The realm of electronics is poised for a transformative shift with the emergence of intrinsically flexible devices capable of seamless adherence to the human body, biological tissues, and curved surfaces6-8. Organic transistors represent basic building blocks of flexible circuits for signal recording, processing, and amplification that can accelerate the evolution of human-machine interfaces, health monitoring, the Internet of Things, etc1,4,9. High amplification coupled with low power consumption is a critical performance requirement for organic transistors deployed in such applications. Reducing the subthreshold swing (SS) of transistors is the necessary premise for achieving high signal amplification efficiency at low power. However, the limitation of thermionic carrier injection fundamentally imposes a lower bound on the SS, prohibiting its reduction below 60 mV dec-1 at room temperature (Extended Discussion 1)10. The advent of tunnel field-effect transistors (TFETs) offer a powerful technique to overcome the Boltzmann thermionic limit and attain subthermionic SS11. Diverging from the conventional thermal carrier injection mechanism, TFETs primarily adopt the cold charge injection mechanism of band-to-band tunnelling (BTBT), whereby charge carriers transport from the valence (conduction) band to the conduction (valence) band12. Although TFETs have recently been implemented on emerging inorganic semiconducting materials12-14, TFETs with intrinsically flexible organic semiconductors (OSCs) as channel materials have not been achieved so far despite their potential for creating flexible electronics with high mechanical robustness.
In typical TFETs, interband BTBT path is accomplished by designing a heterojunction with a highly doped n+(p+)-type semiconductor as the source and a p(n)-type semiconductor as the channel11-13. Since the heavy p- or n-type doping of OSCs is still an open question15, it impedes the realization of all-organic heterojunctions for effective BTBT transmission. An alternative way is the use of heavily doped inorganic materials as source metal electrodes to build inorganic-organic source-to-channel heterojunction. However, a typical inorganic-organic interface is usually plagued by a serious Fermi-level pinning effect, resulting in a high energy barrier at the tunnel junction and diminish the probability of BTBT transmission. Indeed, it is inevitable that several types of deteriorative interaction occur at the interface of two dissimilar materials. First, the high-energy inorganic atoms or clusters could forcefully bombard the delicate OSCs during the deposition of inorganic materials, which damages molecular packing and culminates in surface defects16. Second, the diffusion of atoms or clusters can incite profound strain within crystal lattices and change the band structures of OSCs17. Third, the extended wavefunction from the heavily doped inorganic material seriously perturbs the molecule orbital environment of the OSC due to huge difference in carrier density between them; some molecular levels may be split to form new electronic states within the OSC bandgap17. Consequently, although imbued with promise, the successful demonstration of TFETs with the emerging organic electronic materials remains an arduous endeavor.
Here we demonstrate the first case of subthermionic TFETs based on one benchmark OSC by exploiting a molecule-assisted interface decoupling strategy to create an ideal inorganic-organic heterojunction. Fig. 1a illustrates the organic TFET (OTFET) configuration, in which molybdenum oxide (MoO3) is adopted as the metal-oxide electrode, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) single-crystalline thin film forms the channel and N,N′-bis(2-phenylethyl)perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) molecular layer is decorated at the MoO3/C8-BTBT interface. We chose MoO3 because of its extremely high work function of 6.7-7.0 eV and high conductivity (Extended Data Fig. 1)18, which is pivotal in engendering a hole BTBT path after jointing with p-type C8-BTBT. BPE-PTCDI is selected as the decoupling layer since it not only can form a dense thin film on the C8-BTBT layer to prevent the penetration and bombardment of the high-energy MoO3 clusters, but also has a higher ionization energy (IE) compared to the C8-BTBT (Extended Data Fig. 2), which could facilitate hole injection.
