The authors would like to acknowledge the &#8220;EXCILIGHT&#8221; Project from the European Union&#8217;s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 674990. This work was also developed within the scope of the project i3N, UIDB/50025/2020 &#38; UIDP/50025/2020, financed by national funds through the FCT/MEC.

CAUTION: The following steps involve the use of different solvents and organic materials, so proper care must be taken when handling. Use the fume hood and protective equipment such as lab glasses, face masks, gloves, and lab coats. Weighing of the materials should be done precisely using a high precision scale machine. To ensure cleanliness of the substrates, solution deposition of thin films, and evaporation, it is recommended that all the procedures be performed in a controlled environment or glovebox. Before the use of a spin-coater, micropipettes, thermal evaporators, organic materials, and solvents, all safety data sheets must be consulted.
1. Preparation of host-guest solution
<ol>
	<li>In two small vials (volume between 4&#8211;6 mL, cleaned with isopropanol and dried with nitrogen), weigh host matrix composed of 12 mg of PVK and 8 mg of OXD-7. Start with weighing the OXD-7. Compensate any deviation in the weight using PVK to achieve a final ratio of 6:4 (PVK:OXD-7). In the second vial, weigh 10 mg of 2PXZ-OXD TADF emitter.</li>
	<li>Add 2 mL of chlorobenzene to the vial with host matrix, and 1 mL to the vial with TADF material. If the weights of any vials are not exactly the values described above, adjust the chlorobenzene volume in both vials to achieve a solution with final concentration of 10 mg/mL.</li>
	<li>Leave the solutions stirring with small, cleaned magnetic stir bars for at least 3 h to ensure complete dissolution of the materials. Ensure that the vials are safely covered with respective caps and tightly sealed with organic chemical safe film to avoid any evaporation of solvents.</li>
</ol>
2. Substrate cleaning
NOTE: To handle the substrates, use a pair of tweezers, touching only in a corner (never touch the middle of the substrates). The substrates used here have six pre-patterned ITO pixels (Figure 1A).
<ol>
	<li>Obtain pre-patterned ITO substrates. Clean substrates in an ultrasonic bath containing 1% v/v Hellmanex solution in water, acetone, and 2-propanol (IPA), sequentially, for 15 min in each bath. Perform the first bath at approximately 95 &#176;C and the remaining at room temperature (RT). Finally, dry the substrates using nitrogen flux to remove any cleaning solvent residue.</li>
	<li>Before fabrication, expose the substrates (ITO film facing upwards) to UV ozone treatment for 5 min. Carefully extract the gases and ensure that the ITO patterned face is exposed to the UV. Here, use an ozone cleaner (100 W, 40 kHz). Set the emission wavelength of the UV lamps to 185 nm and 254 nm with a high intensity, low pressure, mercury vapor discharge lamp.</li>
</ol>
3. Spin coating
This is the most important step of this protocol. To ensure uniformity, homogeneity, and absence of pinholes in the thin films, all solvents must be filtered with their respective filter papers. Complete removal of excess solvents from substrates should be ensured to avoid any short in the final device. For the substrates used here, the removal of excess materials from the patterned ITO and cathode is also important to fix the final pixel, and it should be performed with high precision, without disturbing the active area of the pixel. The steps described below should be followed for spin coating of the thin films. The final thickness of the thin film will vary if using a spin coater different than the one used here.
<ol>
	<li>Prepare the spin coater equipment. 
	NOTE: Before using the spin coater, it is necessary to make a curve calibration with the deposition parameters and final thickness obtained for the films. This should be done for each solution employed. The procedure involves making several depositions for the same solution but with different parameters, and the final thickness is measured with a profilometer. Figure 2 shows a typical calibration curve for an active layer.</li>
	<li>Deposit PEDOT:PSS as the first layer on top of ITO. Filter the PEDOT:PSS with a 0.45 &#956;m polyvinylidene fluoride (PVDF) filter. Fill a micropipet with 100 &#956;L of PEDOT:PSS.</li>
	<li>Carefully place the substrate on the spin coater chuck and activate the vacuum system to fix the substrate (Figure 1B,C). Rotate the ITO face-up and adjust to center the substrate area as much as possible. Set the parameters for the spin coating to 5,000 rpm for 30 s. Set an initial step using the spin coater of ~2&#8211;3 s at low rotation (200&#8211;500 rpm). A thickness of 30 nm is expected.</li>
	<li>Keeping the micropipet perpendicular to the substrate (Figure 1D), drop the solution (100 &#956;L) in the middle of the substrate (Figure 1D) and start the spin coater (Figure 1E). 
	NOTE: Do not drop the solution too quickly or slowly to avoid the risk of nonhomogeneous spread of the solution (depending on the viscosity, the contact angle can be nonideal). Usually, dropping the solution in ~1 s is ideal. Do not touch the substrate with the micropipette, and try to synchronize between starting the spin coater and dropping the solution. If a two-step deposition setting (as explained in step 3.3) is not available, consider a static deposition: drop the solution first, then start the spin coater immediately after. Dropping of the solution should be done carefully. All solutions should be dropped at the center of the rotation axis and form a uniform spot to avoid nonuniformities during the process. Be advised that, although these rules are ideal for good film deposition, the spin coating technique is hard to optimize (i.e., requires several preoptimization steps). Furthermore, it depends on solution viscosity, deposition desired area, how the solution is dropped on the substrate, and the start of spinning. An example of good film formation at a microscopic scale can be seen in Figure 3 as an AFM image.</li>
