High Electrical Conductivity and Hole Transport in an Insightfully Engineered Columnar Liquid Crystal for Solution-Processable Nanoelectronics.

Discotic liquid crystals (DLCs) are widely acknowledged as a class of organic semiconductors that can harmonize charge carrier mobility and device processability through supramolecular self-assembly. In spite of circumventing such a major challenge in fabricating low-cost charge transport layers, DLC-based hole transport layers (HTLs) have remained elusive in modern organo-electronics. In this work, a minimalistic design strategy is envisioned to effectuate a cyanovinylene-integrated pyrene-based discotic liquid crystal (PY-DLC) with a room-temperature columnar hexagonal mesophase and narrow bandgap for efficient semiconducting behavior. Adequately combined photophysical, electrochemical, and theoretical studies investigate the structure-property relations, logically correlating them with efficient hole transport. With a low reorganization energy of 0.2 eV, PY-DLC exhibits superior charge extraction ability from the contact electrodes at low values of applied voltage, achieving an electrical conductivity of 3.22 × 10-4 S m-1, the highest reported value for any pristine DLC film in a vertical charge transport device. The columnar self-assembly, in conjunction with solution-processable self-healed films, results in commendably elevated values of hole mobility (≈10-3 cm2 V-1s-1). This study provides an unprecedented constructive outlook toward the development of DLC semiconductors as practical HTLs in organic electronics.

Enhancing the Efficiency and Stability of Perovskite Solar Cells through Defect Passivation and Controlled Crystal Growth Using Allantoin.

In this study, we present a robust approach that concurrently manages crystal growth and defect passivation within the perovskite layer through the introduction of a small molecule additive─allantoin. The precise regulation of crystal growth in the presence of allantoin yields perovskite films characterized by enhanced morphology, larger grain size, and improved grain orientation. Notably, the carbonyl and amino groups present in allantoin passivate under-coordinated Pb2+ and I- defects, respectively, through molecular interactions. Trap density in the perovskite layer is measured, and it is 0.39 × 1016 cm-3 for the allantoin-incorporated device and 0.83 × 1016 cm-3 for the pristine device. This reduction in defects leads to reduced trap-assisted nonradiative recombination, as confirmed by the photoluminescence, transient photo voltage, and impedance measurements. As a result, when these allantoin-incorporated perovskite films are implemented as the active layer in solar cells, a noteworthy efficiency enhancement to 20.63% is attained, surpassing the 18.04% of their pristine counterparts. Furthermore, devices with allantoin exhibit remarkable operational stability, maintaining 80% of their efficiency even after 500 h of continuous illumination, whereas the pristine device degraded to 65% of its initial efficiency in 400 h. Also, allantoin-incorporated devices exhibited exceptional stability against high humidity and elevated temperatures.

Understanding the effects of shape, material and location of incorporation of metal nanoparticles on the performance of plasmonic organic solar cells.

Truncated octahedral gold (Au) nanoparticles (NPs), Au nanocubes (NCs)-, and silver (Ag) NCs are used to study the effect of NPs shape, material and incorporation location on the performance of poly(3-hexylthiophene):[6,6]-phenyl-C71-butyric acid methyl ester (P3HT:PC71BM) based inverted bulk heterojunction (BHJ) organic solar cells (OSCs). Plasmonic OSCs (POSCs) with NPs incorporated as an interfacial layer between zinc oxide (ZnO) and active layer showed highest power conversion efficiency (PCE) and short circuit current density (J sc) values for all kind of shapes and material compared to POSCs with NPs blended into the active layer. Near-field enhancement as well as enhanced forward scattering cross section is attributed for POSC performance improvement. Among the NPs with two shapes, POSCs with truncated octahedral Au NPs exceeded the photovoltaic performance compared to those of POSCs with Au and Ag NCs. Large number of antennas in truncated octahedral Au NPs compared to NC is reasoned to be the cause for this improvement. Even though Ag has better localised surface plasmon resoanance (LSPR) properties compared to Au, the POSCs with Ag NCs showed lower J sc value and is due to reduced number of photons at the blue shifted LSPR wavelength of Ag NCs. The improvement in J sc values of POSCs is confirmed by enhancement in absorption, external quantum efficiency (EQE), exciton generation and exciton dissociation probability measurements and is due to improved LSPR coupling of the NPs with the active layer. The surface enhanced Raman scattering (SERS) and photoluminescence (PL) studies confirm the absorption enhancement in the active layer by NP LSPR coupling and further confirm the enhancement in the photovoltaic performance of POSCs.