To substantiate the effectiveness of the interface decoupling, we mechanically separated the deposited MoO3 metal electrodes and BPE-PTCDI molecular layer from the surface of the C8-BTBT thin film. The underlying C8-BTBT thin film retained its initial sheet-like morphology without any discernible damage (Fig. 1b and Extended Data Fig. 3a,b). In contrast, the absence of the BPE-PTCDI decoupling layer exposed the C8-BTBT to the relentless bombardment of high-energy MoO3 clusters, which infiltrated the crystal lattices (Fig. 1c), leading to considerable defects, strain, and disorder. The MoO3 clusters typically formed strong interactions with the underlying C8-BTBT and fitted closely together. Therefore, when the evaporated MoO3 was mechanically peeled, the underlying C8-BTBT was destroyed (Fig. 1d and Extended Data Fig. 3c,d). This is further clarified by cross-section high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with energy-dispersive X-ray spectroscopy (EDX) mapping characterizations (Fig. 1e,f). It was found that the abundant of evaporated MoO3 clusters penetrated into the surface layer of C8-BTBT, while the MoO3 could not be observed in the C8-BTBT layer after introducing the BPE-PTCDI decoupling layer, resulting in a clean heterojunction interface that is essentially free from disorder and defects. Therefore, few interfacial gap states are induced and Fermi-level pinning effect is alleviated, which leads to a reduction in the tunneling barrier (Fig. 1g, top). Conversely, in direct MoO3 evaporation without the decoupling layer, it formed intense coupling between the energy bands of C8-BTBT and MoO3, generating abundant interfacial gap states in the C8-BTBT bandgap, as well as resulting in a higher tunneling barrier (Fig. 1g, bottom).
Fig. 2a presents transfer characteristics of a typical OTFET. At various drain voltages (VDS), the OTFET has similar electrical performance with a minimum SS of 34.5 mV dec-1, a near-zero threshold voltage (VT), a large on/off ratio over 107, and tiny hysteresis (Fig. 2b and Extended Data Fig. 4). The attainment of sub-60 mV dec-1 at room temperature is a strong indicator that our device breaks the fundamental thermionic limit. Next, the drain-to-source current (IDS) versus SS was extracted, indicating that the point SS is below 60 mV dec-1 for more than one decade of IDS with an average SS lower than 60 mV dec-1 over four decades (Extended Data Fig. 5). Output characteristics of the OTFET show distinct linear and saturation regions with IDS attaining saturation at a low VDS of -1.0 V (Fig. 2c). By contrast, the transistors devoid of the BPE-PTCDI decoupling layer were dominated by conventional thermal carrier injection, resulting in a high SS (613 mV dec-1) and substantial operating voltage (-30 V) (Extended Data Fig. 6). We further extracted the power consumption in the transistor subthreshold regime (Psub-T) from the transfer curves (Extended Discussion 2). The Psub-T was distributed ranging from 2.0 fW to 18.8 pW—more than one order of magnitude reduction in comparison to state-of-the-art organic transistor4 (Extended Data Fig. 7).
Notably, thanks to the ultralow SS, the transistor exhibits a higher signal amplification efficiency (Aeff) of 66.7 S A-1 versus 38.7 S A-1 for thermionic transistors. Such high Aeff signifies a large transconductance at low IDS (Extended Discussion 3), which is a requisite for amplifier circuits to achieve high gain at minimal power. Fig. 2d shows a comparison of the OTFET with previously reported low-SS organic transistors produced by diverse strategies3,4,19-30. Our approach emerges as the frontrunner, manifesting the lowest SS value and the highest Aeff among all the reported organic transistors, both surpassing the theoretical limit of conventional organic transistor technologies. To validate the existence of BTBT in our device, we investigated the temperature-dependent electrical performance (Extended Data Fig. 8). The OTFET exhibited consistent switching characteristics, upholding SS values below 60 mV dec-1 across a wide temperature range from 195 to 300 K. Most strikingly, the SS is almost independent of temperature (Fig. 2e), conclusively substantiating the prevalence of BTBT hole injection, as distinct from linear temperature-related thermal emission mechanism (Extended Discussion 4). Furthermore, a trend towards negative differential resistance (NDR) was also observed in our OTFET (Extended Discussion 5 and Extended Data Fig. 9), further confirming the existence of BTBT13,14,31.
