High-Performance n-Type Polymeric Mixed Ionic-Electronic Conductors: The Impacts of Halogen Functionalization.

Developing high-performance n-type polymer mixed ionic-electronic conductors (PMIECs) is a grand challenge, which largely determines their applications in vaious organic electronic devices, such as organic electrochemical transistors (OECTs) and organic thermoelectrics (OTEs). Herein, two halogen-functionalized PMIECs f-BTI2g-TVTF and f-BTI2g-TVTCl built from fused bithiophene imide dimer (f-BTI2) as the acceptor unit and halogenated thienylene-vinylene-thienylene (TVT) as the donor co-unit are reported. Compared to the control polymer f-BTI2g-TVT, the fluorinated f-BTI2g-TVTF shows lower-positioned lowest unoccupied molecular orbital (LUMO), improved charge transport property, and greater ion uptake capacity. Consequently, f-BTI2g-TVTF delivers a state-of-the-art µC* of 90.2 F cm-1 V-1 s-1 with a remarkable electron mobility of 0.41 cm2 V-1 s-1 in OECTs and an excellent power factor of 64.2 µW m-1 K-2 in OTEs. An OECT-based inverter amplifier is further demonstrated with voltage gain up to 148 V V-1 , which is among the highest values for OECT inverters. Such results shed light on the impacts of halogen atoms on developing high-performing n-type PMIECs.

Cyano-Functionalized Fused Bithiophene Imide Dimer-Based n-Type Polymers for High-Performance Organic Thermoelectrics.

Doped n-type polymers usually exhibit low electrical conductivities and thermoelectric power factors (PFs), restricting the development of high-performance p-n-junction-based organic thermoelectrics (OTEs). Herein, the design and synthesis of a new cyano-functionalized fused bithiophene imide dimer (f-BTI2), CNI2, is reported, which synergistically combines the advantages of both cyano and imide functionalities, thus leading to substantially higher electron deficiency than the parent f-BTI2. On the basis of this novel building block, a series of n-type donor-acceptor and acceptor-acceptor polymers are successfully synthesized, all of which show good solubility, deep-lying frontier molecular orbital levels, and favorable polymer chain orientation. Among them, the acceptor-acceptor polymer PCNI2-BTI delivers an excellent electrical conductivity up to 150.2 S cm-1 and a highest PF of 110.3 µW m-1  K-2 in n-type OTEs, attributed to the optimized polymer electronic properties and film morphology with improved molecular packing and higher crystallinity assisted by solution-shearing technology. The PF value is the record of n-type polymers for OTEs to date. This work demonstrates a facile approach to designing high-performance n-type polymers and fabricating high-quality films for OTE applications.

Understanding molecular surface doping of large bandgap organic semiconductors and overcoming the contact/access resistance in organic field-effect transistors.

The contact resistance (Rc) and the effective carrier mobility (µeff) are considered as the important indicators of the performance of organic field-effect transistors (OFETs). Conventionally, the contact resistance is regarded as the interface effect between the metal electrodes and the organic semiconductors, while the carrier mobility is correlated to the crystallinity and π-π stacking of the organic molecules. In the staggered OFETs, Rc is actually closely correlated to µeff through the channel sheet resistance. Besides, the accuracy of the carrier mobility directly extracted from the non-ideal transfer curves with significant contact effect is always questionable. Herein, a diffusion-lead surface doping approach is employed to improve the contact resistance and mobility issues simultaneously. By suppressing the trap states in the sublimated 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) organic semiconductor with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), we observed a 3-fold increase in the carrier mobility from 0.5 to 1.6 cm2 V-1 s-1, and the Rc also drops remarkably from 25.7 kΩ cm to 5.2 kΩ cm. Moreover, the threshold voltage (VTH), subthreshold swing (SS) and the bias stability of the OFETs are also significantly improved. Based on the detailed characterization of the C8-BTBT film upon surface doping, including X-ray diffraction (XRD) for the film crystallinity, Kelvin probe force microscopy (KPFM) for the surface potential, trap state investigation by density of states (DOS) measurement and electrical circuit modeling for partial doping analysis, we confirmed that the spontaneous charge transfer process due to the diffusion of the F4-TCNQ dopants in the C8-BTBT matrix can lead to an effective trap filling. This technique and findings can be potentially developed into a general approach for the improvement of different performance parameters of OFETs.

Field-Effect Transistors Based on 2D Organic Semiconductors Developed by a Hybrid Deposition Method.

