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Solution processible photovoltaics (PV) are poised to play an important role in scalable manufacturing of low-cost solar cells. Electrospray is uniquely suited for fabricating PVs due to its several desirable characteristics of an ideal manufacturing process such as compatibility with roll-to-roll production processes, tunability and uniformity of droplet size, capability of operating at atmospheric pressure, and negligible material waste and nano structures. This review begins with an introduction of the fundamentals and unique properties of electrospray. We put emphasis on the evaporation time and residence time that jointly affect the deposition outcome. Then we review the efforts of electrospray printing polymer solar cells, perovskite solar cells, and dye sensitized solar cells. Collectively, these results demonstrate the advantages of electrospray for solution processed PV. Electrospray has also exhibited the capability of producing uniform films as well as nanostructured and even multiscale films. So far, the electrospray has been found to improve active layer morphology, and create devices with efficiencies comparable with that of spin-coating. Finally, we discuss challenges and research opportunities that enable electrospray to become a mainstream technique for industrial scale production.

With the power conversion efficiencies of perovskite solar cells (PSCs) exceeding 25%, the PSCs are a step closer to initial industrialization. Prior to transferring from laboratory fabrication to industrial manufacturing, issues such as scalability, material cost, and production line compatibility that significantly impact the manufacturing remain to be addressed. Here, breakthroughs on all these fronts are reported. Carbon‐based PSCs with architecture fluorine doped tin oxide (FTO)/electron transport layer/perovskite/carbon, that eliminate the need for the hole transport layer and noble metal electrode, provide ultralow‐cost configuration. This PSC architecture is manufactured using a scalable and industrially compatible electrospray (ES) technique, which enables continuous printing of all the cell layers. The ES deposited electron transport layer and perovskite layer exhibit properties comparable to that of the laboratory‐scale spin coating method. The ES deposited carbon electrode layer exhibits superior conductivity and interfacial microstructure in comparison to films synthesized using the conventional doctor blading technique. As a result, the fully ES printed carbon‐based PSCs show a record 14.41% power conversion efficiency, rivaling the state‐of‐the‐art hole transporter‐free PSCs. These results will immediately have an impact on the scalable production of PSCs. [ABSTRACT FROM AUTHOR]

Abstract: The challenge of effectively removing high heat flux from microelectronic chips may hinder future advancements in the semiconductor industry. Spray cooling is a promising solution to dissipate high heat flux, but traditional sprays suffer from low cooling efficiency partly because of droplet rebound. Here we show that electrosprays provide highly efficient cooling by completely avoiding the droplet rebound, when the electrically charged droplets are pinned on the heated conducting surface by the electric image force. We demonstrate a cooling system consisting of microfabricated multiplexed electrosprays in the cone-jet mode generating electrically charged microdroplets that remove a heat flux of 96W/cm2 with a cooling efficiency reaching 97%. Scale-up considerations suggest that the electrospray approach is well suited for practical applications by increasing the level of multiplexing and by preserving the system compactness using microfabrication. [Copyright &y& Elsevier]

Abstract: We developed a well-controlled method to generate PLGA microparticles of different morphologies using the electrospray drying route. By judiciously selecting polymer molecular weight, concentration, and solution flow rate, we can control the order in which polymer entanglements and Coulomb fission occur in the droplets and their relative importance, and subsequently govern the morphology of the resulting polymer particles. We show that spherical, monodisperse particles are generated when sufficiently strong polymer entanglements set in the evaporating droplets before they undergo any Coulomb fission. On the other hand, tailed and elongated particles are obtained if the Coulomb fission occurs first and if the droplets/particles are sufficiently evaporated to freeze in their irregular shape. Strictly spherical particles are unachievable for polymer solutions below a critical concentration, because the onset of Coulomb fission always sets in prior to the development of a sufficiently entangled polymer network. An extension of a simple model, originally used to determine the onset of electrospinning of polymer solutions, adequately predicts when non-spherical particles are produced. We conclude by demonstrating the scale-up of this approach to the synthesis of polymer particles using a compact, microfabricated, multiplexed electrospray system, which would make it suitable for practical applications. [Copyright &y& Elsevier]

