A flexible, multifunctional, optoelectronic anticounterfeiting device from high-performance organic light-emitting paper

Counterfeiting remains a growing global issue affecting the economy, health, and environment. Conventional paper-based anticounterfeiting relies largely on photoresponsive features (e.g., dithered patterns, plasma tags, security inks, holograms, watermarks, long-lag phosphors). Such single-stimulus photo-based authentication offers a low security threshold and is vulnerable to cloning. Developing systems with multiple stimuli-responsiveness is therefore highly desirable. Integrating optoelectronic devices into existing paper-based approaches can provide electro- and photo-responsive features simultaneously. OLEDs are particularly attractive as they generate electro-responsive patterns (from electrode overlap) and photo-responsive patterns (from organic emitting area). However, fabricating high-performance OLEDs on paper is challenging due to paper’s porous, fibrous, and rough surface, which can cause breakdowns or short circuits and generally poor device performance. This work addresses these challenges by morphologically modifying commercial paper to serve as a viable substrate for high-performance OLEDs and demonstrates a flexible anticounterfeiting device with multiple stimuli responsiveness and high security.

The paper surveys common anticounterfeiting modalities—dithered patterns, plasma tags, security inks, holograms, watermarks, and phosphors—typically implemented via printing on flexible paper substrates and authenticated by photoresponse alone, leading to low security thresholds. Prior efforts on paper-based OLEDs have faced performance limitations due to substrate roughness and porosity causing leakage and shorting. The authors highlight the absence of high-performance OLED integration onto paper anticounterfeiting products and position OLEDs as ideal candidates due to their dual electro- and photo-responsive capabilities. The work builds on knowledge that reducing substrate roughness and improving morphology are crucial for organic thin-film devices and leverages established encapsulation technologies for operational and storage lifetime considerations.

- Substrate screening and selection: Five commercial papers (stone, art, printing, sulfuric, filter) were imaged by SEM to assess porosity/roughness. Stone paper was chosen for environmental reasons and characteristic morphology. A QR code was laser-printed to ensure preprinted information could be preserved. - Morphological modification: Papers were dip-coated in a PMMA solution (details in Experimental; dip-coating followed by annealing). PMMA infiltrated pores yielding a flat surface (AFM roughness ~21.4 nm). Cross-sectional SEM indicated PMMA film thickness on the order of tens of micrometers. Intrinsic micro/micro-scale morphological features of the paper were preserved to retain identifiable characteristics. - Device fabrication: Top-emitting OLEDs were deposited on the treated (PMMA-infiltrated) stone paper. A patterned emitting layer was defined via a shadow mask (e.g., institution logo). A typical green OLED stack used was: Al (100 nm)/MoO3 (3 nm)/TAPC (30 nm)/TcTa (5 nm)/CBP:10% Ir(ppy)3 (20 nm)/TmPyPB (50 nm)/LiF (0.5 nm)/Mg:Ag (15:1, ~1 nm)/Ag (19 nm). Paper opacity necessitated a top-emitting architecture. Patterning created electro- and photo-responsive features without obscuring the preprinted QR code. - Devices for performance comparison: A paper-based green device with fixed emitting area (3.3 × 3 mm^2), termed TG-P, and a glass-based counterpart with the same stack (TG-G) were fabricated. Additional devices on treated art paper and other treated commercial papers were also made for benchmarking. A PET-based device (TG-PET) with the same structure was prepared for mechanical comparison. - Characterization: Electrical/optical performance (J–V–L, current efficiency–brightness, EQE), EL spectra, and operational lifetime (constant-current operation; acceleration factor 1.7 to estimate LT50 at 100 cd m^-2) were measured. Storage lifetime was assessed after 10 days. Surface morphology was characterized by AFM and SEM; cross-sections by SEM. - Mechanical testing: Repeated bending at 8 mm radius for 100, 500, 1000 cycles; performance variations recorded at 5 V. Sheet resistance evolution of electrodes (Al on paper, Al on PET, ITO/PET) with bending was measured. Flexural properties of substrates (stone paper with and without PMMA dip-coating; PET) were measured by three-point bending to extract flexural modulus and bending strength. Peel-adhesion testing used 3M scotch tape to compare adhesion of Al films on paper vs PET by measuring peel force–displacement and inspecting electrode integrity pre/post peel. - FAC device concept: Patterned electro-responsive OLED emitters and photo-responsive emitters were integrated on paper. Optical microcavity effects were introduced to produce observable color shifts with viewing angle and applied voltage, enabling multi-stimuli (light, electricity, combined) anticounterfeiting responses.

