This work was sponsored by the Petroleum Technology Development Fund (PTDF, Nigeria) and the Engineering and Physical Sciences Research Council, UK, through the Programme Grant MEMPHIS (grant number EP/K003976/1). The authors also appreciate fruitful discussions with Dr. Zhizhao Che.

1. Experimental Rig Setup
NOTE: See Figure 1.
<ol>
	<li>Falling film unit
	<ol>
		<li>Start by cleaning the substrate (glass) surface with a clean, soft cloth. Ensure no dirt is adhered to its surface, which would alter the liquid properties.</li>
		<li>Set the pivot of the glass substrate to the desired inclination angle. An inclination angle, &#946;, of 15&#730; was used in this work.</li>
		<li>Switch on the electric pump and ensure a normal liquid flow on the film surface to further clean the glass substrate. For this work, the test liquid was deionized water.</li>
		<li>Ensure the entire surface of the substrate is wetted.</li>
		<li>Measure the film flow rate using the flow meter. For this work, the flow rate was varied between 1.667 x 10-3 and 10 x 10-3 m3/s with the corresponding film Reynolds number, Re = &#961;q/w&#181;, ranging between 55.5 and 333. w is the falling film width, 0.30 m.</li>
		<li>Gradually adjust the valves on the flow connection to obtain the desired flow rate on the glass substrate.</li>
		<li>Adjust the set of micrometer step at the film inlet to the corresponding Nusselt film thickness value for the chosen flow rate, to avoid a hydraulic jump at the film inlet or a backflow of air into the distribution chamber.</li>
		<li>Manually siphon all air in the distribution chamber to obtain a uniform flow downstream on the film surface.</li>
	</ol>
	</li>
	<li>Film control unit
	<ol>
		<li>Ensure that the function generator is connected to the solenoid valve through a non-latching relay via a data acquisition card (DAC).</li>
		<li>Switch on both the solenoid valve and the function generator.</li>
		<li>Set the function generator to the desired forcing frequency. In this work, frequencies of 2 and 3 Hz were used.</li>
		<li>Choose the desired wave signal (sine wave, sawtooth wave, square wave, etc.). In this work, a sine wave signal was used. Figure 2A and 2B show the contrast between an uncontrolled film and a controlled film.</li>
	</ol>
	</li>
	<li>Droplet generation system
	<ol>
		<li>Attach a clean plastic tubing to a water-filled syringe.</li>
		<li>Insert the syringe into the droplet generator.</li>
		<li>Affix a syringe-needle of a chosen size (depending on the desired droplet diameter) to the other end of the plastic tubing. The droplet diameter range studied was between 0.0023 to 0.0044 m.</li>
		<li>Adjust the fall height of the drop above the film surface. In this work, the drop&#39;s fall height was varied from 0.005 to 0.45 m, giving impact speed between 0.30 &#177; 0.02 - 2.96 &#177; 0.06 m/s.</li>
		<li>Similarly, set the streamwise impact point of the drop from the film inlet. This was set to 0.3 m in this work to ensure the waves are well-formed prior to impact.</li>
		<li>Set a desired flow rate for the syringe pump.</li>
		<li>Adjust the flow rate to achieve a droplet generation frequency greater than the wavelength of the waves formed on the film surface; to ensure drops successively impinge on different regions of the controlled film. See Figure 2C; with an enlargement of a singular waveform in Figure 2D to show the dissimilarities in the flow profile beneath each region19,20.</li>
	</ol>
	</li>
	<li>High-speed imaging setup
	<ol>
		<li>Place the camera on a tripod stand (or any other suitable arrangement).</li>
		<li>Select the macro lens with desired focal length and connect this to the camera.</li>
		<li>Switch on the high-speed camera and ensure direct focus on the film surface. Align the camera at 7&#730; and 12&#730; horizontal and vertical deviations respectively to the film surface. This gives an excellent side-view image of the impact process, resulting in a resolution of 67.5 &#181;m/pixel and 46.6 &#181;m/pixel in the streamwise and spanwise directions, respectively.</li>
		<li>Adjust the focus of the camera-lens (at the largest aperture) using a calibration item&#160;placed exactly on the droplet impact spot.</li>
		<li>Once a sharp focus has been obtained, reduce the aperture to ensure only a small amount of light enters the camera.</li>
		<li>Set the desired frame rate, resolution, and shutter speed of the high-speed camera. A frame rate of 5000 fps, 800 x 600 resolution, aperture size 1/16, and a shutter speed of 1 &#181;s were used in this work.</li>
		<li>Place the light diffuser in front of the light source, as shown in Figure 1C, to ensure the light is uniformly diffused across the imaging region.</li>
		<li>Power on the light source to confirm the uniform spreading of light over the imaging area.</li>
	</ol>
	</li>
</ol>
2. Calibration
NOTE: See Figure 3.
<ol>
	<li>Put a ruler in the film flow direction (exactly on the spot of impact) and obtain snapshots of measured points on the film surface.</li>
	<li>Repeat 2.1 but with the ruler in the spanwise direction.</li>
	<li>Use the above to obtain the spatial resolutions on the film surface.</li>
</ol>
3. Video Recording and Data Acquisition
<ol>
	<li>Once film flow is established on the rig, start the syringe pump and observe the impact of the dripping drops on the film surface.</li>
	<li>Start the function generator and observe the production of spatiotemporally regular waves on the film surface.</li>
	<li>Ensure successive drops are impacting the different regions of the controlled film surface.</li>
	<li>Observe the post-triggering frame number and set this to approximately half of the video length to adequately capture the impact.</li>
	<li>Power on the light source and trigger the image capturing once an impact occurs.</li>
	<li>Power off the light source once image capturing is complete to avoid overheating of the liquid film.</li>
	<li>Visually analyze the obtained snapshot on the computer screen. Check to see if the impact has occurred on one of the flat film, capillary wave, or wave hump regions.</li>
	<li>Trim down the video to the portion showing the impact process and save the frame range in a video/image format.</li>
	<li>Repeat 3.5 - 3.8 and record individual impact on all regions on the film surface, viz. solitary hump, capillary waves, and flat film.</li>
</ol>
4. Image Post-processing and Analysis
<ol>
	<li>Place a ruler in the field of view and calculate the spatial resolution by counting how many pixels fit across 1 cm. Using the calibration image, obtain a scale factor for image dimension measurement.</li>
	<li>Compare the outcomes of the impact process on the different impact regions from the high-speed images. Check to see notable differences.</li>
	<li>Using a suitable MATLAB image-processing routine, measure the characteristics features of the product of the impact process: viz. in the splashing mode, measure the crown height, diameter, wall thickness, tilt angle, crown-facing direction, number and size distribution of ejected secondary droplets.</li>
	<li>Carry out similar quantitative analyses as 4.3 above for the low-Weber impacts. Count the pinch-off time of satellite drops from the time-framed images and measure the apex length and width&#160;of the column formed in partial coalescence before pinch-off of secondary drops. Measure the size of ejected secondary drops. Count the number of cascade in a repeated pinch-off process.</li>
	<li>Observe all qualitative differences in each region.</li>
</ol>

