High-performance Organic Devices Research Articles

Unlocking amplified magneto-photocurrent using multi-field manipulation in nonfullerene organic bulk heterojunction systems.

An effective manipulation of polaron pairs (PPs) for realizing amplified magneto-photocurrent (AMPC) is of critical importance toward the development of low power consumption and high-performance organic spin-optoelectronic devices, for instance magneto-photo-volatile memories. By far, it is challenging and there is a lack of method to reach AMPC. The typical magneto-photocurrent due to the light-matter interanion is primarily for unveiling the spin-dependent electron-hole dissociation in organic solar cells. Herein, we achieved an AMPC of ∼140% in nonfullerene organic bulk heterojunction systems at room temperature. We found that the amplification can be effectively triggered by a multi-field to a large number of photogenerated PPs at intermediate charge transfer states. We further postulate that, at steady state, they may experience a cyclic photophysical process due to the triplet-exciton polaron interaction. This study paves the way for the realization of AMPC in the organic spin-optoelectronic system.

Arylazopyrazole-modulated stable dual-mode phototransistors.

High-performance organic devices with dynamic and stable modulation are essential for building devices adaptable to the environment. However, the existing reported devices incorporating light-activated units exhibit either limited device stability or subpar optoelectronic properties. Here, we synthesize a new optically tunable polymer dielectric functionalized with photochromic arylazopyrazole units with a cis-isomer half-life of as long as 90 days. On this basis, stable dual-mode organic transistors that can be reversibly modulated are successfully fabricated. The trans-state devices exhibit high carrier mobility reaching 7.4 square centimeters per volt per second and excellent optical figures of merit, whereas the cis-state devices demonstrate stable but starkly different optoelectronic performance. Furthermore, optical image sensors are prepared with regulatable nonvolatile memories from 36 hours (cis state) to 108 hours (trans state). The achievement of dynamic light modulation shows remarkable prospects for the intelligent application of organic optoelectronic devices.

Hexylation Stabilises Twisted Backbone Configurations in the Prototypical Low-Bandgap Copolymer PCDTBT.

Conjugated donor-acceptor copolymers hold great potential as materials for high-performance organic photovoltaics, organic transistors and organic thermoelectric devices. Their low optical bandgap is achieved by alternation of donor and acceptor moieties along the polymer chain, leading to a pronounced charge-transfer character of electronic excitations. However, the influence of appended side chains and of chemical defects of the backbone on their photophysical and conformational properties remains largely unexplored on the level of individual chains. Here, we employ room temperature single-molecule photoluminescence spectroscopy on four compounds based on the prototypical copolymer PCDTBT with systematically changed chemical structure. Our results show that an increasing density of statistically added hexyl chains to the TBT comonomer distorts the molecular conformation, likely through the increase of average dihedral angles along the backbone. We find that, although the conformation becomes more twisted with high hexyl density, the side chains appear to stabilize the backbone in this twisted conformation. In addition, we demonstrate that homocoupling defects along the backbone barely influence the PL spectra of single chains, and thus intra-chain electronic properties.

Asymmetric Schottky Barrier in Rubrene Transistor via Monolayer Graphene Insertion toward Self-Powered Imaging

Organic semiconductor materials featuring lightweight, and flexibility may play a significant role in various future applications, such as foldable displays, wearable devices, and artificial skin. For developing high-performance organic devices, organic crystals are highly desired, while a remaining fundamental issue is their contact problem. Here, we have grown a high-quality rubrene single crystal by utilizing a simple in-air sublimation technique. The contact characteristics (barrier height and contact resistance) are detail-studied by resist-free transfer electrodes (Au metal or graphene/Au). The Schottky barrier of the rubrene/graphene interface is lower and can be also modulated by gate bias, which is confirmed by spatial photocurrent mapping. Finally, we demonstrated the zero-bias photocurrent imaging application by constructing the asymmetrical device employing different electrode contacts. Our work would be of significance for studying the contact issue of organic crystals and wireless imaging.

