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Thermally activated delayed fluorescence (TADF) systems exhibit high emissive yield due to efficient back-conversion of nonemissive triplet states to emissive singlet states via reverse intersystem crossing (RISC). In this paper, both the charge carrier and triplet exciton dynamics are explored using transient electroluminescence (TrEL) measurements in the TADF molecule, 2,3,4,6-Tetra(9H-carbazol-9-yl)-5-fluorobenzonitrile (4CzFCN)-based devices. The analysis of the rising edge of the TrEL pulse indicates that the carriers follow multiple trapping, de-trapping, and exciton recombination dynamics. The trailing edge of the TrEL pulse provides insight into the monomolecular and bimolecular exciton dynamics. These studies along with a kinetic model reveal triplet harvesting processes in a 4CzFCN molecule via both RISC and triplet–triplet annihilation (TTA). Furthermore, at high temperatures, the analysis suggests that TADF processes are dominant with negligible contribution from TTA. The presence of bimolecular triplet processes acts as bottlenecks for accessing higher efficiencies in TADF organic light emitting diodes.

High-resolution imaging techniques based on thermography, luminescence imaging, and laser beam induced current (LBIC) probe and inspect solar cells and correlate the observed inhomogeneities to the cell performance. The imaging methods get increasingly qualitative as the size scales from cell level to module dimensions, where several solar cells are connected in series, as in silicon panels. Electroluminescence (EL) imaging is a standard measure of inspecting solar panels and extensively utilized in the manufacturing assembly line. The complementary method of obtaining images from the LBIC scanning technique, however, is rarely utilized at large panel dimensions. We report strategies to implement LBIC imaging at module levels and procedures to analyze in terms of microscopic and circuit parameters. LBIC is inherently quantitative and well suited to identify microscopic defects, since it involves measuring local photo-current. We demonstrate that the contrast features from LBIC in a specific cell region are directly associated with the shunt resistance ( $R_{\text{sh}}$ ) variation of that specific cell. Using a single diode equivalent circuit model, we show that the current contrast of the cell under study is purely a function of its $R_{\text{sh}}$ , and an effective load resistance that includes shunt-resistance of the other series-connected cells. The resulting LBIC-map along with EL then becomes a complete diagnostic tool to quantitatively map and identify the defects prevailing in a solar panel. We overcome the key issue of the slow speed of LBIC based imaging by a modified scan routine for rapid screening of large areas (2 ${\bf m}$ by 1 ${\bf m}$ ) panels under 5 min, at high resolution.

We perform microelectrode array recordings of neonatal chick retina explants and report stimulus-dependent response properties of different retinal ganglion cell (RGC) types to various luminance and contrast adaptation conditions. The isolated single units indicate wavelength-sensitive response properties for all the retinal ganglion cell types recorded from. Responses to different luminance and contrast conditions investigated as a function of wavelength indicates a combination of adaptation and sensitization features in the recorded population of RGCs. We further demonstrate the presence of complementary response properties in most of the RGCs to blue (450 nm) and green (530 nm) light input and infer that luminance, contrast and color information are encoded in a wide variety of metrics such as latency, response event duration and pairwise correlations.

Rapid degradation of bulk heterojunction (BHJ) organic solar cells employing non-fullerene acceptor molecules, especially during the initial period of operation (“burn-in”), poses a major challenge...

We study the differences in electrical charge transport dynamics of the conductivity enhancement of poly(3,4-ethylenedioxythiophene) (PEDOT) derivatives under geometrical confinement. The results of polymer blend poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) and a polymer-monomer blend, poly(3,4- ethylenedioxythiophene):tosylate, highlight the role of dopants and processing conditions of these systems under confinement. The prevailing transport length scales in confined geometry of characteristic dimensions originate from varying disorder in these polymer systems. These observable differences in two different PEDOTs introduced by molecular level reorganization can be utilized to tune conducting polymer systems for efficient electrical and thermoelectric properties. The electrical conductivity {\sigma} of the polymer system, which is a function of the electronic structure at molecular level and a connectivity parameter, has been probed in cylindrical-alumina nanoscaffolds of various channel diameters, at different frequencies {\omega} and temperatures T. The observations also emphasize the role of disorder in these conducting polymer systems., Comment: 19 pages manuscript 12 pages supplementary materials

