Ionic Inhibition Lets a Single Organic Sensor See Like a Retina

Artificial vision systems have long faced an awkward trade-off. Frame-based cameras capture everything, flooding downstream processors with redundant data about scenes that barely change. Event-based sensors, by contrast, fire only when illumination shifts, but they compress those shifts into binary on-off pulses that discard information about how strong the change actually was. The human retina does neither. It detects temporal contrast with exquisite speed while simultaneously adjusting its gain, amplifying what matters and suppressing what does not through coordinated excitatory and inhibitory circuitry. A team of researchers led by Wei Ma of Xi’an Jiaotong University, together with Zhongrui Wang of the Southern University of Science and Technology, now reports in Nature Materials a photosensor that reproduces this dual capability in a single active layer, using nothing more exotic than coupled ionic and electronic transport in an organic semiconductor blend.

The device, which the authors call an ionic-electronic event-driven sensor, or IEES, is built from a 130-nanometre-thick bulk heterojunction of two materials: PgBDT-T, a pi-conjugated polymer that belongs to the family of organic mixed ionic-electronic conductors, and Y6, a well-known non-fullerene acceptor. The blend is coated onto a gold working electrode and immersed in an electrolyte that contacts a counter electrode. What makes the architecture remarkable is its simplicity. Conventional photosensors that need both fast temporal response and adaptive gain typically require multiple transistors per pixel, with separate components handling detection, spike generation and modulation. The IEES achieves all three functions within one photoactive film, because the two blended materials divide the labour naturally.

The operating mechanism hinges on a stark mobility asymmetry between the two phases. Space-charge-limited current measurements show that electrons move through Y6 with a mobility of 7.0 x 10^-4 square centimetres per volt-second, more than two orders of magnitude faster than holes move through PgBDT-T, whose hole mobility is just 5.6 x 10^-6. When light strikes the blend, excitons generated at the heterojunction interface dissociate almost completely, as confirmed by near-total photoluminescence quenching in the optimised 1:1 blend. Electrons race through the Y6 network to the gold electrode, producing a sharp positive photocurrent spike. Meanwhile, the holes left behind in the sluggish PgBDT-T phase accumulate as positive charge.

Here is where the ionic chemistry takes over. Because PgBDT-T is ion-accessible, mobile anions from the electrolyte penetrate the polymer and compensate the accumulating positive charge. This ion-compensated charging weakens the local electric field that extracts photocarriers, promotes recombination and suppresses any further photocurrent. The result is a self-terminating spike: a fast excitatory onset followed by intrinsic inhibitory feedback, functionally analogous to the way inhibitory interneurons in the retina rein in ganglion-cell firing after an initial burst. When the light switches off, the stored charge is released, producing a characteristic negative spike. The device then returns to an electrically quiescent state with no detectable photocurrent, which means near-zero standby power.

The team marshalled an impressive battery of evidence to pin down this mechanism. In operando ultraviolet-visible spectroscopy revealed progressive bleaching of the PgBDT-T absorption near 540 nanometres under increasing illumination, accompanied by a weak sub-gap absorption band, the spectroscopic fingerprint of p-type charging. Kelvin probe force microscopy showed surface-potential shifts in the same direction whether the films were illuminated or electrochemically oxidised. Quartz crystal microbalance measurements documented swelling-associated anion uptake in PgBDT-T while Y6 remained essentially ion-inert. Control devices built from the non-OMIEC blend PTQ10:Y6, tested under identical geometry, electrolyte and illumination, showed only slow monotonic photocurrent decay with no spike at all, confirming that ion-accessible mixed conduction is the essential ingredient. Drift-diffusion simulations reinforced the picture: as the effective charge density rose from 10^16 to 10^18 per cubic centimetre, the simulated short-circuit current collapsed from roughly 95 percent to 17 percent of its neutral value, because bimolecular recombination and transport bottlenecks throttled extraction.

The sensor’s performance figures are striking in their own right. The photocurrent spike amplitude follows a power law in the illumination change, with exponents of about 0.28 for turn-on events and 0.05 for turn-off events across a range spanning 1 to 90 milliwatts per square centimetre. The minimum detectable change is 0.055 milliwatts per square centimetre, roughly 60 lux, comparable to the lower comfort limit for human reading, and the dynamic range exceeds 75 decibels. The device operated with less than 5 percent deviation over 20,000 seconds of continuous cyclic illumination at zero bias in ambient laboratory conditions, suggesting that the ionic feedback is genuinely reversible rather than degrading. Broadband operation was confirmed at wavelengths of 455, 565, 660 and 850 nanometres, and the response tracks the rate of illumination change, with fast ramps producing narrow, high-amplitude spikes that saturate above about 100 milliwatts per square centimetre per second.

