N-type Organic Thin-film Transistors Research Articles

Cyano-Functionalized Pyrazine: A Structurally Simple and Easily Accessible Electron-Deficient Building Block for n-Type Organic Thermoelectric Polymers.

Developing low-cost and high-performance n-type polymer semiconductors is essential to accelerate the application of organic thermoelectrics (OTEs). To achieve this objective, it is critical to design strong electron-deficient building blocks with simple structure and easy synthesis, which are essential for the development of n-type polymer semiconductors. Herein, we synthesized two cyano-functionalized highly electron-deficient building blocks, namely 3,6-dibromopyrazine-2-carbonitrile (CNPz) and 3,6-Dibromopyrazine-2,5-dicarbonitrile (DCNPz), which feature simple structures and facile synthesis. CNPz and DCNPz can be obtained via only one-step reaction and three-step reactions from cheap raw materials, respectively. Based on CNPz and DCNPz, two acceptor-acceptor (A-A) polymers, P(DPP-CNPz) and P(DPP-DCNPz) are successfully developed, featuring deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels, which are beneficial to n-type organic thin-film transistors (OTFTs) and OTEs performance. An optimal unipolar electron mobility of 0.85 and 1.85 cm2  V-1  s-1 is obtained for P(DPP-CNPz) and P(DPP-DCNPz), respectively. When doped with N-DMBI, P(DPP-CNPz) and P(DPP-DCNPz) show high n-type electrical conductivities/power factors of 25.3 S cm-1 /41.4 μW m-1  K-2 , and 33.9 S cm-1 /30.4 μW m-1  K-2 , respectively. Hence, the cyano-functionalized pyrazine CNPz and DCNPz represent a new class of structurally simple, low-cost and readily accessible electron-deficient building block for constructing n-type polymer semiconductors.

Printed organic transistors and complementary ring oscillators operatable at 200 mV

Applications of organic thin-film transistors (OTFTs) include wearable health monitors and next-generation Internet-of-Things systems driven by a small energy-harvesting power supply. Such applications require low voltage and low power consumption organic ICs. In this paper, we demonstrate complementary ICs based on printed p-type and n-type OTFTs operatable at an ultralow supply voltage of 200 mV. For that purpose, threshold voltages were finely tuned by dual-gate structure and self-assembled monolayer. Complementary inverter-based ring oscillators operated at small supply voltages down to 200 mV and exhibited a power consumption as small as 6 pW per stage.

Optimization of thin-film formation of naphthalene tetracarboxylic diimide derivatives with head and tail structure and application to transistors

Herein, we characterized vacuum-evaporated thin-films of several new naphthalene tetracarboxylic diimide derivatives (C12-NTCDI-R) as n-type organic thin-film transistor (TFT) materials. They have head and tail structure with dodecyl chain and phenylalkyl (benzyl or phenylethyl) groups. By introducing fluorine groups into the phenylalkyl groups, the electron transport properties were enhanced. In addition, modifying the substrate surfaces with a fluorine-rich polymer material and annealing at the appropriate temperature allowed the TFT devices with C12-NTCDI-F2Bn (R = 3,5-difluorobenzyl) to reach a maximum electron mobility (μ e) of 0.11 cm2 V−1 s−1. Furthermore, the alteration of the number of methylene carbons in the side chain of phenylalkyl groups in C12-NTCDI-R (namely, phenylethyl was used as R) affected the thermal behavior of their molecules during the thin-film formation. For the TFT device with C12-NTCDI-PhEt (R = phenylethyl), the μ e reached the maximum value of 0.16 cm2 V−1 s−1 by just surface modification, without annealing.

Measurement of 64 organic thin-film transistors in an array test structure using a relay-switch board for efficient evaluation of long-term reliability

We propose an efficient array measurement test structure to measure many devices and to obtain statistical characteristics of organic thin-film-transistors (OTFT) for long-term reliability evaluation of new devices under development in laboratories and those that cannot achieve sufficient yield. We propose to adopt different devices from the device under test for the array control circuit. We implemented a separate control circuit as a dedicated relay-switch board without device consolidation. This relay-switch board can also apply voltage stress to any number of devices under test for negative bias temperature instability evaluation. The interface issue between the chip and board is resolved with a flexible flat cable and an anisotropic conductive film. The proposed measurement system successfully measured the OTFT arrays in (sweep time of source measure unit) × (number of OTFTs) period, and only requires 84 min for 64 nOTFTs (n-type OTFT) (552 points/device), which corresponds to 1.3 min/device in the experiment.

Fine-Tuning the Performance of Ultraflexible Organic Complementary Circuits on a Single Substrate via a Nanoscale Interfacial Photochemical Reaction.

Flexible electronics has paved the way toward the development of next-generation wearable and implantable healthcare devices, including multimodal sensors. Integrating flexible circuits with transducers on a single substrate is desirable for processing vital signals. However, the trade-off between low power consumption and high operating speed is a major bottleneck. Organic thin-film transistors (OTFTs) are suitable for developing flexible circuits owing to their intrinsic flexibility and compatibility with the printing process. We used a photoreactive insulating polymer poly((±)endo,exo-bicyclo[2.2.1]hept-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) to modulate the power consumption and operating speed of ultraflexible organic circuits fabricated on a single substrate. The turn-on voltage (V on) of the p- and n-type OTFTs was controlled through a nanoscale interfacial photochemical reaction. The time-of-flight secondary ion mass spectrometry revealed the preferential occurrence of the PNDPE photochemical reaction in the vicinity of the semiconductor-dielectric interface. The power consumption and operating speed of the ultraflexible complementary inverters were tuned by a factor of 6 and 4, respectively. The minimum static power consumption was 30 ± 9 pW at transient and 4 ± 1 pW at standby. Furthermore, within the tuning range of the operating speed and at a supply voltage above 2.5 V, the minimum stage delay time was of the order of hundreds of microseconds. We demonstrated electromyogram measurements to emphasize the advantage of the nanoscale interfacial photochemical reaction. Our study suggests that a nanoscale interfacial photochemical reaction can be employed to develop imperceptible and wearable multimodal sensors with organic signal processing circuits that exhibit low power consumption.

