H‐Aggregation Strategy in the Design of Molecular Semiconductors for Highly Reliable Organic Thin Film Transistors

Four new quaterthiophene derivatives with end‐groups composed of dicyclohexyl ethyl (DCE4T), dicyclohexyl butyl (DCB4T), cyclohexyl ethyl (CE4T), and cyclohexyl butyl (CB4T) were designed. All materials showed high solubility in common organic solvents. UV–vis absorption measurements showed that the quaterthiophene derivatives with asymmetrically substituted cyclohexyl end‐groups (CE4T and CB4T) preferred H‐type aggregation whereas those with symmetrically substituted cyclohexyl end‐groups (DCE4T and DCB4T) preferred J‐type aggregation. The molecular structure‐dependent packing (H or J) of the new quaterthiophene derivatives was analyzed by grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) measurements. The field‐effect mobilities of devices that incorporated the asymmetrical molecules, CE4T and CB4T, were quite high, above 10^?2 cm² V^?1 s^?1, due to H‐aggregation, whereas the field‐effect mobilities of devices that incorporated symmetrical molecules, DCE4T and DCB4T, were poor, below 10^?4 cm² V^?1 s^?1, due to J‐aggregation. More importantly, H‐aggregation within the thin film provided stable crystalline morphologies in the spin‐coated films, and, thus, thin film transistors (TFTs) using cyclohexylated quaterthiophenes yielded highly reproducible transistor performances. The distributions of measured field‐effect mobilities in transistors based on cyclohexylated quaterthiophenes with H‐aggregation were remarkably narrow.     

Effects of Arylene Diimide Thin Film Growth Conditions on n‐Channel OFET Performance

A series of eight perylene diimide (PDI)‐ and naphthalene diimide (NDI)‐based organic semiconductors was used to fabricate organic field‐effect transistors (OFETs) on bare SiO₂ substrates, with the substrate temperature during film deposition (T_d) varied from 70–130 °C. For the N,N′‐n‐octyl materials that form highly ordered films, the mobility (µ) and current on‐off ratio (I_on/I_off) increase slightly from 70 to 90 °C, and remain relatively constant between 90 and 130 °C. I_on/I_off and µ of dibromo‐PDI‐based OFETs decrease with increasing T_d, while films of N,N′‐1H,1H‐perfluorobutyl dicyanoperylenediimide (PDI‐FCN₂) exhibit dramatic I_on/I_off and µ enhancements with increasing T_d. Increased OFET mobility can be correlated with higher levels of molecular ordering and minimization of film morphology surface irregularities. Additionally, the effects of SiO₂ surface modification with trimethylsilyl and octadecyltrichlorosilyl monolayers, as well as with polystyrene, are investigated for N,N′‐n‐octyl dicyanoperylenediimide (PDI‐8CN₂) and PDI‐FCN₂ films deposited at T_d = 130 °C. The SiO₂ surface treatments have modest effects on PDI‐8CN₂ OFET mobilities, but modulate the mobility and morphology of PDI‐FCN₂ films substantially. Most importantly, the surface treatments result in substantially increased V_th and decreased I_off values for the dicyanoperylenediimide films relative to those grown on SiO₂, resulting in V_th > 0.0 V and I_on/I_off ratios as high as 10⁸. Enhancements in current modulation for these high‐mobility, air‐stable, and solution‐processable n‐type semiconductors, should prove useful in noise‐margin enhancement and further improvements in organic electronics.     

In Situ Tracking of Composition and Morphology of a Diblock Copolymer Film with GISAXS during Exchange of Solvent Vapors at Elevated Temperatures

Solvothermal vapor annealing at elevated temperature is applied to a thin film from a cylinder‐forming polystyrene‐block‐poly(dimethyl siloxane) (PS‐b‐PDMS) diblock copolymer. At this, the film is swollen in the vapor of n‐heptane (highly selective for PDMS). This vapor is stepwise replaced by the vapor of toluene (weakly selective for PS). The morphologies are investigated using in situ, real‐time grazing‐incidence small‐angle X‐ray scattering (GISAXS). The initial cylindrical morphology is transformed into, among others, the lamellar one. This novel type of experiments allows probing a trajectory in the state diagram of the PS‐b‐PDMS/n‐heptane/toluene mixture. To corroborate the morphologies, they are generated by molecular simulations, and the 2D GISAXS maps are calculated using the distorted‐wave Born approximation. To relate the morphologies to the solvent distribution in the two types of nanodomains, the latter is estimated from the intensities of the Bragg reflections in the 2D GISAXS maps along with the swelling ratio of the film. Comparison with the results from a similar experiment carried out at room temperature results in the same sequence of morphologies; however, at elevated temperature, more well‐ordered structures are obtained. This new approach proves to be efficient to achieve a block copolymer thin film having a desired morphology and orientation.     