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L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Drop+impact+dynamics:+Splashing,+spreading,+receding,+bouncing&#8230;.">Drop impact dynamics: Splashing, spreading, receding, bouncing&#8230;.</a> Annual Review of Fluid Mechanics. 38, 159-192 (2006).</li><li>Rein, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Phenomena+of+liquid+drop+impact+on+solid+and+liquid+surfaces.">Phenomena of liquid drop impact on solid and liquid surfaces.</a> Fluid Dynamics Research. 12 (2), 61-93 (1993).</li><li>Liang, G., Mudawar, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Review+of+mass+and+momentum+interactions+during+drop+impact+on+a+liquid+film.">Review of mass and momentum interactions during drop impact on a liquid film.</a> International Journal of Heat and Mass Transfer. 101, 577-599 (2016).</li><li>Dam, D. B., Le Clerc, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+study+of+the+impact+of+an+ink-jet+printed+droplet+on+a+solid+substrate.">Experimental study of the impact of an ink-jet printed droplet on a solid substrate.</a> Physics of Fluids. 16 (9), 3403-3414 (2004).</li><li>Worthington, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> A study of splashes. , (1908).</li><li>Edgerton, H. E., Killian, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Flash! Seeing the unseen by ultra-high-speed photography. , (1954).</li><li>Josserand, C., Thoroddsen, S. 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V., Osheroff, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Splashing+of+drops+on+shallow+liquids.">Splashing of drops on shallow liquids.</a> Science. 158 (3805), 1184-1186 (1967).</li><li>Adomeit, P., Renz, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hydrodynamics+of+three-dimensional+waves+in+laminar+falling+films.">Hydrodynamics of three-dimensional waves in laminar falling films.</a> International Journal of Multiphase Flow. 26 (7), 1183-1208 (2000).</li><li>Blanchette, F., Bigioni, T. 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V., Matar, O. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dynamics+and+stability+of+thin+liquid+films.">Dynamics and stability of thin liquid films.</a> Reviews of Modern Physics. 81 (3), 1131 (2009).</li><li>Adebayo, I. T., Matar, O. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Droplet+impact+on+flowing+liquid+films+with+inlet+forcing:+the+splashing+regime.">Droplet impact on flowing liquid films with inlet forcing: the splashing regime.</a> Soft Matter. 13 (41), 7473-7485 (2017).</li><li>Chang, H. H., Demekhin, E. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Complex wave dynamics on thin films. 14, (2002).</li><li>Liu, J., Gollub, J. 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The authors have nothing to declare.

