Highly Efficient Organic Solar Cells Consisting of Double Bulk Heterojunction Layers

An organic solar cell (OSCs) containing double bulk heterojunction (BHJ) layers, namely, double‐BHJ OSCs is constructed via stamp transferring of low bandgap BHJ atop of mediate bandgap active layers. Such devices allow a large gain in photocurrent to be obtained due to enhanced photoharvest, without suffering much from the fill factor drop usually seen in thick‐layer‐based devices. Overall, double‐BHJ OSC with optimal ≈50 nm near‐infrared PDPP3T:PC₇₁BM layer atop of ≈200 nm PTB7‐Th:PC₇₁BM BHJ results in high power conversion efficiencies over 12%.     

Molecular Structure‐Dependent Charge Injection and Doping Efficiencies of Organic Semiconductors: Impact of Side Chain Substitution

Due to the highly anisotropic nature of π ‐conjugated molecules, the molecular structure of organic semiconductors can significantly affect the device performance of organic optoelectronics. Here, the molecular structure dependence on charge injection and doping efficiencies is investigated by characterizing the typical hole transport material of N,N′‐bis(naphthalen‐1‐yl)‐N,N′‐bis(phenyl)‐benzidine (NPB) and its derivatives N,N′‐bis(naphthalen‐1‐yl)‐N,N′‐bis(phenyl)‐9,9‐dimethyl‐fluorene (DMFL‐NPB) and N,N′‐bis(naphthalen‐1‐yl)‐N,N′‐bis(phenyl)‐9,9‐diphenyl‐fluorene (DPFL‐NPB)]. Using photoelectron spectroscopy data and density functional theory calculation, it is identified that the side chain substitution in NPB and its derivatives plays a crucial role in the intrinsic injection and transport properties, and doping efficiency. The inner twist of the two main benzene rings in NPB is changed from out‐of‐plane to in‐plane due to the alkyl or phenyl side chains of DMFL‐NPB or DPFL‐NPB, which reduces the ionization energies and thus decreases the hole injection barriers at the indium tin oxide/organic interface. The doping efficiency in 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄‐TCNQ) doped systems is also highly dependent on the degree of intermolecular orbital energy hybridization with respect to the side chain substitution. These findings show that the rational design of molecular structures with suitable side chains is crucial for achieving high‐performance organic devices.     

A Semitransparent Inorganic Perovskite Film for Overcoming Ultraviolet Light Instability of Organic Solar Cells and Achieving 14.03% Efficiency

Organic solar cells (OSCs) can be unstable under ultraviolet (UV) irradiation. To address this issue and enhance the power conversion efficiency (PCE), an inorganic‐perovskite/organic four‐terminal tandem solar cell (TSC) based on a semitransparent inorganic CsPbBr₃ perovskite solar cell (pero‐SC) as the top cell and an OSC as bottom cell is constructed. The high‐quality CsPbBr₃ photoactive layer of the planar pero‐SC is prepared with a dual‐source vacuum coevaporation method, using stoichiometric precursors of CsBr and PbBr₂ with a low evaporation rate. The resultant opaque planar pero‐SC exhibits an ultrahigh open‐circuit voltage of 1.44 V and the highest reported PCE of 7.78% for a CsPbBr₃‐based planar pero‐SC. Importantly, the devices show no degradation after 120 h UV light illumination. The related semitransparent pero‐SC can almost completely filter UV light and well maintain photovoltaic performance; it additionally shows an extremely high average visible transmittance. When it is used to construct a TSC, the top pero‐SC acting as a UV filter can utilize UV light for photoelectric conversion, avoiding the instability problem of UV light on the bottom OSC that can meet the industrial standards of UV‐light stability for solar cells, and leading to the highest reported PCE of 14.03% for the inorganic‐perovskite/organic TSC.     

