Adapting to light with a transistor

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In today’s world, adaptation exists almost everywhere. For human societies, all individuals and groups must adjust their behaviors to conform to the norms and values of that particular society. For human beings, people evolved over time to develop the ability to adapt to changes in their environments. For human eyes, visual adaptation allows autonomic responses to stimuli and endows sensory systems with dynamic fitness to varying light irradiations.

Interestingly, when I joined Beijing National Laboratory for Molecular Sciences and the University of the Chinese Academy of Sciences, our group had already started an original work on adaptive devices [Shen, H. et al. Adv. Mater. 2019, 31, 1905018]. An organic adaptive transistor (OAT) was successfully created to mimic sensory adaptation, and this unique device can serve as a promising candidate for encoding dynamic stimuli and achieving multifunctional biomimetic sensing elements. Then an idea comes to my mind, whether it is possible to create a photoadaptive device that can exhibit an autonomic response to dynamic light stimuli?

We started work on photoadaptive devices full of enthusiasm, but challenges aroused as soon as we carried out this project. From the viewpoint of device engineering, an ideal adaptive element should be based on a single device, in order to pursue the simplest geometry and minimized power consumption, for the goal of ‘Less is more’. To satisfy this critical requirement, a device should exhibit a fine-tuned transient response and dynamic adaptation according to the intensity of light stimuli; however, this seems impossible owing to the paradoxical demands of photoexcitation and inhibition of charge transport in the same channel. These questions puzzled us for a long time until we returned to the biological systems to seek inspiration. Interestingly, a similar conundrum has been overcome ingeniously in biological visual systems by using coupled physiological mechanism. The related transmembrane transport of two different ions (Na+ and Ca2+) together with the associated gating dynamics, leading to biological active adaptation. This unique mechanism offers inspiration for the coupled manipulation of charge carriers on an organic thin-film transistor (OTFT) toward photoadaptive transistors.

Consequently, we developed an organic active adaptation transistor (OAAT) that exhibited light intensity-dependent photoadaptation. The knack is to introduce two functionally complementary bulk-heterojunctions into an OTFT. In a model OAAT, the upper bulk-heterojunction serves as the photoresponsive active layer to ensure efficient charge transport and photocarrier generation, whereas the buried bulk-heterojunction in the dielectric layer serves as a floating gate. Upon light irradiation, the device couples the photovoltaic effect with field-effect modulation to allow photo-triggered active adaptation for luminance intensities ranging over six orders of magnitude. Of particular note, the dynamic behavior varied with light intensities in an autonomic manner, indicating the smart feature of the OAAT.

We did create the active adaptation devices, but how to evaluate the performance? The fact is there was no basic parameter, which constitutes a huge obstacle to the research of OAAT. We spent nearly three months on this issue, and further understanding of the visual system guided us to propose the active adaptation index (AAI). Excitingly, the extracted results show high similarity to that of humans, suggesting realization of active visual adaptation with the OAAT. In an effort to improve our work, we are happy to discuss with researchers from various aspects. Thanks to their kind suggestions, we completed the last piece of the puzzle, the comprehensive theoretical model to describe the dynamic relationship between photoadaptative response and environmental stimuli, and found fascinating results. The predicted tendency offers a guideline to design OAATs on-demand in a straightforward way.

These results represent a pioneering work for adaptive devices and may provide an exciting blueprint for exploring the next generation of artificial perception systems.

Curious for more? You can read our full paper “An organic transistor with light intensity-dependent active photoadaptation” published in Nature Electronics: https://www.nature.com/articles/s41928-021-00615-8#citeas.

Zihan He

PhD student, Institute of Chemistry, Chinese Academy of Sciences