To understand the working mechanism of the OTFET, we embarked on density functional theory (DFT) calculations on electronic band structures of MoO3 and C8-BTBT. The total density of states (DOS) of conduction band (CB) of MoO3 overlapped partially with that of highest occupied molecular orbital (HOMO) state of C8-BTBT, forming a staggered heterojunction (Extended Data Fig. 10). This energy band configuration underscores the substantial probability for holes to tunnel from MoO3 to C8-BTBT, thereby facilitating the realization of a low tunneling barrier height. Fig. 2f provides an illustrative depiction of the operation principle for our OTFET using energy band diagrams. When the device works at the off state, HOMO energy level of the C8-BTBT channel lies beneath the CB bottom of the MoO3, so the BTBT is suppressed, leading to very small off-state current. Upon the application of VGS, the HOMO energy level of C8-BTBT is elevated above the CB of MoO3 under device electrostatics and lowers the tunneling barrier. The attendant reduction in the tunneling barrier culminates in the formation of a flat band, marking the inception of BTBT. Further increase of the VGS prompts the tunneling of holes from the energy window into the vacant DOS of C8-BTBT (red arrows in Fig. 2f), triggering an abrupt surge in current. Since the decoupling layer substantially reduces the tunneling barrier height at the source-channel interface, a slight VGS supply can initiate BTBT transmission, achieving the coveted ultralow SS.
To demonstrate the repeatability and robustness of the devices, we have measured the electrical properties of a representative OTFET at different voltage sweep rates. No perceptible variance in electrical properties was discernible at slow, medium and fast sweep speeds (Extended Data Fig. 11). Additionally, the transfer curves have not changed after multiple VGS sweeps at a fixed step (Extended Data Fig. 12), indicative of the inherent repeatability of our OTFET. The long-term stability of our OTFET was also investigated by examining the representative devices over 201 days under ambient conditions. All devices displayed very slightly changes of electrical properties (Extended Data Fig. 13). Furthermore, the OTFETs also have outstanding operational stability under constant bias voltage (Extended Data Fig. 14), endorsing their robust application potential. To evaluate the device variations, we constructed a 5 × 5 OTFET array, showing narrow performance distribution with a low SS value of 35.2 ± 7.6 mV dec-1 and a high Aeff of 68.4 ± 14.7 S A-1 (Extended Data Fig. 15). In addition, the Psub-T of the 25 OTFETs were all distributed in the narrow range (Extended Data Fig. 16). Significantly, our strategy is applicable to other OSCs, such as 2,7-Didecyl[1]benzothieno[3,2-b][1]benzothiophene. Similarly, transistors made from the single-crystalline film of 2,7-Didecyl[1]benzothieno[3,2-b][1]benzothiophene also yielded sub-60 mV dec-1 SS (Extended Data Fig. 17),highlighting the general applicability of our molecule-assisted interface decoupling approach.
Next, we investigated the pivotal factors for achieving low SS within the OTFETs. Firstly, the molecular decoupling layer should have a higher IE compared to the semiconductor layer, thereby facilitating favorable energy-level alignment for hole injection and, by extension, a reduced tunneling barrier height (Fig. 3a). This assertion was corroborated by employing alternative molecules, C8-PTCDI (6.4 eV) and C60 (6.3 eV), with higher IEs relative to C8-BTBT (5.6 eV) as decoupling layers. These transistors likewise achieved subthermionic SS of 40.4 ± 5.3 and 49.7 ± 10.9 mV dec-1, respectively (Fig. 3b and Extended Data Fig. 18). Conversely, when the IE of decoupling layer (2-TNATA, 5.0 eV or Spiro-TAD, 5.3 eV) was lower than that of C8-BTBT, the resulting transistors exhibited SS values of 71.7 ± 11.6 mV dec-1 and 59.1 ± 11.7 mV dec-1, respectively (Fig. 3b and Extended Data Fig. 19). These results are consistent with the expectation that a lower IE of the decoupling layer results in a larger tunneling barrier and requires a higher VGS to trigger BTBT. Additionally, the decoupling molecules need to be capable of forming ultrathin films with a compact morphology on the C8-BTBT layer (Extended Data Fig. 20), which is also a criterion for maximizing the BTBT transmission probability.