	<li>Complete the spin coater step (Figure 1F). Turn off the vacuum, and with a tweezer, remove the substrate. With the help of small cotton bud soaked in water (i.e., the PEDOT:PSS solvent; Figure 1G), remove the excess deposited film around the cathode and corner areas from the substrate, keeping the central pixeled area untouched.</li>
	<li>Keep the substrate in an oven or on a hot plate at 120 &#176;C for 15 min to remove the PEDOT:PSS solvent (water). Remove from the oven or hot plate, move to a glovebox, and leave to cool down to RT (Figure 1H).</li>
	<li>Prepare the solution for the EML. In a new clean vial (see step 1.1), using a micropipette, prepare a new solution composed of 1.8 mL of host solution and 0.2 mL of TADF solution. Before using the solution, filter it with a 0.1 &#956;m PTFE filter.</li>
	<li>Leave the new solution stirring for 15 min at RT.</li>
	<li>Following steps 3.3&#8211;3.5, make the deposition of this second solution in a spin coater in the glovebox. Spin at 2,000 rpm for 60 s. The expected film thickness should be 50 nm. To remove any excess of the second film, use cotton buds soaked in chlorobenzene.</li>
	<li>Leave the substrates on a hot plate inside the glove box at 70 &#176;C for 30 min to completely remove the excess chlorobenzene.</li>
	<li>Remove the substrates from the hot plate and leave to cool to RT.</li>
	<li>For additional precautions, consider some temperature/time (indirectly, evaporation rate) tests for different solvents. The morphology of the final film is strongly dependent on these parameters. A simple AFM test can be useful to confirm that the solvent evaporation rate is adequate. The final structure of the deposited thin films should be more or less similar to the scheme in Figure 1I.</li>
</ol>
4. Evaporation of materials
NOTE: For better evaporation, the minimum vacuum required is typically a pressure lower than 5 x 10-5 mbar. For all organic materials, the evaporation rate should be kept under 2 &#197;/s to reduce the roughness and&#160;uniformity of the layers. For LiF, the evaporation rate should be less than 0.2 &#197;/s. Failure to adhere to this may result in nonuniform emissions. If not already done, program the piezoelectric sensor system (which measures the deposition thickness and evaporation rate) with the required parameters, such as 1) material density, 2) Z-factor: an acoustic coupling of material to the sensor, and 3) tooling factor: geometric calibration of the evaporation crucible vs. sample holder. Before using the evaporator, refer to the equipment specifications on how to perform such calibrations, and consult the materials datasheet for the density and Z-factor values for a specific material. Once programmed, and without any evaporation chamber geometry change (tooling factor), the data can be stored for future use with the same materials.
<ol>
	<li>Insert the substrates (films face-down and after step 3.11 is completed) into the sample holder with the desired evaporation mask (Figure 4A).</li>
	<li>Include the necessary crucibles (the geometry depends on the specific evaporator system) and fill each with the necessary materials (LiF, TmPyPb, and Al). A detailed explanation of the thermal evaporation process in OLEDs development can be found in the literature43 and is discussed further in this report.</li>
	<li>Place the substrate holder with samples in the evaporator sample holder (Figure 4B). Close the chamber and pump down the evaporator chamber. Follow the respective instructions for the evaporator system.</li>
	<li>Evaporate a film of TmPyPb with a thickness of 40 nm. Evaporate 2 nm of LiF and 100 nm of Al, sequentially. For the evaporation, follow the published procedure43. 
	NOTE: The final structure is represented in Figure 4C. In the current work, devices are not encapsulated. For long-term experiments, encapsulation should be performed, which is not the focus here.</li>
</ol>
5. Device characterization
NOTE: To characterize the final device, use a highly sensitive voltage meter, luminance meter, and spectrometer. If there is an integrating sphere, use it. Otherwise, place the luminance meter perpendicularly to the OLED surface emission at a distance indicated by the manufacturer and dependent on the focus lens. If not using an integrating sphere, it can be assumed that the OLED device emission follows a Lambertian profile for the efficiency calculation. Here, the plotted brightness does not correspond to the measured under an integrating sphere (thus, it will be at least &#960; times less).
<ol>
	<li>Insert the fabricated OLED device in the testing holder and make the electrical contacts for the desired pixel. Measure the current (I), applied voltage (V), and brightness (L). Complete details about the experimental setup have been explained previously43.</li>
	<li>With a spectrometer, measure the electroluminescence spectra (EL) at different applied voltages in a range corresponding to the dynamic range of the OLED operation44. Take at least three to four spectra. Here, applied voltages of 5 V, 10 V, and 15 V are used.</li>
	<li>Using the necessary software, calculate the current density (J), current efficiency (&#956;c candela/Ampere), power efficiency (&#951;p, lumen/Watt), and external efficiency (EQE). With the electroluminescence spectra, determine the CIE color coordinates. Suitable information on how to calculate all these figures of merit have been described previously44.</li>
	<li>Plot the indicated data. Perform a critical analysis of the results in terms of efficiencies and brightness. See the electroluminescence spectra and attempt to establish a model to understand the results.&#160;</li>
</ol>