The most consequential feature, however, is voltage-programmable amplitude control. Applying a small positive bias to the working electrode strengthens the ionic feedback pathway through bias-assisted anion compensation, and the amplitude coefficient of the spike response falls exponentially with voltage, with decay constants of 7.2 per volt for turn-on events and 11.1 per volt for turn-off events, while the power-law exponents remain largely untouched. In other words, bias rescales the excitation-inhibition balance without distorting the underlying contrast sensitivity. Background illumination provides a complementary passive adaptation, attenuating spikes exponentially with increasing background level. The team demonstrated the practical payoff with a 4 x 4 pixel array on which dynamic letter patterns were projected: applying 0.2 volts to brightly lit pixels and zero volts to dim ones normalised the responses into a comparable amplitude range, an active gain equalisation performed entirely within the sensor.

Scaled to a 128 x 128 pixel simulation of a video containing three moving participants, the same principle produced something resembling visual attention. Under a uniform 0.1-volt bias, event outputs simply reflected local motion. But when the researchers assigned zero volts to one participant, 0.2 volts to another and 0.1 volts elsewhere, the response from the first participant was enhanced nearly tenfold while the second was strongly suppressed, with the third essentially unchanged. Because the convolutional kernel weights of a neural network can likewise be encoded as pixel-specific bias voltages, the array performs analogue weighting physically, before any data leave the sensor. The authors call this hybrid amplitude-temporal coding: each illumination change generates one analogue event whose magnitude carries both the local contrast change and the programmed weight.

The system-level demonstration is where the efficiency argument becomes concrete. Feeding device-informed event streams from the Weizmann human-action dataset into a lightweight long short-term memory classifier, the IEES-based pipeline achieved 100 percent classification accuracy on four action classes, compared with 94.2 percent for a frame-based network and 75.8 percent for an event-camera-based network using the same downstream architecture. More telling is the energy accounting. Under matched benchmarking conditions across the full 149-video set, the IEES front end consumed 0.064 millijoules, including event generation and in-sensor analogue convolution. The event-camera pipeline required 0.69 joules and the frame-based pipeline 2.80 joules, while an RGB rate-coding baseline demanded 15.70 joules. That is a difference of more than four orders of magnitude, arising because amplitude coding preserves motion-relevant contrast in a single sparse event rather than spending spike counts to represent magnitude.

The platform also shows encouraging generality. Event-driven responses persisted in sodium, calcium and potassium carbonate electrolytes, in solid-state devices using sodium chloride hydrogel, across active areas from 0.24 to 1.0 square centimetre, and in microfabricated pixel arrays with consistent polarity and timing. Alternative OMIEC donors including PgBDT-Se, PTQ-6bO2, HOMO-gDPP, PgDPP-TT and Pg2TE-TT-i all produced clear spikes when blended with Y6, while the closely related but ion-inaccessible P3HT:Y6 blend showed only monotonic decay, underscoring ionic accessibility as the decisive design rule. Much work remains before such electrolyte-interfaced devices can rival mature silicon imagers in integration density and durability, and the amplitude variation across manually dispensed electrolyte droplets hints at the manufacturing challenges ahead. But the conceptual advance is clear and potentially far-reaching: by recruiting mobile ions as an intrinsic inhibitory mechanism, organic mixed conductors offer a materials-level route to vision hardware that senses, adapts and prioritises in one place, the way biology has always done it.

Subject of Research: Organic mixed ionic-electronic conductor photosensors with ionic-mediated inhibitory feedback for event-driven neuromorphic vision

Article Title: Amplitude-controllable event-driven organic photosensors based on ionic-mediated inhibition

Article References: Zhao, C., Su, X., Chen, X., Zhang, S., Yuan, Z., Lin, N., Shen, J., Tao, Y., Wang, S., Wang, B., Li, T., Li, X.-H., Fan, Q., Wang, Z., & Ma, W. (2026). Amplitude-controllable event-driven organic photosensors based on ionic-mediated inhibition. Nature Materials. https://doi.org/10.1038/s41563-026-02747-8

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02747-8

Keywords: organic mixed ionic-electronic conductors, neuromorphic vision, event-driven sensing, bulk heterojunction, ionic-electronic coupling, temporal-contrast detection, PgBDT-T, Y6, spike amplitude modulation, in-sensor computing, retina-inspired sensors, low-power photodetection