Secondly, the minimization of trap state density is essential to enable perfect device electrostatics—a crucial prerequisite for realizing BTBT transmission by means of a small VGS. Trap states mainly originate from grain boundaries within the channel and defects at the semiconductor/gate dielectric interface4. In this study, we adopted a large-sized C8-BTBT single-crystalline film to construct our OTFETs (Extended Discussion 6 and Extended Data Figs. 21-23). We quantitatively evaluated the device electrostatics using the equation: dψ/dVGS, where ψ is the surface potential of the semiconductor channel (Fig. 3c and calculation details in Extended Discussion 7). Strikingly, the electrostatics of the device with the C8-BTBT single-crystalline channel is close to the theoretical minimum value of 1.0 (ref. 14), indicating that the superior capacity of the gate electrode to manipulate its channel potential. For the control device employing the C8-BTBT polycrystalline film with many grain boundaries and defects, the extracted device electrostatics is only 0.28. Consequently, the SS of 66 mV dec-1 exhibited by the polycrystalline device is evidently higher than that of the single-crystalline counterpart (Extended Data Fig. 24). This result firmly establishes the profound dependence of device electrostatics and SS values on the crystallization quality of the semiconductor layer. To reduce interfacial defects, we added an appropriate amount of insulating polystyrene (PS) into the C8-BTBT solution. The polymer not only helped to improve the size of the organic single-crystalline domains but also passivated the interface trap states by vertical phase separation between the small-molecule OSC and polymer32. For the control device without the addition of PS, the SS is 60.8 mV dec-1 (Extended Data Fig. 25), confirming the key role of PS in reducing interfacial traps.
Thirdly, the transistor dimension, including the channel thickness and length, should be optimized to achieve excellent device electrostatics (Extended Discussion 8). In our OTFET, the thickness of the C8-BTBT single-crystalline film is approximately 18 nm. For the control device with a thicker C8-BTBT single-crystalline film (thickness of 78.3 nm), the SS was increased to 62.6 mV dec-1, suggesting that the device electrostatics can be degraded with increasing channel thickness (Extended Data Fig. 26). The channel length-dependent SS and BTBT currents (Fig. 3d and Extended Data Fig. 27) showed that with increasing channel length, the SS first decreased and then increased, and in contrast the BTBT current experienced a rise and decline process. For the short channel lengths, the influence of the source-drain electric field on the potential of the channel predominated over the impact of the gate electric field14. This phenomenon contributed to the degradation of device electrostatics, thereby yielding a larger SS couple with diminished BTBT current. As channel length expanded, the number of defects within the C8-BTBT single-crystalline channel layer increased. Therefore, the ability of gate bias to control the channel (device electrostatics) was weakened (Fig. 3e), which made it impossible to achieve subthermionic SS.