In this section, we provide a few tips necessary to ensure qualitative results are obtained from the protocol. First, the glass substrate on which the liquid film flows must be kept completely dirt-free to ensure the properties of the liquid film are kept uncompromised. This is achievable by regular cleaning (probably using a suitable detergent, and wiped off over a tray to avoid dissolution into the system). Similarly, there should be a regular replacement of the whole test-liquid after some experimental rounds, to guarantee accurate results.
Secondly, the fluid-distribution chamber must be well-meshed and also kept air-tight to ensure the outflowing liquid film is uniform. This can be done by manually siphoning air out of the distribution box before each experiment. The use of micrometer-steps at the film inlet is also advised to set the gap-height at the film inlet to the exact film thickness predicted by the Nusselt estimate of the film flow at the corresponding Reynolds number. This will prevent a hydraulic jump or backflow at the inlet.
The operation of the solenoid valve must also always be checked and ascertained properly. This is because an appropriate pulsation of the flow is required to ensure the production of the forced waves. This could be checked from the regular clicking sound of the solenoid valve as well as a perceived pulsation along the connection pipes. The liquid flow rate into the syringe pump must also be set carefully to ensure the droplets are ejected in a dripping manner, avoiding any pre-acceleration before falling.
Appropriate calibration of the high-speed camera must be ensured to obtain very accurate results. The aperture size must also be carefully chosen considering parameters like the depth of field, exposure time and overall image brightness. For the camera triggering during video recording, users are also required to estimate how many frames should be recorded before triggering. This may vary with individuals, depending on the drop impact time, hence, several trial tests for practicing are recommended before actual measurements. Similarly, the light source must be properly arranged and well-diffused to minimize shadows in the image.
It is important to note and remember that the main focus of the study is the contributions of waves to the impact dynamics of the falling drops, hence the formation of regular wave structures is essential to an accurate study of the underlying physics. In scenarios where the wave structures are observed to quickly transition to three-dimensional structures, it is advised that the substrate inclination angle be reduced14,19 to facilitate a slower transition of the wave structures.
One limitation of the technique is observed in the absence of a measuring device specifying the actual instantaneous film thickness on each region of impact. This would have provided additional details on the overall observed phenomena.
In summary, the procedure outlined in this report can also be used to study simple wave evolution dynamics, while the high-speed imaging system described can be applied to many research fields with fast dynamics such as liquid drop break-up21,22/coalescence23, granular jets24, etc. where important phenomena are observed at a micro timescale.