Electrostatically-sprayed carbon electrodes for high performance organic complementary circuits

Organic thin-film transistors (OTFTs) are promising building blocks of flexible printable electronic devices. Similar to inorganic FETs, OTFTs are heterostructures consisting of metals, insulators, and semiconductors, in which nanoscale interfaces between different components should be precisely engineered. However, OTFTs use noble metals, such as gold, as electrodes, which has been a bottleneck in terms of cost reduction and low environmental loading. In this study, we demonstrate that graphite-based carbon electrodes can be deposited and patterned directly onto an organic single-crystalline thin film via electrostatic spray coating. The present OTFTs exhibited reasonably high field-effect mobilities of up to 11 cm2 V−1 s−1 for p-type and 1.4 cm2 V−1 s−1 for n-type with no significant deterioration during electrostatic spray processes. We also demonstrate two significant milestones from the viewpoint of material science: a complementary circuit, an inverter consisting of p- and n-type OTFTs, and an operatable metal-free OTFT composed of fully carbon-based materials. These results constitute a key step forward in the further development of printed metal-free integrated circuits.

Furan functionalized naphthalenediimide semiconductors with different N-alkyl chains for n-type organic thin-film transistor applications

Development of n-type organic semiconductors (OSCs) is necessary in order to develop complementary electronics based on organic thin-film transistors (OTFTs). In this investigation, we designed and synthesized a series of four novel imide- and core-substituted naphthalene diimide (NDI) molecules. The molecular architecture was altered via introduction of different alkyl side chains (octyl and 2-ethylhexyl) to the imide-N positions and 2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) to the NDI core. The NDI-TCF derivatives are abbreviated as NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B, using a “1” or “2” to indicate the number of TCF subunits at the NDI core, and an “A” or a “B” to indicate an octyl or a 2-ethylhexyl alkyl chain at the imide-N position, respectively. All four compounds displayed similar photophysical properties but exhibited different electrochemical properties with lower LUMO levels of −4.25 eV, −4.33 eV, −4.27 eV and −4.35 eV for NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B, respectively. These LUMO levels suggest a capacity for electron transport. Furthermore, the NDI-TCF chromophores were incorporated into OTFTs and systematically studied on the relationship between the NDI-TCF molecular architecture and electron transport properties. NDI-TCF1A and NDI-TCF1B displayed more consistent OTFT performance than NDI-TCF2A and NDI-TCF2B. NDI-TCF1A and NDI-TCF1B exhibited low threshold voltages (VT) of −0.43 V ± 4.78 V and 6.12 V ± 6.63 V, respectively, electron mobilities (μe) of ca. 3 × 10−5 cm2V−1s−1, and on/off (ION/OFF) ratios of ca. 103. Insofar that TCF facilitates electron transport, the OTFT data suggest that the number of TCF subunits affect OTFT performance via thin-film morphology. The thin-film morphology of NDI-TCF1A, NDI-TCF1B, NDI-TCF2A, and NDI-TCF2B was studied using atomic force microscopy (AFM) technique and supported by X-ray diffraction measurements.

Benchmarking contact quality in N-type organic thin film transistors through an improved virtual-source emission-diffusion model

Due to nonideal behavior, current organic thin film transistor technologies lack the proper models for essential characterization and thus suffer from a poorly estimated parameter extraction critical for circuit design and integration. Organic thin film transistors are often plagued by contact resistance, which is often less problematic in inorganic transistors; consequently, common models used for describing inorganic devices do not properly work with organic thin film transistors. In this work, we fabricate poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} based organic thin film transistors with reduced contact resistance through the introduction of metallic interlayers between the semiconductor and gold contacts. The addition of 10 nm thick manganese interlayer provides optimal organic thin film transistor device performance with the lowest level of contact resistance. Improved organic thin film transistors were characterized using an improved organic virtual-source emission diffusion model, which provides a simple and effective method to extract the critical device parameters. The organic virtual-source emission diffusion model led to nearly perfect prediction using effective gate voltages and a gate dependant contact resistance, providing a significant improvement over common metal–oxide–semiconductor field-effect transistor models such as the Shichman–Hodges model.

Yield and leakage current of organic thin-film transistor logic gates toward reliable and low-power operation of large-scale logic circuits for IoT nodes

This paper reports on a strategy for yield improvement and static leakage current reduction by a standard cell design for large-scale organic thin-film transistor (OTFT) circuits. Printable or flexible devices are suitable for IoT nodes, and digital OTFT circuits comprise the peripheral circuits of such devices. Sufficiently high yields and low static power consumptions are essential for battery operations of IoT nodes having functional digital circuits. Our design method to address the weak n-type OTFT on-current results in improved logic gate yields without any cell area increase. We improved the yield of the inverter, NAND, and NOR gates using a standard cell design, and achieved a 100% yield for the inverter and NOR gates and 88% yield for the NAND gates. Signal propagations with the rail-to-rail operation were measured on test chips. Leakage currents of 585 pA and 2.94 nA were achieved for the inverter and NOR gates, respectively.

Hydrogenated Graphene Based Organic Thin Film Transistor Sensor for Detection of Chloride Ions as Corrosion Precursors

Corrosion monitoring and management has been at the center of structural health monitoring protocols due to its damaging effects on metallic structures. Current corrosion prevention and management programs often fail to include environmental factors such as Cl− ions and surface wetness. Early detection of these environmental factors can prevent the onset of corrosion and reduce repair and maintenance-related expenses. There is growing interest in creating solution-processed thin film environmental sensors with high sensitivity to corrosion precursors, low-cost fabrication, and small footprint, rendering them viable candidates for investigation as potential corrosion sensors that could be easily integrated into existing structures and screen printed or patterned directly into surface coatings. In this work, we have implemented C60-based n-type organic thin film transistors (OTFTs) with functionalized graphene oxide for humidity sensing and functionalized graphene nanoparticles for Cl− ion detection, using low-cost solution processing techniques. The reduced graphene oxide (rGO)-coated OTFT humidity sensor is designed for the qualitative estimation of surface moisture levels and high levels of humidity, and it exhibits a relative responsivity for dry to surface wetness transition of 122.6% to surface wetness, within a response time of 20 ms. We furthermore implemented an in-house synthesized hydrogenated graphene coating in conjunction with a second OTFT architecture for Cl− ions sensing which yielded a sensitivity of 4%/ppm to ultrafine ionic concentrations, over an order of magnitude lower than the range identified to cause corrosion in aircraft structures.

Highlighting the processing versatility of a silicon phthalocyanine derivative for organic thin-film transistors

Silicon phthalocyanine (SiPc) derivatives have recently emerged as promising materials for n-type organic thin-film transistors (OTFTs) with the ability to be fabricated either by solid state or solution processes through axial functionalization.

