Name: fabrication of fully solution processed inorganic nanocrystal photovoltaic devices
Uploaded: 2016-07-08T15:00:00
Description: Watch this Scientific Journal Video about Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices at JoVE.com

The Office of Naval Research (ONR) is gratefully acknowledged for financial support. A portion of this work was conducted while Professor Townsend held a National Research Council (NRC) Postdoctoral Fellowship at the Naval Research Laboratory and is grateful for internal support from St. Mary's College of Maryland.

Note: Please consult all relevant materials safety data sheets (MSDS) before use. Many of the precursor solutions and products are hazardous or carcinogenic. Special consideration should be directed to nanomaterials due to unique safety concerns that arise compared to their bulk counterparts. Proper protective equipment should be worn (safety goggles, face shield, gloves, lab coat, long pants and closed-toed shoes) at all times during this procedure.
1. Synthesis of Nanocrystal Precursor Inks</p...

<ol>
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In summary, this protocol provides guidelines for the key steps involved with building a solution processed electronic device from a spray- or spin-coating deposition. Here, we highlight new methods for solution processing transparent conductive indium tin oxide (ITO) films onto non-conductive glass substrates. After a facile etching procedure, individual electrodes can be formed before spray-depositing the photo-active layers. Using a layer-by-layer technique, CdSe and CdTe nanocrystals can be deposited in air under amb...

Due to their unique emerging properties, inorganic nanocrystal inks have found applications in a wide range of electronic devices including photovoltaics,1-6 light emitting diodes,7,8 capacitors9 and transistors.10 This is due to the combination of the excellent electronic and optical properties of inorganic materials and their solution compatibility on the nanoscale. Bulk inorganic materials are typically not soluble and are therefore limited to high temperature, low pressure vacuum depositions. However, when prepared on the nanoscale with an organic ligand shell, these materials can be dispersed in organic solvents and deposited from solution (drop-, dip-, spin-, spray- coating). This freedom to coat large and irregular surfaces with electronic devices reduces the cost of these technologies while also expanding possible niche applications.6,11,12
Solution processing of cadmium(II) telluride (CdTe), cadmium(II) selenide (CdSe), cadmium(II) sulfide (CdS) and zinc oxide (ZnO) inorganic semiconductor active layers has led to photovoltaic devices reaching efficiencies (&#414;) for metal-CdTe Schottky junction CdTe/Al (&#414; = 5.15%)13,14 and heterojunction CdS/CdTe (&#414; = 5.73%),15 CdSe/CdTe (&#414; = 3.02%),16,17 ZnO/CdTe (&#414; = 7.1%, 12%).18,19 In contrast to vacuum deposition of bulk CdTe devices, these nanocrystal films must undergo ligand exchange following deposition to remove native and insulating long-chain organic ligands which prohibit efficient electron transport through the film. Additionally, sintering Cd- (S, Se, Te) must occur during heating in the presence of a suitable salt catalyst. Recently, it was found that non-toxic ammonium chloride (NH4Cl) can be used for this purpose as a replacement for the commonly used cadmium(II) chloride (CdCl2).20 By dipping the deposited nanocrystal film in NH4Cl:methanol solutions, the ligand exchange reaction occurs simultaneously with exposure to the heat-activated NH4Cl sintering catalyst. These prepared films are heated layer-by-layer to build the desired thickness of the photo-active layers.21
Recent advances in transparent conductive films (metal nanowires, graphene, carbon nanotubes, combustion processed indium tin oxide) and conductive metal nanocrystal inks have led to the fabrication of flexible or curved electronics built on arbitrary non-conductive surfaces.22,23 In this presentation, we demonstrate the preparation of each precursor ink solution including the active layers (CdTe and CdSe nanocrystals), the transparent conducting oxide electrode (i.e., indium doped tin oxide, ITO) and the back metal contact to construct a completed inorganic solar cell entirely from a solution process.24 Here, we highlight the spray process and the device layer patterning architectures on non-conductive glass. This detailed video protocol is intended to aid researchers who are designing and building solution processed solar cells; however, the same techniques described here are applicable to a wide range of electronic devices.