Droplet impact is a very common phenomenon in nature and attracts attention from any curious observer1. It constitutes an active research area due to its numerous applications including spray-cooling, fire-suppression, inkjet-printing, spray-coating, deposition of solder bumps on printed circuit boards, the design of internal combustion engines, surface-cleaning, and cell-printing2. Its application extends also to agriculture, e.g., sprinkling irrigation and crop spraying3,4. Pioneering work dates back to the 19th century, with the work of Worthington5, while major advances have only been recently made due to the emergence of high-speed imaging6. Since then, several studies have been carried out; using different types of impact surfaces ranging from solids7,8, shallow,9 and deep liquid pools10,11 to thin films12,13.
However, despite the large volume of research on droplet impact on liquid surfaces (i.e., shallow and deep pools and quiescent films), impact on flowing thin liquid films has not received as much attention. In addition, hitherto, studies have neglected the contributions of spatial structures of waves to droplet impact dynamics.
In this report, we present a detailed experimental procedure to investigate the droplet impact process on flowing films whose dynamics are influenced by inlet-forcing of the liquid flow rate; below, we refer to them as &#39;controlled&#39; films. We find that these have numerous applications in multiphase industries (e.g., in cooling towers, in distillation columns, and also in the annular flow regime observed in two-phase flows), especially as film control has become an important step in the intensification of both heat and mass transfer in many process industries14. The interested reader is referred to our previous work15 for more details on the results of our research efforts on this.
This application of frequency oscillations of the inlet flow rate results in the formation of regular waves on the film surface. We focus on the solitary wave family, which is essentially characterized by widely-separated narrow peaks and is preceded by a series of front-running capillary waves16,17,18. We study the outcome of the impacts associated with the three main parts of the solitary wave structure: the &#39;flat film&#39;, &#39;wave hump&#39;, and front-running &#39;capillary wave&#39; regions. We also contrast these results with those associated with uncontrolled flowing films. Our results show that the stochastic nature of wave appearance on the uncontrolled film markedly affects the outcome of drop impact, with the separate regions of the controlled film also showing new mechanisms, which we have detailed both qualitatively and quantitatively.
In the previous paper15, using the same procedure, we studied the effect of film control on droplet impact dynamics in the splashing regime. The obtained results showed both quantitative and qualitative differences in the crown morphology (height, diameter, wall thickness, tilt angle, and direction) as well as the number and size distribution of the ejected secondary droplets.
In this report, we describe the designed set-up to understand the critical role played by these spatial structures in the droplet impact dynamics and also present succinct details of our findings not only in the splashing regime but also for other outcomes of droplet impact (viz. bouncing, sliding, partial/total coalescence). By following the standard protocol described below, the effect of film control on the droplet impact dynamics can be studied in a reproducible fashion.