The authors have nothing to disclose.

The protocol used here to fabricate an efficient OLED in a simple device structure is relatively simple. The electrical mobility is not only modulated by the material composition of a device layer but also critically depends on film morphology. Preparation of the solutions and a suitable choice of solvent and concentration are important. No material aggregation can occur, implying complete solubility at the nanometric scale. It is also important to observe the viscosity of the solution. A high viscosity leads to a high c...

The increase in organic electronics used in daily life has become an unsurpassed reality. Among several organic electronic applications, OLEDs are perhaps the most attractive. Their image quality, resolution, and color purity have made OLEDs a primary choice for displays. Moreover, the possibility to achieve large area emission in extremely thin, flexible, lightweight, and easy color-tunable OLEDs has applications in lighting. However, some technological issues associated with the fabrication process in large area emitters have postponed further application.
With the first OLED working at low applied voltages1, new paradigms for solid-state lighting have been designed, though with low external quantum efficiency (EQE). The OLED EQE is obtained by the ratio of emitted photons (light) to injected electrical carriers (electrical current). A simple theoretical estimate for the maximum expected EQE is equal to &#951;out&#160;x &#951;int&#160;2. The internal efficiency (&#951;int) can be approximated by &#951;int&#160;= &#947; x <img alt="Equation 1" src="/files/ftp_upload/61071/61071eq01.jpg" style="margin: auto;" /> x &#934;PL, where &#947; corresponds to the charge balance factor, &#934;PL&#160;is the photoluminescence quantum yield (PLQY), and <img alt="Equation 1" src="/files/ftp_upload/61071/61071eq01.jpg" style="margin: auto;" /> is the efficiency of emissive exciton (electron hole pair) generation. Finally, &#951;out is the outcoupling efficiency2. If outcoupling is not considered, attention is focused on three topics: (1) how efficient the material is in creating excitons that radiatively recombine, (2) how efficient the emissive layers are, and (3) how efficient the device structure is in promoting a well-balanced electrical system3.
A purely fluorescent organic emitter has only 25% internal quantum efficiency (IQE). According to spin rules, the radiative transition from a triplet to a singlet (T&#8594;S) is forbidden4. Therefore, 75% of excited electrical carriers do not contribute to the emission of photons5. This issue was first overcome using transition metals in organic emitter phosphorescence OLEDs6,7,8,9,10, where the IQE was reportedly close to 100%11,12,13,14,15,16. This is due to the spin-orbit coupling between the organic compound and heavy transition metal. The disadvantage in such emitters is their high cost and poor stability. Recently, reports on the chemical synthesis of a pure organic compound with low energy separation between the excited triplet and singlet states (&#8710;EST) by Adachi17,18 have given rise to a new framework. Although not new19, the successful employment of the TADF process in OLEDs have made it possible to obtain high efficiencies without using transition metal complexes.
In such metal-free organic emitters, there is a high probability for the excited carriers in a triplet state to populate to the singlet state; therefore, the IQE can achieve a theoretical limit of 100%5,20,21,22. These TADF materials provide excitons that can radiatively recombine. However, these emitters require dispersion in a matrix host to avoid emission quenching3,20,21,23,24 in a host-guest concept. Additionally, its efficiency depends on how the host (organic matrix) is appropriated to the guest (TADF) material25. Also, it is necessary to idealize the device structure (i.e., thin layers, materials, and thickness) to achieve an electrically balanced device (equilibrium between holes and electrons to avoid loss)26. Achieving the best host-guest system for an electrically balanced device is fundamental to increase the EQE. In TADF-based systems, this is not simple, due to the changes in the electrical carrier mobilities in EML that are not easily tuned.
With TADF emitters, EQE values greater than 20% are easy to obtain26,27,28,29. However, the device structure is typically comprised of three to five organic layers (hole transport/blocking and electron transport/blocking layers, HTL/HBL and ETL/EBL, respectively). Additionally, it is fabricated using a thermal evaporation process that is high in cost, technologically complex, and almost only for display applications. Depending on the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) levels, electrical mobility of carriers, and thickness, each layer can inject, transport, and block electrical carriers and guarantee recombination in the emissive layer (EML).
