Organic single‐crystal light‐emitting transistors with external quantum efficiency over 20%

Organic light‐emitting transistors (OLETs) have attracted increasing attention because of their potential applications in next‐generation displays and high‐energy operating devices. However, the simultaneous achievement of high luminescent efficiency and carrier mobility in organic semiconductors remains challenging because the localized excitons are advantageous for light emission, whereas the delocalized carriers are beneficial for efficient charge migration. Herein, we report an organic single crystal of a cyano‐substituted styrene derivative with balanced mobility yielding a record‐high external quantum efficiency of 20.5% in OLETs. Temperature‐dependent I–V curves and electronic structure analyses based on first‐principles calculations were performed to disclose the underlying mechanism as a band transport, which provides an efficient way to achieve high quantum efficiency in OLETs.


INTRODUCTION
Organic single crystals are highly tunable luminescent platforms that facilitate the engineering of luminescence and mobility and the fabrication of devices. [1] Organic singlecrystal-based electroluminescent devices, essentially organic light-emitting transistors (OLETs), [2] have a geometry similar to that of a three-electrode field-effect transistor (FET): The carriers are injected from the source and drain electrodes to the interface of the crystal/dielectric layer and recombine within the channel to generate excitons. [3] This luminescent efficiency usually requires the materials to have localized excited states, whereas realizing high carrier mobility necessitates the materials to possess delocalized transport properties. This conflict makes simultaneously achieving these two properties in one material extremely challenging; this problem has been highlighted as a bottleneck issue of "critical importance" in recent reviews of this field. [9] According to our previous study, the cyano-substituted styrene derivatives tend to form lamellar organic single crystals, in favor of device fabrication. This is because two strong driving forces, that is, intermolecular hydrogen bonds and π-π interactions, confirmed both theoretically and experimentally. [10] These factors operate in orthogonal directions, along which the crystallization velocities are far superior to those along other directions possessing only weak van der Waal's forces. [11] Based on this observation, we have changed the structures of the skeleton to investigate the optoelectronic properties in solutions, powders, and crystals for seeking competitive organic single crystals for high-performance OLET applications.
Herein we report an organic single crystal that achieves striking EQE over 20% in OLETs. The organic single crystal of (2Z,2′Z)-3,3′-(1,4-phenylene)bis (2-(thiophen-3-yl)acrylonitrile) (β-PBTA) features a linear molecular geometry, very tight π-π packing, and large red-shifted absorption and emission in organic single crystal compared with that in dilute solution, indicating the presence of strong interactions among adjacent molecules. The I-V curve analyses of the OLETs show the high quality of the organic single crystal with less than 1 defect among 4.05 × 10 4 molecules. Temperature-dependent I-V curve analyses reveal band transport in the organic single crystal with an activation energy of only 32.6 meV, in favor of long-range migration of excitons and carriers, which reduces the binding and exchange energy of localized hole-electron pairs, and substantially decreases the difference between singlet and triplet [12] upon electric actuation in a low-defect-density and well-defined molecular packing lattices. Moreover, dispersive band structures with large bandwidth, small effective mass, and high mobility revealed by first-principles calculations further confirm that the band transport is indeed operational in the β-PBTA single crystal.