Atomic layer deposition-processed laminated In2O3:V2O5 nanocompounds: Versatile electrode platforms for high-performance organic photovoltaics and capacitor devices

Despite considerable advancements in transparent conductive electrodes (TCEs), their widespread use in energy storage and conversion devices remains limited. This is primarily attributable to factors such as the work function (WF), surface roughness, and charge mobility limitations. These challenges necessitate comprehensive solutions and innovative approaches to utilize TCEs for energy-related applications. Herein, In2O3:V2O5 nanocompound TCEs were fabricated with excellent transmittance (∼ 90%), high WF (5.33 eV), and large mobility (52.1 cm2/V·s) using atomic layer deposition. The performance of the In2O3:V2O5 nanocompound TCE was evaluated in organic photovoltaics (OPVs) and metal-insulator-metal (MIM) capacitors. In OPVs based on the PM6:Y6 active layer, a low surface roughness and suitable energy level alignment of In2O3:V2O5 nanocompound TCEs with the active layer eliminate the requirement of a hole-transport layer (HTL), resulting in an efficiency of 16.3% and an excellent fill-factor of 71.0% in HTL-free OPVs under simulated 1-sun illumination. Moreover, in MIM capacitor devices that employ the TiO2 dielectric, In2O3:V2O5 nanocompound TCEs exhibit a significantly lower leakage current (> 1800 times) and improved capacitance (2.5 times) compared with those of conventional TiN electrode-based capacitors. The dual functionality of the proposed In2O3:V2O5 nanocompound TCEs validates their potential for use in several optoelectronic devices.

Self-supporting and flexible organic potassium terephthalate with controllable carbon layers in K-ion batteries

Herein, we report a self-supporting and flexible K2TP/MWNCTs/C anode material prepared by electrostatic spinning and controlled medium temperature heat treatment in potassium-ion batteries. The results show that the flexible carbon cladding and stably conductive network structure can facilitate the transport of K+ and electron. The as-synthesized K2TP/MWNCTs/C flexible anode can exhibit a high specific capacity of 150.6 mAh g−1 after 100 cycles at a current density of 50 mA g−1. These results are of significant to explore high-performance organic flexible devices.

Nanographene-Based Heterojunctions for High-Performance Organic Phototransistor Memory Devices.

Organic phototransistors can enable many important applications such as nonvolatile memory, artificial synapses, and photodetectors in next-generation optical communication and wearable electronics. However, it is still a challenge to achieve a big memory window (threshold voltage response ∆Vth ) for phototransistors. Here, a nanographene-based heterojunction phototransistor memory with large ∆Vth responses is reported. Exposure to low intensity light (25.7µWcm-2 ) for 1 s yields a memory window of 35V, and the threshold voltage shift is found to be larger than 140V under continuous light illumination. The device exhibits both good photosensitivity (3.6 × 105 ) and memory properties including long retention time (>1.5 × 105 s), large hysteresis (45.35V), and high endurance for voltage-erasing and light-programming. These findings demonstrate the high application potential of nanographenes in the field of optoelectronics. In addition, the working principle of these hybrid nanographene-organic structured heterojunction phototransistor memory devices is described which provides new insight into the design of high-performance organic phototransistor devices.

Enhancing the Performance of N-Type Thermoelectric Devices via Tuning the Crystallinity of Small Molecule Semiconductors.

In the development of high-performance organic thermoelectric devices, n-type materials, especially with small molecule semiconductors, lags far behind p-type materials. In this paper, three small molecules are synthesized based on electron-deficient naphthalene bis-isatin building blocks bearing different alkyl chains with the terminal functionalized with 3-ethylrhodanine unit and studied their aggregation and doping mechanism in detail. It is found that crystallinity plays an essential role in tuning the doping behavior of small molecules. Molecules with too strong crystallinity tend to aggregate with each other to form large crystalline domains, which cause significant performance degradation. While molecules with weak crystallinity can tolerate more dopants, most of them exhibit low mobility. By tuning the crystallinity carefully, organic thermoelectric devices based on C12NR can maintain high mobility and realize effective doping simultaneously, and a high power factor of 1.07 µW m-1 K-2 at 100°C is realized. This delicate molecular design by modulating crystallinity provides a new avenue for realizing high-performance organic thermoelectric devices.

Ultrafast and Sensitive Self-Powered Photodetector Based on Graphene/Pentacene Single Crystal Heterostructure with Weak Light Detection Capacity.

