Dye Sensitized Solar Cell- TiO2 not sticking to glass

I haven’t forgotten about this thread. I’ve just been busy obtaining and synthesizing the different materials that I need to
proceed.

Vulture’s and watson.fawke’s advice helped get me going in the right direction with this. It turned out that I
was not only using the wrong crystalline form of TiO2, but it was also non-porous. The problem is that this rutile form of
TiO2 is what’s commonly available from pottery supply stores, at least in my area, and I didn’t want to try tracking down an
available source of nano-crystalline anatase. To that end I decided to find a way to use the common everyday TiO2 as a
starting material, since it is both cheap and available.

I appreciate several old posts by plante1999 and blogfast25; these helped me figure out how to synthesize a
soluble form of Ti(IV)+.

These following two references were indispensable in guiding my efforts for the present experiment:

http://link.springer.com/content/pdf/10.1163%2F1568567077792...

http://pubs.acs.org/doi/pdf/10.1021/jp962702%2B

Previously I had attempted to form an adherent titania layer on a glass slide using pottery grade titania. This was mixed with a
small amount of 10% polyvinyl alcohol, and 5% acetic acid. After firing the material between 400-500°C for an hour, and
soaking the resulting titania layer in blackberry juice for an hour, this is what resulted:



If you squint your eyes just right, and use your imagination, you can see a slight pink color from the dye. There was almost no
absorbtion of the dye, and parts of the film came off as soon as it became wet.

The nano-crystalline anatase form of TiO2 is needed, because that is the form that is photo-reactive. Briefly, in order to
obtain this material from coarse rutile TiO2, several steps are needed. First, the TiO2 will need to be dissolved into
something in which it is soluble. Next, an amorphous TiO2 will need to be precipitated back out. Finally, the amorphous
powder needs to be calcined at the proper temperature to allow it to form anatase nano-crystals.

27.5g of NaHSO4.H2O was ground to a fine powder in a mortar, and then shaken together with 6.75g of pottery-grade TiO2 in a
glass jar. After the mixture was thoroughly shaken, it was ground together in a mortar to encourage good mixing. Finally,
the resulting powder was returned to the glass jar. 2.0g of this mixture was then added to a test tube, and heated with an
additional 1.6g of NaHSO4.H2O. As heating was commenced water vapor was boiled away; the mixture turned yellow, and
then orange. Finally the mixture became molten, turning completely transparent as the TiO2 dissolved into the orange
melt. About this point dense clouds of acidic gasses were emitted: be sure to do this in a fume hood. Once the molten
bath becomes tranparent the TiO2 is fully dissolved, and everything can be allowed to cool down. I chose to cool the
tube on its side, to allow easier removal of the solidified salts afterward. Once cooled, the resulting salts were then stirred at
25°C for 45 minutes in 36mL of DI water. Practically everything dissolved. A pH of 1-2 was noted when the solution
was tested with pH paper.

18mL of the resulting solution was then placed into a flask, and boiled on reflux at atmospheric pressure for 2 hours. White,
fine, amorphous powder was suspended as the TiOSO4 hydrolyzed. This powder was then washed several times and
centrifugated, until it was free of sulfate ion. The resulting wet, washed powder was then added to 0.1mL of 5% Na2CO3
and 0.2mL of 5% CH3COOH, and mixed thoroughly with a spatula.

The consistency of the titania at this point reminds me very much of skin lotion. It’s almost thixotrophic, and it has that
white, thick, shiny look of skin lotion. A thin layer of this was spread evenly out across a glass slide (Fisher 12-550-43) that
had been masked with tape. This glass slide had previously been rendered conductive with SnO2, as described in an earlier
post. After drying briefly in warm air, it was noted that this thin film of titania had become practically transparent; it was
very thin when it dried. The masking tape was removed, and then the TiO2 layer was annealed in a kiln from 450-500°C for 1
hour. At this point the kiln was shut down, and the kiln was allowed to cool down to 200°C. At this point the glass slide was
removed from the kiln, and cooled in ambient room air. While noticeably more opaque, the annealed film still retained most of
its transparency. When the film was allowed to soak in fresh blackberry juice, however, a change occurred. The liquid was
rocked back and forth across the TiO2 film, much like you would when making a silver mirror. For the first few minutes
not much change was observed. After this, however, the film started becoming very dark as it soaked up the anthocyanins
from the juice. The film was soaked in the blackberry juice for ten minutes, rinsed off in water and ethyl alcohol, and dried.
This was the result, using the same settings and white balance as the picture of the previous slide:



That is a really big difference from the previous attempt, for sure. Looking up close under a microscope, it was noted that
the film had shrunk upon being annealed, causing thousands of small micro-cracks. This may be obvious in some areas of the
film, as some of these little pieces popped off of the glass. In spite of this, the integrity of the film is much better than the
previous one, and it absorbs the dye very well.

Next I’ll try completing the solar cell, and seeing if it is responsive to light. I don’t expect it to work very well, as my
conductive SnO2 coating is rather poor, and has high resistance. I mainly want to make sure that the TiO2 film is
photo-reactive.


Edit:
Here is a picture of the completed cell:



The glass is the "negative side", and the small piece of graphite paper is the "positive" terminal. A very, very, small bit of KI +
I2 + glycol electrolyte is wicked in between the two electrodes. The cell stays together mostly by capillary action,
even when it is positioned vertically, and the small amount of pressure applied from the piece of copper wire. In front of a 15
watt fluorescent bulb it can deliver about 50mV. That's not impressive at all, but it's about what I expected. Short-circuit
current is about 0.1uA (yes, you read that correctly) due to the extremely high resistance in the cell. The effective shunt
resistance inside the cell is probably many times less than that presented by my electrodes. Also, the small cracks in the TiO2
film that I mentioned earlier probably allow a significant amount of the charges to recombine internally.

Basically, I need to do a better job of making the glass conductive, and I need a better collector than a piece of
graphite with a faint trace of soot on it. I don't think this graphite electrode is very catalytically active. The TiO2 layer is
sensitive to light, however, which indicates that the amorphous titania crystallized at least partly to the anatase form
under anealment. This is mostly what I was looking for.

One other thing that I noticed is that the cell voltage responds slowly to changes in light. It takes maybe ten seconds or so
for it to stabilize when the light intensity changes.


[Edited on 10-12-2013 by WGTR]

Quote: Originally posted by WGTR

18mL of the resulting solution was then placed into a flask, and boiled on reflux at atmospheric pressure for 2 hours. White, fine, amorphous powder was suspended as the TiOSO4 hydrolyzed. This powder was then washed several times and centrifugated, until it was free of sulfate ion. The resulting wet, washed powder was then added to 0.1mL of 5% Na2CO3 and 0.2mL of 5% CH3COOH, and mixed thoroughly with a spatula.

In this case, boiling very dilute solutions of TiOSO4 and H2SO4 give precipitates of powder that crystallize with less catalytic
activity after hydrothermal treatment. I don't claim to fully understand why, and I'm not even doing hydrothermal treatment
anyway. But this is in one of the references listed in my previous post. They actually recommended hydrolyzing a solution with
10% TiOSO4 and 1M H2SO4. The concentrations that I used are more a compromise of catalyst activity vs. ease of accomplishing
the experiment. The acidity is provided by excess NaHSO4 in this case, and Na+ is present in the solution. This could certainly
be improved upon.

I mentioned washing the powder to free the precipitate of sulfate ion; what I should have said is to free it from soluble
sulfate ion. Apparently some of it gets occluded in the precipitate, and this small amount actually encourages the amorphous
powder to give an active anatase form during hydrothermal treatment. I'm not sure what effect it has if the powder is being
annealed in a kiln.

The exact method of preparation of the initial precipitate has a large effect on the resulting product, as shown below:



I should also point out that I did not dehydrate the precipitate before applying it to the slide. Maybe doing this would help minimize
some of the cracking in the film.