<table>
	<tbody>
		<tr>
			<td>Function generator</td>
			<td>GW INSTEK</td>
			<td>AFG 2005 Series, Digital. Geo0852266</td>
			<td>Produces a varied type of wave signals, ranging from sine, square to saw-tooth wave at different frequencies (0.1 Hz - 5 MHz).</td>
		</tr>
		<tr>
			<td>Syringe pump</td>
			<td>Braintree Scientific Inc.</td>
			<td>Bs-8000 /225540</td>
			<td></td>
		</tr>
		<tr>
			<td>Solenoid valve</td>
			<td>SMC-VXD</td>
			<td>2142A. 
			0AE-5001</td>
			<td>Series-pilot-operated-two-port</td>
		</tr>
		<tr>
			<td>Relay</td>
			<td>Takamisara</td>
			<td>A5W-K. 
			154424C-03L</td>
			<td></td>
		</tr>
		<tr>
			<td>Electric pump</td>
			<td>Clarke SP</td>
			<td>SPE1200SS 1</td>
			<td></td>
		</tr>
		<tr>
			<td>Flow meter</td>
			<td>RS Component</td>
			<td>CYNERGY3 UF25B 14011600040110</td>
			<td>Measurement range: 0.2-25 L/min</td>
		</tr>
		<tr>
			<td>Micrometer step</td>
			<td>RS Component</td>
			<td>Micrometer Head</td>
			<td>0.01 mm/0 -13 mm</td>
		</tr>
		<tr>
			<td>High-speed camera</td>
			<td>Olympus</td>
			<td>I-SPEED 3.</td>
			<td>Capable of recording at up to 100, 000 frames per second.</td>
		</tr>
		<tr>
			<td>Light source</td>
			<td>TLC Electrical supplies</td>
			<td>IP54 -black</td>
			<td>Double enclosed halogen floodlight. Rating 500 W.</td>
		</tr>
		<tr>
			<td>Light diffusor</td>
			<td>OptiGraphix</td>
			<td>DFPMET</td>
			<td>250 &#956;m thickness</td>
		</tr>
		<tr>
			<td>Glass substrate</td>
			<td>Instrument Glasses Ltd</td>
			<td>Soda Lime Float Glass; 570 mm x 300 mm x 4 mm</td>
			<td>Flatness tolerance 0.02/0.04.</td>
		</tr>
		<tr>
			<td>Macro-lenses</td>
			<td>(a) Nikon 
			(b) Sigma</td>
			<td>(a) AF-Micro-Nikkor 60 mm f/2.8 D 
			(b) 105 mm f/2.8 Macro-Ex</td>
			<td></td>
		</tr>
		<tr>
			<td>Test-liquid</td>
			<td>De-ionized water from the Imperial College Analytical Lab.</td>
			<td>Standard solution 
			(AnalaR)</td>
			<td></td>
		</tr>
	</tbody>
</table>

film control to study contributions of waves to droplet impact dynamics on thin flowing liquid films

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous studies on flowing liquid films have neglected the contributions of spatial structures of waves to the impact outcome, while this has recently been shown to have a significant influence on the drop impact dynamics. In this report, we outline a step-by-step procedure to investigate the effect of periodic inlet forcing of a flowing liquid film leading to the production of spatiotemporally regular wave structures on drop impact dynamics. A function generator in connection with a solenoid valve is used to excite these spatiotemporally regular wave structures on the film surface while the impact dynamics of uniform-sized droplets are captured using a high-speed camera. Three distinct regions are then studied; viz. the capillary wave region preceding the large wave peak, the flat film region, and the wave hump region. The effects of important dimensionless quantities such as film Reynolds, drop Weber and Ohnesorge numbers parameterized by the film flow rate, drop speed, and drop size are also examined. Our results show interesting, hitherto undiscovered dynamics brought about by this application of film inlet forcing of the flowing film for both low and high inertia drops.

Watch this Scientific Journal Video about Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films at JoVE.com

Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films

A protocol to study the contributions of waves to droplet impact dynamics on flowing liquid films is presented.

Droplet impact is a very common phenomenon in nature and attracts attention due to its aesthetic fascination and wide-ranging applications. Previous ...

film-control-to-study-contributions-waves-to-droplet-impact-dynamics

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7.3K Views.  Imperial College London. A protocol to study the contributions of waves to droplet impact dynamics on flowing liquid films is presented.