<table border="0" cellpadding="0" cellspacing="0" width="589">
	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Oleic acid, 90%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">364525</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">1-octadecene, 90%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">O806</td>
			<td style="width:107px;">Technical grade</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Trioctylphosphine (TOP), 90%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">117854</td>
			<td style="width:107px;">Air sensitive</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;width:204px;">Trimethylsilyl chloride, 99.9%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">92360</td>
			<td style="width:107px;">Air and water sensitive</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Se, 99.5+%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">209651</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:204px;">NH4Cl, 99%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">9718</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:204px;">CdCl2, 99.9%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">202908</td>
			<td style="width:107px;">Highly toxic</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">CdO, 99.99%</td>
			<td style="width:165px;">Strem</td>
			<td style="width:115px;">202894</td>
			<td style="width:107px;">Highly toxic</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Te, 99.8%</td>
			<td style="width:165px;">Strem</td>
			<td style="width:115px;">264865</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:204px;">In(NO3)3.2.85H2O, 99.99%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">326127-50G</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:204px;">SnCl2.2H2O, 99.9%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">431508</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:204px;">NH4OH</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">320145</td>
			<td style="width:107px;">Caustic</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:204px;">NH4NO3, 99%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">A9642</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:204px;">HAuCl4.3H2O, 99.9%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">520918</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;width:204px;">Tetraoctylammonium bromide (TMA-Br)</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">294136</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Toluene, 99.8%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">244511</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Hexanethiol, 95%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">234192</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:204px;">NaBH4, 96%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">71320</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Hexanes, 98.5%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">650544</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Ethanol, 99.5%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">459844</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Methanol, anhydrous, 99.8%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">322415</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">1-propanol, 99.5%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">402893</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">2-propanol, 99.5%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">278475</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;width:204px;">Pyridine, &#62; 99%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">360570</td>
			<td style="width:107px;">Purified by distillation</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Heptane</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">246654</td>
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			<td height="20" style="height:20px;width:204px;">chloroform &#62; 99%</td>
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			<td style="width:115px;">372978</td>
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			<td height="20" style="height:20px;width:204px;">Acetone</td>
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			<td style="width:115px;">34850</td>
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			<td height="40" style="height:40px;width:204px;">Glass microscope slides</td>
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			<td height="20" style="height:20px;width:204px;">Gravity Fed Airbrush</td>
			<td style="width:165px;">Paasche</td>
			<td style="width:115px;">VSR90#1</td>
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			<td height="20" style="height:20px;">Software</td>
			<td>Oriel</td>
			<td>PVIV 2.0</td>
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			<td height="20" style="height:20px;">Round bottom flask</td>
			<td style="width:165px;">Sigma Aldrich</td>
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		<tr height="20">
			<td height="20" style="height:20px;">Round bottom flask</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td>Z418668</td>
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			<td height="48" style="height:48px;width:204px;">Polytetrafluoroethylene (PTFE) syringe filter&#160;</td>
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			<td>Z259926</td>
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			<td height="20" style="height:20px;">Polyamide tape</td>
			<td>Kapton</td>
			<td>KPT-1/8</td>
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			<td height="20" style="height:20px;">Cellophane tape</td>
			<td>Scotch</td>
			<td>810 Tape</td>
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		<tr height="20">
			<td height="20" style="height:20px;">Polypropylene&#160;centrifuge tube</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td>CLS430290</td>
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			<td height="20" style="height:20px;">Silver epoxy</td>
			<td>MG Chemicals</td>
			<td>8331-14G</td>
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fabrication of fully solution processed inorganic nanocrystal photovoltaic devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

Small angle X-ray Diffraction Patterns are used to verify the crystallinity and phase of the annealed nanocrystal film (Figure 1A). If crystallite sizes are below 100 nm, their crystal diameter can be estimated with the Scherrer equation (Eq. 1) and verified with Scanning Electron Microscopy (SEM), 
<img alt="Equation 1" src="/files/ftp_upload/54154/54154eq1.jpg" /> 
where d is the mean crystallite diameter, K is the dimensionless shape factor for the material,&#160;&#946; is the full ...