Reducing the device complexity (e.g., a simple, two layer structure) usually results in a noticeable decrease of EQE, sometimes to less than 5%. This happens due to the different electron and hole mobility in the EML, and the device becomes electrically unbalanced. Thus, instead of the high efficiency of exciton creation, the efficiency of emission in the EML becomes low. Moreover, a noticeable roll-off occurs with a strong decrease of the EQE as the brightness increases, due to the high concentration of excitons at a high applied voltage and long excited lifetimes24,30,31. Overcoming such issues requires a strong capability to manipulate electrical properties of the emissive layer. For a simple OLED architecture using solution-deposited methods, electrical properties of the EML can be tuned by the solution preparation and deposition parameters32.
Solution deposition methods for organic-based devices have been previously used31. OLED fabrication, compared to the thermal evaporation process, is of great interest due to their simplified structure, low cost, and large area production. With high success in transition metal complexes OLEDs, the main goal is to increase the emitting area but keep device structure as simple as possible33. Methods such as roll-to-roll (R2R)34,35,36, inkjet printing37,38,39 and slot-die40 have been successfully applied in multilayer fabrication of OLEDs, which is a possible industrial approach.
Despite solution deposition methods for organic layers serving as a good choice for device architecture simplification, not all desired materials can be easily deposited. Two types of materials are used: small molecules and polymers. In solution deposition methods, small molecules have some drawbacks, such as poor thin film uniformity, crystallization, and stability. Thus, polymers are mostly used due to the ability to form uniform thin films with low surface roughness and on large, flexible substrates. Moreover, the materials should have good solubility in the appropriate solvent (mainly organic ones like chloroform, chlorobenzene, dichlorobenzene, etc.), water, or alcohol derivatives.
Besides the problem of solubility, it is necessary to guarantee that a solvent used in one layer should not act as one for the preceding layer. This allows a multi-layer structure deposited by the wet process; however, there are limitations41. The most typical device structure uses some solution-deposited layers (i.e., the emissive one) and one thermally evaporated layer (ETL). Additionally, thin film homogeneity and morphology strongly depend on the deposition methods and parameters. Electrical charge transport through these layers is completely governed by such morphology. Nevertheless, a tradeoff between the desired final device and compatibilities of the fabrication process should be judiciously established. Adjusting the deposition parameters is a key to success, despite being time-consuming work. For instance, the spin coating is not a straightforward technique. Although it seems simple, there are several aspects of thin film formation from a solution on top of a spinning substrate that require attention.
Besides film thickness optimization, manipulation of spinning velocity, and time (thickness is an exponential decay of both parameters), the experimenter&#8217;s actions must also be adjusted to obtain good results. Correct parameters also depend on the solution viscosity, deposition area, and wettability/contact angle of solution on the substrate. There are no unique sets of parameters. Only basic assumptions with specific adjustments to the solution/substrate yield the desired results. Moreover, the electrical properties that depend on the layer molecular conformation and morphology can be optimized for desired results, following the protocol described here. Once completed, the process is simple and feasible.
Nevertheless, decreasing the device structure complexity leads to a maximum EQE decrease; although, a compromise can be achieved in terms of efficiency vs. brightness. As such a compromise allows practical applications, the surplus of a simple, large area compatible, and low cost process can become a reality. This article describes these requirements and how to develop a recipe to handle the required issues.
The protocol focuses on a green TADF emitter 2PXZ-OXD [2,5-bis(4-(10H-phenoxazin-10-yl)phenyl)-1,3,4-oxadiazole]42 as a guest in a host matrix composed by PVK [poly(N-vinylcarbazole)] and OXD-7 [1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene], which corresponds to the EML. An electron transport layer (ETL) of TmPyPb [1,3,5-Tri(m-pyridin-3-ylphenyl)benzene] is used. Both the anode&#8217;s and cathode&#8217;s work functions are optimized. The anode is comprised of ITO (indium tin oxide) with a high conductive polymer PEDOT:PSS [poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)], and the cathode is comprised of a double layer of aluminum and LiF (lithium fluoride).
Finally, both the PEDOT:PSS and EML (PVK: OXD-7: 2PXZ-OXD) are deposited by spin coating, whereas TmPyPb, LiF, and Al are thermally evaporated. Considering the conductive metal-like nature of PEDOT:PSS, the device is a typical &#8220;two organic layer&#8221; in the simplest structure possible. In the EML, TADF guest (10% wt.) is dispersed in the host (90% wt.) composed of PVK0.6+OXD-70.4.

