Modification of Two-Dimensional Tin-Based Perovskites by Pentanoic Acid for Improved Performance of Field-Effect Transistors

Odd-Even Alkyl Chain Effects on the Structure and Charge Carrier Transport of Two-Dimensional Sn-Based Perovskite Semiconductors

Oscillations in the chemical or physical properties of materials, composed of an odd or even number of connected repeating methylene units, are a well-known phenomenon in organic chemistry and materials science. So far, such behavior has not been reported for the important class of materials, perovskite semiconductors. This work reports a distinct odd-even oscillation of the molecular structure and charge carrier transport properties of phenylalkylammonium two-dimensional (2D) Sn-based perovskites in which the alkyl chains in the phenylalkylammonium cations contain varying odd and even carbon numbers. Density functional theory calculations and grazing-incidence wide-angle X-ray scattering characterization reveal that perovskites with organic ligands containing an alkyl chain with an odd number of carbon atoms display a disordered crystal lattice and tilted inorganic octahedra accompanied by reduced mobilities. In contrast, perovskites with cations of an even number of carbon atoms in the alkyl chain form more ordered crystal structures, resulting in improved charge carrier mobilities. Our findings disclose the importance of minor changes in the molecular conformation of organic cations have an effect on morphology, photophysical properties, and charge carrier transport of 2D layered perovskites, showcasing alkyl chain engineering of organic cations to control key properties, of layered perovskite semiconductors.

Multicolor vision perception of flexible optoelectronic synapse with high sensitivity for skin sunburn warning

The development of flexible synaptic devices with multicolor signal response is important to exploit advanced artificial visual perception systems. The Sn vacancy-dominant memory and narrow gap characteristics of PEA2SnI4 make it suitable as a functional layer in ultraviolet-visible (UV-Vis) light-stimulated synaptic devices. However, such device tends to have high dark current and poor sensitivity, which is not conducive to subsequent information processing. Here, we proposed a self-powered flexible optoelectronic synapse based on PEA2SnI4 films. By introducing the electron transport layer (ETL), the dark current of the device is decreased by 5 orders of magnitude as compared to the Au/PEA2SnI4/ITO device, and the sensitivity is increased from 10.3% to 99.2% at 1.25 mW cm-2 light illumination (520 nm), indicating the vital role of the introduced ETL in promoting the separation of excitons in the interface and inhibiting the free carrier transfer. On this basis, the optoelectronic synaptic functions with integrated sensing, recognition, and memory features were realized. The array device exhibits UV-Vis light sensitivity and tunable synaptic plasticity, enabling its application for multicolor visual sensing and skin sunburn warning. This work provides an effective strategy for fabricating multicolor intelligent sensors and artificial vision systems, which facilitate the practical application of artificial optoelectronic synapses.

Unveiling the role of linear alkyl organic cations in 2D layered tin halide perovskite field-effect transistors

Two-dimensional (2D) tin halide perovskites are promising semiconductors for field-effect transistors (FETs) owing to their fascinating electronic properties. However, the correlation between the chemical nature of organic cations and charge carrier transport is still far from understanding. In this study, the influence of chain length of linear alkyl ammonium cations on film morphology, crystallinity, and charge transport in 2D tin halide perovskites is investigated. The carbon chain lengths of the organic spacers vary from propylammonium to heptanammonium. The increase of alkyl chain length leads to enhanced local charge carrier transport in the perovskite film with mobilities of up to 8 cm2 V-1 s-1, as confirmed by optical-pump terahertz spectroscopy. A similar improved macroscopic charge transport is also observed in FETs, only to the chain length of HA, due to the synergistic enhancement of film morphology and molecular organization. While the mobility increases with the temperature rise from 100 K to 200 K due to the thermally activated transport mechanism, the device performance decreases in the temperature range of 200 K to 295 K because of ion migration. These results provide guidelines on rational design principles of organic spacer cations for 2D tin halide perovskites and contribute to other optoelectronic applications.

Photoinduced Nonvolatile Memory Transistor Based on Lead-Free Perovskite Incorporating Fused π-Conjugated Organic Ligands

Perovskites field-effect transistors (PeFETs) have been intensively investigated for their application in detector and synapse. However, synapse based on PeFETs is still very difficult to integrate excellent charge carrier transporting ability, photosensitivity, and nonvolatile memory effects into one device, which is very important for developing bionic electronic devices and edge computing. Here, two-dimensional (2D) perovskites are synthesized by incorporating fused π-conjugated pyrene-O-ethyl-ammonium (POE) ligands and a systematic study is conducted to obtain enhanced performance and reliable PeFETs. The optimized (POE)2 SnI4 transistors display the hole mobility over 0.3 cm2  V-1  s-1 , high repeatability, and operational stability. Meanwhile, the derived photo memory devices show remarkable photoresponse, with a switching ratio higher than 105 , high visible light responsivity (>4 × 104  A W-1 ), and stable storage-erase cycles, as well as competitive retention performance (104  s). The photoinduced memory behavior can be benefiting from the insulating nature of quantum-well in 2D perovskite under dark and its excellent light sensitivity. The excellent photo memory behaviors have been maintained after 40 days in a N2 atmosphere. Finally, a 2D perovskite-only transistors with a multi-level memory behavior (16 distinct states) is described by controlling incident light pulse. This work provides broader attention toward 2D perovskite and optoelectronic application.