Development of Hybrid Pseudohalide Tin Perovskites for Highly Stable Carbon-Electrode Solar Cells.

Tin-based perovskites degrade rapidly upon interaction with water and oxygen in air because Sn-I bonds are weak. To address this issue, we developed novel tin perovskites, FASnI(3-x)(SCN)x (x = 0, 1, 2, or 3), by employing a pseudohalide, thiocyanate (SCN-), as a replacement for halides and as an inhibitor to suppress the Sn2+/Sn4+ oxidation. The structural and electronic properties of pseudohalide tin perovskites in this series were explored with quantum-chemical calculations by employing the plane-wave density functional theory (DFT) method; the corresponding results are consistent with the experimental results. Carbon-based perovskite devices fabricated with tin perovskite FASnI(SCN)2 showed about a threefold enhancement of the device efficiency (2.4%) relative to that of the best FASnI3-based device (0.9%), which we attribute to the improved suppression of the formation of Sn4+, retarded charge recombination, enhanced hydrophobicity, and stronger interactions between Sn and thiocyanate for FASnI(SCN)2 than those for FASnI3. After the incorporation of phenylethyleneammonium iodide (PEAI, 10%) and ethylenediammonium diiodide (EDAI2, 5%) as coadditives, the FASnI(SCN)2 device gave the best photovoltaic performance with JSC = 20.17 mA cm-2, VOC = 322 mV, fill factor (FF) = 0.574, and overall efficiency of power conversion PCE = 3.7%. Moreover, these pseudohalide-containing devices display negligible photocurrent-voltage hysteresis and great stability in ambient air conditions.

Freestanding Ion Gels for Flexible, Printed, Multifunctional Microsupercapacitors.

Freestanding ion gels (FIGs) provide unique opportunities for scalable, low-cost fabrication of flexible microsupercapacitors (MSCs). While conventional MSCs employ a distinct electrolyte and substrate, FIGs perform both functions, offering new possibilities for device integration and multifunctionality while maintaining high performance. Here, a capillarity-driven printing method is demonstrated to manufacture high-precision graphene electrodes on FIGs for MSCs. This method achieves excellent self-alignment and resolution (width: 50 µm, interdigitated electrode footprint: <1 mm2) and 100% fabrication yield (48/48 devices) and is readily generalized to alternative electrode materials including multiwalled carbon nanotubes (MWCNTs). The devices demonstrate good performance, including high specific capacitance (graphene: 0.600 mF cm-2; MWCNT: 6.64 mF cm-2) and excellent stability against bending, folding, and electrical cycling. Moreover, this strategy offers unique opportunities for device design and integration, including a bifacial electrode structure with enhanced capacitance (graphene: 0.673 mF cm-2; MWCNT: 7.53 mF cm-2) and improved rate performance, print-and-place versatility for integration on diverse substrates, and multifunctionality for light emission and transistor gating. These compelling results demonstrate the potential of FIGs for scalable, low-cost fabrication of flexible, printed, and multifunctional energy storage devices.

Transfer Printing of Sub-5 µm Graphene Electrodes for Flexible Microsupercapacitors.

Printed graphene microsupercapacitors (MSCs) are attractive for scalable and low-cost on-chip energy storage for distributed electronic devices. Although electronic devices have experienced significant scaling to smaller formats, the corresponding miniaturization of energy storage components has been limited, with a typical resolution of ∼30 µm for printed graphene patterns to date. Transfer printing is demonstrated here for patterning graphene electrodes with fine line and spacing resolution less than 5 µm. The resulting devices exhibit an exceptionally small footprint (∼0.0067 mm2), which provides, to the best of our knowledge, the smallest printed graphene MSCs. Despite this, the devices retain excellent performance with a high areal capacitance of ∼6.63 mF/cm2 along with excellent electrochemical stability and mechanical flexibility, resulting from an efficient nonplanar electrode structure and an optimized two-step photoannealing method. As a result, this miniaturization strategy facilitates the on-chip integration of printed graphene MSCs to power emerging electronic devices.

All-Printed, Self-Aligned Carbon Nanotube Thin-Film Transistors on Imprinted Plastic Substrates.