Cyanophenoxy-Substituted Silicon Phthalocyanines for Low Threshold Voltage n-Type Organic Thin-Film Transistors

Silicon phthalocyanines (SiPcs) are a class of n-type or ambipolar organic semiconductors that have been incorporated into organic thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs). Despite a relatively large catalogue of previously reported SiPc materials, fabricated OTFTs with these materials typically have threshold voltages (VT) above 10 V, limiting their usage in commercial devices due to exceedingly high power consumption. Recent studies have suggested that the VT can be reduced in OTFTs prepared from phenoxy-substituted SiPcs by introducing electron-withdrawing groups onto the phenoxy moieties. Herein, we report the synthesis and characterization of three SiPcs with phenoxy axial substituents containing nitrile and fluorine functional groups. These SiPcs, along with 3,5-difluorophenoxy SiPc were evaluated as candidate materials for n-type OTFTs. We found that further increasing the electron-withdrawing character of the pendant phenoxy groups of the SiPc resulted in a significant decrease in average VT with the lowest reported value being 4.8 V, the lowest VT reported for a phenoxy-SiPc-based OTFT exceeding the previous record low of 7.8 V attributed to F10-SiPc. This decrease in VT could be directly correlated to the Hammett parameter of the axial functional groups. Furthermore, it was noted that dewetting occurred when the phenoxy pendant group of the SiPc was substituted at the para position with a nitrile group combined with ortho- or meta-substituted fluorines, which was attributed to interactions at the semiconductor/dielectric interface. Depositing these SiPcs on silane-terminated poly(styrene) brush modified substrates improved long-term stability, demonstrated by a minimal change in surface morphology according to atomic force microscopy (AFM) images.

Separation of bias stress degradation between insulator and semiconductor carrier trapping in organic thin-film transistors

Organic thin-film transistors (OTFTs) are studied intensively for realizing practical applications of flexible or large-area circuits, but rapid degradation of OTFTs due to stress voltage or reaction with water vapor or oxygen in the air limits their lifetime. In order to analyze the cause of rapid bias-stress degradation, we propose a method that separates the cause of threshold voltage (V th) shift into insulator carrier trapping (ICT) and semiconductor carrier trapping components. The experimental results show that the ICT components account for nearly 50% of the total V th shift in n-type OTFTs, while the ICT-induced V th shifts is about 20% of the total V th shift in p-type OTFTs regardless of the insulator materials: SAM or parylene. The experimental results suggest that the short lifetime of the n-type OTFTs with SAM-based insulator is caused by the instability of the SAM-based insulator due to ICT. In addition, the instability of the p-type OTFTs with SAM-based insulator is discussed based on measurement, and as a result, capacitance shift due to ICT may also affect the degradation of highly biased p-type OTFTs.

Alignment of linear polymeric grains for highly stable N-type thin-film transistors