High‐Mobility Air‐Stable Solution‐Shear‐Processed n‐Channel Organic Transistors Based on Core‐Chlorinated Naphthalene Diimides

High charge carrier mobility solution‐processed n‐channel organic thin‐film transistors (OTFTs) based on core‐chlorinated naphthalene tetracarboxylic diimides (NDIs) with fluoroalkyl chains are demonstrated. These OTFTs were prepared through a solution shearing method. Core‐chlorination of NDIs not only increases the electron mobilities of OTFTs, but also enhances their air stability, since the chlorination in the NDI core lowers the lowest unoccupied molecular orbital (LUMO) levels. The air‐stability of dichlorinated NDI was better than that of the tetrachlorinated NDIs, presumably due to the fact that dichlorinated NDIs have a denser packing of the fluoroalkyl chains and less grain boundaries on the surface, reducing the invasion pathway of ambient oxygen and moisture. The devices of dichlorinated NDIs exhibit good OTFT performance, even after storage in air for one and a half months. Charge transport anisotropy is observed from the dichlorinated NDI. A dichlorinated NDI with ?CH₂C₃F₇ side chains reveals high mobilities of up to 0.22 and 0.57 cm² V^?1 s^?1 in parallel and perpendicular direction, respectively, with regard to the shearing direction. This mobility anisotropy is related to the grain morphology. In addition, we find that the solution‐shearing deposition affects the molecular orientation in the crystalline thin films and lowers the d(001)‐spacing (the out‐of‐plane interlayer spacing), compared to the vapor‐deposited thin films. Core‐chlorinated NDI derivatives are found to be highly suitable for n‐channel active materials in low‐cost solution‐processed organic electronics.     

Solvent‐Induced Morphology in Polymer‐Based Systems for Organic Photovoltaics

Studies on the influence of four different solvents on the morphology and photovoltaic performance of bulk‐heterojunction films made of poly(3‐hexylthiophene) (P3HT) and [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM) via spin‐coating for photovoltaic applications are reported. Solvent‐dependent PCBM cluster formation and P3HT crystallization during thermal annealing are investigated with optical microscopy and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) and are found to be insufficient to explain the differences in device performance. A combination of atomic force microscopy (AFM), X‐ray reflectivity (XRR), and grazing‐incidence small‐angle X‐ray scattering (GISAXS) investigations results in detailed knowledge of the inner film morphology of P3HT:PCBM films. Vertical and lateral phase separation occurs during spin‐coating and annealing, depending on the solvent used. The findings are summarized in schematics and compared with the IV characteristics. The main influence on the photovoltaic performance arises from the vertical material composition and the existence of lateral phase separation fitting to the exciton diffusion length. Absorption and photoluminescence measurements complement the structural analysis.     

Role of Side‐Chain Branching on Thin‐Film Structure and Electronic Properties of Polythiophenes

Side‐chain engineering is increasingly being utilized as a technique to impact the structural order and enhance the electronic properties of semiconducting polymers. However, the correlations drawn between structural changes and the resulting charge transport properties are typically indirect and qualitative in nature. In the present work, a combination of grazing incidence X‐ray diffraction and crystallographic refinement calculations is used to determine the precise molecular packing structure of two thiophene‐based semiconducting polymers to study the impact of side‐chain modifications. The optimized structures provide high‐quality fits to the experimental data and demonstrate that in addition to a large difference in interchain spacing between the two materials, there exists a significant disparity in backbone orientation as well. The calculated structures are utilized in density functional theory calculations to determine the band structure of the two materials and are shown to exhibit a dramatic disparity in interchain dispersion which accounts for the large observed difference in charge carrier mobility. The techniques presented here are meant to be general and are therefore applicable to many other highly diffracting semicrystalline polymers.     

Tuning Local Molecular Orientation–Composition Correlations in Binary Organic Thin Films by Solution Shearing

A general impact of solution shearing on molecular orientation correlation is observed in polymer:fullerene organic solar cells in which one of the components forms fibrils or aggregates. Further investigation with polarized soft X‐ray scattering reveals that solution shearing induces more face‐to‐face orientation relative to the interface of two components compared to spin‐coating. This impact is shearing speed dependent, that is, slow shearing speed can induce more face‐to‐face orientation than a fast shearing speed. These results demonstrate that solution shearing is an effective method to control the relative molecular orientation. Solution shearing can also modify the domain size and average composition variations.     