Organic materials exhibit efficient light absorption and low-temperature, large-scale processability, and have stimulated enormous research efforts for next-generation optoelectronics. While, high-performance organic devices with fast speed and high responsivity still face intractable challenges, due to their intrinsic limitationsincluding finite carrier mobility and high exciton binding energy. Here an ultrafast and highly sensitive broadband phototransistor is demonstrated by integrating high-quality pentacene single crystal with monolayer graphene. Encouragingly, the -3dB bandwidth can reach up to 26kHz, which is a record-speed for such sensitized organic phototransistors. Enormous absorption, long exciton diffusion length of pentacene crystal, and efficient interfacial charge transfer enable a high responsivity of >105 AW-1 and specific detectivity of >1011 Jones. Moreover, self-powered weak-light detection is realized using a simple asymmetric configuration, and the obvious zero-bias photoresponses can be displayed even under 750nWcm-2 light intensity. Excellent response speed and photoresponsivity enable high-speed image sensor capability in UV-Vis ranges. The results offer a practical strategy for constructing high-performance self-powered organic hybrid photodetectors, with strong applicability in wireless, weak-light detection, and video-frame-rate imaging applications.

Revealing Three-in-One Nature of Organic Negative Transconductance Transistors

Introducing a negative transconductance (NTC) regime into a switching curve of a field-effect transistor is a promising approach to overcome the fundamental limit of traditional digital logic circuits. However, the operation of NTC devices is only weakly conceptualized up to the present. This study proposes a drastically new way of regarding an NTC transistor as a functional equivalent of three interconnected normal field-effect transistors. The electrical model is directly inspired by the structural and materials characteristics of high-performance organic devices, and it is fully confirmed by measured experimental data. This finding, therefore, rationalizes that the NTC transistors have a unique potential to greatly reduce the transistor count in an integrated circuit, and offers a simple yet universal tool for systematic design and optimization of NTC transistor circuits through simulation-driven engineering.

Effect of Nitrogen Atoms on Structures and Electron Transport of N-heteropentacene Devices.

Pentacene polycyclic aromatic hydrocarbons are the most promising organic semiconductor materials among aromatic compounds due to their potential optoelectronic properties. Introducing nitrogen atoms into the main chain of pentacene is supposed to tune the electronic structure and develop new high-performance organic molecular devices. Herein, we have investigated the electron transport properties of N-heteropentacenes consisting of different numbers, positions, and valence states of N atoms using density functional theory (DFT) and nonequilibrium Green's function (NEGF) method. The results show that the transport properties of N-heteropentacenes are strongly dependent on whether the C-N is a single or double bond. For devices with C-N double bonds, the change of current with voltage is consistent, and its electron transport properties are independent of the number and position of N atoms. In comparison, C-N single-bond devices exhibit an early negative differential resistance (NDR) and significant rectification. Moreover, the threshold voltages exist within certain bias voltages for different numbers of N atoms and might even show a second NDR. These studies would be useful to design performance-enhancing molecular devices by manipulating the molecular structure of N-heteropentacenes.

Highly aligned organic microwire crystal arrays for high-performance polarization-sensitive photodetectors and image sensors

Organic semiconductors with excellent optoelectronic properties are important building blocks for high-performance organic devices. Patterning organic crystals with high precision and accurately positioning them at the target position are major challenges for integrated devices. However, uncontrollable dewetting of the conventional solution method leads to as-prepared micro-nanocrystals with high defect-state density, low crystalline quality, and disordered distribution, which impair the uniformity of the device performance and limit integration. By regulating the solution position with a template and guiding the solution flow direction under gravity, aligned organic microwire arrays and polygonal patterns were fabricated. The polarization-sensitive photodetector exhibited responsivity up to 1234 A W−1, linear dynamic range of 148 dB, Iphoto/Idark of 104, response time as low as 1.1 ms, and dichroic ratio up to 2.1. Given the homogeneity of microwire arrays, the device-to-device variation was reduced to 3.58%, resulting in high-quality imaging. This study provides new insights into organic micro/nanocrystal patterning and device integration.

Organic field-effect transistors with density of states modified by polymer-enhanced solvent vapor annealing

Solvent vapor annealing (SVA) is a post-fabrication process commonly utilized to reduce the structural or energetic disorder of organic semiconductor. In this work, pentacene organic field-effect transistors were treated by different solvent vapors. Methyl ethyl ketone (MEK) vapor was found to substantially improve device performance. The threshold voltage was lowered to be just −0.2 V and the subthreshold swing was steep at 200 mV/dec. Such improvements were interpreted in terms of the density of states. Quantitative analyses indicated that the SVA with MEK reshaped the density of states of the pentacene film; in particular, the localized tail states decreased considerably. Our finding suggests that our SVA method can repair water-induced defects in grain boundaries, which commonly has a negative effect on carrier transport, i.e., lessening charge localization in deep traps and thus ameliorating charge transport in electron devices. Therefore, polymer-enhanced SVA is a powerful post-fabrication technique to manufacturing high-performance organic devices.

