Polymer Thin-film Transistors Research Articles

Poling induced changes in polarization domain structure of polymer ferroelectrics for application in organic thin film transistors

While electrical poling of organic ferroelectrics has been shown to improve device properties, there are challenges in visualizing accompanying structural changes. We observe poling induced changes in ferroelectric domains by applying differential phase contrast (DPC) imaging in the scanning transmission electron microscope, a method that has been used to observe spatial distributions of electromagnetic fields at the atomic scale. In this work, we obtain DPC images from unpoled and electrically poled polyvinylidene fluoride trifluorethylene films and compare their performance in polymer thin film transistors. The vertically poled films show uniform domains throughout the bulk compared to the unpoled film with a significantly higher magnitude of the overall polarization. Thin film transistors comprising a donor–acceptor copolymer as the active semiconductor layer show improved performance with the vertically poled ferroelectric dielectric film compared with the unpoled ferroelectric dielectric film. A poling field of 80–100 MV/m for the dielectric layer yields the best performing transistors; higher than 100 MV/m is seen to degrade the transistor performance. The results are consistent with a reduction in deleterious charge carrier scattering from ferroelectric domain boundaries or interfacial dipoles arising from electrical poling.

Comparative study of physical doping and electrochemical doping in polymer thin-film transistors

Comparative study of physical doping and electrochemical doping in polymer thin-film transistors

High-performance polymer top-contact thin-film transistor with orthogonal photolithographic process

Based on fluorinated photoresist, an orthogonal lift-off process was introduced to directly fabricate patterning metallic source and drain contacts on polymeric thin films to obtain the top-contact structure device. Top-contact polymeric thin-film transistors (TFTs) on the wafer using this orthogonal lift-off process exhibited excellent value of electrical characteristics which was as high as 2.56 cm2V−1s−1 in mobility and >107 in on/off ratio. Compared with Ref. device which was fabricated by shadow mask, this top-contact polymeric TFTs showed the same mobilities but smaller channel length, and the lower contact resistance. In addition, based on fluorinated photoresist, the top-contact polymeric TFTs backplane with photo-patternable expoxy gate insulating layer was also successfully fabricated on glass. The mobility values of this OTFTs with different channel lengths ranged from 0.25 to 0.74 cm2V−1s−1 with an average of 0.48 ± 0.15 cm2V−1s−1 which can be comparable to the Ref. device with epoxy gate insulator using shadow mask method. It was further proved that the availability of such orthogonal photoresists promised to enable the fabrication of high performance OTFT device based the copolymer with long linear alkyl chains.

Effect of Bias Stress in Inkjet-Printed Polymer Thin-Film Transistors: Generation and Annihilation of Subgap Density of States

Here, we report investigations of the effects of bias stress on the density of states (DOS) in polymer thin-film transistors (PTFTs). As the active channel layer, these PTFTs employed an inkjet-printed semiconducting film of P8T2Z-C12 [poly (tetryldodecyloctathiophene-alt-didodecyl bithiazole)]. In positive bias stress tests, the threshold voltage (VT) of the inkjet-printed PTFT shifted in the positive direction. However, this shift was largely recovered when the PTFT was released from the bias stress. We analyzed the effect of the bias stress manifested by the VT shift using the full energy range of the subgap DOS versus the duration of the bias stress, which we obtained by applying various DOS extraction techniques.

Polymer thin film transistor vapor sensor analysis, drift suppression, and response optimization via circuit level strategy

Polymer thin film transistor vapor sensor analysis, drift suppression, and response optimization via circuit level strategy

Realizing Diketopyrrolopyrrole Polymer‐Based Uniform Large‐Area Transistors for Active Circuit via Protonic Acid Mediated Molecular Self‐Assembly

