Ferroelectric Polarization in CsPbI₃/CsSnI₃ Perovskite Heterostructure

Ferroelectricity is believed to be an important origin for the excellent performance of halide perovskite optoelectronic devices, and building halide perovskite heterostructure is an excellent strategy to enhance performance of interfacial ferroelectric polarization. Herein, all-inorganic perovskite CsPbI₃/CsSnI₃ heterostructures are constructed for disclosing the interfacial electrical contacts and the ferroelectricity via first-principles calculations. In CsPbI₃/CsSnI₃ heterostructure, there are four kinds of electrical contacts, i.e., PbI₂–SnI₂, CsI–CsI, PbI₂–CsI, and CsI–SnI₂ interfaces. Large cation–anion displacements along <i>z</i>-direction are observed for all interface compositions, which indicate a strong ferroelectric field effect in the CsPbI₃/CsSnI₃ heterostructure. The net polarization displacements of PbI₂–SnI₂ and CsI–CsI interfaces are smaller than the values of PbI₂–CsI and CsI–SnI₂ interfaces. The interfacial ferroelectricity drives electron extraction from the perovskite and hinders electron–hole recombination by keeping the electrons and holes separated. The intrinsically interfacial ferroelectric polarization results from the difference of work functions of diverse interfaces and the interface charge transfer. This work suggests that such an all-inorganic perovskite heterostructure has significant potential for future optoelectronic applications

Large-Scale Flexible and Highly Conductive Carbon Transparent Electrodes via Roll-to-Roll Process and Its High Performance Lab-Scale Indium Tin Oxide-Free Polymer Solar Cells

A scalable and highly conductive PEDOT:PSS:CNTs transparent electrode (TE) is demonstrated for high performance optoelectronics. The aligned and uniform dispersion of electron conduction favored CNTs in the PEDOT:PSS matrix can achieve the rearrangement of the PEDOT chains with more expended conformation via the π–π interaction between CNTs and PEDOT. As a result, PEDOT:PSS:CNTs electrode presents a high conductivity of 3264.27 S cm^–1 with a high transmittance over 85%, and ITO-free PSCs based on PEDOT:PSS:CNTs electrode achieves a PCE of 7.47% with high stability. Furthermore, a large-scale flexible electrode was obtained by a roll-to-roll technique, which demonstrates an excellent property with a sheet resistance of 17 Ω sq^–1 and 80.7% optical transmittance. Combining the flexible and conductive PEDOT:PSS:CNTs film with the scalable roll-to-roll process, we anticipate that the commercial production of a large-scale transparent electrode, replacing ITO, will be realized in the near future

Large-Scale Flexible and Highly Conductive Carbon Transparent Electrodes via Roll-to-Roll Process and Its High Performance Lab-Scale Indium Tin Oxide-Free Polymer Solar Cells

A scalable and highly conductive PEDOT:PSS:CNTs transparent electrode (TE) is demonstrated for high performance optoelectronics. The aligned and uniform dispersion of electron conduction favored CNTs in the PEDOT:PSS matrix can achieve the rearrangement of the PEDOT chains with more expended conformation via the π–π interaction between CNTs and PEDOT. As a result, PEDOT:PSS:CNTs electrode presents a high conductivity of 3264.27 S cm^–1 with a high transmittance over 85%, and ITO-free PSCs based on PEDOT:PSS:CNTs electrode achieves a PCE of 7.47% with high stability. Furthermore, a large-scale flexible electrode was obtained by a roll-to-roll technique, which demonstrates an excellent property with a sheet resistance of 17 Ω sq^–1 and 80.7% optical transmittance. Combining the flexible and conductive PEDOT:PSS:CNTs film with the scalable roll-to-roll process, we anticipate that the commercial production of a large-scale transparent electrode, replacing ITO, will be realized in the near future

Crystal-Domain Orientation and Boundary in Highly Ordered Organic Semiconductor Thin Film

Conduction of electric charges is often done in polycrystalline materials. Unavoidably, the crystallite size, orientation, and domain boundaries (DBs) affect the transport of the charge carriers. It is particularly so for organic semiconductors known to be highly anisotropic and strongly dependent on DBs. Understanding those effects will have a strong impact on improving the performance of organic electronic and optoelectronic devices. Herein, we report our investigation on the crystal-domain orientation and boundary on the charge transport of operating device with copper phthalocyanine (CuPc) thin films grown on <i>p</i>-sexiphenyl (<i>p</i>-6P) by Kelvin probe force microscopy. In CuPc intradomains, the voltage drop increases as the angle increases between the domain orientation and the source-drain electric field. In DBs, the potential wells and steep voltage drops were observed. The increase of the DBs width and the angle between the orientations of neighboring domains results in the raise of voltage drop across the DBs, which restrict the charge transport in DBs simultaneously. The mobility of CuPc thin films increases with the domain size, resulting from the reduction of the mismatched orientation degree and the number of DBs

Degradation by Exposure of Coevaporated CH₃NH₃PbI₃ Thin Films

Degradation of coevaporated CH₃NH₃PbI₃ thin films were investigated with X-ray photoelectron spectroscopy and X-ray diffraction as the films were subjected to exposure of oxygen, low pressure atmospheric air, atmospheric air, or H₂O. The coevaporated thin films have consistent stoichiometry and crystallinity suitable for detailed surface analysis. The results indicate that CH₃NH₃PbI₃ is not sensitive to oxygen. Even after 10¹³ Langmuir (L, one L equals 10^–6 Torr s) oxygen exposure, no O atoms could be found on the surface. The film is not sensitive to dry air as well. A reaction threshold of about 2 × 10¹⁰ L is found for H₂O exposure, below which no CH₃NH₃PbI₃ degradation takes place, and the H₂O acts as an n-dopant. Above the threshold, the film begins to decompose, and the amount of N and I decrease quickly, leaving the surface with PbI₂, hydrocarbon complex, and O contamination