Abstract: Widespread application of the cone-jet electrospray has been hampered by the low flow rate at which the spray is dispersed, despite the unique ability of this device to disperse quasi-monodispersed particles over a phenomenal size range. Compact multiplexing, that is, with a high number of sources per unit area, is indispensable for dramatically increasing the throughput and reducing the cost per electrospray source. We demonstrate the successful operation of multiplexed electrospray systems with an unprecedented packing density of up to 11,547sources/cm2. The devices were fabricated and operated by implementing three criteria: (a) the extractor electrode configuration should be used to localize the electric field; (b) the viscous pressure drop acting on the liquid should dominate with respect to the electrostatic pulling force by the electric field; and (c) the electric field “driving” the droplets between the extractor electrode and the collector should be sufficiently intense to avoid reversal of the droplet motion near the extractor (satellite trapping). All devices showed excellent droplet size uniformity across the entire spray region, which indicates effective decoupling of the electrospray sources even for closely packed nozzles. The experimental results show that these design criteria provide effective and reliable guidelines for the successful design and operation of multiplexed devices from first principles, that is, based only on knowledge of the suitability of a liquid for electrospray dispersion and of the critical liquid properties such as surface tension, viscosity and electric conductivity. As a result, the design of these devices for a given application is achievable without costly trial-and-error microfabrication trials. This development opens the door for applications requiring flows from multiplexing 106–107 individual electrospray sources from industry-standard 12-in silicon wafers. [Copyright &y& Elsevier]

Abstract: The effect of the space charge of the homopolarly charged droplets in a system of multiplexed electrosprays is investigated. The system consists of a microfabricated liquid distributor with multiple electrospray sources held at a high voltage, a nearby extractor electrode plate held at an intermediate voltage, and a grounded electrode further downstream. The system is very compact, with a spatial density as large as . Experimentally, we observed that the system performance was limited by the occurrence of path reversal of the satellite droplets, accumulating on the extractor electrode, flooding it and ultimately causing the electrospray to cease operation. To prevent this phenomenon, hereafter referred to as satellite trapping, and “sweep” the satellites downstream, the driving field between the extractor electrode and the ground must be increased. The system behavior was modeled by a simple line-of-charge approximation, which was validated by a more detailed Lagrangian model and by experiments on an individual electrospray. Subsequently, the line-of-charge model was extended to the multiplexed system yielding a satisfactory comparison between the computed minimum driving field that is necessary to prevent satellite path reversal and experimental values. Additional analytical approximations of the multilines-of-charge model yielded a scaling law showing the dependence of the space charge on: (i) the primary droplet residence time, (ii) the current emitted per unit area, and (iii) a dimensionless parameter, , that is the ratio of the distance between the extractor electrode and the ground electrode to the distance between neighboring nozzles. As a result, guidelines for the design of multiplexed electrosprays are established on a fundamental basis, without the need for costly and empirical microfabrication prototyping. [Copyright &y& Elsevier]

Abstract: The use of hydrocarbons is very appealing for the realization of high energy density power sources, so long as the fuel can be stored in the liquid phase. As a result, except for the most volatile hydrocarbons, a miniaturized combustor must rely on a good design of the fuel atomizer, which should yield small, rapidly evaporating droplets to generate the fuel vapor promptly, mix it with the oxidizer and subsequently burn it with the attending heat release. To achieve this goal, we relied on the use of multiplexed electrosprays and a catalytic reactor for fuel conversion consisting of a pack of catalyst impregnated meshes (Microliths®). Fuel dispersion was achieved by microfabricating the fuel distributor in Si using deep reactive ion etching. Tests were performed using JP-8 as the liquid fuel. Preliminary experiments in a 0.8cm3 optically accessible combustor, enabling the measurements of droplet size and velocity, revealed that the spreading of the electrospray by Coulombic repulsion is the phenomenon controlling the volume of the mixing/evaporation chamber. Droplet evaporation occurs in the thin (Peclet number dependent) thermal layer preceding the catalytic section of the combustor. Subsequent system optimization in a fully ceramic combustor yielded a volumetric heat release rate as large as 270MW/m3, a value that is of the same order as that of conventional gas turbines. The small overall combustor volume, at only 0.22cm3, suggests that the large volumetric energy density was achieved despite the device large surface-to-volume ratio and attending heat losses. The fuel was fully oxidized, with CO/CO2 ratios well under 1% over a range of equivalence ratios. Inspection of the combustor inner walls after operating continuously for 10h, revealed no traces of deposits. The design has the potential of being scaled either up or down, depending on power needs. [Copyright &y& Elsevier]