AbstractThe solution‐handling characteristics of π‐conjugated polymer semiconductors make them promising candidates for low‐cost flexible electronics. Although homogeneous and stable nonchlorine solvent (NClS) processing is the only way that must be passed for future application, polymer semiconductors still meet the challenges of poor solubility and inhomogeneities in NClS. Here, the authors primp polymer molecules with protonic acid by multiple interactions, that is, proton exchange and hydrogen bond interaction, to achieve scale and model‐constrained PDVT‐10c assembly in NClS. As a result, a uniform near‐amorphous PDVT‐C10c film is developed for polymer thin‐film transistor (PTFT) array, which exhibits uniform performance with much narrower Gaussian standards deviation than pristine PDVT‐C10c one. Finally, a 5 cm × 5 cm PTFT as the active‐matrix circuit for blue organic light‐emitting diode is demonstrated. The results illustrate the great potential of protonic acid‐mediated molecular self‐assembly on homogeneous and stable NClS processing polymer semiconductors for next‐generation flexible electronics.

Completely foldable electronics based on homojunction polymer transistors and logics.

An increase in the demand for completely foldable electronics has motivated efforts for the development of conducting polymer electrodes having extraordinary mechanical stability. However, weak physical adhesion at intrinsic heterojunctions has been a challenge in foldable electronics. This paper reports the completely foldable polymer thin-film transistors (PTFTs) and logic gate arrays. Homojunction-based PTFTs were fabricated by selectively doping p-type diketopyrrolopyrrole-based semiconducting polymer films with FeCl3 to form source/drain electrodes. The doping process caused a gradual work function change with depth, which promoted charge injection to semiconducting regions and provided a low contact resistance. In addition, the interfacial adhesion in the PTFTs was improved by interfacial cross-linking between adjacent component layers. The electrical performance of the resulting PTFTs was maintained without noticeable degradation even after extreme folding, suggesting that the proposed fabrication strategy can further be applied to various semiconducting polymers for the realization of foldable electronics.

(Invited) New Insights on Charge Transport in Organic and Polymer Thin-Film Transistors

There have been many reports of thin-film transistors based on semiconductors with mobilities in the range 1-20 cm2/Vs in the past few decades. The mobilities and mean free paths are simply too low for the application of conventional semiconductor transport theories based on solutions to the Boltzmann transport equation (BTE). We will present a very general solution based on the statistical nature of charge transport and introduction of a factor related to the probability of mean free path exceeding the minimum transport length. We are then able to apply the BTE with appropriate scattering mechanisms and obtain very good results that agree well with experiment. This approach is very well suited to thin-film transistors based on polymer and organic semiconductors with room temperature mobilities in the range 1-20 cm2/Vs. We compare theory with experimental results for DPP-based polymers and also oxides.Charge transport depends on carrier densities and electric fields in complex ways. We describe these dependencies in detail. Scaling down channel dimensions of organic and polymer semiconductor based thin-film transistors to submicron and nanoscale dimensions, required for higher speed operation, presents several challenges. Successful scaling will enable vastly improved device performance and hence the prospects are certainly very enticing. One of the biggest challenges is in making relatively low-resistance contacts. A more thorough understanding of velocity saturation mechanisms and charge transport at high electric fields is also necessary. We describe charge transport in oxide and polymer TFTs with an emphasis on scaling. We show that for some TFTs, scaling of the channel width facilitates scaling down of channel length and a nanostripe or nanogroove array channel geometry has advantages. Finally, we describe hybrid TFTs – with multiple materials that are better suited to scaling down.

