jtot = Jdc + Jsine; the method used for current component determination can be found in Supporting Information). Under the lower voltage of 17.9 V, the DC current density and RMS sine current density are 60.2 and 36.6 mA/cm2, respectively. Compared to higher voltages (18.9, 19.8, 20.6, 21.5, and 22.0 V), the DC current densities are dramatically increased to 82.3, 114.9, 138.5, 164.1, and 184.6 mA/cm2 while relatively constant RMS sinusoidal currents of 37.8, 39.4, 42.1, 43.7, and 45.2 mA/cm2, respectively, are observed. We suggest that the tremendous enhancement of the secondary charge current produces the growth of the 595-nm-wavelength phosphorescence.
In addition, Jrms–L–Vrms characteristics at low frequency (50 Hz) and high frequency (60, 000 Hz) are shown in Fig. 2e, in which the maximum brightnesses, namely, 360 cd/m2 in blue and 600 cd/m2 in red, are of the order that is necessary for devices for personal display use. Figure 2f shows the relation between Jrms and L at the frequencies of 50 Hz and 60 kHz. The 50 Hz curve is fitted by a linear function, which has been widely demonstrated in the standard DC-driven organic light-emitting diodes (OLEDs). The linear fit strongly suggests that the exciton concentration inside the device remains below the level at which multi-exciton effects are dominant, such as Augur, quenching, and dissociation. In contrast, the 60 kHz curve can be fitted not by a linear function but by a first-order exponential function, which indicates an improved exciton recombination efficiency, which rules out the Augur recombination and quenching effects. The exponential line fits the data in both the high- and low-current regimes, which suggests that the phosphorescent exciton recombination efficiency is independent of the exciton density. These triplets are again generated by the free carriers from the e–h dissociation promoted by the AC-magnetic-field-assisted ISC suppression. Therefore, rather than the injection efficiency, the AC field is the main factor that impedes the singlet-to-triplet ISC. In consequence, a higher power efficiency is achieved at 60, 000 Hz (1.5 lm/W; details are shown in Figure S6).
The luminance–frequency characteristics of the color-tunable AC-OEL device are shown in Fig. 3a. The total emission of the device shifts between the fluorescent and phosphorescent contributions, whereas the luminance is relatively stable over the frequency. Corresponding to the F–P shift, the frequency characteristics are also shown with respect to current density in Fig. 3b. Analyzing the low-frequency regime (< 1000 Hz) first, low frequencies lead to dominant blue fluorescence since, as we suspected, the device under low-frequency driving acts more like a carrier-injection-type diode in the forward and reverse biases (Figure S7).
At higher frequencies (> 10, 000 Hz), the current density consists of a sinusoidal contribution and a DC offset (see Figure S7), essentially reflecting both the displacement of the direct current injection and the secondary charge current, respectively. In Fig. 3b, the DC offset component of the current through the device starts at a very low level (13.8 mA/cm2) at 10, 000 Hz and increases to 226.1 mA/cm2 at 45, 000 Hz. In contrast, the RMS value of the sinusoidal component of the current waveform drops from 286.1 mA/cm2 at 10, 000 Hz to 106.8 mA/cm2 at 45, 000 Hz, which suggests a significantly reduced contribution of hot carrier injection to the total current at high frequency. These opposite trends illustrate that an electric field of > 20, 000 Hz applied to the capacitive device is sufficient to generate a magnetic field that is strong enough to yield secondary charge diffusion. The stronger AC magnetic field (due to higher frequency) suppresses ISC between singlet-state and triplet-state e–h pairs in the PFN-Br, thereby resulting in population enhancement of singlet e–h pairs at the F–P interface. The elevated singlet-triplet ratio promotes the generation of secondary charge carriers. The hopping transport of secondary electrons and holes in the organic semiconductor is acutely tied to the generation of radical triplet excitons in Ir(MDQ)2(acac), leading to red phosphorescence (as indicated in Fig. 3a). The coupling of the driver to the capacitive device has been taken into consideration by comparing the dominant capacitance of the device (1–3 nF) with the parallel external capacitance (530 pF) in the driver. Driving frequencies of > 50, 000 Hz cause insufficient carrier injection, resulting in the decrease in the number of e–h pairs and the total current reduction in Fig. 3b.