High Hole Mobility and Thickness‐Dependent Crystal Structure in α,ω‐Dihexylsexithiophene Single‐Monolayer Field‐Effect Transistors

Monolayer‐thickness two‐dimensional layers of α,ω‐dihexylsexithiophene (α,ω‐DH6T) exhibit field‐effect hole mobility of up to 0.032 cm² V^?1 s^?1, higher than previously reported for monolayers of other small‐molecule organic semiconductors. In situ measurements during deposition show that the source‐drain current saturates rapidly after the percolation of monolayer‐high islands, indicating that the electrical properties of α,ω‐DH6T transistors are largely determined by the first molecular monolayer. The α,ω‐DH6T monolayer consists of crystalline islands in which the long axes of molecules are oriented approximately perpendicular to the plane of the substrate surface. In‐plane lattice constants measured using synchrotron grazing‐incidence diffraction are larger in monolayer‐thickness films than the in‐plane lattice constants of several‐monolayer films and of previously reported thick‐film structures. Near‐edge X‐ray absorption fine structure spectroscopy (NEXAFS) reveals that the larger in‐plane lattice constant of single‐monolayer films arises from a larger tilt of the molecular axis away from the surface normal. NEXAFS spectra at the C 1s and S 2p edges are consistent with a high degree of molecular alignment and with the local symmetry imposed by the thiophene ring. The high mobility of holes in α,ω‐DH6T monolayers can be attributed to the reduction of hole scattering associated with the isolation of the thiophene core from the interface by terminal hexyl chains.     

Nanoporous Silver Thin Films: Multifunctional Platforms for Influencing Chain Morphology and Optical Properties of Conjugated Polymers

Disordered nanoporous silver (NPAg) thin films fabricated by a thermally assisted dewetting method are employed as a platform to influence chain alignment, morphology, and optical properties of three well‐known conjugated polymers. Grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) measurements show that the porous structure of the metal induces close π–π stacking of poly(3‐hexylthiophene) (P3HT) chains and extended, planar chain conformations of poly(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl) (PFO) and poly[(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,8‐diyl)] (F8BT). A greater degree of vertically‐oriented P3HT chains are found on NPAg compared with planar Ag. However, PFO and F8BT chain alignment is only affected when pore size is large. The optical properties of NPAg films are investigated by transmission and back‐scattering spectroscopies. Strong back‐scattering is observed for all NPAg morphologies, especially for NPAg with small pore sizes. Photoluminescence spectroscopy of conjugated polymer layers on NPAg showed pronounced emission enhancements (up to factors of 26) relative to layers on glass. The enhancements are attributed primarily to: 1) redistribution of conjugated polymer emission by Ag; 2) redirection of emission by polymer‐filled nanopores; and 3) local electromagnetic field effects. This work demonstrates the potential of NPAg‐thin films to influence molecular chain morphology and to improve light‐extraction in organic optoelectronic devices.     

Fused Thiophene Semiconductors: Crystal Structure–Film Microstructure Transistor Performance Correlations

The molecular packing motifs within crystalline domains should be a key determinant of charge transport in thin‐film transistors (TFTs) based on small organic molecules. Despite this implied importance, detailed information about molecular organization in polycrystalline thin films is not available for the vast majority of molecular organic semiconductors. Considering the potential of fused thiophenes as environmentally stable, high‐performance semiconductors, it is therefore of interest to investigate their thin film microstructures in relation to the single crystal molecular packing and OTFT performance. Here, the molecular packing motifs of several new benzo[d,d′]thieno[3,2‐b;4,5‐b′]dithiophene ( BTDT ) derivatives are studied both in bulk 3D crystals and as thin films by single crystal diffraction and grazing incidence wide angle X‐ray scattering (GIWAXS), respectively. The results show that the BTDT derivative thin films can have significantly different molecular packing from their bulk crystals. For phenylbenzo[d,d′]thieno[3,2‐b;4,5‐b′]dithiophene ( P‐BTDT ), 2‐biphenylbenzo[d,d′]thieno‐[3,2‐b;4,5‐b′]dithiophene ( Bp‐BTDT ), 2 ‐naphthalenyl benzo[d,d′]thieno[3,2‐b;4,5‐b′]dithiophene ( Np‐BTDT ), and bisbenzo[d,d′]thieno[3,2‐b;4,5‐b′]dithiophene ( BBTDT ), two lattices co‐exist, and are significantly strained versus their single crystal forms. For P‐BTDT , the dominance of the more strained lattice relative to the bulk‐like lattice likely explains the high carrier mobility. In contrast, poor crystallinity and surface coverage at the dielectric/substrate interface explains the marginal OTFT performance of seemingly similar PF‐BTDT films.     

