Gaurav Giri

Tuning charge transport in solution-sheared organic semiconductors using lattice strain

Circuits based on organic semiconductors are being actively explored for flexible, transparent and low-cost electronic applications. But to realize such applications, the charge carrier mobilities of solution-processed organic semiconductors must be improved. For inorganic semiconductors, a general method of increasing charge carrier mobility is to introduce strain within the crystal lattice. Here we describe a solution-processing technique for organic semiconductors in which lattice strain is used to increase charge carrier mobilities by introducing greater electron orbital overlap between the component molecules. For organic semiconductors, the spacing between cofacially stacked, conjugated backbones (the π–π stacking distance) greatly influences electron orbital overlap and therefore mobility. Using our method to incrementally introduce lattice strain, we alter the π–π stacking distance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) from 3.33 Å to 3.08 Å. We believe that 3.08 Å is the shortest π–π stacking distance that has been achieved in an organic semiconductor crystal lattice (although a π–π distance of 3.04 Å has been achieved through intramolecular bonding). The positive charge carrier (hole) mobility in TIPS-pentacene transistors increased from 0.8 cm2 V−1 s−1 for unstrained films to a high mobility of 4.6 cm2 V−1 s−1 for a strained film. Using solution processing to modify molecular packing through lattice strain should aid the development of high-performance, low-cost organic semiconducting devices.

Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method

Organic semiconductors with higher carrier mobility and better transparency have been actively pursued for numerous applications, such as flat-panel display backplane and sensor arrays. The carrier mobility is an important figure of merit and is sensitively influenced by the crystallinity and the molecular arrangement in a crystal lattice. Here we describe the growth of a highly aligned meta-stable structure of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) from a blended solution of C8-BTBT and polystyrene by using a novel off-centre spin-coating method. Combined with a vertical phase separation of the blend, the highly aligned, meta-stable C8-BTBT films provide a significantly increased thin film transistor hole mobility up to 43 cm(2) Vs(-1) (25 cm(2) Vs(-1) on average), which is the highest value reported to date for all organic molecules. The resulting transistors show high transparency of >90% over the visible spectrum, indicating their potential for transparent, high-performance organic electronics.

Solution coating of large-area organic semiconductor thin films with aligned single-crystalline domains

Solution coating of organic semiconductors offers great potential for achieving low-cost manufacturing of large-area and flexible electronics. However, the rapid coating speed needed for industrial-scale production poses challenges to the control of thin-film morphology. Here, we report an approach--termed fluid-enhanced crystal engineering (FLUENCE)--that allows for a high degree of morphological control of solution-printed thin films. We designed a micropillar-patterned printing blade to induce recirculation in the ink for enhancing crystal growth, and engineered the curvature of the ink meniscus to control crystal nucleation. Using FLUENCE, we demonstrate the fast coating and patterning of millimetre-wide, centimetre-long, highly aligned single-crystalline organic semiconductor thin films. In particular, we fabricated thin films of 6,13-bis(triisopropylsilylethynyl) pentacene having non-equilibrium single-crystalline domains and an unprecedented average and maximum mobilities of 8.1±1.2 cm(2) V(-1) s(-1) and 11 cm(2) V(-1) s(-1). FLUENCE of organic semiconductors with non-equilibrium single-crystalline domains may find use in the fabrication of high-performance, large-area printed electronics.

High-performance transistors and complementary inverters based on solution-grown aligned organic single-crystals.

Constructing a complementary inverter is technically more complex because both pand n-channel transistors are required to be patterned onto a common substrate. Here, we report a simple solution processing method to fabricate complementary inverters based on n-channel C 60 single crystals and p-channel 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) single crystals. We achieved a signal gain as high as 155. Hence, this work provides a platform to study high-performance complementary circuits based on organic single-crystals. Organic FETs have been widely used for electronic applications such as displays [ 5 , 6 ] and sensors. [ 7–9 ] Organic single-crystals show the best FET performance with the highest charge mobility among organic materials. The p-channel organic single-crystal FETs have exhibited hole mobility as high as 40 cm 2 V − 1 s − 1 , [ 10–14 ]

One-dimensional self-confinement promotes polymorph selection in large-area organic semiconductor thin films

A crystals structure has significant impact on its resulting biological, physical, optical and electronic properties. In organic electronics, 6,13(bis-triisopropylsilylethynyl)pentacene (TIPS-pentacene), a small-molecule organic semiconductor, adopts metastable polymorphs possessing significantly faster charge transport than the equilibrium crystal when deposited using the solution-shearing method. Here, we use a combination of high-speed polarized optical microscopy, in situ microbeam grazing incidence wide-angle X-ray-scattering and molecular simulations to understand the mechanism behind formation of metastable TIPS-pentacene polymorphs. We observe that thin-film crystallization occurs first at the air-solution interface, and nanoscale vertical spatial confinement of the solution results in formation of metastable polymorphs, a one-dimensional and large-area analogy to crystallization of polymorphs in nanoporous matrices. We demonstrate that metastable polymorphism can be tuned with unprecedented control and produced over large areas by either varying physical confinement conditions or by tuning energetic conditions during crystallization through use of solvent molecules of various sizes.