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. 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. The performance of electronic equipment can be affected in many extreme operating environments, such as in the South Pole with a minimum temperature of 191–290 K1Ruhl J.E. Ade P.A.R. Carlstrom J.E. Cho H.M. Crawford T. Dobbs M. Greer C.H. Halverson N.W. Holzapfel W.L. Lanting T.M. et al.The south pole telescope.Proc. SPIE. 2004; 5498: 11-29Crossref Scopus (241) Google Scholar and the Lut desert with a maximum temperature of 253–344 K.2Mildrexler D.J. Zhao M.S. Running S.W. Satellite finds highest land skin temperatures on earth.Am. Meteorol. Soc. 2011; 92: 855-860Crossref Scopus (74) Google Scholar Of the many electronic components currently in use, polymeric field-effect transistors (FETs) have shown promising applications, such as in low-cost logic circuits, sensors, and health detectors.3Klauk H. Zschieschang U. Pflaum J. Halik M. Ultralow-power organic complementary circuits.Nature. 2007; 445: 745-748Crossref PubMed Scopus (1277) Google Scholar,4Schwartz G. Tee B.C.K. Mei J.G. Appleton A.L. Kim D.H. Wang H.L. Bao Z.N. Flexible polymer transistors with high pressure sensitivity for application in electronic skin and health monitoring.Nat. Commun. 2013; 4: 1859Crossref PubMed Scopus (1452) Google Scholar However, reports have shown that when the operating temperatures are reduced from 300 to 240 K, most performances of polymeric transistors will be reduced by about 35%–70%.5Venkateshvaran D. Nikolka M. Sadhanala A. Lemaur V. Zelazny M. Kepa M. 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Two-dimensional charge transport in self-organized, high-mobility conjugated polymers.Nature. 1999; 401: 685-688Crossref Scopus (4204) Google Scholar, 9Pernstich K.P. Rössner B. Batlogg B. Field-effect-modulated Seebeck coefficient in organic semiconductors.Nat. Mater. 2008; 7: 321-325Crossref PubMed Scopus (185) Google Scholar, 10Schott S. Chopra U. Lemaur V. Melnyk A. Olivier Y. Pietro R.D. Romanov I. Carey R.L. Jiao X. Jellett C. et al.Polaron spin dynamics in high-mobility polymeric semiconductors.Nat. Phys. 2019; 15: 814-822Crossref Scopus (31) Google Scholar and influenced by defects.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, 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, 13Fahlman M. Fabiano S. Gueskine V. Simon D. Berggren M. Crispin X. Interfaces in organic electronics.Nat. Rev. Mater. 2019; 4: 627-650Crossref Scopus (130) Google Scholar When the operating temperature is low, the carriers will not have enough energy to overcome the barriers formed by the energetic disorder.5Venkateshvaran D. Nikolka M. Sadhanala A. Lemaur V. Zelazny M. Kepa M. Hurhangee M. Kronemeijer A.J. Pecunia V. Nasrallah I. et al.Approaching disorder-free transport in high-mobility conjugated polymers.Nature. 2014; 515: 384-388Crossref PubMed Scopus (691) Google Scholar,14Luzio A. Nübling F. Martin J. Fazzi D. Selter P. Gann E. McNeill C.R. Brinkmann M. Hansen M.R. Stingelin N. et al.Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors.Nat. Commun. 2019; 10: 3365Crossref PubMed Scopus (22) Google Scholar Moreover, very high temperatures will increase the thermal vibration of molecules, thereby affecting the carrier transport.7Gumyusenge A. Tran D.T. Luo X.Y. Pitch G.M. Zhao Y. Jenkins K.A. Dunn T.J. Ayzner A.L. Savoie B.M. Mei J.G. Semiconducting polymer blends that exhibit stable charge transport at high temperatures.Science. 2018; 362: 1131-1134Crossref PubMed Scopus (104) Google Scholar Also, the thermal energy generated by the contact resistance of interfaces, the resistance of the disordered semiconductor films,14Luzio A. Nübling F. Martin J. Fazzi D. Selter P. Gann E. McNeill C.R. Brinkmann M. Hansen M.R. Stingelin N. et al.Microstructural control suppresses thermal activation of electron transport at room temperature in polymer transistors.Nat. Commun. 2019; 10: 3365Crossref PubMed Scopus (22) Google Scholar and the high-temperature environment can thermally destroy the working stability of organic field-effect transistors (OFETs). Temperature dependence seems unavoidable in polymeric transistors, and because it is difficult to determine the exact factors responsible for the thermal instabilities of these equipment, finding appropriate solutions to the challenge is difficult. However, we believe that factors, such as great crystallinity of the polymer film, ordered molecular packing modes, and reduced defects, can be instrumental to the production of temperature-insensitive polymeric FETs. Several researchers have reported that adding insulating polymers, such as polyacrylonitrile (PAN) to polymeric semiconductors, can improve the stability and mobility of polymeric FETs.7Gumyusenge A. Tran D.T. Luo X.Y. Pitch G.M. Zhao Y. Jenkins K.A. Dunn T.J. Ayzner A.L. Savoie B.M. Mei J.G. Semiconducting polymer blends that exhibit stable charge transport at high temperatures.Science. 2018; 362: 1131-1134Crossref PubMed Scopus (104) Google Scholar,15Dong J. Wang Y. Mori T. Michinobu T. Improving the air-stability of n-type organic thin-film transistors by polyacrylonitrile additive.Jpn. J. Appl. Phys. 2020; 59: SDDC05Crossref Scopus (7) Google Scholar, 16Ford M.J. Wang M. Patel S.N. Phan H. Segalman R.A. Nguyen T.-Q. Bazan G.C. High mobility organic field-effect transistors from majority insulator blends.Chem. Mater. 2016; 28: 1256-1260Crossref Scopus (63) Google Scholar, 17Riera-Galindo S. Leonardi F. Pfattner R. Mas-Torrent M. Organic semiconductor/polymer blend films for organic field-effect transistors.Adv. Mater. Technol. 2019; 4: 1900104Crossref Scopus (57) Google Scholar, 18Lei Y.L. Deng P. Lin M. Zheng X.L. Zhu F.R. Ong B.S. 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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.