Watch this Scientific Journal Video about Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices at JoVE.com

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

The overall goal of this procedure is to deposit spin and spray-coated films of inorganic nanocrystals under ambient conditions to build fully solution-processed solar cell devices after ligand exchange and annealing. This method can help answer key questions in the fields of nanoscience and applied materials interfaces such as can we build functional electronics out of inorganic materials by preparing them on the nanoscale so that we can solution-process entire devices from the bottom up. The main advantage to this technique is that we can start thinking about spraying electronics onto new and unconventional surfaces.This includes spraying large and irregular areas in a short time thanks to the additional freedoms of deposition. Though this method can specifically provide insight into building photovoltaic devices, it can also be applied to other systems such as spray processing of LEDs, transistors and capacitors. When synthesizing the cadmium selenide and cadmium telluride inks, follow the text protocol up to the pyridine synthesis step.Decant the supernatent and add back 5 milliliters of distilled of pyridine and 5 milliliters of 1-propynol. Then, flush the flask with an inert gas and sonicate the mixture at 40 kilohertz for 30 minutes. It is also important to filter the ink through a one micron PTFE syringe filter and measure the concentration of the ink by desiccating 200 microliters.Finally, dilute the ink with pyradine and 1-propynol as needed and store the ink under inert gas. In a 500 milliliter Erlenmeyer flask Stir together gold chloride trihydrate and water. Next, add prepared tetraoctylammonium bromide to the yellow mixture.Next, add the hexanethiol ligand. Separately, mix sodium borohydride into water. Immediately add this bubbling reducing solution dropwise to the reaction flask.After three hours of stirring, separate the organic phase with a separatory funnel. Then, use a rotovap to reduce the volume to 20 milliliters. Wash the collected ink with 50 milliliters of hexanes and 200 milliliters of methanol.Precipitate the solids and decant the colorless supernatent. Then, air dry the solids and redisburse them in chloroform at 70 milligrams per milliliter. Preparing the proper ratios of indium tin solids for the ITO ink is crucial for this electrode to be highly conductive.The oxide film formed after annealing is very sensitive to minor changes in relative concentrations of the precursors. To begin, in a 50 milliliter polypropylene tube combine solid salts of indium nitrate hydrate and tin chloride dihydrate with 10 milliliters of 2-methoxyethanol. To this mixture, add ammonium hydroxide to buffer the pH.Then mix in ammonium nitrate solid to serve as an oxidizer. Sonicate the tube at 40 kilohertz in warm water for 20 to 60 minutes or until the ink changes from hazy and white to colorless and transparent. Cut a square glass slide and clean it with sonication then ethynol and acetone.Next, put the glass into concentrated sodium hydroxide for one minute. Briefly rinse the glass with water and then spin coat ITO ink onto it. Then, immediately transfer the slide to a hot plate set to 400 degrees Celsius.After five minutes, remove it and let it cool to room temperature on a ceramic plate. Continue layering ITO ink onto the slide until the sheet resistance is below 1, 000 ohms as measured by a multimeter or a four point probe. Finally, briefly dip the film in dilute aqua regia and rinse it with distilled water.Once dry, the resistance should be below 500 ohms. Now, using a printed grid as a reference, arrange strips of cellophane tape to mask the layers for etching with acid. Where the tape is placed, the ITO will be retained on the glass.Without the correct pattern, the devices will not have the desired area and the efficiency or current measurements will be incorrect. In addition, patterning assures that neighboring devices are not in contact with each other and this avoids device shorting. Next, soak the film in dilute aqua regia at 60 degrees Celsius to dissolve the exposed ITO.After a quick rinse and dry with water, remove the tape. Then sonicate the film with acetone and ethynol to remove any tape residue. Now, add contact point for taking measurements.Place a small drop of silver epoxy on the edge of each ITO strip at one side of the substrate square. Finally, heat the substrate at 150 degrees Celsius for two minutes and let it cool. Begin with placing the patterned ITO glass substrate on a spin coater and coating it with cadmium selenide crystal ink.Let it dry at 150 degrees Celsius for two minutes and then dip it in an ammonium chloride with methynol solution heated to 60 degrees Celsius for 15 seconds. Then, dip the film into isopropanol. Next, dry the film under inert gas and heat it at 380 degrees Celsius for 25 seconds.Once cooled, rinse off the excess salt with distilled water and dry the substrate under inert gas. Repeat this process until the desired thickness is reached. Unlike spin coating, spray coating has nuances of inconsistencies depending on the person spraying.However, after months of trial and error, we have found a method that works well. Mount the ITO glass substrate vertically with tape or clips onto a flat, solid backing. Load 0.25 to one milliliter of diluted cadmium ink into a gravity-fed airbrush.Set a higher pressure for thin, smooth films. Now, 60 millimeters away from the substrate surface, start spraying nanocrystal ink off the substrate Then, move over the substrate in rapid side to side motions keeping the spray stream perpendicular to the surface. We have much more control over the film thickness, roughness, and overall morphology with spraying.However, due to these added freedoms, one must carefully monitor the spray distance, solution concentration, delivery pressure, and duration of the deposition to achieve consistency. Continue adding layers of nanocrystal ink until the desired thickness is achieved. Then, tape pattern the active layers.Finally, a film of gold nanocrystals can be sprayed onto the substrate. Scanning electron microscopy was used to monitor the extent of grain growth in the annealed films. After depositing a single layer of cadmium dye and heating in the presence of ammonium chloride, the grain size was optimized by adjusting the temperature and duration of heating, the ink concentration, the spray pressure and duration, or the spin speed.Typically, larger grains indicate devices with higher short circuit currents. UV-Vis spectroscopy is used to estimate the nanocrystal's size based on absorbence peak correlation with quantum confinement effects. Crystal size was tuned by modifying the concentration of precursors, reaction temperature, and the duration of the ink sythesis.Optical profilometry was used to measure the film thickness and roughness. This was used on a single layer of each material and on completed devices. Fourier transform infrared spectra was taken to monitor the degress of ligand exchange during the ammonium chloride methynol treatment as measured by the disappearance of the C-H alchol stretching bands at 2, 924 and 2, 852.Current voltage characteristics were obtained in the dark and under simulated one sun illumination from a calibrated solar simulator. A detailed explanation is provided in the text protocol. After watching this video, you should have a good understanding of how to spray deposit inorganic nanocrystals which are not typically soluble onto non-conductive substrates to build working electronic devices simply using an airbrush, prepared inks, and a heat source.Once mastered, this technique can be done in one to two hours if it is performed properly. While attempting this procedure, it's important to remember to slowly cool your device substrates after heating to avoid cracking the glass. Don't forget that working with nanomaterials can be extremely hazardous.Always take precautions such as conducting all spray processes in a fume hood while wearing personal protective equipment.