Optoelectronic semiconducting polymer material interfaced with a blind-developing chick-retina (E13–E18) in subretinal configuration reveals a response to full-field flash stimulus that resembles an elicited response from natural photoreceptors in a neonatal chick retina. The response manifests as evoked-firing of action potentials and was recorded using a multi-electrode array in contact with the retinal ganglion layer. Characteristics of increasing features in the signal unfold during different retina-development stages and highlight the emerging network mediated pathways typically present in the vision process of the artificial photoreceptor interfaced retina.

Hybrid organic-inorganic perovskites (HOIPs) have emerged as a versatile class of semiconductors for numerous optoelectronic applications. Here, we demonstrate light-excitation-dependent two-dimensional (2D) position-sensitive detectors (PSDs) using a mixed-phase perovskite, FA0.83Cs0.17Pb(I0.9Br0.1)3, as the active semiconductor, incorporated within a five-terminal device geometry. The light-induced lateral photovoltage, which is initiated by selective charge transfer across the metal-perovskite barrier interface, is utilized to achieve the excitation-position-dependent electric response. The 2D PSD devices exhibit a spatially dependent linear variation of the photosignal with sensitivity >50 μV mm-1 and a low position detection error (1-2%), making them suitable for applications such as quadrant detectors. Further, it is observed that the device architecture plays a key role in controlling the dynamics and linearity of the HOIP PSDs. The large active area devices (up to ∼2 cm × 2 cm) exhibit a distinct spatial variation of the photosignal. We utilize the functionality of the PSD device for light-tracking applications by implementing a continuous detection scheme.

Unprecedented ambient triplet-mediated emission in core-substituted naphthalene diimide (cNDI) derivatives is unveiled via delayed fluorescence and room temperature phosphorescence. Carbazole core-substituted cNDIs, with a donor-acceptor design, showed deep-red triplet emission in solution processable films with high quantum yield. This study, with detailed theoretical calculations and time-resolved emission experiments, enables new design insights into the triplet harvesting of cNDIs; an important family of molecules which has been, otherwise, extensively been investigated for its n-type electronic character and tunable singlet fluorescence.

We demonstrate a single-step fabrication process of highly stable and luminescent polymer fibers embedded with quantum dots (QDs) of the organic-inorganic hybrid perovskite (OIP) (CH3NH3PbBr3) using the electrospinning process. The fiber (∼2 μm diameter) primarily consists of poly(methyl methacrylate) dispersed with clusters of OIP quantum dots. The OIP clusters are radially distributed, normal to the fiber axis. The photoluminescence quantum yield (PLQY) is high (∼80%) and comparable to that of conventional QDs. The emission maxima are tunable by varying the concentration of OIP precursor in the electrospinning solution. Submicron emission maps show an isotropic and continuous emission along the fiber, suggesting uniform distribution of QD clusters. Temperature-dependent PL response indicates features which are a function of the particle size. For small QDs, the PLQY(T) maxima are close to the ambient temperature, whereas the PLQY(T) maxima shift sizably to T < 50 K for larger QDs. Significant waveguiding of QDs emission and amplified spontaneous emission, a prerequisite for lasing, were observed in the fiber confined OIP system at room temperature.

We probe the electrical transport in the mixed conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a function of confinement. The scaling of electronic conduct...

Leakage-free, thin molecular dielectric layers with sizable macroscopic polarization are valuable for organic field-effect transistor applications. Molecules that are designed for large polarizabil...