Effect of High-Speed Blade Coating on Electrical Characteristics in Polymer Based Transistors.

We explore the effect of high-speed blade coating on electrical characteristics of conjugated polymer-based thin-film transistors (TFTs). As the blade-coating speed increased, the thickness of the polymer thin-film was naturally increased while the surface roughness was found to be unchanged. Polymer TFTs show two remarkable tendencies on the magnitude of field-effect mobility with increasing blade-coating speed. As the blade-coating speed increased up to 2 mm/s, the fieldeffect mobility increased to 4.72 cm²V-1s-1. However, when the coating speed reached 6 mm/s beyond 2 mm/s, the field-effect mobility rather decreased to 3.18 cm²V-1s-1. The threshold voltage was positively shifted from 2.09 to 8.29 V with respect to increase in blade-coating speed.

Low-Voltage, Dual-Gate Organic Transistors with High Sensitivity and Stability toward Electrostatic Biosensing.

High levels of performance and stability have been demonstrated for conjugated polymer thin-film transistors in recent years, making them promising materials for flexible electronic circuits and displays. For sensing applications, however, most research efforts have been focusing on electrochemical sensing devices. Here we demonstrate a highly stable biosensing platform using polymer transistors based on the dual-gate mechanism. In this architecture a sensing signal is transduced and amplified by the capacitive coupling between a low-k bottom dielectric and a high-k ionic elastomer top dielectric that is in contact with an analyte solution. The new design exhibits a high signal amplification, high stability under bias stress in various aqueous environments, and low signal drift. Our platform, furthermore, while responding expectedly to charged analytes such as the protein bovine serum albumin, is insensitive to changes of salt concentration of the analyte solution. These features make this platform a potentially suitable tool for a variety of biosensing applications.

Universal three-dimensional crosslinker for all-photopatterned electronics

All-solution processing of large-area organic electronics requires multiple steps of patterning and stacking of various device components. Here, we report the fabrication of highly integrated arrays of polymer thin-film transistors and logic gates entirely through a series of solution processes. The fabrication is done using a three-dimensional crosslinker in tetrahedral geometry containing four photocrosslinkable azide moieties, referred to as 4Bx. 4Bx can be mixed with a variety of solution-processable electronic materials (polymer semiconductors, polymer insulators, and metal nanoparticles) and generate crosslinked network under exposure to UV. Fully crosslinked network film can be formed even at an unprecedentedly small loading, which enables preserving the inherent electrical and structural characteristics of host material. Because the crosslinked electronic component layers are strongly resistant to chemical solvents, micropatterning the layers at high resolution as well as stacking the layers on top of each other by series of solution processing steps is possible.