fabrication-fully-solution-processed-inorganic-nanocrystal

Research

JoVE Journal

Engineering

Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young's modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC61BM system correspond well with literature data. The P3HT:PC61BM blend separates onto PC61BM-rich and P3HT-rich domains. Mechanical properties of PC61BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution.

Organic photovoltaic  (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale  inhomogeneity of OPV materials affects performance ...

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene&#8217;s charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene&#39;s electronic structure in the presence of a charged impurity.&#160;The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene&#8217;s electronic properties.1-8 Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge2 and/or molecular5 states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies.2-5 These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

This paper details the fabrication process of a gate-tunable graphene device, decorated with Coulomb impurities for scanning tunneling microscopy studies. Mapping the spatially dependent electronic structure of graphene in the presence of charged impurities unveils the unique behavior of its relativistic charge carriers in response to a local Coulomb potential.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees ...

Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve the solar conversion efficiency of a concentrated photovoltaic system. The proposed system will also lower the solar cell cost in the concentrated photovoltaic system by replacing the expensive tandem solar cells with the cost-effective single junction solar cells. The structures and the parameters of high contrast gratings for the dispersive elements were numerically optimized. The large-area fabrication of high contrast gratings was experimentally demonstrated using nanoimprint lithography and dry etching. The quality of grating material and the performance of the fabricated device were both experimentally characterized. By analyzing the measurement results, the possible side effects from the fabrication processes are discussed and several methods that have the potential to improve the fabrication processes are proposed, which can help to increase the optical efficiency of the fabricated devices.

The fabrication of high contrast gratings as the parallel spectrum splitting dispersive element in a concentrated photovoltaic system is demonstrated. Fabrication processes including nanoimprint lithography, TiO2 sputtering and reactive ion etching are described. Reflectance measurement results are used to characterize the optical performance.

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve ...