•Fast deposition of polymer thin films with aligned grains and molecules•The polyacrylonitrile can tune the size of the pre-aggregates in polymer solution•Microscopic structures of polymers affected the thermal stability of transistors•The electron mobilities of transistors were >3.5 cm2V–1s–1 from 200 to 460 K For polymeric thin-film transistors, the ordered arrangements of polymer backbones, the flat face-on crystallites, and long linear grains can simultaneously improve the carrier transport efficiency. These strategies reduce pinholes, grain boundaries, and barriers in the charge transport paths. The aligned films have smaller extent of microscopic structural disorders, fewer localized states, and lower structural energies, thereby improving the thermal stability of devices. Temperature-insensitive property is very important for transistors. Based on the aligned polymeric thin films, we prepared the transistors with great thermal stability. The electron mobilities based on top-gate transistors were more than 3.5 cm2V–1s–1 at temperatures between 200 and 460 K. Temperature-insensitive properties are attractive for most electronics, including polymeric semiconducting devices. Especially, polymeric field-effect transistors (FETs) with high mobility have been important research targets due to their broad applications. However, polymeric FETs with stable charge transport operating at extremely cold or hot zones are faced with enormous challenges. In this study, the polyacrylonitrile was found to significantly tune the sizes of pre-aggregates of polymers in solutions and the crystallinity of the polymeric films. The orientation of 5–25 μm linear grains in the films were prepared through the bar-coating process with polyacrylonitrile as an additive, which stabilized the electron mobility over a wide range of temperatures. The linear-grain morphology of the film contributed to reducing the holes and grain boundaries in the transport paths of carriers. Typically, the top-gate FETs based on P(NDI2OD-T2) displayed a stable electron transporting behavior from 200 to 460 K, with mobility greater than 3.5 cm2V−1s−1. Temperature-insensitive properties are attractive for most electronics, including polymeric semiconducting devices. Especially, polymeric field-effect transistors (FETs) with high mobility have been important research targets due to their broad applications. However, polymeric FETs with stable charge transport operating at extremely cold or hot zones are faced with enormous challenges. In this study, the polyacrylonitrile was found to significantly tune the sizes of pre-aggregates of polymers in solutions and the crystallinity of the polymeric films. 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Macroscopic and high-throughput printing of aligned nanostructured polymer semiconductors for MHz large-area electronics.Nat. Commun. 2015; 6: 8394Crossref PubMed Scopus (237) Google Scholar In this work, PAN was added in poly{[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} (P(NDI2OD-T2)) semiconductor solution to prepare highly crystalline linear-grain polymer films through the bar-coating approach (Figure 1). We found that PAN remarkably increased the sizes of the pre-aggregates in the solution, thereby increasing the sizes of the grains in the solid films and improving the crystallinity of the polymer molecules. Furthermore, the bar-coating method controlled the arrangement of P(NDI2OD-T2) molecular backbones and the stacking grain direction. The aligning backbone and grain arrangements contributed to reduce the grain boundaries and defects in the transport paths of the carriers,11Wang C.L. Dong H.L. Hu W.P. Liu Y.Q. Zhu D.B. Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.Chem. Rev. 2012; 112: 2208-2267Crossref PubMed Scopus (2731) Google Scholar,27Bucella S.G. Luzio A. Gann E. Thomsen L. McNeill C.R. Pace G. Perinot A. Chen Z.H. Facchetti A. Caironi M. Macroscopic and high-throughput printing of aligned nanostructured polymer semiconductors for MHz large-area electronics.Nat. Commun. 2015; 6: 8394Crossref PubMed Scopus (237) Google Scholar, 28Noriega R. Rivnay J. Vandewal K. Koch F.P.V. Stingelin N. Smith P. Toney M.F. Salleo A. A general relationship between disorder, aggregation and charge transport in conjugated polymers.Nat. Mater. 2013; 12: 1038-1044Crossref PubMed Scopus (1436) Google Scholar, 29Diao Y. Shaw L. Bao Z.N. Mannsfeld S.C.B. Morphology control strategies for solution-processed organic semiconductor thin films.Energy Environ. Sci. 2014; 7: 2145-2159Crossref Google Scholar, 30Xu X.M. Yao Y.F. Shan B. Gu X. Liu D.Q. Liu J.Y. Xu J.B. Zhao N. Hu W.P. Miao Q. Electron mobility exceeding 10 cm2V−1s−1 and band-like charge transport in solution-processed n-channel organic thin-film transistors.Adv. Mater. 2016; 28: 5276-5283Crossref PubMed Scopus (149) Google Scholar which resulted in stable charge transport and high mobility in the polymer film (Figure 1A). Notably, the carrier transport paths were limited by linear grains, so the carrier transport was anisotropic in the aligned film. At the same time, vapor-deposited parylene-C was used as the insulating layer. Top-gate FETs were constructed, and electron mobility above 3.5 cm2V−1s−1 was attained between 200–460 K. Figures 1B and 1C are the arrangements of P(NDI2OD-T2) molecules in the P(NDI2OD-T2)/PAN film prepared by bar-coating and the molecular arrangements calculated from Grazing-incidence wide-angle X-ray scattering (GIWAXS) data, respectively. Figure S1 is the photo of the actual equipment. The tapping mode atomic force microscopy (AFM) images show that the spin-coated P(NDI2OD-T2)/PAN films had longer and more ordered grains (Figure 1F) than those of the P(NDI2OD-T2) films (Figure 1D). Moreover, the bar-coating method was employed to prepare the aligned films on 40°C glass substrates in air, at a coating speed of 120 mm/s, and all bars moved in the direction indicated by the blue arrow in Figure 1E. After drying, the film was annealed at 190°C for 30 min under nitrogen atmosphere. The grains in the aligned P(NDI2OD-T2) film were observed to be small and closely connected with each other (Figure 1E), whereas the aligned P(NDI2OD-T2)/PAN film had long independently linear grains (Figure 1G). Most of the grains in the aligned P(NDI2OD-T2)/PAN film were 5–25 μm long and 150–350 nm wide (Figures S2 and S3). Moreover, the examination of the film morphologies revealed that the aligned P(NDI2OD-T2)/PAN films were anisotropic, and there were few grain boundaries in the parallel direction of the bar movement, whereas many grain boundaries in perpendicular direction. It is noteworthy to state that linearly aligned grains limit carrier transport paths. To verify the universality of PAN, PAN was added to PCII2T and DPPT-TT, and it was observed that PAN improved both the crystallinity of the polymer semiconductor films and the grain sizes. Basically, the success rate of the bar-coating method for the preparation of the aligned films was significantly improved by the presence of PAN. Root-mean-square (RMS) analysis of image height was used to investigate the roughness of the films, and the results show that the spin-coated P(NDI2OD-T2) film, aligned P(NDI2OD-T2) film, spin-coated P(NDI2OD-T2)/PAN film, and aligned P(NDI2OD-T2)/PAN film exhibited RMS surface roughness values of 1.10, 0.672, 2.57, and 3.92 nm, respectively. Additionally, the AFM