Polymer/Graphene oxide nanocomposite thin film for NO2 sensor: An in situ investigation of electronic, morphological, structural, and spectroscopic properties

The higher operating temperature of metal oxide and air instability of organic based NO2 sensor causes extremely urgent for development of a reliable low cost sensor to detect NO2 at room temperature. Therefore, we present a fabrication of large area Polymer/GO nano hybrid thin film for polymer thin film transistors (PTFTs) based NO2 sensors assisted via facile method named ‘spreading-solidifying (SS) method’, grown over air/liquid interface and successive investigation of effect of NO2 on film via several characterizations. The PTFTs sensor has demonstrated swift and high response towards low concentration of NO2 gas with air stability and provided real time non-invasive type NO2 sensor. Herein, we are reporting the nanohybrid PBTTT/GO composite based PTFT sensor with good repeatability and sensor response for low concentration NO2. The thin film grown via SS technique has reported very good adsorption/desorption of target analyte having response/recovery time of 75 s/523 s for 10 ppm concentration of NO2 gas. It has been observed that % change in drain current (sensor response) saturated with increasing concentration of NO2. The transient analysis demonstrates the fast sensor response and recovery time. Furthermore, in order to understand the insight of high performance of sensor, effect of NO2 on nanohybrid film and sensing mechanism, an in situ investigations was conducted via multiple technique viz. spectral, electronic, structural, and morphological characterization. Finally, the performance of sensor and the site of adsorption of NO2 at polymer chains were argued using schematic diagram. This work shows the simple fabrication process for mass production, low cost and room temperature operated gas sensors for monitoring the real-time environment conditions and gives an insight about the sensing mechanism adsorption site of NO2.

Density-of-States-Based Physical Model for Ink-Jet Printed Thiophene Polymeric TFTs

We proposed a physical model for ink-jet printed polymeric thin-film transistors (PTFTs) all over the sub- and above-threshold regions by using an effective carrier density. The nonlinearity under the low lateral electric field in the printed thiophene PTFTs was reproduced by applying the back-to-back Schottky diode model based on simple Poole–Frenkel (PF) mobility formalism. The analytical ${I}{-}\!{V}$ model supplemented with ${C}{-}\!{V}$ model in a single framework was also verified by successfully reproducing the measured characteristics of TFTs with three different thiophene polymeric channel materials. Additionally, we applied the physics-based analytical model on the inkjet-printed PTFT-based inverter and confirmed that the proposed models could predict the inverter circuit characteristics of the gain and static noise margin (SNM) based on the physical parameters.

Use of surface photo-reactive nanometal printing for polymer thin-film transistors: contact resistance and short-channel effects

Abstract

P3HT Nanofibrils Thin-Film Transistors by Adsorbing Deposition in Suspension.

A novel film preparation method utilizing polymer suspension, entitled adsorbing deposition in suspensions (ADS), has been proposed. The poly(3-hexylthiophene) (P3HT) toluene solution forms P3HT nanofibrils dispersed suspension by aging. P3HT nanofibrils are highly crystallized with sharp vibronic absorption spectra. By the ADS method, only P3HT nanofibrils in suspension can be deposited on the substrate surface without any disordered fraction from the dissolved P3HT in suspension. Formed ADS film contains only the nanostructured conjugated polymer. Fabricated polymer thin-film transistor (TFT) utilizing ADS P3HT film shows good TFT performances with low off current, narrow subthreshold swing and large on/off current ratio.