Charge Carrier Polarity Modulation in Diketopyrrolopyrrole-Based Low Band Gap Semiconductors by Terminal Functionalization.

Organic semiconductors with variable charge carrier polarity are required for optoelectronic applications. Herein, we report the synthesis of three novel diketopyrrolopyrrole (DPP)-based D-A molecules having three different terminal groups (amide, ester, and dicyano) and study their electronic properties. An increase in electron acceptor strength from amide to dicyano leads to a bathochromic shift in absorption. Photoconductivity and field effect transistor (FET) measurements confirmed that a small increase in acceptor strength can result in a large change in the charge transport properties from p-type to n-type. The molecule with an amide group, DPP-amide, exhibited a moderate p-type mobility (1.3 × 10-2 cm2 V-1 s-1), whereas good n-type mobilities were observed for molecules with an ester moiety, DPP-ester (1.5 × 10-2 cm2 V-1 s-1), and with a dicyano group, DPP-DCV (1 × 10-2 cm2 V-1 s-1). The terminal functional group modification approach presented here is a simple and efficient method to alter the charge carrier polarity of organic semiconductors.

Surface-Treated Poly(dimethylsiloxane) as a Gate Dielectric in Solution-Processed Organic Field-Effect Transistors.

Poly(dimethylsiloxane) (PDMS) is a transparent and flexible elastomer which has a myriad of applications in various fields including organic electronics. However, the inherent hydrophobic nature and low surface energy of PDMS prevent its direct use in many applications. It is seldom utilized as a gate dielectric in solution-processed organic field effect transistors (OFETs). In this work, we demonstrate a simple method, extended ultraviolet-ozone (UVO) treatment, to modify the PDMS surface and effectively employ it in solution-processed OFETs as a gate dielectric material. The modified PDMS surface shows enhanced wettability and adherence to both polar and nonpolar liquids, which is contrary to the generally observed hydrophilic nature of UVO-treated PDMS surfaces because of the creation of polar functional groups. The morphological changes happening on the PDMS surface as a result of extended UVO treatment play a major role in making the surface suitable for all type of solvents discussed here. The contact angle measurements are used to give qualitative evidence for this observation. The modified PDMS is then used as a gate dielectric in solution-processed n- and p-channel OFETs using [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) and regioregular poly(3-hexylthiophene) (rr-P3HT) semiconductors, respectively.

Semiconducting Fabrics by In†Situ Topochemical Synthesis of Polydiacetylene: A New Dimension to the Use of Organogels.

A diyne functionalized 4,6-O-benzylidene ß-d-galactopyranoside gelator, which can align its diyne motifs upon self-assembly (gelation) have been synthesized. The organogel formed by this gelator undergoes topochemical polymerization to polydiacetylene (PDA) under photoirradiation. This strategically designed gelator has been used to make semi-conducting fabrics. By developing the organogel on the fabrics, the gelator molecules were made not only to self-assemble on the fibers, but also to adhere to fabrics through hydrogen bonding. UV irradiation of the gel-coated fabric/fiber resulted in the formation of PDA on fibers. The benzylidene motif could be deprotected to get PDA with pendant free sugars that strongly bind to the cotton fibrils through multiple hydrogen bonds. Conductivity measurements revealed the semiconducting nature of these fabrics.

Squaraine based solution processed inverted bulk heterojunction solar cells processed in air.

Inverted bulk heterojunction solar cells based on low temperature solution processed squaraine (SQ) and [6,6]-phenyl C71 butyric acid methyl-ester (PC71BM) with varying blend ratios were made in air. An optimized bulk heterojunction device of SQ and PC71BM (with a blend ratio of 1 : 6) showed a power conversion efficiency (PCE) of 2.45% with an incident photon to current conversion efficiency of 65% at 680 nm and a spectral window extending to the NIR region. The devices also showed an enhanced PCE value of 4.12% upon continuous illumination from an AM1.5G light source of intensity 1 Sun. The intensity dependent photocurrent studies showed a monomolecular recombination mechanism in the photovoltaic device performance. The device stored in air showed reasonable stability for a period of one month.

Efficient organic-inorganic hybrid perovskite solar cells processed in air.

Organic-inorganic hybrid perovskite solar cells with fluorine doped tin oxide/titanium dioxide/CH3NH3PbI3-xClx/poly(3-hexylthiophene)/silver were made in air with more than 50% humidity. The best devices showed an open circuit voltage of 640 mV, a short circuit current density of 18.85 mA cm(-2), a fill factor of 0.407 and a power conversion efficiency of 5.67%. The devices showed external quantum efficiency varying from 60 to 80% over a wavelength region of 350 nm to 750 nm of the solar spectrum. The morphology of the perovskite was investigated using scanning electron microscopy and it was found to be porous in nature. This study provides insights into air-stability of perovskite solar cells.