Solution-processed 2D organic semiconductors (OSCs) have drawn considerable attention because of their novel applications from flexible optoelectronics to biosensors. However, obtaining well-oriented sheets of 2D organic materials with low defect density still poses a challenge. Here, a highly crystallized 2,9-didecyldinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (C10-DNTT) monolayer crystal with large-area uniformity is obtained by an ultraslow shearing (USS) method and its growth pattern shows a kinetic Wulff's construction supported by theoretical calculations of surface energies. The resulting seamless and highly crystalline monolayers are then used as templates for thermally depositing another C10-DNTT ultrathin top-up film. The organic thin films deposited by this hybrid approach show an interesting coherence structure with a copied molecular orientation of the templating crystal. The organic field-effect transistors developed by these hybrid C10-DNTT films exhibit improved carrier mobility of 14.7 cm2 V-1 s-1 as compared with 7.3 cm2 V-1 s-1 achieved by pure thermal evaporation (100% improvement) and 2.8 cm2 V-1 s-1 achieved by solution sheared monolayer C10-DNTT. This work establishes a simple yet effective approach for fabricating high-performance and low-cost electronics on a large scale.

Solution-Phase Epitaxial Growth of Perovskite Films on 2D Material Flakes for High-Performance Solar Cells.

The quality of perovskite films is critical to the performance of perovskite solar cells. However, it is challenging to control the crystallinity and orientation of solution-processed perovskite films. Here, solution-phase van der Waals epitaxy growth of MAPbI3 perovskite films on MoS2 flakes is reported. Under transmission electron microscopy, in-plane coupling between the perovskite and the MoS2 crystal lattices is observed, leading to perovskite films with larger grain size, lower trap density, and preferential growth orientation along (110) normal to the MoS2 surface. In perovskite solar cells, when perovskite active layers are grown on MoS2 flakes coated on hole-transport layers, the power conversion efficiency is substantially enhanced for 15%, relatively, due to the increased crystallinity of the perovskite layer and the improved hole extraction and transfer rate at the interface. This work paves a way for preparing high-performance perovskite solar cells and other optoelectronic devices by introducing 2D materials as interfacial layers.

Inch-Scale Grain Boundary Free Organic Crystals Developed by Nucleation Seed-Controlled Shearing Method.

Crystals of organic semiconductors are excellent candidates for flexible and array-based electronics. Large-scale synthesis of organic crystals in a controllable way while maintaining homogeneous single-crystal property has been a great challenge. The existence of grain boundaries and small crystal domains, however, restrict the device performance and limit the access to commercially viable organic electronics in the industry. Herein, we report the inch-scale synthesis of highly oriented 2,7-dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) organic single crystal by nucleation seed-controlled shearing method. The organic field-effect transistors developed from such single crystal have excellent carrier mobility as high as 14.9 cm2 V-1 s-1 and uniformity (standard deviation is 1.3 cm2 V-1 s-1) of 225 devices. We also found that the rotation of the principal axis in the crystal is governed by the orientations of seeds and the possible mechanism behind this phenomenon is proposed based on the density functional theory calculations. We anticipate that this proposed approach will have great potential to be developed as a platform for the growth of organic crystals with high crystallinity on a large scale.

A High-Performance Optical Memory Array Based on Inhomogeneity of Organic Semiconductors.

Organic optical memory devices keep attracting intensive interests for diverse optoelectronic applications including optical sensors and memories. Here, flexible nonvolatile optical memory devices are developed based on the bis[1]benzothieno[2,3-d;2',3'-d']naphtho[2,3-b;6,7-b']dithiophene (BBTNDT) organic field-effect transistors with charge trapping centers induced by the inhomogeneity (nanosprouts) of the organic thin film. The devices exhibit average mobility as high as 7.7 cm2 V-1 s-1 , photoresponsivity of 433 A W-1 , and long retention time for more than 6 h with a current ratio larger than 106 . Compared with the standard floating gate memory transistors, the BBTNDT devices can reduce the fabrication complexity, cost, and time. Based on the reasonable performance of the single device on a rigid substrate, the optical memory transistor is further scaled up to a 16 × 16 active matrix array on a flexible substrate with operating voltage less than 3 V, and it is used to map out 2D optical images. The findings reveal the potentials of utilizing [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives as organic semiconductors for high-performance optical memory transistors with a facile structure. A detailed study on the charge trapping mechanism in the derivatives of BTBT materials is also provided, which is closely related to the nanosprouts formed inside the organic active layer.

A pentacene monolayer trapped between graphene and a substrate.

A self-assembled pentacene monolayer can be fabricated between the solid-solid interface of few-layered graphene (FLG) and the mica substrate, through a diffusion-spreading method. By utilizing a transfer method that allows us to sandwich pentacene between graphene and mica, followed by controlled annealing, we enabled the diffused pentacene to be trapped in the interfaces and led to the formation of a stable monolayer. We found that the formation of a monolayer is kinetically favored by using a 2D Ising lattice gas model for pentacene trapped between the graphene-substrate interfaces. This kinetic Monte Carlo simulation results indicate that, due to the graphene substrate enclosure, the spreading of the first layer proceeds faster than the second layer, as the kinetics favors the filling of voids by molecules from the second layer. This graphene assisted monolayer assembly method provides a new avenue for the fabrication of two-dimensional monolayer structures.