Nerve restoration and repair in the central nervous system is complicated and requires several factors to be considered while designing the scaffolds like being bioactive as well as having neuroinductive, neuroconductive and antioxidant properties. Aligned electrospun nanofibers provide necessary guidance and topographical cues required for directing the axonal and neurite outgrowth during regeneration. Conduction of nerve impulses is a mandatory feature of a typical nerve. The neuro-conductive property can be imparted by blending the biodegradable, bioactive polymers with conductive polymers. This will provide additional features, i.e., electrical cues to the already existing topographical and bioactive cues in order to make it a more multifaceted neuroregenerative approach. Hence in the present study, we used a combination of silk fibroin and melanin for the fabrication of random and aligned electrospun nanofibrous composite scaffolds. We performed the physico-chemical characterization and also assessed their antioxidant properties. We also evaluated their neurogenic potential using human neuroblastoma cells (SH-SY5Y) for their cellular viability, proliferation, adhesion and differentiation levels. Designed nanofibrous scaffolds had adequate physical properties suitable as neural substrates to promote neuronal growth and regeneration. They stimulated the neuroblastoma cell attachment and viability indicating their biocompatible nature. Silk/melanin composite scaffolds have specifically exhibited high antioxidant nature proven by the radical scavenging activity. Additionally, the melanin incorporated aligned silk fibroin scaffolds promoted the cell differentiation into neurons and orientation along their axis. Our results confirmed the potential of melanin incorporated aligned silk fibroin scaffolds as the promising candidates for effective nerve regeneration and recovery.

Direct bandgap perovskite and indirect bandgap Si, which form the two active layers in a tandem solar cell configuration, have different optoelectronic properties and thicknesses. The charge-carrier dynamics of the two-terminal perovskite-on-Si tandem solar cell in response to a supercontinuum light pulse is studied using transient photocurrent (TPC) measurements. Spectral dependence of TPC lifetime is observed and can be classified into two distinct timescales based on their respective carrier generation regions. The faster timescale (∼500 ns) corresponding to the spectral window (300-750 nm) represents the top-perovskite sub-cell, while the slower timescale regime of ∼25 μs corresponds to the bottom-Si sub-cell (>700 nm). Additionally, under light-bias conditions, the transient carrier dynamics of the perovskite sub-cell is observed to be coupled with that of the Si sub-cell. A sharp crossover from the fast-response to a slow-response of the device as a function of the light-bias intensity is observed. These results along with a model based on transfer matrix formulation highlight the role of charge-carrier dynamics in accessing higher efficiencies in tandem solar cells. The carrier transit times and lifetimes in addition to their optical properties need to be taken into account for optimizing the performance.

Blended films of insulating polymer-ferroelectrics with a dilute proportion of polymer semiconductors in a device structure exhibit memristor features. High current magnitudes during ON and OFF states of memory operation are critical when these devices are scaled down to nano dimensions. However, current in these devices is restricted by a limited proportion of low-mobility semiconducting transport channels bridging the top and bottom electrodes within the insulating ferroelectric-polymer matrix. We introduce electric field-assisted thermal annealing treatment during the blend film formation of memristors to overcome current limitations. This strategy enables improvement in the bridging fraction and the effective charge carrier mobility of the semiconducting regions. We present these studies for PVDF-TrFE: PFO and PVDF-TrFE: P3HT model systems.

We utilize ordered mesoporous alumina templates for solution processable electronics and demonstrate massively parallel organization of connected three-terminal vertical transistors. The vertical transistor device consists of a connected organic nanotriode array obtained using porous anodized alumina membranes of pore density ≈ 109 pores/cm2. In this structure, a collector–emitter diode gives rise to a space charge limited current, which can be controlled by a third intermediate porous base electrode to give transistor-like characteristics. We study the response characteristics along with 2D device simulations of this novel structure to indicate key parameters involved in the underlying mechanism. Device operation at single transistor level is verified by conductive atomic force microscopy, and the inherent short switching time scales of the vertical structure device is also demonstrated.