Decoupling the Effects of Self‐Assembled Monolayers on Gold,Silver, and Copper Organic Transistor Contacts

In bottom‐contact organic field‐effect transistors (OFETs), the functionalization of source/drain electrodes leads to a tailored surface chemistry for film growth and controlled interface energetics for charge injection. This report describes a comprehensive investigation into separating and correlating the energetic and morphological effects of a self‐assembled monolayers (SAMs) treatment on Au, Ag, and Cu electrodes. Fluorinated 5,11‐bis(triethylsilylethynyl) anthradithiophene (diF‐TES‐ADT) and pentafluorobenzenethiol (PFBT) are employed as a soluble small‐molecule semiconductor and a SAM material, respectively. Upon SAM modification, the Cu electrode devices benefit from a particularly dramatic performance improvement, closely approaching the performance of OFETs with PFBT‐Au and PFBT‐Ag. Ultraviolet photoemission spectroscopy, polarized optical microscopy, grazing‐incidence wide‐angle X‐ray scattering elucidate the metal work function change and templated crystal growth with high crystallinity resulting from SAMs. The transmission‐line method separates the channel and contact properties from the measured OFET current–voltage data, which conclusively describes the impact of the SAMs on charge injection and transport behavior.     

Stability of pentacene organic field effect transistors with a low-k polymer/high-k oxide two-layer gate dielectric

This paper reports on the processing and the characterization of pentacene organic field effect transistors (OFETs) with a two-layer gate dielectric consisting of a polymer (PMMA) on a high-k oxide (Ta₂O₅). This dielectric stack has been designed in view to combine low voltage operating devices, by the use of a high-k oxide which increases the charge in the accumulation channel and the gate capacitance, and highly stable devices which generally could be achieved with polymer dielectrics but not necessarily with strongly polar high-k oxides. Bi-layer dielectric devices were compared to those with only Ta₂O₅ or PMMA gate insulators. Bias stress at room temperature was used to assess the electrical stability. A very low operating voltage was achieved with Ta₂O₅ but these devices exhibit hysteresis and degraded characteristics upon bias stress. OFETs with PMMA revealed very stable but operate at rather a high voltage due to the low dielectric constant of PMMA. Reasonably stable devices operating at about 10 V could have been obtained with PMMA/Ta₂O₅ two-layer dielectric. The origin of observed threshold voltage shift and mobility decrease upon bias stress are discussed.     

High‐Performance Inorganic Perovskite Quantum Dot–Organic Semiconductor Hybrid Phototransistors

All‐inorganic lead halide perovskite quantum dots (IHP QDs) have great potentials in photodetectors. However, the photoresponsivity is limited by the low charge transport efficiency of the IHP QD layers. High‐performance phototransistors based on IHP QDs hybridized with organic semiconductors (OSCs) are developed. The smooth surface of IHP QD layers ensures ordered packing of the OSC molecules above them. The OSCs significantly improve the transportation of the photoexcited charges, and the gate effect of the transistor structure significantly enhances the photoresponsivity while simultaneously maintaining high I _photo/I _dark ratio. The devices exhibit outstanding optoelectronic properties in terms of photoresponsivity (1.7 × 10⁴ A W^?1), detectivity (2.0 × 10¹⁴ Jones), external quantum efficiency (67000%), I _photo/I _dark ratio (8.1 × 10⁴), and stability (100 d in air). The overall performances of our devices are superior to state‐of‐the‐art IHP photodetectors. The strategy utilized here is general and can be easily applied to many other perovskite photodetectors.     

Gas Sensors Based on Nano/Microstructured Organic Field‐Effect Transistors

Benefiting from the advantages of organic field‐effect transistors (OFETs), including synthetic versatility of organic molecular design and environmental sensitivity, gas sensors based on OFETs have drawn much attention in recent years. Potential applications focus on the detection of specific gas species such as explosive, toxic gases, or volatile organic compounds (VOCs) that play vital roles in environmental monitoring, industrial manufacturing, smart health care, food security, and national defense. To achieve high sensitivity, selectivity, and ambient stability with rapid response and recovery speed, the regulation and adjustment of the nano/microstructure of the organic semiconductor (OSC) layer has proven to be an effective strategy. Here, the progress of OFET gas sensors with nano/microstructure is selectively presented. Devices based on OSC films one dimensional (1D) single crystal nanowires, nanorods, and nanofibers are introduced. Then, devices based on two dimensional (2D) and ultrathin OSC films, fabricated by methods such as thermal evaporation, dip‐coating, spin‐coating, and solution‐shearing methods are presented, followed by an introduction of porous OFET sensors. Additionally, the applications of nanostructured receptors in OFET sensors are given. Finally, an outlook in view of the current research state is presented and eight further challenges for gas sensors based on OFETs are suggested.     