Enhanced Performance Consistency in Nanoparticle/TIPS Pentacene‐Based Organic Thin Film Transistors

In this study, inorganic silica nanoparticles are used to manipulate the morphology of 6,13‐bis(triisopropylsilylethynyl)‐pentacene (TIPS pentacene) thin films and the performance of solution‐processed organic thin‐film transistors (OTFTs). This approach is taken to control crystal anisotropy, which is the origin of poor consistency in TIPS pentacene based OTFT devices. Thin film active layers are produced by drop‐casting mixtures of SiO₂ nanoparticles and TIPS pentacene. The resultant drop‐cast films yield improved morphological uniformity at ～10% SiO₂ loading, which also leads to a 3‐fold increase in average mobility and nearly 4 times reduction in the ratio of measured mobility standard deviation (μ_Stdev) to average mobility (μ_Avg). Grazing‐incidence X‐ray diffraction, scanning and transmission electron microscopy as well as polarized optical microscopy are used to investigate the nanoparticle‐mediated TIPS pentacene crystallization. The experimental results suggest that the SiO₂ nanoparticles mostly aggregate at TIPS pentacene grain boundaries, and 10% nanoparticle concentration effectively reduces the undesirable crystal misorientation without considerably compromising TIPS pentacene crystallinity.     

Enhancement of Interconnectivity in the Channels of Pentacene Thin‐Film Transistors and Its Effect on Field‐Effect Mobility

With the aim of improving the field‐effect mobility of transistors by promoting the interconnectivity of the grains in pentacene thin films, deposition conditions of the pentacene molecules using one‐step (total thickness of layer 50 nm: 0.1 Å s^–1) and two‐step (first layer 10 nm: 0.1 Å s^–1, second layer 40 nm: 4.0 Å s^–1) depositions are controlled. Significantly, it is found that the continuities of the pentacene thin films vary with the deposition conditions of the pentacene molecules. Specifically, a smaller number of voids is observed at the interface for the two‐step deposition, which results in field‐effect mobilities as high as 1.2 cm² V^–1 s^–1; these are higher by more than a factor of two than those of the pentacene films deposited in one step. This remarkable increase in field‐effect mobility is due in particular to the interconnectivity of the pentacene grains near the insulator substrate.     

Enhancing the Charge Transport in Solution‐Processed Perylene Di‐imide Transistors via Thermal Annealing of Metastable Disordered Films

The introduction of side chains in π‐conjugated molecules is a design strategy widely exploited to increase molecular solubility thus improving the processability, while directly influencing the self‐assembly and consequently the electrical properties of thin films. Here, a multiscale structural analysis performed by X‐ray diffraction, X‐ray reflectivity, and atomic force microscopy on thin films of dicyanoperylene molecules decorated with either linear or branched side chains is reported. The substitution with asymmetric branched alkyl chains allows obtaining, upon thermal annealing, field‐effect transistors with enhanced transport properties with respect to linear alkyl chains. Branched chains induce molecular disorder during the film growth from solution, effectively favouring 2D morphology. Post‐deposition thermal annealing leads to a structural transition towards the bulk‐phase for molecules with branched chains, still preserving the 2D morphology and allowing efficient charge transport between crystalline domains. Conversely, molecules with linear chains self‐assemble into 3D islands exhibiting the bulk‐phase structure. Upon thermal annealing, these 3D islands keep their size constant and no major changes are observed in the organic field effect transistor characteristics. These findings demonstrate that the disorder generated by the asymmetric branched chains when the molecule is physisorbed in thin film can be instrumental for enhancing charge transport via thermal annealing.     

Boosting Direct X‐Ray Detection in Organic Thin Films by Small Molecules Tailoring

The attention focused on the application of organic electronics for the detection of ionizing radiation is rapidly growing among the international scientific community, due to the great potential of organic technology to enable large‐area conformable sensor panels. However, high‐energy photon absorption is challenging as organic materials are constituted of atoms with low atomic numbers. Here it is reported how, by synthesizing new solution‐processable organic molecules derived from 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene) and 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene, with Ge‐substitution in place of the Si atoms to increase the material atomic number, it is possible to boost the X‐ray detection performance of organic thin films on flexible plastic substrates. Bis(triisopropylgermylethynyl)‐pentacene based flexible organic thin film transistors show high electrical performance with higher mobility (0.4 cm² V^?1 s^?1) and enhanced X‐ray sensitivity, up to 9.0 × 10⁵ µC Gy^?1 cm^?3, with respect to TIPS‐pentacene‐based detectors. Moreover, similar results are obtained for 5,11‐bis(triethylgermylethynyl)anthradithiophene devices, confirming that the proposed strategy, that is, increasing the atomic number of organic molecules by chemical tailoring to improve X‐ray sensitivity, can be generalized to organic thin film detectors, combining high X‐ray absorption, mechanical flexibility, and large‐area processing.     