micrographs of PCII2T monolayer films, 20 nm PCII2T/PAN films, and 20 nm DPPT-TT/PAN films, prepared by the bar-coating method, showed that the films contained long linearly aligned grains (Figure S4). Since GIWAXS is a powerful way to explain the correlation between molecular packing and the performance of transistors, 2D-GIWAXS was utilized in the analysis of the aligned bar-coated P(NDI2OD-T2)/PAN films (Figures 2A and 2 B), spin-coated P(NDI2OD-T2)/PAN films (Figure 2C), bar-coated P(NDI2OD-T2) films (Figures 2D and 2E), and spin-coated P(NDI2OD-T2) films (Figure 2F). The direction of the GIWAXS light source was parallel (Figures 2A and 2D) and perpendicular (Figures 2B and 2E) to the bar-coating direction. The line cuts of the P(NDI2OD-T2) and P(NDI2OD-T2)/PAN films are shown in Figure S5. The detailed crystallographic parameters were calculated and presented in Tables S1 and S2. Also, the 2D-GIWAXS diagrams and line cuts of PCII2T, DPPT-TT, and PAN films are shown in Figures S6–S11, while their corresponding detailed crystallographic parameters are listed in Tables S3–S6. According to the 2D-GIWAXS, the (010) peaks for all the films can be seen in the out-of-plane direction (Figure 2), and the weak in-plane (010) peak appeared in the aligned P(NDI2OD-T2)/PAN (parallel) and aligned P(NDI2OD-T2) films (parallel) (Figures 2A and 2D). The result also shows that in the spin-coated P(NDI2OD-T2)/PAN and spin-coated P(NDI2OD-T2) films, neat P(NDI2OD-T2) molecules adopted face-on π-stacking crystallites arrangements, as shown in Figure S12. Most of the P(NDI2OD-T2) molecules in the aligned P(NDI2OD-T2)/PAN and aligned P(NDI2OD-T2) films mainly adopted a face-on π-stacking crystallites arrangement, while a few adopted an edge-on crystallites arrangement. Furthermore, the intensities of the (002) signals were stronger than those of the (001) signals (Figures 2B, 2C, and 2E), suggesting that for two adjacent P(NDI2OD-T2) molecules, the NDI units of one molecule tend to stack on the T2 units of the other molecule.31Brinkmann M. Gonthier E. Bogen S. Tremel K. Ludwigs S. Hufnagel M. Sommer M. Segregated versus mixed interchain stacking in highly oriented films of naphthalene diimidebithiophene copolymers.ACS Nano. 2012; 6: 10319-10326Crossref PubMed Scopus (126) Google Scholar Additionally, the signal intensity and quantity of P(NDI2OD-T2)/PAN films were obviously better than those of P(NDI2OD-T2), which means the crystallinity of the P(NDI2OD-T2)/PAN films was significantly improved. For the parallel direction of P(NDI2OD-T2)/PAN film, the (010) signals in the out-of-plane direction showed two symmetrical peaks (Figure 2A), and the planes of the conjugated backbone of P(NDI2OD-T2), calculated from the signal positions, formed an angle of 14°–15° with the substrate, as shown in the inset of Figure 1A. According to the differences in the (010) signals for the parallel and perpendicular directions of the aligned P(NDI2OD-T2)/PAN film (Figures 2A and 2B), it was observed that the direction of the P(NDI2OD-T2) backbones was parallel to the bar-coating direction. The same phenomenon was observed for the (001) and (002) peaks in the perpendicular direction of P(NDI2OD-T2)/PAN film (Figure 2B). Alternatively, the (010) peak in the P(NDI2OD-T2) films was not divided into two symmetrical peaks (Figures 2D–2F). This may be due to the fact that the planes of the conjugated backbone of P(NDI2OD-T2) in the aligned P(NDI2OD-T2) films were parallel to the substrate. Together with the results from the AFM analysis, the undivided (010) signal may also have resulted from the poor crystallinity of the aligned P(NDI2OD-T2) film. According to the detailed crystallographic parameters (Table S3), the in-plane lamellar stacking correlation lengths of the aligned P(NDI2OD-T2)/PAN films were 325 Å (parallel) and 270 Å (perpendicular), which were longer than the 251 Å (parallel) and 141 Å (perpendicular) of the aligned P(NDI2OD-T2) films. Meanwhile, the in-plane backbone stacking correlation length of the aligned P(NDI2OD-T2)/PAN film (181 Å) was similar to that of the aligned P(NDI2OD-T2) film (193 Å), indicating that the addition of PAN resulted in the formation of larger crystallites. The out-of-plane π-stacking correlation lengths of the aligned P(NDI2OD-T2)/PAN films were smaller than those of the spin-coated films, which indicates that the molecules in the aligned P(NDI2OD-T2)/PAN films prefer stacking in the lamellar direction to the π direction. According to GIWAXS analysis, the addition of PAN was observed to markedly improve the film crystallinity, and the polymer backbones from bar coating were positioned in the same direction as the linear grains. Meanwhile, the small out-of-plane π-stacking correlation lengths were favorable to the in-plane carrier transport along the linear grains. This can limit the out-of-plane transport and reduce the chances of lattice scattering. In addition to the effects of the processing methods, the polymer film morphology is also closely related to the pre-aggregates in solution.32Zheng Y.Q. Yao Z.F. Lei T. Dou J.H. Yang C.Y. Zou L. Meng X.Y. Ma W. Wang J.Y. Pei J. Unraveling the solution-state supramolecular structures of donor–acceptor polymers and their influence on solid-state morphology and charge-transport properties.Adv. Mater. 2017; 29: 1701072Crossref Scopus (78) Google Scholar,33Li M.M. Mangalore D.K. Zhao J.B. Carpenter J.H. Yan H.P. Ade H. Yan H. Müllen K. Blom P.W.M. Pisula W. et al.Integrated circuits based on conjugated polymer monolayer.Nat. Commun. 2018; 9: 451Crossref PubMed Scopus (50) Google Scholar UV-vis spectroscopy was used to analyze the P(NDI2OD-T2)/PAN and P(NDI2OD-T2) solutions (Figure 3A). The test concentration of P(NDI2OD-T2) was 0.02 mg/mL, while PAN was 0.004 mg/mL, and tetralin was used as the solvent. Obvious changes in absorption spectra were observed during the heating process. From 283 to 393 K, the absorbance of the 0–0 peak at 718 nm decreased continuously and then disappeared, while the 0–1 peak at 601 nm gradually appeared. These results demonstrate the presence of strong pre-aggregation of P(NDI2OD-T2) in the low-temperature solution. The pre-aggregates were broken up since polymer chains separate from each other under heating conditions.34Chen Z.H. Cai P. Chen J.W. Liu X.C. Zhang L.J. Lan L.F. Peng J.B. Ma Y.G. Cao Y. Low band-gap conjugated polymers with strong interchain aggregation and very high hole mobility towards highly efficient thick-film polymer solar cells.Adv. Mater. 2014; 26: 2586-2591Crossref PubMed Scopus (343) Google Scholar There was no obvious change in the spectrum after the addition of PAN, which indicates that there was no charge transfer between PAN and P(NDI2OD-T2). At the same time, the ultraviolet photoelectron spectroscopy (UPS) of the spin-coated P(NDI2OD-T2) and spin-coated P(NDI2OD-T2)/PAN films were the same, confirming that there was no charge transfer between PAN and P(NDI2OD-T2).12Nikolka M. Nasrallah I. Rose B. Ravva M.K. Broch K. Sadhanala A. Harkin D. Charmet J. Hurhangee M. Brown A. et al.High operational and environmental stability of high-mobility conjugated polymer field-effect transistors through the use of molecular additives.Nat. Mater. 