All‐Solution‐Processed Metal‐Oxide‐Free Flexible Organic Solar Cells with Over 10% Efficiency

All‐solution‐processing at low temperatures is important and desirable for making printed photovoltaic devices and also offers the possibility of a safe and cost‐effective fabrication environment for the devices. Herein, an all‐solution‐processed flexible organic solar cell (OSC) using poly(3,4‐ethylenedioxythiophene):poly‐(styrenesulfonate) electrodes is reported. The all‐solution‐processed flexible devices yield the highest power conversion efficiency of 10.12% with high fill factor of over 70%, which is the highest value for metal‐oxide‐free flexible OSCs reported so far. The enhanced performance is attributed to the newly developed gentle acid treatment at room temperature that enables a high‐performance PEDOT:PSS/plastic underlying substrate with a matched work function (≈4.91 eV), and the interface engineering that endows the devices with better interface contacts and improved hole mobility. Furthermore, the flexible devices exhibit an excellent mechanical flexibility, as indicated by a high retention (≈94%) of the initial efficiency after 1000 bending cycles. This work provides a simple route to fabricate high‐performance all‐solution‐processed flexible OSCs, which is important for the development of printing, blading, and roll‐to‐roll technologies.     

Low Schottky Barrier Black Phosphorus Field‐Effect Devices with Ferromagnetic Tunnel Contacts

Black phosphorus (BP) has been recently unveiled as a promising 2D direct bandgap semiconducting material. Here, ambipolar field‐effect transistor behavior of nanolayers of BP with ferromagnetic tunnel contacts is reported. Using TiO₂/Co contacts, a reduced Schottky barrier <50 meV, which can be tuned further by the gate voltage, is obtained. Eminently, a good transistor performance is achieved in the devices discussed here, with drain current modulation of four to six orders of magnitude and a mobility of μ_h ≈ 155 cm² V^?1 s^?1 for hole conduction at room temperature. Magnetoresistance calculations using a spin diffusion model reveal that the source–drain contact resistances in the BP device can be tuned by gate voltage to an optimal range for injection and detection of spin‐polarized holes. The results of the study demonstrate the prospect of BP nanolayers for efficient nanoelectronic and spintronic devices.     

The Design of 3D‐Interface Architecture in an Ultralow‐Power,Electrospun Single‐Fiber Synaptic Transistor for Neuromorphic Computing

Synaptic electronics is a new technology for developing functional electronic devices that can mimic the structure and functions of biological counterparts. It has broad application prospects in wearable computing chips, human–machine interfaces, and neuron prostheses. These types of applications require synaptic devices with ultralow energy consumption as the effective energy supply for wearable electronics, which is still very difficult. Here, artificial synapse emulation is demonstrated by solid‐ion gated organic field‐effect transistors (OFETs) with a 3D‐interface conducting channel for ultralow‐power synaptic simulation. The basic features of the artificial synapse, excitatory postsynaptic current (EPSC), paired‐pulse facilitation (PPF), and high‐pass filtering, are successfully realized. Furthermore, the single‐fiber based artificial synapse can be operated by an ultralow presynaptic spike down to ?0.5 mV with an ultralow reading voltage at ?0.1 mV due to the large contact surface between the ionic electrolyte and fiber‐like semiconducting channel. Therefore, the ultralow energy consumption at one spike of the artificial synapse can be realized as low as ≈3.9 fJ, which provides great potential in a low‐power integrated synaptic circuit.     

Novel Tunnel‐Contact‐Controlled IGZO Thin‐Film Transistors with High Tolerance to Geometrical Variability

Thin insulating layers are used to modulate a depletion region at the source of a thin‐film transistor. Bottom contact, staggered‐electrode indium gallium zinc oxide transistors with a 3 nm Al₂O₃ layer between the semiconductor and Ni source/drain contacts, show behaviors typical of source‐gated transistors (SGTs): low saturation voltage (V_{D_SAT} ≈ 3 V), change in V_{D_SAT} with a gate voltage of only 0.12 V V^?1, and flat saturated output characteristics (small dependence of drain current on drain voltage). The transistors show high tolerance to geometry: the saturated current changes only 0.15× for 2–50 µm channels and 2× for 9‐45 µm source‐gate overlaps. A higher than expected (5×) increase in drain current for a 30 K change in temperature, similar to Schottky‐contact SGTs, underlines a more complex device operation than previously theorized. Optimization for increasing intrinsic gain and reducing temperature effects is discussed. These devices complete the portfolio of contact‐controlled transistors, comprising devices with Schottky contacts, bulk barrier, or heterojunctions, and now, tunneling insulating layers. The findings should also apply to nanowire transistors, leading to new low‐power, robust design approaches as large‐scale fabrication techniques with sub‐nanometer control mature.     