Inherent spontaneous polarization in ferroelectric-dielectric polymer PVDF-TrFE (Poly[(vinylidenefluoride- co -trifluoroethylene]) and an external electric field induced surface modification procedure are utilized to enhance organic field effect transistor (OFET) characteristics. The increase in the carrier mobility of the electric-field (EF) treated device correlates with the EF magnitude and evolution of dielectric microstructure and exhibits an enhancement beyond 300%. The enhanced interfacial transport property appears to have its origin in the dipolar orientation and nanostructure evolution at the interface.

We present experimental evidence showing that the effective carrier diffusion length ($L_d$) and lifetime ($\tau$) depend on the carrier density in MAPbBr$_3$ single crystals. Independent measurements reveal that both $L_d$ and $\tau$ decrease with an increase in photo-carrier density. Scanning photocurrent microscopy is used to extract the characteristic photocurrent $I_{ph}$ decay-length parameter, $L_d$, which is a measure of effective carrier diffusion. The $L_d$ magnitudes for electrons and holes were determined to be ~ 13.3 $\mu$m and ~ 13.8 $\mu$m respectively. A marginal increase in uniform light bias ($\leq 5 \times 10^{15}$ photons/cm$^2$) increases the modulated photocurrent magnitude and reduces the $L_d$ parameter by a factor of two and three for electrons and holes respectively, indicating that the recombination is not monomolecular. The $L_d$ variations were correlated to the features in photoluminescence lifetime studies. Analysis of lifetime variation shows intensity-dependent monomolecular and bimolecular recombination trends with recombination constants determined to be ~ 9.3 $\times 10^6$ s$^{-1}$ and ~ 1.4 $\times 10^{-9}$ cm$^{3}$s$^{-1}$ respectively. Based on the trends of $L_d$ and lifetime, it is inferred that the sub-band-gap trap recombination influences carrier transport in the low-intensity excitation regime, while bimolecular recombination and transport dominate at high intensity., Comment: 19 pages, 6 figures

We report smart color-sensing devices of two-dimensional lead halide perovskites that exhibit a graded band gap across the film. We observe that the device short-circuit photocurrent is strongly dependent on excitation wavelength λ, and this arises through photoabsorption at different depths in the sample due to the graded bandgaps present. This λ signature in the response of the device is observed in case of steady-state excitation when incident from the high bandgap side of the film, where a complete reversal in the polarity of the photocurrent

Geometrical isomers of a set of new ferrocene-benzo[1,2-b:4,5-b′]difuran-2,6-(3H,7H)-dione dyads were synthesized and their optical, nonlinear optical (NLO) and electrochemical properties were investigated. The compounds were fully characterized by spectroscopic data and single crystal X-ray analysis in few cases. The second-order nonlinear polarizabilities were measured in chloroform using a femtosecond hyper-Rayleigh scattering (HRS) method at 1300 nm. The dyads exhibit structure-dependent NLO properties, which could be rationalized by correlation with electrochemical and theoretical data. The (Z)-6 chromophore recorded higher βHRS values than the corresponding (E)-6 isomer, in the donor–acceptor (D–A) type dyads. Similarly, in D–A–D chromophore 7, the βHRS values of isomers follow the order (Z,Z) > (E,Z) > (E,E), which matches with the trend of the wavelength of their maximum absorption as well as an increasing band gap in that order. Owing to the inbuilt structural features, two of these D–A compounds were also explored as semiconducting materials in vacuum deposited top contact bottom gated thin film organic field-effect transistors (OFETs). Defying steric restrictions of the ferrocene unit, the triad (Z,Z)-7 depicted an unbalanced (μe: 3 × 10−3 cm−2 V−1 s−1 and μh: 7 × 10−5 cm−2 V−1 s−1) charge transport behaviour. Evaluation of the isomers of the type studied in this investigation constitutes one of very few reports, in particular this study reveals for the first time the semiconducting behaviour of thermally stable redox active Fc based semiconductors.