RESULTS AND DISCUSSION
We synthesized β-PBTA (Figures S1-S6) and obtained many pieces of its lamellar organic single crystals by the physical vapor transport (PVT) method. To obtain high-quality crystals, we designed a tailor-made PVT instrument capable of precise control over gas pressure, flow rate, and temperature. High-quality lamellar crystals with the size of several hundreds of micrometers and thickness of several hundreds of nanometers were eventually harvested by comprehensively exploring and controlling the above parameters ( Figure S7, Tables S2). The crystal adopted a triclinic lattice and space group of P-1 (Table S3). In the crystal, β-PBTA molecules with a planar configuration packed in an H-aggregation mode with a slip angle of 80.46 • and the intermolecular π-π distance of only 3.4 Å ( Figure 1A-C), indicative of strong π-π interaction. Upon excitation with 366-nm UV light, the β-PBTA dilute solution showed blue luminescence with two peaks at 428 and 448 nm ( Figure 2A) and a weak photoluminescence quantum yield (PLQY) of 5% (Table S4), whereas the PLQY of the β-PBTA powder was less than 7%. By contrast, the β-PBTA crystal exhibited a substantially red-shifted emission by 92 nm (Figure 2A) without vibration splitting but significantly higher PLQY ( Figure S32). In addition, the photoluminescence lifetime in the crystal is fitted perfectly with a double-exponential model, yielding two fluorescence lifetimes ( Figure 2B and Table S4).
To obtain high-performance OLETs, a special sample miniature holder ( Figure S8) was designed to deposit the electrodes sequentially and independently onto the crystal ( Figure 1D), namely, the asynchronous evaporation method, [11c] by which it was feasible to respectively modify the interfaces between the crystal and the electrodes ( Figure S9b). This deposition method substantially improved the injectability of electrons and holes and was consequently critical to improving device performance ( Figure S10). The measurement system consisted of a high-definition microscope, an output coupler of light, and a high-sensitive grating spectrometer, [13] as traditional half-sphere integrating sphere did not have enough sensitivity. The microscope and sample stage were free to rotate and move in multiple directions, which made it possible to collect the emission signals of OLETs from any direction. All the detail of light collection and EQE calculation had been described in Ref. [13]. As the emission area of the crystal-based OLETs was too small to be measured reliably, all the parameters of OLETs associated with the emission area, for example, luminance, were not taken into the discussion here. After calibrating the responsivity of the system, the quantum efficiency, emission power, and other important parameters were extracted. Compared with the transistors fabricated by the traditional method ( Figure  S11), the β-PBTA crystal exhibited a much better ambipolar carrier-transport property with a hole/electron mobility of 0.73/4.48 cm 2 V −1 s −1 (Figures 2C,D and S12) and smaller threshold voltage for both N-and P-channels, which revealed an excellent performance among ambipolar transistors.
When it operated under the ambipolar mode, the OLETs exhibited broad and weak emission with a peak at 542 nm under a low gate voltage (V g ) of 30 V and drain voltage (V d ) of 45 V. By controlling the drain and gate biases, the emission photos from crystal surface ( Figure 3A) and edge (Figure S13) were located in the center of the channel. The emission intensity was significantly increased ( Figure 3B), and the spectra were narrowed upon increasing V g to 80 V ( Figure S14). The emission spectra of the OLET under high V g were similar to the photoluminescence spectrum of the single crystal, whereas the EL spectra had some red-shift at the right side of the emission peak ( Figure S15). Under the effect of optical waveguiding, the intensity of the edge emission was 2.7-folds of that from the crystal surface ( Figure  S16). In the repetitive experiments, we found that the spectra of edge emission were related to the shape of different pieces of crystals and the nuance of intermolecular interaction inside the crystals (Figures S17-S21). The zigzag signal of the electroluminescence spectra in some of the OLETs suggested strong light resonance before being emitted out of the crystal ( Figure S18). The OLET exhibited a maximum EQE as high as 20.5% ± 0.9% ( Figure 3C,D) at an irradiation power of 1.04 nW. The channel length also affected the maximum EQE ( Figures S22 and S23), which was expected to rely on the F I G U R E 1 (A) Single-crystal structure of the β-PBTA crystal, revealing the multiple hydrogen bonds (sky-blue dashed lines). (B) The length of the hydrogen bonds (in red) and the molecular twisted angle (in green) in the crystal. (C) The π-π distance (in red) and the slipping angle (in sky-blue) in the crystal. (D) Asynchronous evaporation method to fabricate the asymmetric electrodes in the organic light-emitting transistors (OLETs). The shadow mask was aligned to one side of a crystal, and CsF and calcium were subsequently deposited. Then, the shadow mask was moved to the other side of the crystal, and MoO 3 and gold were subsequently deposited.  Figure S14 and Table S6. difficulty to control the recombination zones. Such EQE value is one order of magnitude higher than all the results reported in the literature on OLETs based on organic crystals (Table S4) and polymers. [14] The EQE was still higher than 8% when the current was larger than 218 nA. The maximum power efficiency of the OLET was 42.6 lm W −1 when I = 1.9 nA and still reached 5.5 lm W −1 at I = 218 nA. The exciton usage efficiency in the OLET was calculated to be 35.2% ± 3.2% even if we use the highest PLQY of the crystal (58.3%) for calculation, which substantially exceeded the limitation of photoluminescence under electrical-driven. The maximum irradiation power was 156 nW in the transistors with a channel width of only 40 μm and the input recombination current of 218 nA, the highest value of crystalbased OLETs. Though higher irradiation power was reported in OLETs based on polymer films and tooth-shaped source and drain electrodes, with a channel width up to 1000 μm, [15] the maximum irradiance density in our devices was superior as the irradiance power of the similar emission spectrum is proportional to the channel width of the OLETs. The devices have substantial thermal degeneration under high current injection, but their electrical luminescence spectra could recover most performance after cooling for 20-30 min ( Figure S24).