Edit: I was not fully awake when I wrote the previous paragraph. Generally the precipitate is washed thoroughly, and then
dried in an oven at 120°C. Then afterwards, a hydrothermal treatment, or a deposition onto a glass slide with the amorphous powder.
That's what I've understood in the literature so far. What I meant is that I skipped this drying step after washing, and kept the
precipitate wet from its synthesis up until after it was applied to the slide. One thing that I can try differently is to dry the powder
and do a bulk annealment in the kiln, then mix up the crystallized powder with the acetic acid and Na2CO3 and then apply this to
the glass slide. It will still probably crack as it shrinks (artifact of the crystallization), but maybe it won't be as extensive.

[Edited on 10-12-2013 by WGTR]

Quote: Originally posted by WGTR

I should also point out that I did not dehydrate the precipitate before applying it to the slide. Maybe doing this would help minimize some of the cracking in the film. [Edited on 10-12-2013 by WGTR]

After reading through this paper:

http://essay.utwente.nl/62126/1/BSc_J_Sch%C3%A4ffer.pdf

I figured out a way to prevent cracking in the TiO2 film. This is the latest result, using the same slides as before (Fisher). This
time there is no conductive coating on the glass:



This picture was taken after the TiO2 layer had been dyed with blackberry juice for ten minutes. The pen markings that you
see are actually on the paper beneath the slide, showing how transparent the TiO2 coating is.

The leftover TiO2 gel that was used in the previous experiment was added to 11mL of DI water and 11mL of isopropyl alcohol.
3.5mL of glacial acetic acid was then added. This was then sonicated for about 30 minutes, after which 4.5g of 50%
polyvinyl alcohol was then added and stirred in. After the PVA dissolved fully, the solution was sonicated for another 30
minutes.

A small amount of this solution was applied to a glass slide that had been degreased and polished with zirconium silicate, and
this film was dried gently on a hot plate. Unfortunately, the film is a bit thicker around the edges, due to the masking tape
gathering up some of the solution.

The film was annealed at 500°C for 1 hour, and then cooled down to 200°C in the kiln. Then, the slide was removed and
left to cool in ambient air.

Anyway, the small imperfections in the film where there when the slide came out of the kiln. Some bits of brick dust fell on
the slide while it was in the kiln, and some of the thicker coating around the edges developed some cracks. Other than
that, the film is continuous and did not show the "mud cracking" that the previous experiment did. According to the
aforementioned literature, the high PVA concentration is vital to separate the individual particles of TiO2 from each other
while the firing process begins. This keeps them spaced out evenly on the glass slide, and helps prevent the particles from
agglomerating during annealment.

When the film was soaked in blackberry juice, rinsed in both water and ethyl alcohol, and dried with compressed air; it was
noted that none of the film had loosened or come off.

While the results look good with this, I need to make some changes. For one, I'm going to start with a fresh batch of
TiO2, as the batch of TiO2 that I was using this time had some of the Na2CO3 that I had added in the previous experiment.
Second, I need to duplicate this experiment on conductive glass. Thirdly, I may spin-coat the slide, in an attempt to get a
more even coating. Fourthly, I need to fire the TiO2 film for a longer period of time (4 hours) at 500°C, and then check it for
activity in the solar cell.

OK, after dialyzing sample B for a few hours, the pH of the sample was measured at 3, using cheap pH paper. At this
point I tried centrifuging the sample, and had no trouble getting the precipitate to settle out. Even washing and
centrifuging the sample for five times total, using room temperature DI water each time, 2000 rpm at 6 minutes was
sufficient to settle the precipitate. Overall, the washing provided about a 5000:1 dilution ratio. I'll have to retry this
whole thing again later to see if the initial problem with the precipitate not settling out was related to temperature or pH (or
both).

After washing the sample, it was diluted out to about 25mL. About 5mL of this was pipetted into a small glass vial, which
was then inserted carefully into this high-priced and handsomely constructed autoclave:



It was constructed of 1" x 3" galvanized pipe, with caps on either end. White PTFE tape was used to seal the threads.
Clamping one cap into a bench vise, I tightened the assembly together using a 15" crescent wrench. It took all the weight I
could put into it to get it to tighten together sufficiently.