Reduction of threshold voltage fluctuation for organic field effect transistors by increase of insulator capacitance

Fluctuation of threshold voltages for top contact organic field effect transistors with pentacene active layers were successfully reduced by increase of gate insulator capacitance. Average fluctuation of threshold voltages of devices with 1000-nm-SiO₂-insulator was approximately 4 V, however it became only 0.5 V when the SiO₂ thickness was reduced to 100 nm. Since a vast number of transistors with constant threshold voltage are needed for application to displays, increase of capacitance would be a useful method for improving the reliability of OFETs.     

Surpassing 10% Efficiency Benchmark for Nonfullerene Organic Solar Cells by Scalable Coating in Air from Single Nonhalogenated Solvent

The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale‐up. Despite the rapid increase in power conversion efficiency (PCE) of spin‐coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high‐performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT‐M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin‐coated FTAZ:IT‐M devices using a single halogen‐free solvent. More importantly, chlorine‐free, blade coating of FTAZ:IT‐M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X‐ray scattering results reveal that large π–π coherence length, high degree of face‐on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low‐cost, low‐toxicity, and high‐efficiency OSCs by high‐throughput production.     

Coherent Electron Transport in Air-Stable,Printed Single-Crystal Organic Semiconductor and Application to Megahertz Transistors

Organic semiconductors (OSCs) have attracted growing attention for optoelectronic applications such as field-effect transistors (FETs), and coherent (or band-like) carrier transport properties in OSC single crystals (SCs) have been of interest as they can lead to high carrier mobilities. Recently, such p-type OSC SCs compatible with a printing technology have been used to achieve high-speed FETs; therefore, developments of n-type counterparts may be promising for realizing high-speed complementary organic circuits. Herein, coherent electron transport properties in a printed SC of a state-of-the-art, air-stable n-type OSC, PhC₂−BQQDI, by means of variable-temperature gated Hall effect measurements and X-ray single-crystal diffraction analyses in conjunction with band structure calculations, are reported. Furthermore, the SC FET is tested for high-speed operations, which obtains a cutoff frequency of 4.3 MHz at an operation voltage of 20 V in air. Thus, PhC₂−BQQDI is shown as a new candidate for practical applications of SC-based, organic complementary devices.     

Flexible high capacitance nanocomposite gate insulator for printed organic field-effect transistors

Ceramic-polymer nanocomposite dielectric consisting of an epoxy solution with propylene glycol methyl ether acetate as the solvent and barium titanate nanoparticles with capacitance in excess of 60 pF/mm² was developed and utilized as the gate insulator for organic field-effect transistors (OFETs). The high relative permittivity (κ = 35), bimodal nanocomposite utilized had two different filler particle sizes 200 nm and 1000 nm diameter particles. Bottom gate organic filed-effect transistors were demonstrated using a commercially available printing technology for material deposition. A metal coated plastic film was used as the flexible gate substrate. Solution processable, p-type arylamine-based amorphous organic semiconductor was utilized as the active layer. Fabricated OFETs with the solution processed nanocomposite dielectric had a high field-induced current and a low threshold voltage; these results suggest that the low operating voltage was due to the high capacitance gate insulator. In this paper, we review the characteristics of the nanocomposite dielectric material and discuss the processing and performance of the printed organic devices.     

Change in electronic states in the accumulation layer at interfaces in a poly(3-hexylthiophene) field-effect transistor and the impact of encapsulation

The electrical properties of organic field-effect transistors (OFETs) are largely determined by the accumulation layer that extends only a few molecular layers away from the gate dielectric/organic semiconductor interface. To understand degradation processes that occur within the device structure under ambient conditions, it is thus essential to probe the interface using an architecture that minimizes the effects of bulk transport of contaminating species through upper layers of material in a thick film device. Using FETs designed with multiple voltage probes along the conducting channel and an ultrathin film of the active material, we found that the charge carrier density and the FET mobility decrease, and further, the contact and channel properties are strongly correlated. FET devices prepared with an ultrathin film of P3HT become significantly contact limited in air due to a hole diffusion barrier near the drain electrode. Encapsulation of the device with a layered organic/inorganic barrier material consisting of parylene and Al(2)O(3) appreciably retarded diffusion of molecular species from ambient air into P3HT.     