We propose an optimum low-temperature-based annealing procedure for semicrystalline donor-fullerene solar cells that is well-suited for plastic and flexible substrates. This proposed alternate strategy utilizes an external electric field (EF) across the bulk heterojunction (BHJ) film during processing at a desired temperature. This processing technique is studied for different molecular weights of the donor in the BHJ blend films. The films indicate an increase in interchain interactions of the semicrystalline polymer chains and an enhancement in hole mobility with EF-assisted annealing treatment. Besides being a controlled method, this processing technique is capable of yielding solar cell devices with performance equivalent to or better than those obtained using plain thermal procedures.

The optical and electronic properties of nanoparticles/nanocrystals (NC) of methylammonium lead tribromide perovskite (MAPbBr3) have been studied in detail. We observe the effect of quantum confinement in particles of an average diameter of ∼6 nm and smaller, in the form of an increase in excitonic nature with decrease in particle size. The differences in the photophysical properties in bulk and NC forms of MAPbBr3 are clearly observed in the temperature dependent measurements, and provide insight into the length scales prevalent in this system. We demonstrate devices consisting of active layers of NC in conjunction with low band gap polymer semiconductors which exhibit the dual functionality of a light emitting diode in the forward bias and a photodetector in the reverse bias.graphic.

We present a bioinspired design strategy to effectively tailor the assembly of naphthalenediimides (NDIs) into a wide variety of architectures by functionalizing with amino acid derivatives. This bioinspired process of custom designing and engineering molecular assemblies is termed "bioinspired architectonics". By employing minute structural mutations in the form of α-substituents of amino acids, we successfully engineered molecular assembly of NDIs into zero-dimensional (0D, spheres), one-dimensional (1D, fibers), and two-dimensional (2D, sheets) architectures. The 2D sheets of phenylalanine methylester appended NDI 1 showed remarkable bulk electron mobility of up to 1 cm(2) V(-1)s(-1). With the aid of photophysical, diffraction, and microscopy techniques we rationalize the effect of molecular structure with their ordering and electronic properties in an effort to find structure-property correlations via a bioinspired modular approach.

A 3D fluidic device (3D-FD) is designed and developed with the capability of auto bubble guidance via a helical pathway in a 3D geometry. This assembly is integrated to a multi-electrode array (MEA) to maintain secondary cell lines, primary cells and primary retinal tissue explants of chick embryos for continuous monitoring of the growth and electrophysiology recording. The ability to maintain the retinal tissue explant, extracted from day 14 (E-14) and day 21 (E-21) chick embryos in an integrated 3D-FD MEA for long duration (>100 h) and study the development is demonstrated. The enhanced duration of monitoring offered by this device is due to the controlled laminar flow and the maintenance of a stable microenvironment. The spontaneous electrical activity of the retina, including the spike recordings from the retinal ganglion layer, was monitored over a long duration. Specifically, the spiking activity in embryonic chick retinas of different days (E-14 to 21) is studied, and the presence of light-stimulated firings along with a distinct electroretinogram for E-21 mature retina provides the evidence of a stable microenvironment over a sustained period.

Introduction of interface layers can maximize the performance of certain organic solar cells. We demonstrate that high efficiency non-fullerene acceptor based solar cells can be further improved with the insertion of PC70BM as an interlayer between the electron transport layer and the active layer. The combination of ZnO and PC70BM layers between a cathode and a bulk heterojunction active layer appears to serve as a better selective contact by reducing charge transport barrier and recombination. The enhanced short-circuit current density (JSC) is characterized by a low series-resistance (

