Tianjin University: Developing environmentally friendly and biodegradable organic neuromorphic visual sensors

With the advancement of electronic technology, people's material and spiritual lives have been greatly enriched. However, due to the accelerated pace of electronic product updates, the increase in electronic waste cannot be underestimated, which will have adverse effects on the ecological environment and human health. Faced with this thorny issue, transient electronic devices provide a clean and pollution-free way to eliminate the increasingly serious "electronic pollution" problem in the environment.Therefore, it is considered a candidate for the next generation of electronic devices needed to build a sustainable future. In such devices, degradable organic thin film transistors (OTFTs) have attracted widespread research interest due to their potential applications in display drivers, smart cards, and RFID tags.Existing reports have conducted extensive research on degradable electrodes, degradable semiconductors, degradable dielectric materials, and degradable substrates; However, the currently reported degradable devices have not shown excellent optoelectronic performance, which greatly hinders their further application in optoelectronic devices such as organic neuromorphic visual sensors (ONeuVS).

Recently, Professor Hu Wenping Ji Deyang's team and collaborators from Tianjin University used an environmentally friendly degradable polycarbonate material as a dielectric layer to construct high photoelectric performance degradable ONeuVSs. Through research, it was found that the mobility of the device is 2.74 cm2 V-1 s-1 and the current switching ratio is greater than 109.In addition, excellent optical performance has been achieved, with a maximum photosensitivity (Pmax) of 8.7 × 108 and a maximum detection rate (D * max) of 9.42 × 1016 Jones, which is the best value in transient electronic devices. In addition, the ONeuVS array can also achieve static image recognition and high pass filtering functions.After completing functional applications, the device is controllably degraded into acidic or alkaline environments without causing secondary pollution. This not only contributes to balancing device stability and environmentally friendly degradation, but also provides new insights for optimizing device performance, promoting the development of more practical transient electronics.

【Polycarbonate material】

Polycarbonate can be synthesized through the copolymerization of carbon dioxide and epoxides, which can effectively utilize greenhouse gases and achieve carbon neutrality. Typically, life cycle assessments of carbon dioxide based polycarbonates indicate that adding 20% carbon dioxide to polyether carbonates can reduce greenhouse gas emissions by 11% -19% and fossil resource consumption by 13% -16%.Therefore, it is considered a highly promising biodegradable polymer material. In this study, a degradable polymer material, polycarbonate cyclohexene ester (PCHC), was synthesized using CO2 and epoxy cyclohexene as raw materials using a one-step method as the dielectric layer. Thermogravimetric analysis (TGA) shows that PCHC has good thermal stability.Furthermore, the PCHC film prepared by spin coating method has a smooth surface with a root mean square (RMS) roughness of 0.37 nm. Moreover, it has excellent transparency in the visible light region. In addition, the capacitance and breakdown electric field strength of PCHC thin films at different frequencies (less than 178 MV/m) were determined using Si/PCHC/Ag sandwich structure. The above results indicate that PCHC polymer materials as gate insulators are ideal materials for constructing the next generation of degradable OTFTs.

【Biodegradable organic phototransistors】

In order to investigate the effect of polymer dielectric materials with different molecular weights on the electrical properties of OTFTs, OTFTs with bottom to top contact structures (C10-DNTT as organic layer) were constructed using PCHC dielectric materials with molecular weights of 9.7 kDa, 12.1 kDa, and 26.3 kDa, respectively.The results showed that the OTFT constructed based on PCHC dielectric material with a molecular weight of 12.1 kDa exhibited a high mobility of 2.74 cm2 V-1 s-1 and a current switching ratio higher than 109. In addition, the device with the best performance can still exhibit excellent stability even after being placed in an atmospheric environment for 8 months and undergoing 5700 consecutive switching operations.Based on excellent electrical performance, the optical performance of the device was explored. Through optical performance parameter evaluation, the device showed the best value among the reported degradable optoelectronic devices.

Degradable phototransistors: A OTFT array schematic diagram; B. The capacitance of PCHC polymer dielectric material (molecular weight 12.1 kDa) at different frequencies; C. XRD spectra of ITO, ITO/C10-DNTT, and ITO/PCHC (molecular weights 9.7 kDa, 12.1 kDa, and 26.3 kDa, respectively)/C10-DNTT;AFM images of D-F. C10-DNTT grown on PCHC dielectric layers with molecular weights of 12.1 kDa, 9.7 kDa, and 26.3 kDa, respectively; G. Device mobility distribution map; H. Comparison of migration rate in our work with reported degradable OTFT; L. The stability of ITO/PCHC (molecular weight 12.1 kDa)/C10-DNTT/Au structured OTFT after being placed in air for 8 months; J. P-values under different light intensities.

【Biodegradable neurovisual sensors】

Degradable photonic synaptic transistors have advantages such as low latency, fast response, and high bandwidth, and are considered to have the potential to solve the von Neumann bottleneck. In this work, we constructed degradable ONeuVSs excited by the retina and simulated the process of artificial vision system detecting light signals and converting them into electrical signals. These signals are transmitted from the optic nerve to the brain through synapses.Under light stimulation, excitatory neurotransmitters are released from the presynaptic membrane to the postsynaptic membrane, forming excitatory postsynaptic current (EPSC). By adjusting different light intensities, light duration, and the number of light pulses stimulated, the device successfully simulated synaptic plasticity.In addition, the device also has static recognition function and high pass filtering characteristics. This provides new ideas and insights for developing transient electronic devices with low power consumption and versatility.

【 Degradation mechanism based on PCHC thin film 】

Degradable electronic devices can decompose into small structural fragments after achieving their own functional applications, and are considered an effective way to protect the ecological environment and human health. In this study, chemical degradation was used to decompose electronic devices due to the decomposition of some components of the devices in alkaline or acidic solutions, including polyvinyl alcohol (PVA), aluminum (Al), and PCHC.Using water-soluble material PVA as the substrate and Al electrode as the gate. Gold is used as a source electrode and a drain electrode, which can be recycled and reused without causing any environmental pollution. During the degradation process, the PCHC film is ultimately decomposed into CO2 and cyclohexanediol through nucleophilic addition reactions in alkaline solutions.In PCHC, hydroxide ions (OH −) attack carbonyl groups in ester groups, which undergo nucleophilic addition reactions to form tetrahedral intermediates. Subsequently, the elimination process produces carbonate and alkoxy compounds. Carbonate groups are thermodynamically unstable and easily decompose into alcohol oxides, releasing CO2. Finally, alkoxy compounds extract protons from water to obtain cyclohexanediol.In addition, flexible arrays based on PCHC dielectric materials can also be degraded in hydrochloric acid solution (pH 5). During this degradation process, water molecules attack protonated ester groups and form positively charged tetrahedral intermediates through nucleophilic addition reactions. After proton transfer, intermediate products are eliminated, generating carbonates and alcohol oxides. Unstable carbonate groups can easily decompose into alcohol oxides and release CO2. Therefore, the degradation process confirms that polycarbonate based dielectric materials have environmental advantages.

Summary: This study developed a novel biodegradable PCHC dielectric material and applied it to phototransistors and synaptic transistors. After the functional application of the device, it is controllable to degrade in the environment without generating secondary pollution. This work provides ideas for the development of multifunctional applications in transient electronics.

Source: Sensor Expert Network