<table border="0" cellpadding="0" cellspacing="0" style="width:999px;" width="998">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">2PXZ-OXD (2,5-bis(4-(10H-phenoxazin-10-yl)phenyl)-1,3,4-oxadiazole)</td>
			<td style="width:207px;">Lumtec ltd</td>
			<td style="width:119px;">1447998-13-1</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Aluminum (99.999%)</td>
			<td style="width:207px;">Alfa Aesar</td>
			<td style="width:119px;">7429-90-5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Acetone (99.9%)</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:119px;">67-64-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Hellmanex</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">7778-53-2</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Isopropyl alcohol</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:119px;">67-63-0</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">ITO patterned substrates</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">65997-17-3</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">Lithium Fluoride (99.99%)</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:119px;">7789-24-4</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">OXD-7 (1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene)</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">138372-67-5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">PEDOT: PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate)</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">155090-83-8</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">PVK (Polyvinlycarbazole) (average Mn 25,000-50,000)</td>
			<td style="width:207px;">Sigma Aldrich</td>
			<td style="width:119px;">25067-59-8</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:507px;">TmPyPb (1,3,5-Tri(m-pyridin-3-ylphenyl)benzene)</td>
			<td style="width:207px;">Ossila</td>
			<td style="width:119px;">138372-67-5</td>
			<td></td>
		</tr>
	</tbody>
</table>