The advent of a new class of high-mobility semiconducting polymers opens up a window to address fundamental issues in electrical transport mechanism such as transport between localized states versus extended state conduction. Here, we investigate the origin of the ultralow degree of disorder $({E}_{a}\ensuremath{\sim}16\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ and the ``bandlike'' negative temperature ($T$) coefficient of the field effect electron mobility: ${\ensuremath{\mu}}_{\mathrm{FET}}^{e}(T)$ in a high performance $({\ensuremath{\mu}}_{\mathrm{FET}}^{e}g2.5\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$ diketopyrrolopyrrole based semiconducting polymer. Models based on the framework of mobility edge with exponential density of states are invoked to explain the trends in transport. The temperature window over which the system demonstrates delocalized transport was tuned by a systematic introduction of disorder at the transport interface. Additionally, the Hall mobility $({\ensuremath{\mu}}_{\mathrm{Hall}}^{e})$ extracted from Hall voltage measurements in these devices was found to be comparable to field effect mobility $({\ensuremath{\mu}}_{\mathrm{FET}}^{e})$ in the high $T$ bandlike regime. Comprehensive studies with different combinations of dielectrics and semiconductors demonstrate the effectiveness of rationale molecular design, which emphasizes uniform-energetic landscape and low reorganization energy.

Perylene diimide (PDI)-based organic photovoltaic devices can potentially deliver high power conversion efficiency values provided the photon energy absorbed is utilized efficiently in charge transfer (CT) reactions instead of being consumed in nonradiative energy transfer (ET) steps. Hitherto, it remains unclear whether ET or CT primarily drives the photoluminescence (PL) quenching of the PDI excimer state in PDI-based blend films. Here, we affirm the key role of the thermally assisted PDI excimer diffusion and subsequent CT reaction in the process of PDI excimer PL deactivation. For our study we perform PL quenching experiments in the model PDI-based composite made of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2-6-diyl] (PBDTTT-CT) polymeric donor mixed with the N,N′-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) acceptor. Despite the strong spectral overlap between the PDI excimer PL emission and UV-vis absorption of PBDTTT-CT, two main observations indicate that no significant ET component operates in the overall PL quenching: the PL intensity of the PDI excimer (i) increases with decreasing temperature and (ii) remains unaffected even in the presence of 10 wt % content of the PBDTTT-CT quencher. Temperature-dependent wide-angle X-ray scattering experiments further indicate that nonradiative resonance ET is highly improbable due to the large size of PDI domains. The dominance of the CT over the ET process is verified by the high performance of devices with an optimum composition of 30:70 PBDTTT-CT:PDI. By adding 0.4 vol % of 1,8-diiodooctane we verify the plasticization of the polymer side chains that balances the charge transport properties of the PBDTTT-CT:PDI composite and results in additional improvement in the device efficiency. The temperature-dependent spectral width of the PDI excimer PL band suggests the presence of energetic disorder in the PDI excimer excited state manifold.

Herein, a meta‐analysis of the device performance and transient spectroscopic results are undertaken for various donor:acceptor blends, employing three different donor polymers and seven different acceptors including nonfullerene acceptors (NFAs). From this analysis, it is found that the primary determinant of device external quantum efficiency (EQE) is the energy offset driving interfacial charge separation, ΔECS. For devices employing the donor polymer PffBT4T blended with NFA and fullerene acceptors, an energy offset ΔECS = 0.30 eV is required to achieve near unity charge separation, which increases for blends with PBDTTT‐EFT and P3HT to 0.36 and ≈1.2 eV, respectively. For blends with PffBT4T and PBDTTT‐EFT, a 100 meV decrease in the LUMO of the acceptor is observed to result in an approximately twofold increase in EQE. Steady state and transient optical data determine that this energy offset requirement is not associated with the need to overcome the polymer exciton binding energy and thereby drive exciton separation, with all blends studied showing efficient exciton separation. Rather, the increase in EQE with larger energy offset is shown to result from suppression of geminate recombination losses. These results are discussed in terms of their implications for the design of donor/NFA interfaces in organic solar cells, and strategies to achieve further advances in device performance.