Abstract: The development, fabrication, and testing of a compact multiplexed system of electrosprays are presented with the dual goal of increasing by orders of magnitude the liquid flow rate to be dispersed and of retaining the quasi-monodispersity of the generated droplets. The system was microfabricated as an array of nozzles etched in silicon, with a density of 250sources/cm2. Although the operation of a single electrospray is rather forgiving with respect to the electrode geometry, successful performance of the multiplexed system is critically dependent on a careful selection of the electrode configuration, which in the present work entails an extractor electrode mounted at a distance from the spray sources that is comparable to the distance between sources (on the order of 0.5mm). The electrode has the dual function of limiting electric field cross-talk between neighboring sources and minimizing space charge feedback from the spray cloud. Measurements of current and droplet size as a function of flow rate and of droplet size distribution using ethanol demonstrated that the system may be optimized to produce uniform droplets simultaneously from all parallelized electrosprays, each one operating as an isolated spray in the quasi-monodisperse cone-jet mode. Ease of operation and uniformity in size from spray to spray require strategies to increase the pressure drop in the liquid flow path and/or to uniformize the electric field at the spray sources. [Copyright &y& Elsevier]

Abstract: The electrospray of conducting liquids operated in the cone-jet mode is well known to have the unique ability of generating droplets uniform in size over a phenomenal range of sizes depending primarily on the liquid flow rate and physical properties. Since there is a monotonic dependence of size on flow rate, the liquid flow rates that can be dispersed are modest if the goal is to produce very small (below a few micrometers in diameter) droplets. Yet, this is precisely the application niche for which few, if any, atomization alternatives are available. Multiplexing the spray source is indispensable for the electrospray capabilities to have an impact in high-value-added applications. We report here on a novel approach to multiplexing based on a well-known, but hitherto unexploited, regime of operation, the multi-jet mode. Ordinarily, such a mode is rather unsteady and the range of flow rates at which appreciable multiplexing is achieved is small. However, if the multi-jet mode is anchored by some sharp features (e.g., grooves, ridges, etc.) machined at the outlet of the atomizer, to intensify the electric field at discrete points around its perimeter, then the cone-jets are simultaneously anchored at these features and a stable mode of operation is identified over several hundreds of volts and a broad range of flow rates. Most importantly, so long as the machining is accurately reproduced from point to point, droplets generated do not vary significantly in size from spray to spray. As a result, a compact, inexpensive and versatile multiplexing system is realized without sacrificing droplet monodispersity. [Copyright &y& Elsevier]

Electrospray (ES) has emerged as an attractive approach to fabricate thin-film organic photovoltaic devices. However, the morphological properties and their implication to electrical characteristics of polymer:fullerene blends deposited by this method have not been clearly understood yet. In order to uncover the interplay of nanoscale morphology and electrical properties of ES-deposited films, both the residues of single droplets and thin films from the overlapping of multiple droplets were investigated by various characterization techniques. The surface morphology was demonstrated in details by the optical microscope (OM) and atom force microscope (AFM). The ordering structures and aggregation of the polymer were identified by the Raman and UV–vis spectroscopic investigations. The spatially distribution map of the P3HT aggregation states was analyzed by fitting the C˭C mode of P3HT with Lorentzian functions. The conductive atomic force microscopy (C-AFM) was used to quantify local currents and reveal the correlation between the nanostructure and charge-transport mobility. Both surface morphology and aggregation of the P3HT were found to strongly depend on the overlapping boundaries formed by the dry residues of individual droplets. More P3HT aggregation present at the boundaries. However, boundaries also show significantly higher charge-transport resistance thus lower current. Thin films deposited with less droplet evaporation exhibit more homogenous morphology, more uniform phase segregation, and consequently higher charge mobility. [ABSTRACT FROM AUTHOR]