Fabrication and Operation of Acoustofluidic Devices Supporting Bulk Acoustic Standing Waves for Sheathless Focusing of Particles

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must be smaller than the incident wavelength of sound and the width of the fluidic channels are typically tens to hundreds of micrometers across, acoustofluidic devices typically use ultrasonic waves generated from a piezoelectric transducer pulsating at high frequencies (in the megahertz range). At characteristic frequencies that depend on the geometry of the device, it is possible to induce the formation of standing waves that can focus particles along desired fluidic streamlines within a bulk flow. Here, we describe a method for the fabrication of acoustophoretic devices from common materials and clean room equipment. We show representative results for the focusing of particles with positive or negative acoustic contrast factors, which move towards the pressure nodes or antinodes of the standing waves, respectively. These devices offer enormous practical utility for precisely positioning large numbers of microscopic entities (e.g., cells) in stationary or flowing fluids for applications ranging from cytometry to assembly.

Acoustofluidic devices use ultrasonic waves within microfluidic channels to manipulate, concentrate and isolate suspended micro and nanoscopic entities. This protocol describes the fabrication and operation of such a device supporting bulk acoustic standing waves to focus particles in a central streamline without the aid of sheath fluids.

Acoustophoresis refers to the displacement of suspended objects in response to directional forces from sound energy. Given that the suspended objects must ...

Morphology Control for Fully Printable Organic&#8211;Inorganic Bulk-heterojunction Solar Cells Based on a Ti-alkoxide and Semiconducting Polymer

The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting material. However, fullerene derivatives are air-sensitive; therefore, air-stable material is needed as an alternative. In the present study, we propose and describe the properties of Ti-alkoxide as an alternative electron-accepting material to fullerene derivatives to create highly air-stable BHJ solar cells. It is well-known that controlling the morphology in the photoactive layer, which is constructed with fullerene derivatives as the electron acceptor, is important for obtaining a high overall efficiency through the solvent method. The conventional solvent method is useful for high-solubility materials, such as fullerene derivatives. However, for Ti-alkoxides, the conventional solvent method is insufficient, because they only dissolve in specific solvents. Here, we demonstrate a new approach to morphology control that uses the molecular bulkiness of Ti-alkoxides without the conventional solvent method. That is, this method is one approach to obtain highly efficient, air-stable, organic-inorganic bulk-heterojunction solar cells.

A method for fully printable, fullerene-free, highly air-stable, bulk-heterojunction solar cells based on Ti alkoxides as the electron acceptor and the electron-donating polymer fabrication is described here. Moreover, a method for controlling the morphology of the photoactive layer through the molecular bulkiness of the Ti-alkoxide units is reported.

The photoactive layer of a typical organic thin-film bulk-heterojunction (BHJ) solar cell commonly uses fullerene derivatives as the electron-accepting ...

Solvent Bonding for Fabrication of PMMA and COP Microfluidic Devices

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each thermoplastic of interest. Solvent bonding is a simple and versatile method that can be used to fabricate devices from a variety of plastics. An appropriate solvent is added between two device layers to be bonded, and heat and pressure are applied to the device to facilitate the bonding. By using an appropriate combination of solvent, plastic, heat, and pressure, the device can be sealed with a high quality bond, characterized as having high bond coverage, bond strength, optical clarity, durability over time, and low deformation or damage to microfeature geometry. We describe the procedure for bonding devices made from two popular thermoplastics, poly(methyl-methacrylate) (PMMA), and cyclo-olefin polymer (COP), as well as a variety of methods to characterize the quality of the resulting bonds, and strategies to troubleshoot low quality bonds. These methods can be used to develop new solvent bonding protocols for other plastic-solvent systems.

Solvent bonding is a simple and versatile method for fabricating thermoplastic microfluidic devices with high quality bonds. We describe a protocol to achieve strong, optically clear bonds in PMMA and COP microfluidic devices that preserve microfeature details, by a judicious combination of pressure, temperature, an appropriate solvent, and device geometry.

Thermoplastic microfluidic devices offer many advantages over those made from silicone elastomers, but bonding procedures must be developed for each ...