2017; 16: 356-362Crossref PubMed Scopus (272) Google Scholar For comparison, temperature-dependent absorption spectra was used to analyze P(NDI2OD-T2)/PAN and P(NDI2OD-T2) in o-dichlorobenzene (Figure S13), and the film spectra are shown in Figure S14. To further understand the effect of PAN on the formation of the pre-aggregates, dynamic light-scattering (DLS) technique was utilized in the size analysis of the pre-aggregates in low concentration solutions, while small-angle neutron scattering (SANS) technique was used to analyze the pre-aggregates in high concentration solutions. Figure 3B displays the DLS-generated results of the polymer particle sizes of P(NDI2OD-T2)/PAN and P(NDI2OD-T2) in solutions where tetralin and o-dichlorobenzene were the employed solvents, respectively. For the P(NDI2OD-T2)/PAN solution, the concentrations of P(NDI2OD-T2) and PAN were 0.01 and 0.002 mg/mL, respectively, whereas the concentration of P(NDI2OD-T2) in the P(NDI2OD-T2) solution was 0.012 mg/mL. The diameters of the particles in the tetralin P(NDI2OD-T2) solution were 67–125 nm, which was larger than those in the o-dichlorobenzene P(NDI2OD-T2) solution (37–78 nm). This indicates that the P(NDI2OD-T2) chains formed larger pre-aggregates in tetralin. Correspondingly, the diameters of the particles in the tetralin solution containing PAN were in the range of 127–236 nm and were larger than the particles for the tetralin solution without PAN, indicating that PAN promoted the increase in particle size even in extremely dilute solutions. The results of SANS analysis for P(NDI2OD-T2)/PAN and P(NDI2OD-T2) solutions are displayed in Figures 3C and 3D. The concentrations of P(NDI2OD-T2) and PAN in the P(NDI2OD-T2)/PAN solution were 6 and 1.2 mg/mL, respectively, while the P(NDI2OD-T2) concentration in the P(NDI2OD-T2) solution was 6 mg/mL. To improve the signal-to-noise ratio, deuterated 1,2-dichlorobenzene-d4 (ODCB-d4) was used as the solvent to reduce incoherent scattering.35Le Cœur C.L. Combet S. Carrot G. Busch P. Teixeira J. Longeville S. Conformation of the poly(ethylene glycol) chains in DiPEGylated hemoglobin specifically probed by SANS: correlation with PEG length and in vivo efficiency.Langmuir. 2015; 31: 8402-8410Crossref PubMed Scopus (22) Google Scholar The power exponent in the P(NDI2OD-T2)/PAN solution was 1.93, while it was 1.82 in the P(NDI2OD-T2) solution (Figure 3D). These indicate that the pre-aggregates in the two solutions adopted large rod-like structures.32Zheng Y.Q. Yao Z.F. Lei T. Dou J.H. Yang C.Y. Zou L. Meng X.Y. Ma W. Wang J.Y. Pei J. Unraveling the solution-state supramolecular structures of donor–acceptor polymers and their influence on solid-state morphology and charge-transport properties.Adv. Mater. 2017; 29: 1701072Crossref Scopus (78) Google Scholar,35Le Cœur C.L. Combet S. Carrot G. Busch P. Teixeira J. Longeville S. Conformation of the poly(ethylene glycol) chains in DiPEGylated hemoglobin specifically probed by SANS: correlation with PEG length and in vivo efficiency.Langmuir. 2015; 31: 8402-8410Crossref PubMed Scopus (22) Google Scholar From the fitted line,36Chen W.R. Butler P.D. Magid L.J. Incorporating intermicellar interactions in the fitting of SANS data from cationic wormlike micelles.Langmuir. 2006; 22: 6539-6548Crossref PubMed Scopus (148) Google Scholar the radius of the pre-aggregates in the P(NDI2OD-T2) solution was 1.6 nm, and the Kuhn length was 26 nm, whereas the radius and Kuhn length of the pre-aggregates in the P(NDI2OD-T2)/PAN solution were 1.6 and 280 nm, respectively. The lamella direction length was calculated by density functional theory (DFT), and its vale for the P(NDI2OD-T2) molecule was 3.22 nm. According to the fitting results of the experimental data generated from the SANS analysis, it was observed that the addition of PAN increased the length and rigidity of the pre-aggregates.37Pedersen J.S. Schurtenberger P. Scattering functions of semiflexible polymers with and without excluded volume effects.Macromolecules. 1996; 29: 7602-7612Crossref Scopus (474) Google Scholar Long rod-like aggregates can facilitate the formation of long linear grains in solid films. Molecular dynamics (MD) simulation was used to gain understanding to the relationship between the addition of PAN and the size increase of pre-aggregates. MD simulations can only reflect intermolecular accumulation and conformation. The MD simulation was carried out with the Gromacs-4.6.7 package based on the general Amber force field (see Supplemental information for details).38Hess B. Kutzner C. van der Spoel D. Lindahl E. GROMACS 4: algorithms for highly efficient, load-balanced, and scalable molecular simulation.J. Chem. Theor. Comput. 2008; 4: 435-447Crossref PubMed Scopus (11893) Google Scholar,39Wang J.M. Wolf R.M. Caldwell J.W. Kollman P.A. Case D.A. Development and testing of a general amber force field.J. Comput. Chem. 2004; 25: 1157-1174Crossref PubMed Scopus (10866) Google Scholar To match the experimental mass ratio of 5:1 for P(NDI2OD-T2) to PAN during the solvent evaporation process (Figure S15), the root-mean-square end-to-end distance (RMSD), the backbone radius of gyration, and the number of backbone aggregation of P(NDI2OD-T2), all increased upon PAN addition (Figure S16), indicating that the addition of PAN can indeed enhance the backbone ductility and aggregation ability of P(NDI2OD-T2). To illustrate the role of PAN in the self-assembly of P(NDI2OD-T2) in solvents, snapshots were taken from the MD trajectory and presented in Figures 3E, 3F, and S17. Figure 3E shows that P(NDI2OD-T2) chains aggregated around PAN, and PAN acted as the nucleation center of P(NDI2OD-T2). At the same time, some PAN served as a bridge for P(NDI2OD-T2) and increased the cluster sizes (Figure 3F). Combined with the results from SANS and DFT calculations, MD simulations show that the diameter of the pre-aggregate (3.2 nm) was close to the length of the lamella direction of the P(NDI2OD-T2) molecule (3.22 nm), so in the actual P(NDI2OD-T2)/PAN solution, the arrangement of P(NDI2OD-T2) molecules may be as shown in Figure 3F; that is, P(NDI2OD-T2) molecules were twining round the PAN molecules and PAN molecules served as a bridge for P(NDI2OD-T2). These calculation results are consistent with the experimental observation that the aggregate size of P(NDI2OD-T2) increased upon the addition of a small amount of PAN. FETs with top-gate bottom-contact (TGBC) configuration were fabricated to characterize charge carrier transport in the P(NDI2OD-T2) and P(NDI2OD-T2)/PAN films (Figure 4A). The parallel gold source-drain electrodes were modified with cesium carbonate (Cs2CO3), and the RMS of the Cs2CO3 film prepared by bar coating on the glass substrate was 0.27 nm (Figure S18). The P(NDI2OD-T2) and P(NDI2OD-T2)/PAN semiconductor films were prepared by bar coating, and for comparison, P(NDI2OD-T2) and P(NDI2OD-T2)/PAN films were also prepared by spin coating. Parylene-C (200 nm) was deposited as an insulating layer, and it also exhibited a packaging effect. Aluminum was used as a gate electrode. In the following description, the test mode is denoted as “para” when the direction of the electric field from the source electrode to the drain electrode was parallel to the direction in which the bar moved, and it was denoted as “perp” when the two directions were perpendicular to each other. The electron mobilities in the parallel direction for the 36-transistor array based on the aligned P(NDI2OD-T2)/PAN films were counted and shown in Figure 4B. Table 1 shows the device performance tested in air and at room temperature (295 K), and Figure