The development of high-performance multifunctional polymer-based electronic circuits is a major step towards future flexible electronics. Here, we demonstrate a tunable approach to fabricate such devices based on rationally designed dielectric super-lattice structures with photochromic azo-benzene molecules. These nanodielectrics possessing ionic, molecular, and atomic polarization are utilized in polymer thin-film transistors (TFTs) to realize high performance electronics with p-type field-effect mobility exceeding 2 cm^2/(V.s). A crossover in the transport mechanism from electrostatic dipolar disorder to ionic-induced disorder is observed in the transistor characteristics over a range of temperatures. The facile supramolecular design allows the possibility to optically control the extent of molecular and ionic polarization in the ultra-thin nanodielectric. Thus, we demonstrate a three-fold increase in the capacitance from 0.1 uF/cm^2 to 0.34 uF/cm^2, which results in a 200% increase in TFT channel current.

With recent progress in flexible electronics, developing facile one-step techniques for fabricating stretchable conductors and interconnects remain essential. It is also desirable for these processes to have a small number of processing steps, incorporate micropatterning, and be capable of being effortlessly implemented for manufacturing of wearable logic circuits. A low vacuum flash evaporation of Au nanoclusters is proposed as a facile method to fabricate highly stretchable conductors capable of fulfilling all such requirements. High metal-elastomer adhesion on textured substrates ensures low surface resistances (100% strain ≈ 25 Ω-sq–1) where thin film Au accommodate strain like a “bellow”. Stretchability for conductors deposited on non-prestretched textured substrates up to 150% and smooth PDMS substrates up to 200% are shown. The system is modeled on a microscopic system calculating 2-D current continuity equations. Devising low cost techniques for fabricating stretchable conductors remains essential...

Ferroelectric materials find extensive applications in the fabrication of compact memory devices and ultra-sensitive multifunctional detectors. Face-to-face alternate stacking of electron donors and acceptors effectuate long-range unidirectional ordering of charge transfer (CT) dipoles, promising tunable ferroelectricity. Herein we report a new TTF-quinone system - an emerald green CT complex consisting pillar[5]quinone (P5Q) and tetrathiafulvalene (TTF). The CT crystals, as determined by single crystal synchrotron X-ray diffraction, adopt a 1:1 mixed-stack arrangement of donor and acceptor with alternating dimers of TTF and 1,4-dioxane encapsulated P5Q. The TTF-P5Q.dioxane crystal possesses a macroscopic polarization axis giving rise to ferroelectricity at room temperature. The CT complex manifests ferroelectric features such as optical polarization rotation, temperature-dependent phase transition and piezoelectric response in single crystals. Ferroelectric behavior observed in P5Q-based CT complex widens the scope for further work on this structurally intriguing and readily accessible cyclic pentaquinone.

Nanostructured supramolecular donor-acceptor assemblies were formed when an unsymmetrical N-substituted pyridine functionalized perylenebisimide (UPBI-Py) was complexed with oligo(p-phenylenevinylene) (OPVM-OH) complementarily functionalized with hydroxyl unit and polymerizable methacrylamide unit at the two termini. The resulting supramolecular complex [UPBI-Py (OPVM-OH)]1.0 upon polymerization by irradiation in the presence of photoinitiator formed well-defined supramolecular polymeric nanostructures. Self-assembly studies using fluorescence emission from thin film samples showed that subtle structural changes occurred on the OPV donor moiety following polymerization. The 1:1 supramolecular complex showed red-shifted aggregate emission from both OPV (∼500 nm) and PBI (∼640 nm) units, whereas the OPV aggregate emission was replaced by intense monomeric emission (∼430 nm) upon polymerizing the methacrylamide units on the OPVM-OH. The bulk structure was studied using wide-angle X-ray diffraction (WXRD). Complex formation resulted in distinct changes in the cell parameters of OPVM-OH. In contrast, a physical mixture of 1 mol each of OPVM-OH and UPBI-Py prepared by mixing the powdered solid samples together showed only a combination of reflections from both parent molecules. Thin film morphology of the 1:1 molecular complex as well as the supramolecular polymer complex showed uniform lamellar structures in the domain range10 nm. The donor-acceptor supramolecular complex [UPBI-Py (OPVM-OH)]1.0 exhibited space charge limited current (SCLC) with a bulk mobility estimate of an order of magnitude higher accompanied by a higher photoconductivity yield compared to the pristine UPBI-Py. This is a very versatile method to obtain spatially defined organization of n and p-type semiconductor materials based on suitably functionalized donor and acceptor molecules resulting in improved photocurrent response using self-assembly.

