Self‐Assembly of π‐Conjugated Amphiphiles: Free Standing,Ordered Sheets with Enhanced Mobility

Oligo(p‐phenylenevinylenes) (OPVs) with amphiphilic character are synthesized and their self‐assembly characteristics studied. Careful studies point at two morphologically different states of assemblies, with one being two dimensional sheets and the other as rolled tubes. This is also the first time that self‐assembled sheets are achieved for OPVs. Morphological and photo‐physical studies reveal a unique aggregate to aggregate transition between rolled tubes and two dimensional sheets, which is outlined as a more thermodynamic aggregate. The thermodynamic aggregate (2D sheet) is better ordered and consists of chromophores that are better excitonically coupled. The mobilities of these aggregates are also studied for a field effect transistor device and as expected sheets supersede rolled tubes by a couple of orders. More interestingly, the mobility values obtained for the well ordered chromophores in sheets is three orders higher than any other self‐assembled OPV previously reported. It is hypothesized that the better π interactions enforced by the amphiphilic design and the resultant supramolecular organization is a prime factor for such a remarkable rise in mobilities.     

Strategies for Fast‐Switching in All‐Polymer Field Effect Transistors

Low‐cost printable field effect transistors (FETs) are typically associated with slow switching characteristics. Dynamic response of polymer field effect transistors (PFETs) is a manifestation of time scales involved in processes such as dielectric polarization, structural relaxation, and transport via disordered‐interfacial states. A range of dielectrics and semiconductors are studied to arrive at a parameter which serves as a figure of merit and quantifies the different processes contributing to the switching response. A cross‐over from transport limiting factors to dielectric limiting factors in the dynamics of PFETs is observed. The dielectric limited regime in the PFET dynamics is tapped in to explore high speed processes, and an enhancement of switching speed by three orders of magnitude (from 300 μs to 400 ns) is observed at channel lengths which can be accessed by low cost printing methods. The device structure utilizes polymer‐ferroelectrics (FE) as the dielectric layer and involves a fabrication‐procedure which assists in circumventing the slow dynamics within the bulk of FE. This method of enhancing the dynamic response of PFETs is universally applicable to all classes of disordered‐FE.     

Investigation of Electrode Electrochemical Reactions in CH3NH3PbBr3 Perovskite Single‐Crystal Field‐Effect Transistors

Optoelectronic devices based on metal halide perovskites, including solar cells and light‐emitting diodes, have attracted tremendous research attention globally in the last decade. Due to their potential to achieve high carrier mobilities, organic–inorganic hybrid perovskite materials can enable high‐performance, solution‐processed field‐effect transistors (FETs) for next‐generation, low‐cost, flexible electronic circuits and displays. However, the performance of perovskite FETs is hampered predominantly by device instabilities, whose origin remains poorly understood. Here, perovskite single‐crystal FETs based on methylammonium lead bromide are studied and device instabilities due to electrochemical reactions at the interface between the perovskite and gold source–drain top contacts are investigated. Despite forming the contacts by a gentle, soft lamination method, evidence is found that even at such “ideal” interfaces, a defective, intermixed layer is formed at the interface upon biasing of the device. Using a bottom‐contact, bottom‐gate architecture, it is shown that it is possible to minimize such a reaction through a chemical modification of the electrodes, and this enables fabrication of perovskite single‐crystal FETs with high mobility of up to ≈15 cm² V^?1 s^?1 at 80 K. This work addresses one of the key challenges toward the realization of high‐performance solution‐processed perovskite FETs.     

Linking Glass-Transition Behavior to Photophysical and Charge Transport Properties of High-Mobility Conjugated Polymers

The measurement of the mechanical properties of conjugated polymers can reveal highly relevant information linking optoelectronic properties to underlying microstructures and the knowledge of the glass transition temperature (T_g) is paramount for informing the choice of processing conditions and for interpreting the thermal stability of devices. In this work, we use dynamical mechanical analysis to determine the T_g of a range of state-of-the-art conjugated polymers with different degrees of crystallinity that are widely studied for applications in organic field-effect transistors. We compare our measured values for T_g to the theoretical value predicted by a recent work based on the concept of effective mobility ζ. The comparison shows that for conjugated polymers with a modest length of the monomer units, the T_g values agree well with theoretically predictions. However, for the near-amorphous, indacenodithiophene–benzothiadiazole family of polymers with more extended backbone units, values for T_g appear to be significantly higher, predicted by theory. However, values for T_g are correlated with the sub-bandgap optical absorption suggesting the possible role of the interchain short contacts within materials’ amorphous domains.     