S19 shows the device performance tested in air as a function of time. The average electron mobility of the spin-coated P(NDI2OD-T2) films at room temperature was 0.23 ± 0.07 cm2V−1s−1; whereas, the parallel electron mobility (μelec,∥) of the aligned P(NDI2OD-T2) films was 1.6 ± 0.4 cm2V−1s−1, while its perpendicular electron mobility (μelec,⊥) was 0.25 ± 0.05 cm2V−1s−1, μelec,∥/μelec,⊥= 5–10. Comparatively, the electron mobility of spin-coated P(NDI2OD-T2)/PAN films was 0.45 ± 0.1 cm2V−1s−1, the parallel electron mobility of aligned P(NDI2OD-T2)/PAN films was 4.85 ± 0.3 cm2V−1s−1, and its perpendicular mobility was 0.02 ± 0.01 cm2V−1s−1, μelec,∥/μelec,⊥= 150–520. The results show the correlation between electron mobility and film morphology. The better the grains order, the larger the μelec,∥, the higher the ratio of μelec,∥ and μelec,⊥, and the greater the anisotropy of the carrier transport. In addition, the performance of the FETs is also related to the energy level of the semiconductor layers (Figure S20).Table 1OFET parameters calculated from the saturation regime (Vds = 10 V)Depositionμe (cm2V–1s–1)aData are represented as mean ± SD.Von (V)Ion:Ioff (Log10)Vt (V)P(NDI2OD-T2)spin-coat0.23 ± 0.07254–5bar-coat (para)1.6 ± 0.424–55–6bar-coat (perp)0.25 ± 0.05242–3P(NDI2OD-T2)+PANspin-coat0.45 ± 0.1–151–2bar-coat(para)4.85 ± 0.3–0.574–5bar-coat (perp)0.02 ± 0.01–0.540–1The FETs in this table are tested in air at room temperature (295 K).a Data are represented as mean ± SD. Open table in a new tab The FETs in this table are tested in air at room temperature (295 K). The relationships between the device performance and the operating temperatures in vacuum environment were evaluated and the results are shown in Figures 4C, S21, and S22. It should be emphasized that the dielectric constant of the insulating layer was affected by temperature, as shown in Figure S23. The electron mobility increased with increasing temperature (Figures 4D and S24), and after 360 K, the mobility of the transistors with different semiconductor layers decreased to different degrees. As temperature increased, the spin-coated P(NDI2OD-T2)/PAN FETs exhibited mobility degradation around 360 K. The mobility degradation may be related to the disorder and large pinholes of the semiconductor films. The relationship between the carrier mobility and temperature was analyzed by the generalized Einstein relation (GER) method,40Liu C. Huang K. Park W.T. Li M. Yang T. Liu X. Liang L. Minari T. Noh Y.Y. A unified understanding of charge transport in organic semiconductors: the importance of attenuated delocalization for the carriers.Mater. Horiz. 2017; 4: 608-618Crossref Google Scholar as shown in Figure 4D, which is generally applicable to describe charge transport of various microscopic mechanisms, such as band-like hopping, multiple-trapping and releasing, variable range hopping, etc. Generally, the GER method completely fits the mobility data, as shown in Figure 4D, except the data from the devices with spin-coated films above 360 K, due to the degraded performance. Two characteristics parameters (the density of states [DOS] width, ΔE, and the delocalization degree parameter, ΔD) were listed in the figure captions and, in general, a smaller value of ΔE signifies less dispersed energy distribution in the Gaussian-like DOS and a larger value of ΔD signifies ordered charge transport (the maximum is 1 corresponding to the classic band transport in single crystals). Importantly, the transistor with aligned P(NDI2OD-T2)/PAN exhibited the smallest ΔE (50 meV) and the largest ΔD (0.7), signifying a much smaller extent of microscopic structural disorders and fewer localized states as compared with that of the spin-coated P(NDI2OD-T2)/PAN film (ΔE = 90 meV and ΔD = 0.42). The results are highly consistent with those of the AFM and GIWAXS analysis, showing that the former film features a much smaller number of grain boundaries or pinholes and a higher extent of chain alignment as compared with the latter film. Notice that, in the low-temperature region (< 200 K), the addition of PAN in the bar-coating process, sufficiently eliminated disorders in charge transport and enhanced mobility, but this was not the case in spin-coating films. This may be attributed to the fact that the bar-coating process provided a longer timescale to lower structural energy and to reach alignment with minimal disorders. According to the transfer characteristics of transistors (Figure S22), when the operating temperature was 440 K, the hysteresis voltage of the spin-coated P(NDI2OD-T2)/PAN FETs was 5 V, and this may be related to the morphology of the films. Although PAN improved the crystallinity of the P(NDI2OD-T2) films, there were many large irregular gaps in the P(NDI2OD-T2)/PAN films according to the AFM micrographs. This observation illustrates that the presence of too many static physical defects, such as grain boundaries and pinholes, may cause thermal instability of the device performance at high temperatures. The electron mobilities of the aligned P(NDI2OD-T2)/PAN FETs had high thermal stability. The value measured at 300–460 K was 5.5 ± 0.5 cm2V−1s−1, and at 200 K, the electron mobility was still above 3.5 cm2V−1s−1. In addition, within the temperature range from 200 to 460 K, the hysteresis voltage was small (Figure S22A), which may indicate that the P(NDI2OD-T2)/PAN films had few defects and holes in the carrier transport paths, therefore the fewer localized states and low structural energies may be the keys to ensure the thermal stability of devices. When the operating temperature increased from 160 to 460 K, the threshold voltage of the aligned P(NDI2OD-T2)/PAN transistor showed the least change compared with other transistors (Figure 4E). Additionally, the turn-on voltage of the aligned P(NDI2OD-T2)/PAN FETs was –0.5 V, which is smaller than that of other FETs. Meanwhile, the Ioff of the aligned P(NDI2OD-T2)/PAN transistors exhibited the least variation compared with the other transistors. The OFET channel current (Vds = 10 V, Vgs = 20 and 0 V) versus temperature is shown in Figure S25. For the aligned P(NDI2OD-T2)/PAN FETs, the increase in dielectric constant (Figure S23) of the insulating layer with increasing temperature was an important factor that affected the Ioff (Figure 4F). The study showed that controlling the charge transport paths by multiple strategies can effectively improve the mobility and thermal stability of polymer FETs. Also, the ordered arrangements of polymer backbones, the flat face-on crystallites, and long linear grains can simultaneously limit the carrier transport paths. These strategies reduce pinholes, grain boundaries, and barriers in the charge transport paths, making films have smaller extent of microscopic structural disorders, fewer localized states, and lower structural energies, thereby improving the thermal stability of devices. Vapor-deposited insulating parylene-C layers can provide great encapsulation and further ensure device stability. In addition, we found that adding PAN to polymer semiconductors increased the size of the pre-aggregates in solution, the grain sizes, and the crystallinity of films. To sum up, highly crystalline polymer films with aligned linear grains were successfully prepared by the bar-coating method. The electron mobilities based on top-gate FETs were 3.5–6 cm2V−1s−1 at 200–460 K and 5–6 cm2V−1s−1 at 300–460 K.