development of efficient oleds from solution deposition

The use of highly efficient organic emitters based on the thermally activated delayed fluorescence (TADF) concept is interesting due to their 100% internal quantum efficiency. Presented here is a solution deposition method for the fabrication of efficient organic light-emitting diodes (OLEDs) based on a TADF emitter in a simple device structure. This fast, low-cost, and efficient process can be used for all OLED emissive layers that follow the host-guest concept. The fundamental steps are described along with necessary information for further reproduction. The goal to establish a general protocol that can be easily adapted for principal organic emitters currently under study and development.

Figure 5 shows the main results for the fabricated device. The turn-on voltage was extremely low (~3 V), which is an interesting result for a two organic layer device. The maximum brightness was around 8,000 cd/m2 without using an integrating sphere. The maximum values for &#951;c, &#951;p, and EQE were around 16 cd/A, 10 lm/W, and 8%, respectively. Although the results are not the best figures of merits for this TADF emitter, they were the best found in such...

Watch this Scientific Journal Video about Development of Efficient OLEDs from Solution Deposition at JoVE.com

Development of Efficient OLEDs from Solution Deposition

Presented here is a protocol for the fabrication of efficient, simple, solution-deposited organic light-emitting diodes with low roll-off.

The use of highly efficient organic emitters based on the thermally activated delayed fluorescence (TADF) concept is interesting due to their 100% ...

development-of-efficient-oleds-from-solution-deposition

Research

JoVE Journal

Engineering

2.0K Views.  University of Aveiro. Presented here is a protocol for the fabrication of efficient, simple, solution-deposited organic light-emitting diodes with low roll-off.

Video: Development of Efficient OLEDs from Solution Deposition

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

Mechanical Expansion of Steel Tubing as a Solution to Leaky Wellbores

Wellbore cement, a procedural component of wellbore completion operations, primarily provides zonal isolation and mechanical support of the metal pipe (casing), and protects metal components from corrosive fluids. These are essential for uncompromised wellbore integrity. Cements can undergo multiple forms of failure, such as debonding at the cement/rock and cement/metal interfaces, fracturing, and defects within the cement matrix. Failures and defects within the cement will ultimately lead to fluid migration, resulting in inter-zonal fluid migration and premature well abandonment. Currently, there are over 1.8 million operating wells worldwide and over one third of these wells have leak related problems defined as Sustained Casing Pressure (SCP)1. 
The focus of this research was to develop an experimental setup at bench-scale to explore the effect of mechanical manipulation of wellbore casing-cement composite samples as a potential technology for the remediation of gas leaks. 
The experimental methodology utilized in this study enabled formation of an impermeable seal at the pipe/cement interface in a simulated wellbore system. Successful nitrogen gas flow-through measurements demonstrated that an existing microannulus was sealed at laboratory experimental conditions and fluid flow prevented by mechanical manipulation of the metal/cement composite sample. Furthermore, this methodology can be applied not only for the remediation of leaky wellbores, but also in plugging and abandonment procedures as well as wellbore completions technology, and potentially preventing negative impacts of wellbores on subsurface and surface environments.