A few mL of DI water had been added inside the metal tube, but outside of the sample vial, to help provide heat transfer. After
autoclaving the sample at 125°C for 4 hours in an oven, the oven was turned off and allowed to cool slowly. If it cools too
fast, the water inside the sample vial will boil out into the main tube assembly. Here's what the sample looked like after being
removed from the tube:



While being autoclaved, the cap was NOT on the vial. It would have melted; and besides, you need to allow the pressure to
equalize while it's being heated or cooled.

I measured the pH of this solution again after it was given the hydrothermal treatment, and it was about 1. This is a pretty
significant increase in acidity; but according to this reference this is to be expected. Due to the rather low pH here, I'll
probably go through some more washing and centrifuge cycles. Then, the sample can be hydrothermally treated for several
more hours if needed.

It was interesting (and encouraging) to note that after the hydrothermal treatment in the autoclave, the precipitate now
settles to the bottom on its own within about 10 minutes, even if it has been vigorously shaken up, leaving a water-clear
solution on top. The main reason that the TiO2 is treated hydrothermally, is to cause the amorphous powder to crystallize
into anatase form TiO2, and to help remove defects in the crystalline powder. If the amorphous powder is instead
crystallized the normal way, i.e. by dry calcination in a kiln for several hours, the powder becomes denser and has less surface
area (and lower activity). So the idea here is to minimize the amount of time that the powder needs to spend at high kiln
temperatures.



[Edited on 21-12-2013 by WGTR]

Thanks, blogfast25.

I don't yet know what kind of pressure it can handle. To be honest, I'm not sure that I want to find out through personal experience!
But where's my sense of adventure, right? The standard ratings on some of the galvanized pipe that I've looked at varied from
100 to 150 psi, assuming that water was being used. I could (and probably will) raise the temperature up to 150*C next time, which
should give me around 75-100psi in the tube. I don't plan on heating it at a higher temperature than that. Another issue is that
with increased pressure and temperature comes the increased likelihood of leaks. The pipe itself could probably handle several
hundred psi behind it, but the threads would probably leak long before that. It wouldn't be a lot of fun to come back after
12 hours to find that all of the water had boiled away from the tube.

Mission Creep...

Qualitative Test for TiO2 Activity

Out of the possible crystalline forms of TiO2, there are three relatively common ones: Anatase, rutile, and brookite. Out of
those three, the anatase form is the one that is photocatalytically active (band gap of 3.23V). In a dye-sensitized solar cell
(DSSC), it is the dye itself that responds to the light, so technically the TiO2 doesn't have to be photosensitive. However,
the anatase TiO2 also anneals into a structure that has more surface area and better conveys electrons from the dye to
the conductive glass that the other forms, so it is still the one most commonly used in DSSCs. Other than putting a solar
cell together and seeing if it works, it would sure be nice to have a method of characterizing the TiO2 itself, without having
to deal with all of the other variables in the solar cell (i.e., the electrolyte, counter electrode, dye, conductive glass quality,
etc.).

I've previously dealt with the issue of transforming generic TiO2 of any form (amorphous, rutile, etc.) into the anatase form.
But short of borrowing the use of some very expensive analytical equipment, how is it possible to know that you have
successfully made anatase TiO2?

In the following article, TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide, the
focus of the research was in making nanocomposite material. When reducing graphene oxide in solution, the single layers
tend to restack, giving low surface area. Since the idea of using reduced graphene oxide (rGO) is to take advantage of its
massive surface area, much research has been done exploring the different ways of accomplishing this. In this case anatase
TiO2 particles were used to space out the individual layers of rGO. The added bonus of the TiO2 in solution is that it can be
used to photocatalytically reduce the graphene oxide with alcohol.