We present a self-aligned process for printing thin-film transistors (TFTs) on plastic with single-walled carbon nanotube (SWCNT) networks as the channel material. The SCALE (self-aligned capillarity-assisted lithography for electronics) process combines imprint lithography with inkjet printing. Specifically, inks are jetted into imprinted reservoirs, where they then flow into narrow device cavities due to capillarity. Here, we incorporate a composite high- k gate dielectric and an aligned conducting polymer gate electrode in the SCALE process to enable a smaller areal footprint than prior designs that yields low-voltage SWCNT TFTs with average p-type carrier mobilities of 4 cm2/V·s and ON/OFF current ratios of 104. Our work demonstrates the promising potential of the SCALE process to fabricate SWCNT-based TFTs with favorable I- V characteristics on plastic substrates.

Ga-Doped Pt-Ni Octahedral Nanoparticles as a Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction.

Bimetallic PtNi nanoparticles have been considered as a promising electrocatalyst for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) owing to their high catalytic activity. However, under typical fuel cell operating conditions, Ni atoms easily dissolve into the electrolyte, resulting in degradation of the catalyst and the membrane-electrode assembly (MEA). Here, we report gallium-doped PtNi octahedral nanoparticles on a carbon support (Ga-PtNi/C). The Ga-PtNi/C shows high ORR activity, marking an 11.7-fold improvement in the mass activity (1.24 A mgPt-1) and a 17.3-fold improvement in the specific activity (2.53 mA cm-2) compared to the commercial Pt/C (0.106 A mgPt-1 and 0.146 mA cm-2). Density functional theory calculations demonstrate that addition of Ga to octahedral PtNi can cause an increase in the oxygen intermediate binding energy, leading to the enhanced catalytic activity toward ORR. In a voltage-cycling test, the Ga-PtNi/C exhibits superior stability to PtNi/C and the commercial Pt/C, maintaining the initial Ni concentration and octahedral shape of the nanoparticles. Single cell using the Ga-PtNi/C exhibits higher initial performance and durability than those using the PtNi/C and the commercial Pt/C. The majority of the Ga-PtNi nanoparticles well maintain the octahedral shape without agglomeration after the single cell durability test (30,000 cycles). This work demonstrates that the octahedral Ga-PtNi/C can be utilized as a highly active and durable ORR catalyst in practical fuel cell applications.

High-Resolution Transfer Printing of Graphene Lines for Fully Printed, Flexible Electronics.

Pristine graphene inks show great promise for flexible printed electronics due to their high electrical conductivity and robust mechanical, chemical, and environmental stability. While traditional liquid-phase printing methods can produce graphene patterns with a resolution of ∼30 µm, more precise techniques are required for improved device performance and integration density. A high-resolution transfer printing method is developed here capable of printing conductive graphene patterns on plastic with line width and spacing as small as 3.2 and 1 µm, respectively. The core of this method lies in the design of a graphene ink and its integration with a thermally robust mold that enables annealing at up to ∼250 °C for precise, high-performance graphene patterns. These patterns exhibit excellent electrical and mechanical properties, enabling favorable operation as electrodes in fully printed electrolyte-gated transistors and inverters with stable performance even following cyclic bending to a strain of 1%. The high resolution coupled with excellent control over the line edge roughness to below 25 nm enables aggressive scaling of transistor dimensions, offering a compelling route for the scalable manufacturing of flexible nanoelectronic devices.

Surface Modification of TiO2 Photoanodes with Fluorinated Self-Assembled Monolayers for Highly Efficient Dye-Sensitized Solar Cells.

Dye aggregation and electron recombination in TiO2 photoanodes are the two major phenomena lowering the energy conversion efficiency of dye-sensitized solar cells (DSCs). Herein, we introduce a novel surface modification strategy of TiO2 photoanodes by the fluorinated self-assembled monolayer (F-SAM) formation with 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFTS), blocking the vacant sites of the TiO2 surface after dye adsorption. The F-SAM helps to efficiently lower the surface tension, resulting in efficient repelling ions, e.g., I3(-), in the electrolyte to decrease the electron recombination rate, and the role of F-SAM is characterized in detail by impedance spectroscopy using a diffusion-recombination model. In addition, the dye aggregates on the TiO2 surface are relaxed by the F-SAM with large conformational perturbation (i.e., helix structure) seemingly because of steric hindrance developed during the SAM formation. Such multifunctional effects suppress the electron recombination as well as the intermolecular interactions of dye aggregates without the loss of adsorbed dyes, enhancing both the photocurrent density (11.9 → 13.5 mA cm(-2)) and open-circuit voltage (0.67 → 0.72 V). Moreover, the combined surface modification with the F-SAM and the classical coadsorbent further improves the photovoltaic performance in DSCs.