Video: Film Control to Study Contributions of Waves to Droplet Impact Dynamics on Thin Flowing Liquid Films

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Nanomoulding of Functional Materials, a Versatile Complementary Pattern Replication Method to Nanoimprinting

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding combined with layer transfer enables the replication of arbitrary surface patterns from a master structure onto the functional material. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes. In particular we demonstrate the fabrication of patterned transparent zinc oxide electrodes for light trapping applications in solar cells.

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes.

We describe a nanomoulding technique which  allows low-cost nanoscale patterning of functional materials, materials stacks  and full devices. Nanomoulding ...

Fast Imaging Technique to Study Drop Impact Dynamics of Non-Newtonian Fluids

In the field of fluid mechanics, many dynamical processes not only occur over a very short time interval but also require high spatial resolution for detailed observation, scenarios that make it challenging to observe with conventional imaging systems. One of these is the drop impact of liquids, which usually happens within one tenth of millisecond. To tackle this challenge, a fast imaging technique is introduced that combines a high-speed camera (capable of up to one million frames per second) with a macro lens with long working distance to bring the spatial resolution of the image down to 10 &#181;m/pixel. The imaging technique enables precise measurement of relevant fluid dynamic quantities, such as the flow field, the spreading distance and the splashing speed, from analysis of the recorded video. To demonstrate the capabilities of this visualization system, the impact dynamics when droplets of non-Newtonian fluids impinge on a flat hard surface are characterized. Two situations are considered: for oxidized liquid metal droplets we focus on the spreading behavior, and for densely packed suspensions we determine the onset of splashing. More generally, the combination of high temporal and spatial imaging resolution introduced here offers advantages for studying fast dynamics across a wide range of microscale phenomena.

Drop impact of non-Newtonian fluids is a complex process since different physical parameters influence the dynamics over a very short time (less than one tenth of a millisecond). A fast imaging technique is introduced in order to characterize the impact behaviors of different non-Newtonian fluids.

In the field of fluid mechanics, many dynamical processes not only occur over a very short time interval but also require high spatial resolution for ...

Speciation and Bioavailability Measurements of Environmental Plutonium Using Diffusion in Thin Films

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can potentially contribute to the exposure of biota and humans. The diffusive gradients in thin films technique is introduced here for in-situ measurements of Pu bioavailability and speciation. A diffusion cell constructed for laboratory experiments with Pu and the newly developed protocol make it possible to simulate the environmental behavior of Pu in model solutions of various chemical compositions. Adjustment of the oxidation states to Pu(IV) and Pu(V) described in this protocol is essential in order to investigate the complex redox chemistry of plutonium in the environment. The calibration of this technique and the results obtained in the laboratory experiments enable to develop a specific DGT device for in-situ Pu measurements in freshwaters. Accelerator-based mass-spectrometry measurements of Pu accumulated by DGTs in a karst spring allowed determining the bioavailability of Pu in a mineral freshwater environment. Application of this protocol for Pu measurements using DGT devices has a large potential to improve our understanding of the speciation and the biological transfer of Pu in aquatic ecosystems.

The technique of diffusive gradients in thin films (DGT) is proposed for speciation studies of plutonium. This protocol describes diffusion experiments probing the behavior of Pu(IV) and Pu(V) in presence of organic matter. DGTs deployed in a karstic spring allow assessment of the bioavailability of Pu.

The biological uptake of plutonium (Pu) in aquatic ecosystems is of particular concern since it is an alpha-particle emitter with long half-life which can ...

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 ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...

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

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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 ...