In comparison to inorganic TFETs, our OTFETs with organic semiconductor layer provide competitive advantages such as low-temperature solution-processing, scalable and cost-effective fabrication, as well as remarkable mechanical flexibility for emerging wearable electronics. To showcase these advantages, we developed a fabrication protocol to create 10-μm-thick and flexible OTFETs over a large scale (Extended Data Fig. 28 for the fabrication details). Using this process, we successfully obtained an ultraflexible array of 5 × 6 OTFETs, displaying a 100% device yield and excellent performance uniformity. The SS values of all the devices are below 49.6 mV dec-1 and the variation in SS is less than 15.5% (Extended Data Fig. 29). Subsequently, the mechanical properties of the ultrathin OTFETs were studied systematically. A series of transfer curves of the flexible OTFET under sustaining different bending radii exhibited imperceptible variation (Extended Data Fig. 30a), confirming the excellent mechanical stability of our device even after bending at the minimum bending radius of 0.1 mm. Representative device parameters of the OTFET, including SS and Psub-T, were extracted from the transfer curves and plotted as a function of the bending radius (Extended Data Fig. 30b). The two parameters remained almost invariant under the bending radii from 1 to 0.1 mm. When the device was bent at the minimum bending radius of 0.1 mm, the change in the SS was only 14.8%, and the Psub-T fluctuations amounted to less than 10.8%. The bending durability of the OTFET was proven by performing a bending cycle test. After sustaining 1000 bending cycles, no noticeable degradation in the electrical properties was observed and the SS value of the device had no change (Extended Data Fig. 31), suggesting an extremely excellent bending durability.
The excellent high-performance uniformity and bending stability of the OTFETs enabled the development of flexible circuits—the core of wearable electronics. By integrating one pair of OTFETs, we created a flexible amplification circuit (Fig. 4a) to demonstrate its potential for emerging wearable technologies. Thanks to the record-high Aeff and ultralow Psub-T of our OTFETs, the amplifier presented steep output voltage (Vout) characteristics with an impressive voltage gain of 537 V/V (Fig. 4b) and ultralow power consumption of <0.8 nW at the peak (Fig. 4c). As a proof-of-concept demonstration, we harnessed such a high-gain and low-power amplifier to monitor human electrooculographic (EOG) signals, reflecting the potential difference between the retina and the cornea of human eyes. Two commercial gel electrodes were mounted on the right and left sides of a subject’s eyes and then wired to the amplification circuit (Fig. 4d). To match the amplifying region of the amplifier, a bias voltage was applied on one of the gel electrodes. Fig. 4e presents theamplified EOG signals with repeated left and right eye movements, showing regular voltage variations with amplitudes of ~684 mV and a signal-to-noise ratio (SNR) of 71.5 dB, both of which exceed previously reported results (Extended Table 1)4,33-40. For comparison, the unconditioned EOG signals transmitted directly from the gel electrodes displayed very low amplitudes of around 1.3 mV. This striking enhancement, facilitated by our OTFET-based ultralow-power amplifier, achieved a signal amplification of approximately 526 times, rendering the EOG signals several hundred millivolts in magnitude and amenable to wireless transmission. Furthermore, taking advantage of the high signal amplification capacity, the amplifier has the ability to track lots of sophisticated and subtle eye movements (Fig. 4f,g), which is of great significance for human-machine interfaces and medical diagnosis. Remarkably, the recording of these EOG signals only consumes very low power consumption, suggesting that the OTFET-based amplifier proposed in this work has great practicality for emerging wearable technologies.
In summary, we have broken the Boltzmann’s tyranny in organic transistors and achieved the first case of subthermionic OTFETs operating with a quantum BTBT mechanism by decoupling the metal oxide and organic semiconductor heterojunction interface through molecular decoupling layers. The molecule-assisted interface decoupling approach solved the undesired damage and defect-induced gap states at the contact during aggressive and high-energy deposition process to enable an ideal heterojunction between robust inorganic material and delicate OSC. The resulting OTFETs exhibit subthermionic SS (with an average SS lower than 60 mV dec-1 over four decades of current), the highest Aeff, and the lowest Psub-T among all the reported organic transistors. We further demonstrate that the OTFETs can address both the weak physiological signal amplification and energy-efficiency requirements. Our study marks a critical turning point in the field of organic electronics and will advance the development of the Internet of Things, bioelectronics, wearable technologies, and so on.