Ligand-free Cu-doped CdS nanocrystals (NCs) have been synthesized to elucidate their surface electronic structure. The Cu-doped ligand-free NCs unlike their undoped counterparts are shown to be luminescent. We used this Cu-related emission as a probe to study the nature of the surface trap states that results in negligible luminescence in the undoped NCs. The concentration of the sulfide ligands is shown to play a crucial role in the surface passivation of the NCs. Electrical conductivity of these NCs was also studied, and they were shown to exhibit significant conductivity of ∼10(-4) S cm(-1). Further we have shown that the electrical conductivity is closely correlated to the surface charge and hence the trap states of the individual NCs have far-reaching consequences in the device optimization.

The photoresponse of a metal-semiconductor-metal (MSM) device using single silicon nanowire (Si NW) (~50-nm diameter) with responsivity approaching 3 A/W and external quantum efficiency ~900% at a moderate bias of 1.0 V is reported. The device exhibits a rapid switching of the current when the light of wavelength 405 nm is turned ON/OFF even at zero bias. The dark and illuminated current-voltage characteristics are studied using the MSM device model. The analysis indicates that a dominant contribution to the photoresponse arises from the reduction of the barrier at the contact regions along with photoconductive response in the strand of the Si NW.

The advent of optically functional materials with low-intensive processing methods is accompanied by a growing need for high resolution imaging to probe the inherent inhomogeneities in the underlying microstructure. Atomic force microscopy based techniques are typically utilized for imaging the surface of organic thin films, quantum dots and other nanomaterials with ultrahigh resolution. Several modes like conductive, Kelvin, electrostatic amongst others have been particularly successful in imaging the local current, potential and charge distribution of variety of systems. However, the functionality of photoconduction in these materials cannot be directly imaged by these modes alone. There is a requirement for a local excitation source or collection arrangement that is compatible with scanning microscopy techniques followed by a current monitoring mechanism. Near-field scanning optical microscopy (NSOM) possesses all the advantages of scanning microscopy and is capable of local excitation that overcomes the diffraction limit faced by conventional optical microscopes. Additionally, NSOM can be carried out on actual photoconductive two terminal and three terminal device structures to image local optoelectronic properties. In this review, we present the various geometries that have been demonstrated to perform photoconductive NSOM (p-NSOM). We highlight a representative set of important results and discuss the implications of photocurrent imaging in macroscopic device performance.

A series of alternating and random donor (D)–acceptor (A) copolymers based on naphthalene diimide (NDI) as the acceptor and oligo(p-phenylenevinylene) (OPV) or benzobisoxazole (BBO) as the strong and weak donor, respectively, were designed and synthesized by Suzuki coupling and Horner–Wadsworth–Emmons polymerization. The effect of the varying donor strength of OPV and BBO on the photophysical, electrochemical, and semiconducting properties of the polymers was investigated. Absorption and emission spectra recorded for dilute chloroform solution and thin film showed increased intramolecular charge transfer for NDI-alt-OPV polymer compared to NDI-alt-BBO polymer. Cyclic voltammetry studies along with DFT (density functional theory) studies at the B3LYP/6-31g* level gave insight into the energy level (HOMO/LUMO) and molecular orientation of donor and acceptor along the polymer backbone. NDI-alt-OPV polymer exhibited rigid coplanar structure with extended π-conjugation which induced backbone planarity and crys...

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.