High‐Performance Solution‐Processed Organo‐Metal Halide Perovskite Unipolar Resistive Memory Devices in a Cross‐Bar Array Structure

Resistive random access memories can potentially open a niche area in memory technology applications by combining the advantages of the long endurance of dynamic random‐access memory and the long retention time of flash memories. Recently, resistive memory devices based on organo‐metal halide perovskite materials have demonstrated outstanding memory properties, such as a low‐voltage operation and a high ON/OFF ratio; such properties are essential requirements for low power consumption in developing practical memory devices. In this study, a nonhalide lead source is employed to deposit perovskite films via a simple single‐step spin‐coating method for fabricating unipolar resistive memory devices in a cross‐bar array architecture. These unipolar perovskite memory devices achieve a high ON/OFF ratio up to 10⁸ with a relatively low operation voltage, a large endurance, and long retention times. The high‐yield device fabrication based on the solution‐process demonstrated here will be a step toward achieving low‐cost and high‐density practical perovskite memory devices.     

Controllable Organic Resistive Switching Achieved by One‐Step Integration of Cone‐Shaped Contact

Conductive filaments (CFs)‐based resistive random access memory possesses the ability of scaling down to sub‐nanoscale with high‐density integration architecture, making it the most promising nanoelectronic technology for reclaiming Moore's law. Compared with the extensive study in inorganic switching medium, the scientific challenge now is to understand the growth kinetics of nanoscale CFs in organic polymers, aiming to achieve controllable switching characteristics toward flexible and reliable nonvolatile organic memory. Here, this paper systematically investigates the resistive switching (RS) behaviors based on a widely adopted vertical architecture of Al/organic/indium‐tin‐oxide (ITO), with poly(9‐vinylcarbazole) as the case study. A nanoscale Al filament with a dynamic‐gap zone (DGZ) is directly observed using in situ scanning transmission electron microscopy (STEM) , which demonstrates that the RS behaviors are related to the random formation of spliced filaments consisting of Al and oxygen vacancy dual conductive channels growing through carbazole groups. The randomicity of the filament formation can be depressed by introducing a cone‐shaped contact via a one‐step integration method. The conical electrode can effectively shorten the DGZ and enhance the localized electric field, thus reducing the switching voltage and improving the RS uniformity. This study provides a deeper insight of the multiple filamentary mechanisms for organic RS effect.     

Device Physics of Solution‐Processed Organic Field‐Effect Transistors

Field‐effect transistors based on solution‐processible organic semiconductors have experienced impressive improvements in both performance and reliability in recent years, and printing‐based manufacturing processes for integrated transistor circuits are being developed to realize low‐cost, large‐area electronic products on flexible substrates. This article reviews the materials, charge‐transport, and device physics of solution‐processed organic field‐effect transistors, focusing in particular on the physics of the active semiconductor/dielectric interface. Issues such as the relationship between microstructure and charge transport, the critical role of the gate dielectric, the influence of polaronic relaxation and disorder effects on charge transport, charge‐injection mechanisms, and the current understanding of mechanisms for charge trapping are reviewed. Many interesting questions on how the molecular and electronic structures and the presence of defects at organic/organic heterointerfaces influence the device performance and stability remain to be explored.     

Advances in Solution‐Processed Multijunction Organic Solar Cells

The efficiency of organic solar cells can benefit from multijunction device architectures, in which energy losses are substantially reduced. Herein, recent developments in the field of solution‐processed multijunction organic solar cells are described. Recently, various strategies have been investigated and implemented to improve the performance of these devices. Next to developing new materials and processing methods for the photoactive and interconnecting layers, specific layers or stacks are designed to increase light absorption and improve the photocurrent by utilizing optical interference effects. These activities have resulted in power conversion efficiencies that approach those of modern thin film photovoltaic technologies. Multijunction cells require more elaborate and intricate characterization procedures to establish their efficiency correctly and a critical view on the results and new insights in this matter are discussed. Application of multijunction cells in photoelectrochemical water splitting and upscaling toward a commercial technology is briefly addressed.     

Solution‐Processed Wide‐Bandgap Organic Semiconductor Nanostructures Arrays for Nonvolatile Organic Field‐Effect Transistor Memory

In this paper, the development of organic field‐effect transistor (OFET) memory device based on isolated and ordered nanostructures (NSs) arrays of wide‐bandgap (WBG) small‐molecule organic semiconductor material [2‐(9‐(4‐(octyloxy)phenyl)‐9H‐fluoren‐2‐yl)thiophene]3 (WG₃) is reported. The WG₃ NSs are prepared from phase separation by spin‐coating blend solutions of WG₃/trimethylolpropane (TMP), and then introduced as charge storage elements for nonvolatile OFET memory devices. Compared to the OFET memory device with smooth WG₃ film, the device based on WG₃ NSs arrays exhibits significant improvements in memory performance including larger memory window (≈45 V), faster switching speed (≈1 s), stable retention capability (>10⁴ s), and reliable switching properties. A quantitative study of the WG₃ NSs morphology reveals that enhanced memory performance is attributed to the improved charge trapping/charge‐exciton annihilation efficiency induced by increased contact area between the WG₃ NSs and pentacene layer. This versatile solution‐processing approach to preparing WG₃ NSs arrays as charge trapping sites allows for fabrication of high‐performance nonvolatile OFET memory devices, which could be applicable to a wide range of WBG organic semiconductor materials.