Many people like to jot down their fresh ideas with pen and paper. In the future, they could draw those ideas in the form of custom LEDs on party balloons, clothing or even personalized medical sensors.
Researchers in the United States have developed a system for handwriting LEDs and photodetectors onto many ordinary substrates, from paper to rubber and fabric (Nat. Photon., doi: 10.1038/s41566-023-01266-1). The team filled regular ballpoint pens with a special “ink” containing tiny metal wires and nanocrystals of perovskites—the versatile semiconductors that light up brightly under the right stimulation. Potential applications range from flexible, disposable wearable sensors to customizable textiles and smart packaging, according to the researchers.
“The writing experience mirrors the natural flow of everyday writing,” says lead author Junyi Zhao, a doctoral candidate in the laboratory of Chuan Wang, an engineering professor at Washington University in St. Louis, USA.
Simplifying optoelectronic fabrication
The time-tested techniques for fabricating optoelectronic devices include spin coating, evaporation and etching, usually involving vacuum chambers or other specialized equipment. Although some scientists—including Wang and his colleagues—have tried inkjet printing and other simpler deposition systems as substitutes, cleaning and aligning the inkjet printheads can be a hassle.
Two years ago, Zhao and Wang debuted an organic–inorganic compound optoelectronic ink consisting of perovskite crystals embedded in a flexible polymer matrix, and they printed circuits by feeding the compound into an inkjet printer. A humble ballpoint pen, filled with the liquid LEDs instead of its original ink, turned out to be an even simpler delivery mechanism, but the perovskite ink formula first needed tweaking.
Zhao says he and his colleagues carefully tuned the ink’s rheology, or ability to flow, and its wetting ability to produce uniform lines on many different surfaces. The team also had to customize the solvents so that drawing multiple layers of the optoelectronic ink on the same spot would not cause the upper layers to dissolve or compromise those below.
“In terms of the writing experience, we thoroughly examined the impact of applied force on the pen during the writing process, defined as ‘soft writing’ and ‘hard writing,’” Zhao says. “Notably, the force applied to the pen won’t compromise the functionality of our optoelectronic devices. Interestingly, both soft and hard writing techniques effectively influence the width of the writing path. Soft writing yields a narrower path, whereas hard writing yields a wider path. This versatility in writing pressure contributes to customizable patterning resolutions in the final device.”
Considerations for LED inks
Because the hand-drawn LEDs have a vertical sandwich structure, the researchers worked hard to keep the layers sufficiently separated and uniform in thickness to prevent current leakage between the top and bottom electrodes, Wang says. “This is easier on planar and nonabsorbing surfaces, such as glass or plastic films, but becomes especially challenging on fibrous and porous substrates, such as paper and textile,” he adds.
According to Zhao, the researchers needed to consider the drying time of their LED inks because any design value of the drawn images would be lost if the ink got smeared. To expedite drying, the team incorporated various low-boiling-point solvents as additives, including isopropanol and toluene. “These solvents effectively lowered the boiling point of our functional inks,” Zhao says. “Consequently, the drying rate of certain layers, such as the perovskite light-emitting layer, polyethylenimine buffer layer and silver nanowire top electrode, was notably accelerated. In fact, in some cases, the drying occurred rapidly or immediately, eliminating the need for any waiting period.”
To demonstrate the strength and versatility of the handwritten LEDs, the researchers drew designs on the interior surface of a glass vial and on both sides of a piece of paper. For the latter, the team attached a coin-style battery and demonstrated the on-off operation of a drawing of a traffic light. Group members drew a pattern on the surface of a rubber balloon and subjected it to several dozen inflation–deflation cycles with a biaxial stretch of more than 40%, and they reported that the performance did not noticeably degrade.
The researchers also found that the brightness of the handwritten perovskite LEDs could go as high as 15,225 cd/m2, while the maximum responsivity of the handwritten perovskite photodetectors was 132 mA/W.
Next, Wang and colleagues plan to incorporate the writable ink into disposable electronics for potential biomedical applications. According to Wang, the researchers will also fine-tune the ink formulas to substitute lead-free perovskites to reduce or eliminate the ink’s toxicity.