Large‐Area Assembly of Densely Aligned Single‐Walled Carbon Nanotubes Using Solution Shearing and Their Application to Field‐Effect Transistors

Dense alignment of single-walled carbon nanotubes over a large area is demonstrated using a novel solution-shearing technique. A density of 150-200 single-walled carbon nanotubes per micro-meter is achieved with a current density of 10.08 μA μm(-1) at VDS = -1 V. The on-current density is improved by a factor of 45 over that of random-network single-walled carbon nanotubes.

Tuning the Morphology of Solution-Sheared P3HT:PCBM Films

UNLABELLED Organic bulk heterojunction (BHJ) solar cells are a promising alternative for future clean-energy applications. However, to become attractive for consumer applications, such as wearable, flexible, or semitransparent power-generating electronics, they need to be manufactured by high-throughput, low-cost, large-area-capable printing techniques. However, most research reported on BHJ solar cells is conducted using spin coating, a single batch fabrication method, thus limiting the reported results to the research lab. In this work, we investigate the morphology of solution-sheared films for BHJ solar cell applications, using the widely studied model blend P3HT:PCBM. Solution shearing is a coating technique that is upscalable to industrial manufacturing processes and has demonstrated to yield record performance organic field-effect transistors. Using grazing incident small-angle X-ray scattering, grazing incident wide-angle X-ray scattering, and UV-vis spectroscopy, we investigate the influence of solvent, film drying time, and substrate temperature on P3HT aggregation, conjugation length, crystallite orientation, and PCBM domain size. One important finding of this study is that, in contrast to spin-coated films, the P3HT molecular orientation can be controlled by the substrate chemistry, with PEDOT PSS substrates yielding face-on orientation at the substrate-film interface, an orientation highly favorable for organic solar cells.

Large-area formation of self-aligned crystalline domains of organic semiconductors on transistor channels using CONNECT

Significance Solution-processed organic electronics are expected to pave the way for low-cost large-area electronics with new and exciting applications. However, realizing solution-processed organic electronics requires densely packed transistors with patterned and precisely registered organic semiconductors (OSCs) within the transistor channel with uniform electrical properties over a large area, a task that remains a significant challenge. To address such a challenge, we have developed an innovative technique that generates self-patterned and self-registered OSC film with low variability in electrical properties over a large area. We have fabricated highest density of transistors with a yield of 99%, along with various logic circuits. This work significantly advances organic electronics field to enable large-scale circuit fabrication in a facile and economical manner. The electronic properties of solution-processable small-molecule organic semiconductors (OSCs) have rapidly improved in recent years, rendering them highly promising for various low-cost large-area electronic applications. However, practical applications of organic electronics require patterned and precisely registered OSC films within the transistor channel region with uniform electrical properties over a large area, a task that remains a significant challenge. Here, we present a technique termed “controlled OSC nucleation and extension for circuits” (CONNECT), which uses differential surface energy and solution shearing to simultaneously generate patterned and precisely registered OSC thin films within the channel region and with aligned crystalline domains, resulting in low device-to-device variability. We have fabricated transistor density as high as 840 dpi, with a yield of 99%. We have successfully built various logic gates and a 2-bit half-adder circuit, demonstrating the practical applicability of our technique for large-scale circuit fabrication.

p-Channel Field-Effect Transistors Based on C60 Doped with Molybdenum Trioxide

Fullerene (C60) is a well-known n-channel organic semiconductor. We demonstrate that p-channel C60 field-effect transistors are possible by doping with molybdenum trioxide (MoO3). The device performance of the p-channel C60 field-effect transistors, such as mobility, threshold voltage, and on/off ratio is varied in a controlled manner by changing doping concentration. This work demonstrates the utility of charge transfer doping to obtain both n- and p-channel field-effect transistors with a single organic semiconductor.

Probing the interfacial molecular packing in TIPS-pentacene organic semiconductors by surface enhanced Raman scattering

In organic thin film transistors (OTFTs), the molecular structure of the first few monolayers at the semiconductor–dielectric interface is crucial to the device performance. The assumption of homogeneous molecular packing throughout the thickness of the film is not always valid considering interfacial effects. However, it remains challenging to unambiguously determine the molecular packing at both the top surface and the buried bottom interface, due to the lack of a nanoscopic tool. Here we show that a combination of Raman spectroscopy and surface enhanced Raman scattering (SERS) provides a means for effective characterization of the interfacial packing in 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) films. We observed that the TIPS-pentacene crystal lattices assume a non-equilibrium packing state near the substrate interface, which gradually relaxes towards equilibrium packing near the top interface. Our investigation suggests the existence of non-equilibrium molecular packing for TIPS-pentacene.