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>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">chloroform &#62; 99%</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">372978</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:204px;">Acetone</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td style="width:115px;">34850</td>
			<td style="width:107px;"></td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;width:204px;">Glass microscope slides</td>
			<td style="width:165px;">Fisher</td>
			<td style="width:115px;">12-544-4</td>
			<td style="width:107px;">Cut with glass cutter</td>
		</tr>
		<tr height="20">
			<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>
			<td style="width:107px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Syringe needle</td>
			<td style="width:165px;">Fisher</td>
			<td>CAD4075</td>
			<td></td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:204px;">Solar Simulator Testing Station</td>
			<td>Newport</td>
			<td>PVIV-1A</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Software</td>
			<td>Oriel</td>
			<td>PVIV 2.0</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Round bottom flask</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td>Z723134</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Round bottom flask</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td>Z418668</td>
			<td></td>
		</tr>
		<tr height="48">
			<td height="48" style="height:48px;width:204px;">Polytetrafluoroethylene (PTFE) syringe filter&#160;</td>
			<td>Sigma Aldrich</td>
			<td>Z259926</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Polyamide tape</td>
			<td>Kapton</td>
			<td>KPT-1/8</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Cellophane tape</td>
			<td>Scotch</td>
			<td>810 Tape</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Polypropylene&#160;centrifuge tube</td>
			<td style="width:165px;">Sigma Aldrich</td>
			<td>CLS430290</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Silver epoxy</td>
			<td>MG Chemicals</td>
			<td>8331-14G</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

Research

JoVE Journal

Engineering

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

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

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

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