Towards flexible all-carbon electronics: Flexible organic field-effect transistors and inverter circuits using solution-processed all-graphene source/drain/gate electrodes

Flexible organic field-effect transistors (OFETs) using solution-processable functionalized graphene for all the electrodes (source, drain, and gate) have been fabricated for the first time. These OFETs show performance comparable to corresponding devices using Au electrodes as the source/drain electrodes on SiO₂/Si substrates with Si as the gate electrode. Also, these devices demonstrate excellent flexibility without performance degradation over severe bending cycles. Furthermore, inverter circuits have been designed and fabricated using these all-graphene-electrode OFETs. Our results demonstrate that the long-sought dream for all-carbon and flexible electronics is now much closer to reality.     

Polar Side Chains Enhance Processability,Electrical Conductivity,and Thermal Stability of a Molecularly p‐Doped Polythiophene

Molecular doping of organic semiconductors is critical for optimizing a range of optoelectronic devices such as field‐effect transistors, solar cells, and thermoelectric generators. However, many dopant:polymer pairs suffer from poor solubility in common organic solvents, which leads to a suboptimal solid‐state nanostructure and hence low electrical conductivity. A further drawback is the poor thermal stability through sublimation of the dopant. The use of oligo ethylene glycol side chains is demonstrated to significantly improve the processability of the conjugated polymer p(g₄2T‐T)—a polythiophene—in polar aprotic solvents, which facilitates coprocessing of dopant:polymer pairs from the same solution at room temperature. The use of common molecular dopants such as 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) is explored. Doping of p(g₄2T‐T) with F4TCNQ results in an electrical conductivity of up to 100 S cm^?1. Moreover, the increased compatibility of the polar dopant F4TCNQ with the oligo ethylene glycol functionalized polythiophene results in a high degree of thermal stability at up to 150 °C.     

Relieving the Photosensitivity of Organic Field-Effect Transistors

It is generally believed that the photoresponse behavior of organic field-effect transistors (OFETs) reflects the intrinsic property of organic semiconductors. However, this photoresponse hinders the application of OFETs in transparent displays as driven circuits due to the current instability resulting from the threshold voltage shift under light illumination. It is necessary to relieve the photosensitivity of OFETs to keep the devices stable. 2,6-diphenyl anthracene thin-film and single-crystal OFETs are fabricated on different substrates, and it is found that the degree of molecular order in the conducting channels and the defects at the dielectric/semiconductor interface play important roles in determining the phototransistor performance. When highly ordered single-crystal OFETs are fabricated on polymeric substrates with low defects, the photosensitivity (P) decreases by more than 10⁵ times and the threshold voltage shift (ΔV_T) is almost eliminated compared with the corresponding thin-film OFETs. This phenomenon is further verified by using another three organic semiconductors for similar characterizations. The decreased P and ΔV_T of OFETs ensure a good current stability for OFETs to drive organic light-emitting diodes efficiently, which is essential to the application of OFETs in flexible and transparent displays.     

Scalable Fabrication of Highly Crystalline Organic Semiconductor Thin Film by Channel‐Restricted Screen Printing toward the Low‐Cost Fabrication of High‐Performance Transistor Arrays

Control over the morphology and crystallinity of small‐molecule organic semiconductor (OSC) films is of key importance to enable high‐performance organic optoelectronic devices. However, such control remains particularly challenging for solution‐processed OSC devices because of the complex crystallization kinetics of small‐molecule OSC materials in the dynamic flow of inks. Here, a simple yet effective channel‐restricted screen‐printing method is reported, which uses small‐molecule OSCs/insulating polymer to yield large‐grained small‐molecule OSC thin‐film arrays with good crystallization and preferred orientation. The use of cross‐linked organic polymer banks produces a confinement effect to trigger the outward convective flow at two sides of the channel by the fast solvent evaporation, which imparts the transport of small‐molecule OSC solutes and promotes the growth of small‐molecule OSC crystals parallel to the channel. The small‐molecule OSC thin‐film array produced by screen printing exhibits excellent performance characteristics with an average mobility of 7.94 cm² V^?1 s^?1 and a maximum mobility of 12.10 cm² V^?1 s^?1, which are on par with its single crystal. Finally, screen printing can be carried out using a flexible substrate, with good performance. These demonstrations bring this robust screen‐printing method closer to industrial application and expand its applicability to various flexible electronics.