- Viable paper substrate: PMMA dip-coating reduced surface roughness (AFM ~21.4 nm) sufficient for OLED deposition without sacrificing preprinted information or intrinsic paper features. - High performance OLEDs on paper: The paper-based green device TG-P achieved maximum brightness of 71,346 cd m^-2 and maximum current efficiency of 64 cd A^-1, comparable spectra to glass-based TG-G. On treated art paper, performance reached 110,000 cd m^-2 and 90 cd A^-1, the highest reported among paper-based OLEDs summarized by the authors. - Lifetime: With an acceleration coefficient of 1.7, LT50 at 100 cd m^-2 for TG-P is estimated >4,000 h. Storage tests showed >90% luminance retention after 10 days (1 mA drive). Encapsulation maturity suggests suitability for commercial lifetimes. - Mechanical robustness: Paper-based OLEDs are bendable, foldable, twistable, and tailorable. After 100, 500, and 1000 bending cycles at 8 mm radius, TG-P exhibited only slight changes in current density and brightness and minimal change in current efficiency. In contrast, TG-PET degraded significantly, with some areas failing to turn on after 1000 cycles. - Degradation mechanisms: Bending primarily increased electrode sheet resistance, reducing current density and brightness; organic functional layers maintained efficiency. Microscopy revealed far fewer cracks in TG-P than TG-PET post-bending. - Flexural properties: Paper substrates had flexural modulus and bending strength roughly one-quarter of PET at similar thickness, indicating higher flexibility and lower bending-induced stress, thus less device damage. - Adhesion: Al adheres more strongly to paper than to PET. Peel tests required lower force on PET; Al on PET showed damage post-peel, while Al on paper remained intact, attributed to higher nanoscale roughness of paper aiding adhesion. - Anticounterfeiting functionality: The FAC device integrated patterned electro-responsive and photo-responsive emitters on paper and used microcavity effects to create discernible color shifts with viewing angle and voltage. The device shows multi-stimuli responsiveness (to light, electricity, and their combination), multiple patterns/colors, and unclonable behaviors while preserving preprinted security features (e.g., QR code).

The study demonstrates that morphological modification of commercial paper via PMMA dip-coating transforms it into a suitable substrate for high-performance OLEDs despite the inherent porosity and roughness of paper. The resulting top-emitting paper-based OLEDs deliver brightness and efficiency on par with or exceeding prior art and approach glass-based device performance, though with somewhat higher leakage due to residual roughness. The improved mechanical flexibility compared with PET-based devices arises from the lower flexural modulus and bending strength of paper, which reduce bending-induced stress, and from stronger electrode–substrate adhesion on paper. These mechanical advantages translate to superior durability under repeated bending. Integrating OLEDs with patterned electro- and photo-responsive regions and leveraging microcavity-induced angle/voltage-dependent color shifts create multiple, easily verifiable, and difficult-to-clone security features. The combination of high optoelectronic performance, multi-stimuli responsiveness, and retention of intrinsic paper identifiers (microstructure, preprinted codes) addresses the low-security threshold of traditional photo-only anticounterfeiting methods and offers a practical pathway toward robust, unclonable anticounterfeiting devices.

The work introduces a facile, low-cost process to produce high-performance organic light-emitting paper and a flexible, multifunctional optoelectronic anticounterfeiting (FAC) device. By PMMA dip-coating and subsequent OLED fabrication, the devices preserve preprinted information and intrinsic paper morphology, achieve record-high brightness and efficiency for paper-based OLEDs, and exhibit strong durability (LT50 > 4000 h at 100 cd m^-2) and mechanical flexibility under repeated bending. The FAC device provides multi-stimuli responsiveness (light, electricity, and combined), angle- and voltage-dependent color shifts via microcavities, and unclonable patterns/colors, significantly elevating security beyond traditional photoresponsive methods. Future research could optimize surface planarization to further reduce leakage and close the performance gap to glass, explore broader color/emitter sets and microcavity designs for richer authentication schemes, integrate robust encapsulation for long-term field use, and scale patterning/printing processes for mass production and variable data security.

- Residual surface roughness of treated paper remains higher than glass, leading to increased leakage currents and somewhat lower efficiency under the same operating conditions. - The opaque nature of paper necessitates a top-emitting device architecture, which may constrain optical design choices compared to transparent substrates. - Lifetime estimates for LT50 rely on an acceleration factor; long-term field data under varied environmental conditions were not reported in detail. - While storage tests showed >90% retention after 10 days, extended storage and environmental robustness (humidity, temperature cycling) with thin-film encapsulation were not fully elaborated. - Detailed quantitative metrics of the multi-stimuli anticounterfeiting demonstrations (e.g., exact color shift values vs angle/voltage, cloning attack analyses) were only qualitatively described in the provided text.