The striking device performance inspired us to clarify the underlying mechanisms. An important factor for highperformance OLET was the quality of the crystal, which was quantitatively evaluated by measuring the subthreshold swing (S) of the devices under low-drain voltage. The results reveal that S in the OLETs was only 0.068 and 0.104 V dec −1 when operated in the N-and P-regions, respectively, which were near the limitation of that from FETs of metal-insulator-semiconductor structure [16] and were close to the reported best result of organic FETs based on rubrene crystal (0.065 V dec −1 ). [17] Accordingly, the defect densities for holes and electrons at the crystal/insulator interface were calculated to be 1.12 × 10 10 and 5.98 × 10 10 cm −2 , respectively, indicating that there was less than 1 defect among 4.05 × 10 4 molecules ( Table S5). The low-trap density avoided the carrier capture by traps, which made it easier to adjust the recombination zone to the center of the channel, thereby avoiding the quenching by electrodes and defects. However, only the low-trap density fails to explain the breakthrough of spin-statistical rules limitation in our OLET, and more essential reasons inside the organic single crystal itself are expected.
Theoretical calculations reveal that the free single molecule of β-PBTA had a planar molecular configuration with small twist angles ( Figure S25), similar to those in the crystal. Therefore, the largely red-shifted emission spectrum of the single crystal compared with that in the dilute solution was caused by the strong orbital overlaps between the adjacent molecules. According to the crystal structure, the intermolecular couplings in the tight-packing single crystal are strong, resulting in band transport, so we could treat them as similar methods in the delocalized systems.
We further performed temperature-dependent I-V measurements for the same OLET, in which the mobility increased with the increasing temperature ( Figures 4D and  S26). [18] The activation energies of the electrons and the holes calculated according to the Arrhenius law were 32.6 and 63.6 meV, respectively. [19] The activation energy of the electron is very close to 1 k B T (25.7 meV at room temperature), comparable with that of rubrene crystals. [20] On the other side, the OFET with symmetric gold as its source and drain electrodes was observed decrease of mobility along with the increase of temperature ( Figure S27), a typical phenomenon of band transport. This indicates that it is easy to excite electrons from the relatively localized states into the delocalized states, similar to the case of rubrene crystals. [12b] Therefore, the charge-transport mechanism in the β-PBTA single crystal can be described as a combined band and hopping transport by using the multiple traps and release model. [21] This is supported by the results of our first-principles calculations, in which well-defined band structures were obtained for the β-PBTA single crystal. As shown in Figure 4A,B, both the valence band (VB) and the conduction band (CB) are substantially dispersive, with the bandwidth of about 0.8 and 0.4 eV, respectively, which are larger than those of both rubrene and pentacene crystals (0.44 and 0.59 eV for VB, respectively). [22] The dispersive bands lead to effective masses for holes and electrons as small as 0.9 m 0 and 2.4 m 0 (with m 0 the electron rest mass, Figure S28), respectively. Assuming one-dimensional charge transport for simplicity, the effective electronic couplings corresponding to the VB and the CB are about 0.2 and 0.1 eV, respectively, which are also much larger than the activation energies. The calculated electron mobility decreases as a function of increasing temperature ( Figure 4C), which is a typical feature of band transport. These results strongly suggest that band transport has a significant contribution to charge transport in the β-PBTA crystal. Moreover, the top of the VB and the bottom of the CB have the same wave vector, a characteristic of direct-bandgap semiconductors, which agrees with the high PLQY in the H-aggregated crystal. The calculated results of electron and hole mobility of the crystal along the π-stacking direction by taking both band and hopping transport into account are 2.4 and 1.0 cm 2 V −1 s −1 , respectively, which are comparable to the experimental results. In the crystals with band transport properties, the excitons' binding and exchange energies were comparable with the thermal energy of room temperature, which implies that the difference between singlets and triplets was almost eliminated. Consequently, the β-PBTA single crystal can overcome the limits of maximum electroluminescent limitation. [23]

CONCLUSION
In summary, we have demonstrated an efficient singlecrystal-based OLET achieving maximum electroluminescent limitation with unprecedented EQE of 20.5%, which results from a high-quality crystal with balanced mobility and band transport property. Moreover, the classical Kasha with longrange dipole-dipole interaction is not enough to explain the high PLQY of crystals with strong H-aggregates due to the neglect of short-range charge-transfer interactions. This rationale can provide a new paradigm for electroluminescent devices. CCDC 1547514 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

A C K N O W L E D G M E N T S
We thank J. Zhou (South China University of Technology) for support on X-ray diffraction experiments. Y. Wang and P. Lu (Jilin University) are acknowledged for support of OLET equipment setup and material synthesis. This work was supported by Financial support was received from the National Key R&D Program of China (2020YFA0714604), the Natural Science Foundation of China (21975078, 91833304, 21973081, 51521002, 51703065, 62174100)

C O N F L I C T S O F I N T E R E S T
The authors declare no conflicts of interest.