Bulk and Thin Film Synthesis of Compositionally Variant Entropy-stabilized Oxides

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (Cu variant) entropy-stabilized oxides. Phase pure and chemically homogeneous (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (x = 0.20, 0.27, 0.33) and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O (x = 0.11, 0.27) ceramic pellets are synthesized and used in the deposition of ultra-high quality, phase pure, single crystalline thin films of the target stoichiometry. A detailed methodology for the deposition of smooth, chemically homogeneous, entropy-stabilized oxide thin films by pulsed laser deposition on (001)-oriented MgO substrates is described. The phase and crystallinity of bulk and thin film materials are confirmed using X-ray diffraction. Composition and chemical homogeneity are confirmed by X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The surface topography of thin films is measured with scanning probe microscopy. The synthesis of high quality, single crystalline, entropy-stabilized oxide thin films enables the study of interface, size, strain, and disorder effects on the properties in this new class of highly disordered oxide materials.

The synthesis of high quality bulk and thin film (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O and (Mg0.25(1-x)Co0.25(1-x)Ni0.25(1-x)CuxZn0.25(1-x))O entropy-stabilized oxides is presented.

Here, we present a procedure for the synthesis of bulk and thin film multicomponent (Mg0.25(1-x)CoxNi0.25(1-x)Cu0.25(1-x)Zn0.25(1-x))O (Co variant) and ...

Radio Frequency Magnetron Sputtering of GdBa2Cu3O7&#8722;&#948;/ La0.67Sr0.33MnO3 Quasi-bilayer Films on SrTiO3 (STO) Single-crystal Substrates

Here, we demonstrate a method of coating ferromagnetic La0.67Sr0.33MnO3 (LSMO) nanoparticles on (001) SrTiO3 (STO) single-crystal substrates by radio frequency (RF) magnetron sputtering. LSMO nanoparticles were deposited with diameters from 10 to 20 nm and heights between 20 and 50 nm. At the same time, (Gd) Ba2Cu3O7&#8722;&#948; ((Gd) BCO) films were fabricated on both undecorated and LSMO nanoparticle decorated STO substrates using RF magnetron sputtering. This report also describes the properties of GdBa2Cu3O7&#8722;&#948;/ La0.67Sr0.33MnO3 quasi-bilayer films structures (e.g., crystalline phase, morphology, chemical composition); magnetization, magneto-transport, and superconducting transport properties were also evaluated.

Radio Frequency Magnetron Sputtering of GdBa2Cu3O7ÃƒÆ’Ã‚Â¢Ãƒâ€¹Ã¢â‚¬Â 'ÃƒÆ’Ã…Â½ Â´/ La0.67Sr0.33MnO3 Quasi-bilayer Films on SrTiO3 STO Single-crystal Substrates

Here, we present a protocol to grow LSMO nanoparticles and (Gd) BCO films on (001) SrTiO3 (STO) single-crystal substrates by radio frequency (RF)-sputtering.

Here, we demonstrate a method of coating ferromagnetic La0.67Sr0.33MnO3 (LSMO) nanoparticles on (001) SrTiO3 (STO) single-crystal substrates by radio ...

Determining Tribocorrosion Rate and Wear-Corrosion Synergy of Bulk and Thin Film Aluminum Alloys

The increasing complexity and severity of service conditions in areas, such as aerospace and marine industries, nuclear systems, microelectronics, batteries, and biomedical devices, etc., impose great challenges on the reliable performance of alloys exposed to extreme conditions where mechanical and electrochemical attack co-exist. Finding ways for alloys to mitigate the combined attack of wear and corrosion (i.e., tribocorrosion) under such extreme conditions is thus highly critical for improving their reliability and service lifetime when used in such conditions. The challenge lies in the fact that wear and corrosion are not independent of each other, but rather work synergistically to accelerate the total material loss. Thus, a reliable method to evaluate the tribocorrosion resistance of metals and alloys is needed. Here, a protocol for measuring the tribocorrosion rate and wear-corrosion synergy of Al-based bulk and thin film samples in a corrosive environment under room temperature is presented.

Here, we present a protocol to measure the tribocorrosion rate and wear-corrosion synergy of thin film and bulk Al alloys in simulated sea water at room temperature.