In Fig. 3c, the EL spectrum of the AC-OEL device shows a dramatic color change when the driving field frequency varies from 50; 100; 500; 1000; 10, 000; 20, 000; 30, 000; 40, 000; 50, 000 Hz, up to 60, 000 Hz. The 474 nm emission band is dominant at the low frequency of 50 Hz. As the driving frequency increases, the peaks at 430, 474, and 531 nm (due to PFN-Br's fluorescent emission) are weakened; in contrast, the 595 nm peak (which corresponds to Ir(MDQ)2(acac)'s phosphorescent emission) grows rapidly and becomes the dominant emission band. All spectra are measured under 100 cd/m2, integrated for 500 ms and averaged over 5 runs. The operating pixel images in Fig. 3d provide us with a clearer picture of the color change with frequency in the forward and reverse sweeps. As indicated by the marked circles, the CIE coordinates in Fig. 3e start from (0.23, 0.34) at 50 Hz, cross the white zone, and reach (0.53, 0.40) in the red zone at 60, 000 Hz. Another graphic display of the AC-OEL pixel color change with driving frequency is shown in Movie S2. We also observed non-homogenous emission from the pixel, which is mainly attributed to two major mechanisms: the non-homogenous thin film of PFN-Br and the uniformity of the AC magnetic field coupling. The thin PFN-Br layer at the substrate edge has an e–h generation zone close to the F–P interface, which facilitates the drift of the secondary charges, resulting in red emission. The AC field coupling is also spatially dependent, as shown in Fig. 1f, which suggests a stronger magnetic field at the substrate edge than at the center. Therefore, the spatially dependent magnetic field also leads to the red color shift that first occurs in the pixel area near the substrate edge. Further analysis of the non-homogenous emission can be found in Supplemental Information.
To further study the energy transition at the F–P interface, photoluminescence (PL) spectroscopy is performed, and the results are shown in Fig. 4a, b. The absorption of PFN-Br (peaks at 398 nm) shows a large overlap with the PL spectra of the host PVK (which show a wide peak at 404 nm) in Fig. 4a. This implies an efficient Fӧrster energy transfer route between PVK and PFN-Br (Fӧrster resonance energy transfer efficiency ~21.1%, calculated from Figure S8). In the PL spectra for PVK:3 wt% Ir(MDQ)2(acac)/PFN-Br shown in Fig. 4b, an extra 585 nm peak due to Ir(MDQ)2(acac) was detected when the sample was excited by 347 nm (PVK's strongest absorption); in contrast, under 380 nm excitation (PFN-Br's strongest absorption), this 585 nm peak is absent. This suggests that direct energy transfer between PFN-Br and Ir(MDQ)2(acac) is not allowed. The Fӧrster energy transfer from PVK to PFN-Br and the forbidden transfer between PFN-Br and Ir(MDQ)2(acac) explains the blue florescence that is due to PFN-Br in hot carrier injection.
Because of the full ionization of poly[(9, 9-bis(3'-(N, N-dimethylamino)propyl)-2, 7-fluorene)-alt-2, 7-(9, 9-dioctylfluorene)] (PFN-DOF) by Br atoms, PFN-Br possesses many moveable negative charges among the main polymers. Figure 4c shows the electron mobility enhancement by the diffusive Br-negative ions compared with PFN-DOF. The ionic conductivity is the most important property of solid polymer electrolyte. By introducing the Br ions into the PFN to form an electrolyte system, the magnitude of the conductivity will be enhanced. The conductivity of PFN-Br (~7.8 × 10−7 S/cm) is significantly improved compared to that of PFN-DOF (~3.1 × 10−9 S/cm), which is due to the Br-anion movements under the electric field. Because of the long-distance diffusion, the movable Br ions are able to populate the free secondary electrons, which leads to the dissociation of more excitons, thereby facilitating carrier diffusion to the phosphorescent emission layer. For comparison with PFN-Br devices, we studied the spectral shift in PFN-DOF devices between a magnetic-field-free device (50 Hz) and an AC-magnetic-field-coupled device (60, 000 Hz) in the same architecture. In Fig. 4d, the fluorescence of PFN-DOF (at 423 and 476 nm) is promoted, which implies the promotion of singlet-state excitons by suppressing ISC of PFN-DOF. However, the color change is almost unnoticeable in PFN-DOF, which suggests that the secondary carriers are unable to reach the phosphorescent sites without the aid of Br ions. More work about the magnetic field effect on PFN-DOF can be found in supplementary information and Figure S9. In the absence of a PFN-Br layer, the excitons still can be formed and recombined at the p–n interface. However, the extremely low radiative recombination rate of TPBi facilitates exciton decay directly at the Ir(MDQ)2(acac) phosphorescent sites and the PVK fluorescent host, which was demonstrated by the EL spectra in Figure S10.