자기조립 단분자막의 알킬사슬 길이에 따른 고분자 박막 트랜지스터의 전하트랩 밀도 제어

자기조립 단분자막의 알킬사슬 길이에 따른 고분자 박막 트랜지스터의 전하트랩 밀도 제어

RETRACTED: Study of dielectric and optical constants of Benzobis(thiazole)-based copolymer thin films for designing optoelectronic devices

Abstract A new series of benzobisthiadiazole (BBT)-based donor–acceptor copolymers, namely, PBBT-FT, PBBT-T-FT, and PBBT-Tz-FT, with different π-conjugated bridges have been developed for polymer thin film transistors (TFTs). It was found that inserting different π-conjugated bridges into the backbone of the polymer allowed tailoring of opto-electrical properties, molecular organizations, and accordingly, ambipolar transport of TFTs. Copolymer PBB-T with benzo[1,2-d:4,5-d’]bis(thiazole) (BBT) as the accepting unit and benzodithiophene (BDT) as the donor unit is a promising candidate for high-performance non-fullerene polymer solar cells (PSCs). As much as we know the optical and dielectric constants of the PBB-T are not fully known. PBB-T was synthesized and subsequently thin films were properly prepared. By using the Kramers-Kronig relations and the transmission spectra of these thin films, the dielectric and optical constants were calculated and analyzed, including the absorption coefficient, dielectric constant, extinction coefficient, and refractive index of the PBB-T thin films. The results show that these dielectric and optical constants of the PBB-T thin films are quasi-independent on the thicknesses of the thin films, indicating our results are reliable. The features of the dielectric and optical constants show the PBB-T thin films are very promising candidates for high-performance non-fullerene PSCs and potential cut-off filter only permitting red and near-infrared light pass. These results are significant for designing optoelectronic devices related to the PBB-T thin films.

Transient Response and Charge Transport in Organic and Polymer Thin-Film Transistors

Organic thin-film transistors (TFTs) are very promising for several large-area applications particularly when mechanical flexibility is important. A detailed model that directly correlates the TFT time-domain transient response with charge transport and material properties is very desirable for circuit applications. Temperature-dependent measurements of the field effect transistor (FET) mobility have provided numerous insights on the characteristics of the charge carriers and nature of transport phenomena. However, in disordered semiconductors such as organics and polymers, the FET mobility μFET is very different from the trap-free mobility μ as most of the carriers remain trapped in the band-tail states. In such systems, determining the drift mobility becomes very important. To study this drift mobility, we analyze the time-domain response of the transistor. The time-domain response of transistors can be obtained through channel formation time measurements. In such measurements, carriers are injected at the source of the transistor using a voltage pulse and extracted at the drain. A large-signal time domain analysis of these electronic time-of-flight measurements was first done by J.R. Burns [1] nearly 50 years ago for metal-oxide-semiconductor (MOS) transistors and was later extended to organic thin-film transistors by D. Basu et al. [2]. However, the mathematical models used in both works assumed that mobility is insensitive to the variance in carrier density and temperature. Therefore, these models cannot accurately predict complex time-domain response in organic and polymer FETs, hence there exists the need for more comprehensive models that account for mobility dependence on carrier concentration and temperature. Indeed, the main mechanisms of charge transport in organic and polymer semiconductors are commonly accepted to be variable-range hopping (VRH) and multiple-trap and release (MTR). In MTR, trapped carriers are thermally excited into the band, where charge transport takes place. Both charge transport models predict a mobility that depends on the trap density of states of the material, carrier density, and lattice temperature and, thus a complete model must account for these parameters. This paper presents an extension to the work of Burns and Basu et al. that considers the effect of the multiple trap and release charge transport model on mobility. With the underlying assumption that the trap density of states in most high-mobility organic and polymer transistors can be modeled by a single exponential function, we can write a closed form non-linear partial differential equation that describes the time and spatial evolution of current and voltage in an organic or polymer FET. In this way, we account for relevant material and device operation parameters such as trap density of states, carrier density, and temperature and therefore present a much more complete model of the time-domain response of organic and polymer thin-film transistors while retaining ease of use. Given sufficient knowledge of material parameters, we are able to accurately model electronic time-of-flight measurements in organic and polymer thin film transistors across different device operation regimes and temperatures. This model can also be used to extract material parameters from channel formation measurements. We apply this more complete time-domain dynamic response model to experimental data from high-mobility donor-acceptor polymer based thin-film transistors measured in our group. [1] J. R. Burns, RCA Review, 30, 15, 1969. [2] Basu, Debarshi & Dodabalapur, Ananth. (1970). Drift Velocity and Drift Mobility Measurement in Organic Semiconductors Using Pulse Voltage. 10.1007/12_2009_4. Figure 1