High‐Mobility Naphthalene Diimide and Selenophene‐Vinylene‐Selenophene‐Based Conjugated Polymer: n‐Channel Organic Field‐Effect Transistors and Structure–Property Relationship

Interdependence of chemical structure, thin‐film morphology, and transport properties is a key, yet often elusive aspect characterizing the design and development of high‐mobility, solution‐processed polymers for large‐area and flexible electronics applications. There is a specific need to achieve >1 cm² V^?1 s^?1 field‐effect mobilities (μ) at low processing temperatures in combination with environmental stability, especially in the case of electron‐transporting polymers, which are still lagging behind hole transporting materials. Here, the synthesis of a naphthalene‐diimide based donor–acceptor copolymer characterized by a selenophene vinylene selenophene donor moiety is reported. Optimized field‐effect transistors show maximum μ of 2.4 cm² V^?1 s^?1 and promising ambient stability. A very marked film structural evolution is revealed with increasing annealing temperature, with evidence of a remarkable 3D crystallinity above 180 °C. Conversely, transport properties are found to be substantially optimized at 150 °C, with limited gain at higher temperature. This discrepancy is rationalized by the presence of a surface‐segregated prevalently edge‐on packed polymer phase, dominating the device accumulated channel. This study therefore serves the purpose of presenting a promising, high‐electron‐mobility copolymer that is processable at relatively low temperatures, and of clearly highlighting the necessity of specifically investigating channel morphology in assessing the structure–property nexus in semiconducting polymer thin films.     

High Tg Cyclic Olefin Copolymer Gate Dielectrics for N,N′‐Ditridecyl Perylene Diimide Based Field‐Effect Transistors: Improving Performance and Stability with Thermal Treatment

A novel application of ethylene‐norbornene cyclic olefin copolymers (COC) as gate dielectric layers in organic field‐effect transistors (OFETs) that require thermal annealing as a strategy for improving the OFET performance and stability is reported. The thermally‐treated N,N′‐ditridecyl perylene diimide (PTCDI‐C13)‐based n‐type FETs using a COC/SiO₂ gate dielectric show remarkably enhanced atmospheric performance and stability. The COC gate dielectric layer displays a hydrophobic surface (water contact angle = 95° ± 1°) and high thermal stability (glass transition temperature = 181 °C) without producing crosslinking. After thermal annealing, the crystallinity improves and the grain size of PTCDI‐C13 domains grown on the COC/SiO₂ gate dielectric increases significantly. The resulting n‐type FETs exhibit high atmospheric field‐effect mobilities, up to 0.90 cm² V^?1 s^?1 in the 20 V saturation regime and long‐term stability with respect to H₂O/O₂ degradation, hysteresis, or sweep‐stress over 110 days. By integrating the n‐type FETs with p‐type pentacene‐based FETs in a single device, high performance organic complementary inverters that exhibit high gain (exceeding 45 in ambient air) are realized.     

Orientation Regulation of Tin‐Based Reduced‐Dimensional Perovskites for Highly Efficient and Stable Photovoltaics

Tin‐based perovskites have exhibited high potential for efficient photovoltaics application due to their outstanding optoelectrical properties. However, the extremely undesired instabilities significantly hinders their development and further commercialization process. A novel tin‐based reduced‐dimensional (quasi‐2D) perovskites is reported here by using 5‐ammoniumvaleric acid (5‐AVA⁺) as the organic spacer. It is demonstrated that by introducing appropriate amount of ammonium chloride (NH₄Cl) as additive, highly vertically oriented tin‐based quasi‐2D perovskite films are obtained, which is proved through the grazing incidence wide‐angle X‐ray scattering characterization. In particular, this approach is confirmed to be a universal method to deliver highly vertically oriented tin‐based quasi‐2D perovskites with various spacers. The highly ordered vertically oriented perovskite films significantly improve the charge collection efficiency between two electrodes. With the optimized NH₄Cl concentration, the solar cells employing quasi‐2D perovskite, AVA₂FA_n?1Sn_nI_3n+1 (<n> = 5), as light absorbers deliver a power conversion efficiency up to 8.71%. The work paves the way for further employing highly vertically oriented tin‐based quasi‐2D perovskite films for highly efficient and stable photovoltaics.