Light-Induced Degradation of CH₃NH₃PbI₃ Hybrid Perovskite Thin Film

The stability of CH₃NH₃PbI₃ was investigated by observing the degradation in a coevaporated film irradiated by a blue laser in ultrahigh vacuum. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were employed to investigate the effects of irradiation on the surface. The core levels of CH₃NH₃PbI₃ were observed to shift toward a higher binding energy (BE) during the irradiation, suggesting that the surface became more n-type. A new metallic Pb component in the XPS spectrum appeared after 120 min of irradiation, indicating that the film had started to decompose. The decomposition saturated after about 480 min of irradiation when the ratio of metallic Pb to total Pb was about 33%. Furthermore, the film was no longer continuous after irradiation, as the elements gold and oxygen from the substrate were detected by XPS. SEM images also show a roughened surface after irradiation. The results strongly indicate that CH₃NH₃PbI₃ is sensitive to the laser irradiation and that the light induced decomposition is a self-limiting process

Artificial Retina Based on Organic Heterojunction Transistors for Mobile Recognition

The flicker frequency of incident light constitutes a critical determinant in biology. Nevertheless, the exploration of methods to simulate external light stimuli with varying frequencies and develop artificial retinal neurons capable of responsive behavior remains an open question. This study presents an artificial neuron comprising organic phototransistors. The triggering properties of neurons are modulated by optical input, enabling them to execute rudimentary synaptic functions, emulating the biological characteristics of retinal neurons. The artificial retinal neuron exhibits varying responses to incoming light frequencies, allowing it to replicate the persistent visual behavior of the human eye and facilitating image discrimination. Additionally, through seamless integration with circuitry, it can execute motion recognition on a machine cart, preventing collisions with high-speed obstacles. The artificial retinal neuron offers a cost-effective and energy-efficient route for future mobile robot processors

Artificial Synapses Based on in-Plane Gate Organic Electrochemical Transistors

Realization of biological synapses using electronic devices is regarded as the basic building blocks for neuromorphic engineering and artificial neural network. With the advantages of biocompatibility, low cost, flexibility, and compatible with printing and roll-to-roll processes, the artificial synapse based on organic transistor is of great interest. In this paper, the artificial synapse simulation by ion-gel gated organic field-effect transistors (FETs) with poly­(3-hexylthiophene) (P3HT) active channel is demonstrated. Key features of the synaptic behaviors, such as paired-pulse facilitation (PPF), short-term plasticity (STP), self-tuning, the spike logic operation, spatiotemporal dentritic integration, and modulation are successfully mimicked. Furthermore, the interface doping processes of electrolyte ions between the active P3HT layer and ion gels is comprehensively studied for confirming the operating processes underlying the conductivity and excitatory postsynaptic current (EPSC) variations in the organic synaptic devices. This study represents an important step toward building future artificial neuromorphic systems with newly emerged ion gel gated organic synaptic devices

Coplanar Multigate MoS₂ Electric-Double-Layer Transistors for Neuromorphic Visual Recognition

Spatial coordinate and visual orientation recognition in cortical cells play important roles in the visual system. Herein, spatiotemporally processed visual neurons are mimicked by a facile coplanar multigate two-dimensional (2D) MoS₂ electric-double-layer transistor with proton-conducting poly­(vinyl alcohol) electrolytes as laterally coupled gate dielectrics. Fundamental neuromorphic behaviors, e.g., excitatory postsynaptic current and paired-pulse facilitation, were successfully mimicked. For the first time, a proof-of-principle artificial visual neural network system for mimicking spatiotemporal coordinate and orientation recognition was experimentally demonstrated in such devices. The experimental results provide a promising opportunity for adding intelligent spatiotemporally-processed functions in emerging brain-like neuromorphic nanoelectronics

Iodine and Chlorine Element Evolution in CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> Thin Films for Highly Efficient Planar Heterojunction Perovskite Solar Cells

Highly efficient planar heterojunction perovskite solar cells (PHJ–PSCs) with a structure of ITO/PEDOT:PSS/CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>/PCBM/C₆₀/Ag was fabricated, in which the compact and pinhole-free CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> perovskite thin film was obtained using a mixture of precursors containing lead iodide (PbI₂), lead chloride (PbCl₂), and methylammonium iodide (CH₃NH₃I) at an optimized ratio of 1:1:4. The morphology and formation process of CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> thin film was closely related to the annealing temperature and time, which would result in the controllable performance for the PHJ–PSC devices. The morphology, crystallization process, and element analysis suggested that the chlorine gradually diffused and sublimated from the film surface while the iodine moved to the surface, together with the removal of the pinholes in the film. The PHJ–PSCs with the as-prepared CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> thin film showed good performance and excellent repeatability. The power conversion efficiency (PCE) up to 14.03% was achieved without obvious hysteresis under different scanning conditions. The understanding of the iodine and chlorine element evolving process during the thermal treatment is beneficial to develop a more efficient scalable one-step solution processing method for fabricating large-area, highly efficient CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>-based PSCs