Filament theory based WORM memory devices using aluminum/poly(9-vinylcarbazole)/aluminum structures.

Spin coated poly(N-vinylcarbazole) (PVK) sandwiched between thermally evaporated aluminum (Al) electrodes on a glass substrate showed unipolar Write Once Read Many times (WORM) characteristics. The pristine devices were in the low resistance ON state exhibiting ohmic behavior and at a voltage near -2 V, they switched abruptly to the high resistance OFF state showing space charge limited current (SCLC). We suggest that the rupturing of metallic filaments due to Joule heating may explain the effect. The WORM devices exhibited an ON/OFF ratio of 10(8), a retention of 1000 s and an endurance of ∼10(6) cycles in both ON and OFF states.

On the Uniqueness of Ideality Factor and Voltage Exponent of Perovskite-Based Solar Cells.

Perovskite-based solar cells have attracted much recent research interest with efficiency approaching 20%. While various combinations of material parameters and processing conditions are attempted for improved performance, there is still a lack of understanding in terms of the basic device physics and functional parameters that control the efficiency. Here we show that perovskite-based solar cells have two universal features: an ideality factor close to two and a space-charge-limited current regime. Through detailed numerical modeling, we identify the mechanisms that lead to these universal features. Our model predictions are supported by experimental results on solar cells fabricated at five different laboratories using different materials and processing conditions. Indeed, this work unravels the fundamental operation principle of perovskite-based solar cells, suggests ways to improve the eventual performance, and serves as a benchmark to which experimental results from various laboratories can be compared.

Fabrication of a novel biomaterial with enhanced mechanical and conducting properties.

Conducting polymers have the combined advantages of metal conductivity with ease in processing and biocompatibility; making them extremely versatile for biosensor and tissue engineering applications. However, the inherent brittle property of conducting polymers limits their direct use in such applications which generally warrant soft and flexible material responses. Addition of fillers increases the material compliance, but is achieved at the cost of reduced electrical conductivity. To retain suitable conductivity without compromising the mechanical properties, we fabricate an electroactive blend (dPEDOT) using low grade PEDOT:PSS as the base conducting polymer with polyvinyl alcohol as filler and glycerol as a dopant. Bulk dPEDOT films show a thermally stable response till 110 °C with over seven fold increase in room temperature conductivity as compared to 0.002 S cm-1 for pristine PEDOT:PSS. We characterize the nonlinear stress-strain response of dPEDOT, well described using a Mooney-Rivlin hyperelastic model, and report elastomer-like moduli with ductility ∼ fives times its original length. Dynamic mechanical analysis shows constant storage moduli over a large range of frequencies with corresponding linear increase in tan(δ). We relate the enhanced performance of dPEDOT with the underlying structural constituents using FTIR and AFM microscopy. These data demonstrate specific interactions between individual components of dPEDOT, and their effect on surface topography and material properties. Finally, we show biocompatibility of dPEDOT using fibroblasts that have comparable cell morphologies and viability as the control, which make dPEDOT attractive as a biomaterial.

The mechanical properties of individual, electrospun fibrinogen fibers.

We used a combined atomic force microscopic (AFM)/fluorescence microscopic technique to study the mechanical properties of individual, electrospun fibrinogen fibers in aqueous buffer. Fibers (average diameter 208 nm) were suspended over 12 microm-wide grooves in a striated, transparent substrate. The AFM, situated above the sample, was used to laterally stretch the fibers and to measure the applied force. The fluorescence microscope, situated below the sample, was used to visualize the stretching process. The fibers could be stretched to 2.3 times their original length before breaking; the breaking stress was 22 x 10 (6)Pa. We collected incremental stress-strain curves to determine the viscoelastic behavior of these fibers. The total stretch modulus was 17.5 x 10 (6)Pa and the relaxed elastic modulus was 7.2 x 10 (6)Pa. When held at constant strain, electrospun fibrinogen fibers showed a fast and slow stress relaxation time of 3 and 55 s. Our fibers were spun from the typically used 90% 1,1,1,3,3,3-hexafluoro-2-propanol (90-HFP) electrospinning solution and re-suspended in aqueous buffer. Circular dichroism spectra indicate that alpha-helical content of fibrinogen is approximately 70% higher in 90-HFP than in aqueous solution. These data are needed to understand the mechanical behavior of electrospun fibrinogen structures. Our technique is also applicable to study other nanoscopic fibers.