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

Scalable Solution-processed Fabrication Strategy for High-performance, Flexible, Transparent Electrodes with Embedded Metal Mesh

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a polymer film. This paper also presents a low-cost, vacuum-free fabrication method for this novel TE; the approach combines lithography, electroplating, and imprint transfer (LEIT) processing. The embedded nature of the EMTEs offers many advantages, such as high surface smoothness, which is essential for organic electronic device production; superior mechanical stability during bending; favorable resistance to chemicals and moisture; and strong adhesion with plastic film. LEIT fabrication features an electroplating process for vacuum-free metal deposition and is favorable for industrial mass production. Furthermore, LEIT allows for the fabrication of metal mesh with a high aspect ratio (i.e., thickness to linewidth), significantly enhancing its electrical conductance without adversely losing optical transmittance. We demonstrate several prototypes of flexible EMTEs, with sheet resistances lower than 1 &#937;/sq and transmittances greater than 90%, resulting in very high figures of merit (FoM) &#8211; up to 1.5 x 104 &#8211; which are amongst the best values in the published literature.

This protocol describes a solution-based fabrication strategy for high-performance, flexible, transparent electrodes with fully-embedded, thick metal mesh. Flexible transparent electrodes fabricated by this process demonstrate among the highest reported performances, including ultra-low sheet resistance, high optical transmittance, mechanical stability under bending, strong substrate adhesion, surface smoothness, and environmental stability.

Here, the authors report the embedded metal-mesh transparent electrode (EMTE), a new transparent electrode (TE) with a metal mesh completely embedded in a ...

Flow-assisted Dielectrophoresis: A Low Cost Method for the Fabrication of High Performance Solution-processable Nanowire Devices

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of nanowires. DEP is used for nanowire analysis, characterization, and for solution-based fabrication of semiconducting devices. The method works by applying an alternating electric field between metallic electrodes. The nanowire formulation is then dropped onto the electrodes which are on an inclined surface to create a flow of the formulation using gravity. The nanowires then align along the gradient of the electric field and in the direction of the liquid flow. The frequency of the field can be adjusted to select nanowires with superior conductivity and lower trap density. 
In this work, flow-assisted DEP is used to create nanowire field effect transistors. Flow-assisted DEP has several advantages: it allows selection of nanowire electrical properties; control of nanowire length; placement of nanowires in specific areas; control of orientation of nanowires; and control of nanowire density in the device. 
The technique can be expanded to many other applications such as gas sensors and microwave switches. The technique is efficient, quick, reproducible, and it uses a minimal amount of dilute solution making it ideal for the testing of novel nanomaterials. Wafer scale assembly of nanowire devices can also be achieved using this technique, allowing large numbers of samples for testing and large-area electronic applications.

In this paper, flow assisted dielectrophoresis is demonstrated for the self-assembly of nanowire devices. The fabrication of a silicon nanowire field effect transistor is shown as an example.

Flow-assisted dielectrophoresis (DEP) is an efficient self-assembly method for the controllable and reproducible positioning, alignment, and selection of ...

Atmospheric Pressure Fabrication of Large-Sized Single-Layer Rectangular SnSe Flakes

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for applications in two-dimensional nanoelectronics devices. Although many methods to synthesize SnSe nanocrystals have been developed, a simple way to fabricate large-sized single-layer SnSe flakes remains a great challenge. Herein, we show the experimental method to directly grow large-sized single-layer rectangular SnSe flakes on commonly used SiO2/Si insulating substrates using a straightforward two-step fabrication method in an atmospheric pressure quartz tube furnace system. The single-layer rectangular SnSe flakes with an average thickness of ~6.8 &#197; and lateral dimensions of about 30 &#181;m &#215; 50 &#181;m were fabricated by a combination of vapor transport deposition technique and nitrogen etching route. We characterized the morphology, microstructure, and electrical properties of the rectangular SnSe flakes and obtained excellent crystallinity and good electronic properties. This article about the two-step fabrication method can help researchers grow other similar two-dimensional, large-sized, single-layer materials using an atmospheric pressure system.

A protocol is presented demonstrating a two-step fabrication technique to grow large-sized single-layer rectangular shaped SnSe flakes on low-cost SiO2/Si dielectrics wafers in an atmospheric pressure quartz tube furnace system.

Tin selenide (SnSe) belongs to the family of layered metal chalcogenide materials with a buckled structure like phosphorene, and has shown potential for ...