Trap-Free Charge Transport Approaches in Polymer Thin Film Transistors

Polymer-based thin film transistors are being investigated for use in large-area, flexible applications due to their ideal solution processability and low processing costs [1-4]. The performance of these materials has greatly improved over the past two decades with some charge carrier mobilities > 4 cm2/Vs [5-8]. However, charge carrier trapping in these disordered materials is still limiting the charge carrier mobility as shown in numerous polymer thin film transistor studies [9-10]. We will compare the experimental results of several approaches to reduce the density of charge carrier trapping occurring in the polymer thin film and the subsequent improvement in transistor performance. Thin film transistors have been fabricated by blending semiconducting polymers with a wide variety of small organic molecules and then measured to observe any changes in the charge carrier mobility of the polymers. The small molecules are being used to solubilize alkyl chains dangling off the polymer backbone and to improve the ordering of the polymer chains, which reduces the amount of trap states caused by crystalline disorder. The polymer/small molecule blend films have already achieved comparable output characteristics, transfer characteristics, and charge carrier mobilities to those seen in the neat polymer films. The performance can also be tuned based off the polymer concentration with respect to that of the small molecule. We will also present studies on the effects of this blending approach on other key figures of merit for thin film transistors such as threshold voltage, subthreshold swing, on-off current ratio, contact resistance, device stability, and device-to-device variation. [1] Bucella, S. G.; Luzio, A.; Gann, E.; Thomsen, L.; McNeill, C. R.; Pace, G.; Perinot, A.; Chen, Z.; Facchetti, A.; Caironi, M. Nat. Commun. 2015, 6, 1-10. [2] Drury, C. J.; Mutsaers, C. M. J.; Hart, C. M.; Matters, M.; de Leeuw, D. M. Appl. Phys. Lett. 1998, 73(1), 108-110. [3] Fix, W.; Ullmann, A.; Ficker, J.; Clemens, W. Appl. Phys. Lett. 2002, 81(9), 1735-1737. [4] Gelink, G. H.; Huitema, H. E. A.; van Veenendaal, E.; Cantatore, E.; Schrijnemakers, L.; van der Putten, J. B. P. H.; Geuns, T. C. T.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B.-H.; Meijer, E. J.; Benito, E. M.; Touwslager, F. J.; Marsman, A. W.; van Rens, B. J. E.; de Leeuw, D. M. Nat. Mater. 2004, 3(2), 106-110. [5] Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69, 4108-4110. [6] Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000, 77, 406-408. [7] Ha, T.-J.; Sonar, P.; Dodabalapur, A. ACS Appl. Mater. Interfaces 2014, 6(5), 3170-3175. [8] Sirringhaus, H. Adv. Mater. 2014, 26, 1319-1335. [9] Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Oliview, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Nature 2014, 515, 384-388. [10] Nikola, M.; Nasrallah, I.; Rose, B.; Ravva, M. K.; Broch, K.; Sadhanala, A.; Harkin, D.; Charmet, J.; Hurhangee, M.; Brown, A.; Illig, S.; Too, P.; Jongman, J.; McCulloch, I.; Bredas, J.-L.; Sirringhaus, H. Nat. Mater. 2016, 16, 356-362.