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>
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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>Z723134</td>
			<td></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>
			<td></td>
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		<tr height="48">
			<td height="48" style="height:48px;width:204px;">Polytetrafluoroethylene (PTFE) syringe filter&#160;</td>
			<td>Sigma Aldrich</td>
			<td>Z259926</td>
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			<td height="20" style="height:20px;">Polyamide tape</td>
			<td>Kapton</td>
			<td>KPT-1/8</td>
			<td></td>
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		<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 ...

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