Conjoint use of Naphthalene Diimide and Fullerene Derivatives to Generate Organic Semiconductors for n-type Organic Thin Film Transistors.

In this paper, we described the design, synthesis, and characterization of two novel naphthalene diimide (NDI) core‐based targets modified with terminal fullerene (C60) yield – so called S4 and S5, in which NDI bearing 1 and 2 molecules of C60, respectively. The absorption, electrochemical and thin‐film transistor characteristics of the newly developed targets were investigated in detail. Both S4 and S5 displayed broad absorption in the 450–500 nm region, owing to the effect of conjugation due to fullerene functionalities. The electrochemical measurement suggested that the HOMO and the LUMO energy levels can be altered with the number of C60 units. Both S4 and S5 were employed as organic semiconductor materials in n‐channel transistors. The thin film transistor based on S4 exhibited superior electron mobility (μe) values ranging from 1.20×10−4 to 3.58×10−4 cm2 V−1 s−1 with a current on‐off ratio varying from 102 to 103 in comparison with the performance of S5 based transistor, which exhibited μe ranging from 8.33×10−5 to 2.03×10−4 cm2 V−1 s−1 depending on channel lengths.

Modeling of two complementary ambipolar organic thin film transistors: application to organic inverter

In this paper, we have study two types of thin-film organic transistors and their application to release the organic inverter. For manufacturing p-type and n-type organic thin film transistors (OTFT), pentacene and N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13H27) have been used as organic semiconductors active layers. These organic thin film transistors have been shown excellent ambipolar operation. We proceeded initially to model and study these complementary organic transistors individually. Therefore we present the various electrical parameters resulting from the modeling of these two types of organic transistors (n-type and p-type) as well as the various parameters characterizing the organic inverter. Very good agreement is obtained between the experimental electrical characteristics of the two types of organic transistors and the characteristics obtained by the analytical model, as well as the experimental characteristics of the organic inverter thus produced.

Cyano-Functionalized Bithiophene Imide-Based n-Type Polymer Semiconductors: Synthesis, Structure–Property Correlations, and Thermoelectric Performance

n-Type polymers with deep-positioned lowest unoccupied molecular orbital (LUMO) energy levels are essential for enabling n-type organic thin-film transistors (OTFTs) with high stability and n-type organic thermoelectrics (OTEs) with high doping efficiency and promising thermoelectric performance. Bithiophene imide (BTI) and its derivatives have been demonstrated as promising acceptor units for constructing high-performance n-type polymers. However, the electron-rich thiophene moiety in BTI leads to elevated LUMOs for the resultant polymers and hence limits their n-type performance and intrinsic stability. Herein, we addressed this issue by introducing strong electron-withdrawing cyano functionality on BTI and its derivatives. We have successfully overcome the synthetic challenges and developed a series of novel acceptor building blocks, CNI, CNTI, and CNDTI, which show substantially higher electron deficiencies than does BTI. On the basis of these novel building blocks, acceptor-acceptor type homopolymers and copolymers were successfully synthesized and featured greatly suppressed LUMOs (-3.64 to -4.11 eV) versus that (-3.48 eV) of the control polymer PBTI. Their deep-positioned LUMOs resulted in improved stability in OTFTs and more efficient n-doping in OTEs for the corresponding polymers with a highest electrical conductivity of 23.3 S cm-1 and a power factor of ∼10 μW m-1 K-2. The conductivity and power factor are among the highest values reported for solution-processed molecularly n-doped polymers. The new CNI, CNTI, and CNDTI offer a remarkable platform for constructing n-type polymers, and this study demonstrates that cyano-functionalization of BTI is a very effective strategy for developing polymers with deep-lying LUMOs for high-performance n-type organic electronic devices.

Thin-Film Engineering of Solution-Processable n-Type Silicon Phthalocyanines for Organic Thin-Film Transistors.

Metal and metalloid phthalocyanines are an abundant and established class of materials widely used in the dye and pigment industry as well as in commercial photoreceptors. Silicon phthalocyanines (SiPcs) are among the highest-performing n-type semiconductor materials in this family when used in organic thin-film transistors (OTFTs) as their performance and solid-state arrangement are often increased through axial substitution. Herein, we study eight axially substituted SiPcs and their integration into solution-processed n-type OTFTs. Electrical characterization of the OTFTs, combined with atomic force microscopy (AFM), determined that the length of the alkyl chain affects device performance and thin-film morphology. The effects of high-temperature annealing and spin coating time on film formation, two key processing steps for fabrication of OTFTs, were investigated by grazing-incidence wide-angle X-ray scattering (GIWAXS) and X-ray diffraction (XRD) to elucidate the relationship between thin-film microstructure and device performance. Thermal annealing was shown to change both film crystallinity and SiPc molecular orientation relative to the substrate surface. Spin time affected film crystallinity, morphology, and interplanar d-spacing, thus ultimately modifying device performance. Of the eight materials studied, bis(tri-n-butylsilyl oxide) SiPc exhibited the greatest electron field-effect mobility (0.028 cm2 V-1 s-1, a threshold voltage of 17.6 V) of all reported solution-processed SiPc derivatives.

Silicon Phthalocyanines for n-Type Organic Thin-Film Transistors: Development of Structure–Property Relationships

Silicon phthalocyanines (SiPcs) have shown great potential as n-type or ambipolar organic semiconductors in organic thin-film transistors (OTFTs) and organic photovoltaics. Although properly design...

Acceptor–acceptor-type conjugated polymer for use in n-type organic thin-film transistors and thermoelectric devices

The development of n -type conjugated polymers (CPs) forming an acceptor–acceptor (A–A)-type structure are quite challenging due to the synthetic limitation of imposing a functional group on the electron deficient aromatic structure. The diketopyrrolopyrroles (DPP) structure exhibits n -type characteristics; however, the extended π-structure enables stannylation at the terminal position. The stannylated DPP is beneficial for making n -type CPs, but its use in the development of n -type CPs and their application in n -type organic thin-film transistors and organic thermoelectric devices have been not extensively studied. In the present study, we synthesized the A–A-type conjugated polymer, poly [((2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo [3,4- c ]pyrrole-1,4-dione)-5,5′-diyl)- alt -((2,7-bis(2-ethylhexyl)benzo [lmn] [3,8]phenanthroline-1,3,6,8(2 H ,7 H )-tetraone)-4,9-diyl))] (PDPP-NDI), by Stille coupling of distannylated DPP and dibrominated naphthalene diimide (NDI). The alternating copolymerization of DPP and NDI moieties led to a low-lying LUMO energy level of −4.4 eV, suitable for electron transport. The n -type organic thin-film transistors were fabricated with PDPP–NDI, and a substantial average electron mobility of 3.78 × 10 −2 cm 2 V −1 s −1 was obtained after thermal annealing. The n -type organic thermoelectric devices were developed via n -doping, and PDPP–NDI exhibited n -type thermoelectric properties, with an average power factor of 1.82 × 10 −3 μW m −1 K −2 . • An acceptor–acceptor-structured n -type alternating copolymer, PDPP–NDI, was developed. • A small bandgap of 1.03 eV and deep LUMO energy level of −4.4 eV were achieved. • The n -type characteristics of PDPP–NDI were successfully utilized in n -type OTFTs and OTE devices. • The device characteristics of both OTFT and OTE devices were fully studied and discussed.