Hierarchically-structured mesoporous TiO 2 films are prepared by the electrospray (E-spray) method. The microstructures of TiO 2 films can be feasibly tuned as spheres or nanocrystalline-mixed spheres, rendering in situ preparation of TiO 2 dual-layer. With these dual-layer TiO 2 films as photoanode and N719 as sensitizer, the dye-sensitized solar cell achieves a power conversion efficiency (PCE) of 8.96%, which is among the best of N719 devices so far, and constituting up to 19% improvement relative to single-layer TiO 2 photoanode based devices. To disclose the mechanism for the improvement, corresponding samples are prepared and investigated by scanning electron microscopy, electrochemical impedance spectroscopy, diffuse reflectance spectra. It was found that the dual-layer photoanode can simultaneously incorporate high dye loading and low charge transfer resistance, leading to high short current density and thus high PCE. Different to other reported dual-layer photoanodes, which often need two or more preparing procedures, only one step is needed in this work. To our best knowledge, it is the first time to employ only one step to in situ prepare photoanodes with dual-layer structures, which greatly simplifies the experimental processes. [ABSTRACT FROM AUTHOR]

The incorporation of quantum dots (QDs) into chalcogenide glass films is attractive for their luminescent properties. Such QD-doped glass structures could serve as a compact, on-chip light source for planar photonic devices. Typical processing methods such as spin coating have limitations of excessive material waste, little control on the pattern of the film and difficulty for scale-up. To overcome these limitations, this study introduces electrospray as a more versatile processing method and has deposited QD-doped chalcogenide glass thin films. The parameters of the electrospray process are prudently chosen to ensure one or none QD is enclosed per liquid droplet. The transmission electron microscopy imaging of resultant films confirm enhanced QD dispersion with reduced aggregations. Absorption and photoluminescence characterization shows the QD-doped chalcogenide glass films prepared by electrospray maintain signature spectra of QDs from the manufacturer. [ABSTRACT FROM AUTHOR]

Solution-based chalcogenide glass films, traditionally deposited by spin-coating, are attractive for their potential use in chip-based devices operating in the mid-infrared and for ease of nanostructure incorporation. To overcome limitations of spin-coating such as excessive material waste and difficulty for scale-up, this paper introduces electrospray as a film deposition technique for solution-based chalcogenide glasses. Electrospray is shown to produce Ge 23 Sb 7 S 70 films with similar surface quality and optical properties as films deposited by spin-coating. The advantages of electrospray deposition for nanoparticle dispersion, scalable and continuous manufacturing with little material waste, and comparable film quality to spin-coating make electrospray a promising deposition method for practical applications of chalcogenide glass films. [ABSTRACT FROM AUTHOR]

The electrospray (ES) process has emerged as a scalable and efficient fabrication method for organic photovoltaic cells (OPVs). However, the ES process could often involve numerous parameters, which impose a major challenge on uncovering the interplay among process, morphology of active layers, and device performance. This work attempts to reduce the parameter space and capture the essence of the ES process using the Damkhöler ( Da ) number of evaporation, which is the ratio of the droplet residence time over evaporation time. We first derived an explicit equation for Da that links nine different parameters affecting the process. Experimental results indicate that Da number shows strong effect on morphology and crystallinity of the active layer composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Most remarkably, the power conversion efficiency exhibits monotonically dependence on Da values spanning more than one order of magnitude, e.g., from 0.13 to 1.52. This finding suggests that Da number analysis can reduce the large parameter space in electrospray deposition, thus provides a simple way to control and predict the morphology of active layer and the performance of the solar cells. [ABSTRACT FROM AUTHOR]