Understanding the Role of Grain Boundaries on Charge-Carrier and Ion Transport in Cs2AgBiBr6 Thin Films

Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air, and recent demonstrations of long charge-carrier lifetimes that can exceed 1 µs. In particular, Cs₂AgBiBr₆ is the subject of many investigations in photovoltaic devices. However, the efficiencies of solar cells based on this double perovskite are still far from the theoretical efficiency limit of the material. Here, the role of grain size on the optoelectronic properties of Cs₂AgBiBr₆ thin films is investigated. It is shown through cathodoluminescence measurements that grain boundaries are the dominant nonradiative recombination sites. It also demonstrates through field-effect transistor and temperature-dependent transient current measurements that grain boundaries act as the main channels for ion transport. Interestingly, a positive correlation between carrier mobility and temperature is found, which resembles the hopping mechanism often seen in organic semiconductors. These findings explain the discrepancy between the long diffusion lengths >1 µm found in Cs₂AgBiBr₆ single crystals versus the limited performance achieved in their thin film counterparts. This work shows that mitigating the impact of grain boundaries will be critical for these double perovskite thin films to reach the performance achievable based on their intrinsic single-crystal properties.     

Detection of X‐Rays by Solution‐Processed Cesium‐Containing Mixed Triple Cation Perovskite Thin Films

Materials and technology development for designing innovative and efficient X‐ray radiation detectors is of utmost importance for a wide range of applications ranging from security to medical imaging. Here, highly sensitive direct X‐ray detectors based on novel cesium (Cs)‐based triple cation mixed halide perovskite thin films are reported. Despite being in a thin film form, the devices exhibit a remarkably high X‐ray sensitivity of (3.7 ± 0.1) µC Gy^?1 cm^?2 under short‐circuit conditions. At a small reverse bias of 0.4 V, the sensitivity further increases by orders of magnitude reaching a record value of (97 ± 1) µC Gy^?1 cm^?2 which surpasses state‐of‐the‐art inorganic large‐area detectors (a‐Se and poly‐CZT). Based on detailed structural, electrical, and spectroscopic investigations, the exceptional sensitivity of the triple cation Cs perovskite is attributed to its high ambipolar mobility‐lifetime product as well as to the formation of a pure stable perovskite phase with a low degree of energetic disorder, due to an efficient solution‐based alloying of individual n‐ and p‐type perovskite semiconductors.     

Synthesis,characterization, and OFET characteristics of 3,4-diaryl substituted poly(thienylene vinylene) derivatives

Poly(thienylene vinylene) derivatives bearing aryl substituents at 3,4-positions have been synthesized in good yield by Stille-type polycondensation. Two types of aryl substituents, either a 4-octylphenyl or a 5-octyl-2-thienyl, were investigated in this study. The polymers were characterized by ¹H NMR, GPC, TGA, UV–vis absorption, and photoluminescence spectroscopy. The polymers (P1–P4) showed good to excellent solubility in common organic solvents, and UV–vis absorption spectra in solution exhibit maxima in the range of 511–595 nm. GPC analysis using PPP standards showed a number average molecular weight range of 6.59–8.98 × 10³ g mol⁻¹ for the various polymers. The field effect transistors based on these polymers were studied, and the results obtained are correlated to their molecular structure.     

Size effects on electrical properties of sol–gel grown chromium doped zinc oxide nanoparticles

In this communication, we report the results of the studies on electrical properties of Zn_(0.95)Cr_(0.05)O nanoparticles synthesized using sol–gel method. X–ray diffraction(XRD) and transmission electron microscopy(TEM) measurements were performed for the structural and microstructural behaviors of the nanoparticles. Rietveld analysis was carried out to confirm the single phasic nature. High resolution TEM(HRTEM) confirms the nanoscale nature and polycrystalline orientations in the samples. Dielectric response has been understood in the context of universal dielectric response(UDR) model along with the Koop's theory and Maxwell – Wagner(M–W) mechanism. Variation in ac conductivity with frequency has been discussed in detail in terms of power law fits. Results of the impedance measurements have been explained on the basis of crystal cores and crystal boundary density. Cole – cole behavior has been studied for the impedance data. For potential application of nanoparticles, average normalized change(ANC) in impedance has been estimated and discussed in the light of size effects and oxygen vacancies.     

FDTD algorithm to achieve absolute stability in performance analysis of SWCNT interconnects

Crosstalk analysis in very-large-scale (VLSI) interconnects is usually carried out using finite-difference time-domain (FDTD) methods. However, the stability of FDTD is limited by the Courant–Friedrichs–Lewy (CFL) stability criteria. In this work, we propose an innovative FDTD algorithm to overcome the stability limitations due to the CFL criteria and make the analysis absolutely stable for all times. Using the proposed FDTD method, we analyze the response of coupled interconnects for symmetric and asymmetric inputs with both in-phase and out-of-phase switching. We further analyzed the functional switching and determined how to reduce noise peaks due to crosstalk. For all responses, we compared our new algorithm with HSPICE results, revealing greatly enhanced accuracy and central processing unit (CPU) runtime compared with conventional FDTD. Finally, relative and absolute stability analyses of the proposed FDTD method were carried out using Nyquist and Routh–Hurwitz (R–H) criteria.