Low-Temperature-Processed High-Performance Pentacene OTFTs with Optimal Nd-Ti Oxynitride Mixture as Gate Dielectric

When processed at a low temperature of 200 °C, organic thin-film transistors (OTFTs) with pentacene channel adopting high-k Neodymium-Titanium oxynitride mixtures (NdTiON) with various Ti contents as gate dielectrics are fabricated. The Ti content in the NdTiON is varied by co-sputtering a Ti target at 0 W, 10 W, 20 W and 30 W, respectively, while fixing the sputtering power of an Nd target at 45 W. High-performance OTFT is obtained for the 20 W-sputtered Ti, including a small threshold voltage of −0.71 V and high carrier mobility of 1.70 cm2/V·s. The mobility improvement for the optimal Ti content can be attributed to smoother dielectric surface and resultant larger overlying pentacene grains as reflected by Atomic Force Microscopy measurements. Moreover, this sample with the optimal Ti content shows much higher mobility than its counterpart processed at a higher temperature of 400 °C (0.8 cm2/V·s) because it has a thinner gate-dielectric/gate-electrode interlayer for stronger screening on the remote phonon scattering by the gate electrode. In addition, a high dielectric constant of around 10 is obtained for the NdTiON gate dielectric that contributes to a threshold voltage smaller than 1 V for the pentacene OTFT, implying the high potential of the Nd-Ti oxynitride in future high-performance organic devices.

Asymmetric Hybrid Siloxane Side Chains for Enhanced Mobility and Mechanical Properties of Diketopyrrolopyrrole-Based Polymers.

High performance organic field effect transistor devices based on intrinsically scalable materials are of great significance in wearable electronics. In this work, an exclusive approach is reported to rationale the carrier mobility and stretchability of the conjugate polymers (CPs) by modifying the symmetry of the side chains species. Semiconductor CPs with symmetrical alkyl side chains (P(C-C)), symmetrical siloxane side chains (P(Si-Si)), and asymmetrical silicon-carbon side chains (P(C-Si)) are synthesized to investigate the influence of these side chains on the carrier mobility and mechanical behavior. The result shows that silicon-carbon asymmetric side chains can modulate the aggregation degree of polymer chains with a coherence length of 134 Å and maintain the mobility at 0.90 cm2 V-1 s-1 . P(C-Si) exhibits superior tensile properties that even elongation up to 100% the value of mobility retains a majority properties. The main reason is that the lowest coherence length of P(C-Si) polymer leads to an increased proportion of amorphous zones in its polymer film, which efficiently dissipates mechanical stresses. This study provides an efficient strategy for the design and synthesis of the CPs with high carrier transport properties-mechanical stability.

Ultrasmooth Organic Films Via Efficient Aggregation Suppression by a Low-Vacuum Physical Vapor Deposition.

Organic thin films with smooth surfaces are mandated for high-performance organic electronic devices. Abrupt nucleation and aggregation during film formation are two main factors that forbid smooth surfaces. Here, we report a simple fast cooling (FC) adapted physical vapor deposition (FCPVD) method to produce ultrasmooth organic thin films through effectively suppressing the aggregation of adsorbed molecules. We have found that thermal energy control is essential for the spread of molecules on a substrate by diffusion and it prohibits the unwanted nucleation of adsorbed molecules. FCPVD is employed for cooling the horizontal tube-type organic vapor deposition setup to effectively remove thermal energy applied to adsorbed molecules on a substrate. The organic thin films prepared using the FCPVD method have remarkably ultrasmooth surfaces with less than 0.4 nm root mean square (RMS) roughness on various substrates, even in a low vacuum, which is highly comparable to the ones prepared using conventional high-vacuum deposition methods. Our results provide a deeper understanding of the role of thermal energy employed to substrates during organic film growth using the PVD process and pave the way for cost-effective and high-performance organic devices.