Densely packed siloxane barrier for blocking electron recombination in dye-sensitized solar cells.

A challenge in developing photovoltaic devices is to minimize the loss of electrons, which can seriously deteriorate energy conversion efficiency. In particular, minimizing this negative process in dye-sensitized solar cells (DSCs) is imperative. Herein, we use three different kinds of siloxanes, which are adsorbable to titania surfaces and polymerizable in forming a surface passivation layer, to reduce the electron loss. The siloxanes used are tetraethyl orthosilicate (TEOS or compound A), 1-(3-(1H-imidazol-1-yl)propyl)-3-(3-triethoxysilyl) propyl) urea (compound B), and N-(3-triethoxysilylpropyl)-N'[3-(3-methyl-1H-imidazol-3-ium) propyl] urea iodide (compound C). Titania surface passivation by either compound B or C was comparatively more effective in increasing the electron lifetime than TEOS. In the case of small-sized TEOS combined with either large-sized compound B or C, a thinner and denser passivation layer was presumably developed, thus increasing electron lifetime further. Intriguingly, device AB shows the longest electron lifetime, whereas device AC has the highest energy conversion efficiency among these experimental conditions. These results suggest that, in this special case, the electron lifetime may not be a dominant parameter in determining the energy conversion efficiency.

Tetrathiafulvalene as an electron acceptor for positive charge induction on the surface of silver nanoparticles for facilitated olefin transport.

Tetrathiafulvalene (TTF), a well-known electron donor, can also behave as an electron acceptor after being adsorbed on the surface of silver nanoparticles (Ag NPs), thereby inducing a partial positive charge on the Ag NPs surface. The Ag NPs activated by TTF help propylene transport much faster than propane, i.e., facilitated olefin transport, resulting in extremely high separation performance for propylene-propane mixtures.

Toward Higher Energy Conversion Efficiency for Solid Polymer Electrolyte Dye-Sensitized Solar Cells: Ionic Conductivity and TiO2 Pore-Filling.

Even though the solid polymer electrolyte has many intrinsic advantages over the liquid electrolyte, its ionic conductivity and mesopore-filling are much poorer than those of the liquid electrolyte, limiting its practical application to electrochemical devices such as dye-sensitized solar cells (DSCs). Two major shortcomings associated with utilizing solid polymer electrolytes in DSCs are first discussed, low ionic conductivity and poor pore-filling in mesoporous photoanodes for DSCs. In addition, future directions for the successful utilization of solid polymer electrolytes toward improving the performance of DSCs are proposed. For instance, the facilitated mass-transport concept could be applied to increase the ionic conductivity. Modified biphasic and triple-phasic structures for the photoanode are suggested to take advantage of both the liquid- and solid-state properties of electrolytes.

Successful demonstration of an efficient I(-)/(SeCN)2 redox mediator for dye-sensitized solar cells.

A new I(-)/(SeCN)(2) redox mediator has favorable properties for dye-sensitized solar cells (DSCs) such as less visible light absorption, higher ionic conductivity, and downward shift of redox potential than I(-)/I(3)(-). It was then applied for DSCs towards increasing energy conversion efficiency, giving a new potential for improving performance.

Synergistic catalytic effect of a composite (CoS/PEDOT:PSS) counter electrode on triiodide reduction in dye-sensitized solar cells.

Inorganic/organic nanocomposite counter electrodes comprised of sheetlike CoS nanoparticles dispersed in polystyrenesulfonate-doped poly(3,4-ethylenedioxythiophene (CoS/PEDOT:PSS) offer a synergistic effect on catalytic performance toward the reduction of triiodide for dye-sensitized solar cells (DSSCs), yielding 5.4% power conversion efficiency, which is comparable to that of the conventional platinum counter electrode (6.1%). The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry measurements revealed that the composite counter electrodes exhibited better catalytic activity, fostering rate of triiodide reduction, than that of pristine PEDOT: PSS electrode. The simple preparation of composite (CoS/PEDOT:PSS) electrode at low temperature with improved electrocatalytic properties are feasible to apply in flexible substrates, which is at most urgency for developing novel counter electrodes for lightweight flexible solar cells.