This technical note documents the use of ballpoint pen tips as robust electrospray emitters. The embedded microsphere helps stabilize Taylor cones by effectively reducing the height of the liquid meniscus and significantly decreasing the viscous damping time of the Taylor cone. The microsphere also provides a mechanism for the Taylor cone to continuously adapt its base diameter to match the voltage applied. As a result, the ballpoint pen electrospray emitter noticeably expands the operation envelope in the flow rate-voltage space. [ABSTRACT FROM AUTHOR]

Abstract: Electrospray (ES) as a thin film deposition method that is uniquely suited for manufacturing organic photovoltaic cells (OPVs) with desired characteristics of atmospheric pressure fabrication, roll-to-roll compatibility, less material loss, and possible self-organized nanostructures. The additional solvent with high electrical conductivity plays an important role in ES deposition process to fabricate OPVs with active layer composed of polymer mixture poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PC61BM). Here we introduced acetic acid, which possesses high electrical conductivity, as additive solvent in ES process. The dependence of device performance on the concentration of acetic acid was investigated, and optimal ratio was obtained. To further demonstrate the influence of additive solvents with different electrical conductivity, OPV devices with active layer deposited by ES method using solutions containing acetic acid, acetone or acetonitrile were fabricated. The characteristics of active layers were revealed by optical microscope, atomic force microscopy, UV–vis spectroscopy and X-ray diffraction. Compared with additive solvents of acetone and acetonitrile, the active layer formed by electrospraying solvent containing acetic acid demonstrated enhanced vertical segregation distribution and improved P3HT crystallinity, which resulted in better device performance. OPV device using acetic acid as additive achieved power convention efficiency (PCE) of 2.99±0.08% under AM 1.5 solar simulation, which is on par with that of the spin coated device (PCE 3.12±0.07%). [Copyright &y& Elsevier]

Abstract: Multiplexed electrospray is a promising aerosol technique to achieve both high throughput and unique advantages of the electrospray such as monodispersity and small droplet size down to nanometer range. Here we study the interaction and overall deposition pattern of multiplexed electrosprays. First, we derived an approximate model for the profile of a single electrospray subjected to its own space charge field as well as external driving electric field. Then the spray profile model was generalized to describe the deposition pattern for multiple electrosprays. Numerically, we investigated the interaction of charged droplets generated by up to 91 electrospray sources using a full Lagrangian model. The highly intensive computational load was successfully handled by implementing the numerical code on a desktop supercomputer with 10 Tera FLOPS of computational power delivered by Graphics Processing Units (GPUs). The results from the spray profile model and numerical simulation were compared to experimental data and good agreements were demonstrated. The simplicity of the spray profile model can conveniently guide the design and operation of multiplexed electrosprays for a range of promising applications. [Copyright &y& Elsevier]

Thin films deposited by conventional electrospray of volatile solutions often give rise to rough morphology consisting of "coffee rings" formed by individual sessile droplets. This is due to the long average inter-droplet distance and low merging probability of deposited droplets within the sprayed circular area. We present a quadrupole configuration to form a squeezing electric field that compresses the circular electrospray pattern into a much slenderer elliptical shape. The design is validated by tracking the trajectories of all sprayed droplets using the Lagrangian method. Moreover, the statistical study of the deposited droplet merging process and experimental results prove that the quadrupole configuration can dramatically decrease inter-droplet distance and increase the merging probability of deposited droplets. The simple apparatus offers precise controls of the aspect ratio of the pattern and provides a feasible approach to produce smooth thin films without pin holes. • A quadrupole configuration is presented to produce the deposited patterns of high aspect ratios in electrospray. • Trajectories of sprayed charged droplets are simulated using the Lagrangian model. • The quadrupole configuration can dramatically increase droplet merging probability for producing smooth thin films. [ABSTRACT FROM AUTHOR]