A single organic device reconfigures as transistor, rectifier and logic gate, offering compact circuits with higher functional density for flexible and lightweight electronics.
(Nanowerk Spotlight) Electronics have advanced by shrinking their basic building block, the transistor. Modern chips now contain billions of these switches, each one measured in nanometers, a scale comparable to the spacing between individual molecules. At such scales, the rules of operation start to shift. Switches leak current instead of turning fully off, and driving them requires more energy. Manufacturing also grows more complex and expensive the further this approach is pushed.
Silicon has enabled this era of miniaturization. Its crystalline structure can be etched into precise patterns, making it the foundation of computing power in phones, laptops, and servers. But progress is slowing. Building ever smaller silicon devices is increasingly costly, and the benefits are diminishing. That is one reason researchers are searching for alternatives that can expand electronics into new areas rather than relying on continued miniaturization.
One path involves organic semiconductors, which are made from carbon-based molecules. These materials can be deposited on flexible or lightweight surfaces, opening the door to bendable displays, medical sensors that conform to skin, and low-cost devices printed on plastics. But organic circuits face their own barrier. They cannot be patterned at the same tiny scales as silicon, so their devices remain larger and harder to pack densely. That makes complex systems difficult to build, even as the appeal of flexible and wearable electronics continues to grow.
A shift in strategy is taking place. If organic transistors cannot simply be made smaller, perhaps they can be made more versatile. The idea is to create reconfigurable devices that can change function depending on how they are operated. Instead of filling a circuit with multiple specialized components, a single element could take on several roles. Early attempts required complicated electrode layouts that consumed power and space. More recently, advances in ultrathin crystalline films have enabled a different approach, using carefully designed junctions where two materials meet to steer current in new ways.
Structure and versatility of the 2D reconfigurable organic heterojunction device. (a) A schematic shows the device layout, where two ultrathin organic crystals overlap at one electrode to form the active junction. (b) An optical microscope image captures the physical scale of the device. (c) An atomic force microscopy scan demonstrates the crystals’ uniform thickness and clean overlap. (d) Electron diffraction data confirm that both layers are highly ordered single crystals, ensuring a sharp interface. (e) A conceptual diagram highlights how the same junction can serve different roles, operating as a transistor, a rectifier, or a logic gate depending on applied conditions. (Image: Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
The design is based on a reconfigurable asymmetric heterojunction, an interface where two different crystals meet. One crystal, C6-DPA, conducts positive charges, known as holes. The other, TFT-CN, conducts negative charges, or electrons. A layer of C6-DPA spans the source and drain electrodes, while a single molecular layer of TFT-CN sits only at the drain side. Gold contacts are applied in a way that preserves the integrity of the crystals.
This geometry places the junction at one end of the device and makes it extremely thin. That turns out to be critical. When a voltage is applied with the drain positive, the current flows through the junction by tunneling, a quantum effect where carriers pass directly through a barrier. Cooling the device has little impact, confirming that the process does not rely on heat. When the drain is negative, the story changes. Carriers must overcome an energy barrier, so the current falls sharply as the device is cooled. In this case, charges are injected from the source and recombine at the junction, a process that depends on thermal energy.
The same structure therefore supports two distinct transport mechanisms depending on bias. With positive drain voltage, tunneling dominates. With negative drain voltage, thermally activated flow takes over. The ability to toggle between such different behaviors comes directly from placing a monolayer junction at the drain. Thicker junctions do not show this effect.
As a transistor, the device operates with strong performance. It switches cleanly, with very little leakage current and a large difference between on and off states. These qualities arise from the molecularly thin crystals and clean interfaces, which allow precise control of carriers.
The rectification behavior is especially striking. Rectification means allowing current to pass more easily in one direction than the other, as in a diode. By adjusting the gate voltage, Wu and colleagues were able to tune this effect over eight orders of magnitude. At certain gate settings, the device favored current in one direction strongly. At others, it reversed preference. The gate effectively acts as a control knob, switching the rectifying behavior on demand.
Light provides another control. When illuminated with violet light, the device shows a large increase in current under positive drain bias, but very little under negative bias. This difference arises because light generates electron-hole pairs in the crystal, which can be separated efficiently under one polarity but tend to recombine under the other. The photoresponse is unusually strong, with the device able to detect weak signals when biased in the right way.
With both gate voltage and light available as inputs, the researchers configured the device to perform logic operations. Under one polarity, it acted like an AND gate, producing output only when both inputs were present. Under the opposite polarity, it acted like an OR gate, producing output when either input was present. Switching between these functions required only reversing the drain voltage, not changing the device structure.
This multifunctionality is significant. A conventional circuit would need multiple transistors to perform the same tasks. Combining them into one element increases functional density, a crucial metric when miniaturization is not feasible. The device also proved stable during repeated tests in air and continued working when built on flexible substrates, showing practical resilience.
The researchers note that their prototypes required relatively high operating voltages because they used a thick layer of silicon dioxide as the gate insulator. In practical circuits, thinner insulators would lower the required voltages. The essential behaviors, such as tunneling and reconfigurable logic, stem from the heterojunction itself rather than the substrate.
Fabrication plays an important role here. The team grew the heterojunction through a single crystallization step in solution, which produced clean, defect-free interfaces. This process avoids the contamination and misalignment that can occur when crystals are stacked manually. The result is a structure thin enough and clean enough to support tunneling and precise control of carrier flow.
The broader significance lies in shifting the design philosophy for organic electronics. Instead of competing with silicon on size, this approach leverages the unique properties of molecularly thin crystals to achieve multiple functions in one component. For flexible electronics, wearable sensors, or lightweight diagnostic tools, this kind of reconfigurable element could reduce part counts and add capabilities without increasing bulk. Circuits could integrate sensing, rectification, and logic using fewer devices, making them lighter and easier to manufacture.
Wu and colleagues show that a two-dimensional organic heterojunction can serve as a transistor, a diode, and a logic gate, depending on how it is biased and illuminated. The work demonstrates a different way forward for organic electronics, where careful control of thickness and geometry allows a single device to perform tasks that usually require several. It highlights a strategy where smarter design, rather than smaller size, defines progress.
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