This article reports on a laboratory scale investigation of an existing field procedure and its adaptation for sealing of leaky wellbores. It consists of mechanical expansion of metal pipe, which results in an improved metal/cement bond, ultimate sealing of hydraulic pathways and prevention of gas leaks caused by the presence of a microannular channel.

Wellbore cement, a procedural component of wellbore completion operations, primarily provides zonal isolation and mechanical support of the metal pipe ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Formation of Thick Dense Yttrium Iron Garnet Films Using Aerosol Deposition

Aerosol deposition (AD) is a thick-film deposition process that can produce layers up to several hundred micrometers thick with densities greater than 95% of the bulk. The primary advantage of AD is that the deposition takes place entirely at ambient temperature; thereby enabling film growth in material systems with disparate melting temperatures. This report describes in detail the processing steps for preparing the powder and for performing AD using the custom-built system. Representative characterization results are presented from scanning electron microscopy, profilometry, and ferromagnetic resonance for films grown in this system. As a representative overview of the capabilities of the system, focus is given to a sample produced following the described protocol and system setup. Results indicate that this system can successfully deposit 11 &#181;m thick yttrium iron garnet films that are &#160;&#62; 90% of the bulk density during a single 5 min deposition run. A discussion of methods to afford better control of the aerosol and particle selection for improved thickness and roughness variations in the film is provided.

This report describes the use of a custom-built system to perform aerosol deposition of thick films of yttrium iron garnet onto sapphire substrates at RT. The deposited films are characterized using scanning electron microscopy, profilometry, and ferromagnetic resonance to give a representative overview of the capabilities of the technique.

Aerosol deposition (AD) is a thick-film deposition process that can produce layers up to several hundred micrometers thick with densities greater than 95% ...

Research and Development of High-performance Explosives

Developmental testing of high explosives for military applications involves small-scale formulation, safety testing, and finally detonation performance tests to verify theoretical calculations. For newly developed formulations, the process begins with small-scale mixes, thermal testing, and impact and friction sensitivity. Only then do subsequent larger scale formulations proceed to detonation testing, which will be covered in this paper. Recent advances in characterization techniques have led to unparalleled precision in the characterization of early-time evolution of detonations. The new technique of Photonic Doppler Velocimetry (PDV) for the measurement of detonation pressure will be shared and compared with traditional fiber-optic detonation velocity and plate-dent calculation of detonation pressure. In particular, the role of aluminum in explosive formulations will be discussed. Recent developments led to the development of explosive formulations that result in reaction of aluminum very early in the detonation product expansion. This enhanced reaction leads to changes in the detonation velocity and pressure due to reaction of the aluminum with oxygen in the expanding gas products.

Developmental testing of high explosives for military applications involves small-scale formulation, safety testing, and finally detonation performance tests to verify theoretical calculations. This paper will share typical development tests associated with the measurement of detonation velocity and detonation pressure.

Developmental testing of high explosives for military applications involves small-scale formulation, safety testing, and finally detonation performance ...