Investigation of spin polarization in Gd-doped ZnO films for high-performance organic spintronic devices

In this study, Gd-doped ZnO films with different Gd concentrations were synthesized by sol–gel spin coating technique. The intrinsic ferromagnetism and the existence of spin-polarized carriers in Gd-doped ZnO films were investigated using Hall effect and MFM. The Hall effect and MFM measurement detect the AHE signal and the magnetic domain structure, respectively, proving intrinsic FM and the existence the spin polarization in the films. XRD and FTIR analysis showed that the crystal structure of Gd-doped ZnO films is a hexagonal wurtzite structure. The optical band gap is found red-shifted from 3.265 to 3.227 eV with the increase in Gd doping concentration, indicating the existence of strong spin-exchange interaction. EDX analysis confirmed the presence of Gd in the ZnO matrix. Based on the finding, we proposed 8 at. % Gd-doped ZnO film is optimal, potentially used as a spin injector for organic spintronic devices.

Diketopyrrolopyrrole based highly conjugated small molecules for organic solar cells

A series of three donor molecules (DPP-B, DPP-N and DPP-P) based on diketopyrrolopyrrole (DPP) sharing the similar backbone of D-π-A-π-D have been investigated. In these molecules, substituents such as pyrene, naphthalene and benzene act as the electron donating end groups, DPP as the central core unit and thiophene has been used as a bridge between donor and acceptor fragments. Theoretical calculations have been carried out with the help of density functional theory (DFT) and time-dependent functional theory (TD-DFT). For the optimization of geometry of investigated molecules, DFT functional B3LYP/6-31g(d) has been used and TD-B3LYP/6-31g(d) has been used to obtain the best results of calculations inexcited state. DPP-P has been considered a suitable donor molecule among all investigated molecules as it manifests the suitable value of Eg of 2.24 eV and showed the stronger absorption λmaxof 611 nm. Hence, this study reveals that investigated donor molecules are suitable for high performance organic solar cell devices

Large thermoelectric power factor in wafer-scale free-standing single-walled carbon nanotube films

Single-walled carbon nanotubes (SWCNTs) have the potential for application in thermoelectric energy generators owing to their advantages, such as good charge-carrier transport properties, mechanical flexibility and robustness, and tunability of polarity. However, the fabrication of SWCNTs still remains a problem due to its complexity and high cost. In this paper, we propose an approach for the direct formation of free-standing SWCNT films from as-grown SWCNT mats without any dispersion or separation processes. We used this approach to develop high-performance SWCNT-based thermoelectric leg materials. The as-grown SWCNT mats were synthesized by an enhanced direct injection pyrolytic synthesis (eDIPS) method. The selectivity of the tube diameter for the eDIPS method clarified the dependence of the thermoelectric performance of the free-standing SWCNT films on the tube diameter. The Seebeck coefficients and thermal conductivities were found to correlate with the tube diameter and agreed with the theoretical predictions. Owing to the dispersion-free film formation, our SWCNT films afforded large thermoelectric power factors. In particular, a power factor of 350 μW/(m K2) was obtained for the mean tube diameter of 1.7 nm without any semiconductor extraction or doping treatments. Our approach allowed the fabrication of thermoelectric legs with an arbitrary size; thus, it offers a useful strategy for the simpler, cheaper, and low-waste manufacturing of high-performance organic thermoelectric devices.

Materials Strategies for Organic Neuromorphic Devices

Neuromorphic computing is becoming increasingly prominent as artificial intelligence (AI) facilitates progressively seamless interaction between humans and machines. The conventional von Neumann architecture and complementary metal-oxide-semiconductor transistor scaling are unable to meet the highly demanding computational density and energy efficiency requirements of AI. Neuromorphic computing aims to address these challenges by using brain-like computing architectures and novel synaptic memories that coallocate information storage and computation, thereby enabling low latency at high energy efficiency and high memory density. Though various emerging memory devices have been extensively studied to emulate the functionality of biological synapses, there is currently no material/device system that encompasses both the needed metrics for high-performance neuromorphic computing and the required biocompatibility for potential body-computer integration. In this review, we aim to equip the reader with general design principles and materials requirements for realizing high-performance organic neuromorphic devices. We use instructive examples from recent literature to discuss each requirement, illustrating the challenges as well as future research opportunities. Though organic devices still face many challenges to become major players in neuromorphic computing, mostly due to their lack of compliance with back-end-of-line processes required for integration with digital logic, we propose that their biocompatibility and mechanical conformability give them an advantage for creating adaptive biointerfaces, brain-machine interfaces, and biology-inspired prosthetics.