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to neutral gold Au(0) by reducing agents, such as sodium citrate or sodium borohydride. The extension of this method to decorate iron oxide or similar nanoparticles with gold nanoparticles to create multifunctional hybrid Fe2O3-Au nanoparticles is straightforward. This approach yields fairly good control over Au nanoparticle dimensions and loading onto Fe2O3. Additionally, the Au metal size, shape, and loading can easily be tuned by changing experimental parameters (e.g., reactant concentrations, reducing agents, surfactants, etc.). An advantage of this procedure is that the reaction can be done in air or water, and, in principle, is amenable to scaling up. The use of such optically tunable Fe2O3-Au nanoparticles for hyperthermia studies is an attractive option as it capitalizes on plasmonic heating of gold nanoparticles tuned to absorb light strongly in the VIS-NIR region. In addition to its plasmonic effects, nanoscale Au provides a unique surface for interesting chemistries and catalysis. The Fe2O3 material provides additional functionality due to its magnetic property. For example, an external magnetic field could be used to collect and recycle the hybrid Fe2O3-Au nanoparticles after a catalytic experiment, or alternatively, the magnetic Fe2O3 can be used for hyperthermia studies through magnetic heat induction. The photothermal experiment described in this report measures bulk temperature change and nanoparticle solution mass loss as functions of time using infrared thermocouples and a balance, respectively. The ease of sample preparation and the use of readily available equipment are distinct advantages of this technique. A caveat is that these photothermal measurements assess the bulk solution temperature and not the surface of the nanoparticle where the heat is transduced and the temperature is likely to be higher.

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

We describe the synthesis and properties of multifunctional Fe2O3-Au nanoparticles produced by a wet chemical approach and investigate their photothermal properties using laser irradiation. The composite Fe2O3-Au nanoparticles retain the properties of both materials, creating a multifunctional structure with excellent magnetic and plasmonic properties.

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to ...

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.

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 ...

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two chalcogenide compositions are demonstrated: Ge23Sb7S70 and As40S60, which have both been studied extensively for planar mid-infrared (mid-IR) microphotonic devices. In this approach, uniform thickness films are fabricated through the use of computer numerical controlled (CNC) motion. Chalcogenide glass (ChG) is written over the substrate by a single nozzle along a serpentine path. Films were subjected to a series of heat treatments between 100 &#176;C and 200 &#176;C under vacuum to drive off residual solvent and densify the films. Based on transmission Fourier transform infrared (FTIR) spectroscopy and surface roughness measurements, both compositions were found to be suitable for the fabrication of planar devices operating in the mid-IR region. Residual solvent removal was found to be much quicker for the As40S60 film as compared to Ge23Sb7S70. Based on the advantages of electrospray, direct printing of a gradient refractive index (GRIN) mid-IR transparent coating is envisioned, given the difference in refractive index of the two compositions in this study.

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

A method of uniform thickness solution-derived chalcogenide glass film deposition is demonstrated using computer numerical controlled motion of a single-nozzle electrospray.

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Fabrication of Ultra-thin Color Films with Highly Absorbing Media Using Oblique Angle Deposition

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present an advanced method for fabricating ultra-thin color films with improved characteristics. The proposed process addresses several fabrication issues, including large area processing. Specifically, the protocol describes a process for fabricating ultra-thin color films using an electron beam evaporator for oblique angle deposition of germanium (Ge) and gold (Au) on silicon (Si) substrates.&#160; Film porosity produced by the oblique angle deposition induces color changes in the ultra-thin film. The degree of color change depends on factors such as deposition angle and film thickness. Fabricated samples of the ultra-thin color films showed improved color tunability and color purity. In addition, the measured reflectance of the fabricated samples was converted into chromatic values and analyzed in terms of color. Our ultra-thin film fabricating method is expected to be used for various ultra-thin film applications such as flexible color electrodes, thin film solar cells, and optical filters. Also, the process developed here for analyzing the color of the fabricated samples is broadly useful for studying various color structures.

We present a detailed method for fabricating ultra-thin color films with improved characteristics for optical coatings. The oblique angle deposition technique using an electron beam evaporator allows improved color tunability and purity. Fabricated films of Ge and Au on Si substrates were analyzed by reflectance measurements and color information conversion.

Ultra-thin film structures have been studied extensively for use as optical coatings, but performance and fabrication challenges remain.&#160; We present ...