Ethyl alcohol was used as a solvent in the original research, but I found that graphene oxide does not disperse well in it. It
does, however, disperse quite well in isopropyl alcohol, so that is what I used as a solvent/reductant. Some water came into
the reaction with the graphene oxide solution, but it was fairly minimal. A very small amount of the hydrothermally-treated
TiO2 (mentioned in the previous post) was added to the isopropanol/graphene oxide (GO) solution, and everything was
ultrasonicated for a few minutes to make sure that the GO and titania were fully dispersed. The headspace in the glass vial was
flushed thoroughly with nitrogen, to remove as much oxygen as possible. After this the closed vial was placed under a 100W
mercury vapor lamp, and illuminated for about 4 hours. The lamp had a UV-B bandpass filter on it, so all that could be seen
was a vague bluish glow. When the contents of the vial were inspected afterward, the pale-yellowish appearance of the GO
had changed to the dark appearance of rGO. It was interesting to note that since the solution was not stirred while it
was illuminated, most of the reduction took place at the surface of the solution, where the light was hitting it. In this way you
could see how far the UV light was penetrating the solution. I took the partially-reduced solution and shook it up, giving the
results shown below. In the following picture I have placed side-by-side two identical samples. The one on the right was
illuminated with UV-B for 4 hours, and the one on the left was kept in the dark. The illuminated sample was found to be
slightly warm after the experiment (30-35°C), but nowhere near hot enough to affect the results:





Based on this alone, I would feel confident that the photocatalytically-active anatase form of TiO2 was synthesized. To make
the results more quantitative, it would be necessary to measure out the exact concentration of GO in solution, weigh out the
titania, etc. The samples would have to be stirred, and illuminated in a duplicable manner. In this way the relative activity
between samples could be judged, by determining how fast the color change occurs.

If someone wants to try this without the mercury vapor lamp, just put the sample outside in the sun all day, and see what
happens. Ideally a quartz container should be used, but I think both soda/lime and borosilicate glasses are both optically
transparent enough to UV-B.

[Edited on 3-1-2014 by WGTR]

So you wanted some Indium Tin Oxide conductive glass…

You checked around, and although the prices have come down in recent years, you still noticed that 1” x 1” piece of coated
glass will set you back several bucks, plus shipping.

“Why is it so dang expensive? What a ripoff. Heck, I’ll betcha I could make it myself for 25¢. ”

So you decided to make your own. It can’t be that hard, right…?

This is where I found myself recently. Though I can afford to just buy the glass, the real fun in chemistry is actually making
things yourself in the lab. In my quest for a complete DIY solar cell solution, I’m going to try documenting how I worked out a
method of coating soda-lime silicate glass (SLS) with Indium Tin Oxide (ITO).

Before getting too carried away, an important issue to mention is that you will find an ITO layer on SLS only if the glass surface
has first been passivated with a barrier to sodium ions. This barrier could be a thin film of silica, for example. When SLS is
heated up, the sodium ions become very mobile. This is the reason that the experiment mentioned in this post works the
way it does; the sodium ions travel right through the glass.

So who cares if the sodium ions can move around? It wouldn’t be a problem, except that the migration of alkali ions into the
ITO layer causes the conductivity of the film to deteriorate rapidly. This was a problem that frustrated me to no end when
I tried making this conductive film for the first time.

This reference (Attachment: The Diffusion of Sodium Ions into Tin Oxide Thin Films from Glass Substrates.pdf.pdf (319kB)
This file has been downloaded 615 times) deals specifically with Sn/Sb oxide coated glass, and details the dramatic effect that
sodium ions have on the film conductivity. From page 55, a bleak prognosis is given:





In the search for a practical way (for the home experimenter) to solve this problem, I encountered US patent 2,455,719. I’ve
uploaded the pages of the patent below:





In brief, the patent describes a process wherein clay was used to leach sodium ions out of SLS at high temperatures. During
the firing schedule, sodium ions migrate from the glass, and into the clay.

In this process soda-lime glass was first coated with a slip of alkali-free kaolin clay, which was then fired at practical
temperatures ranging from 250-550°C for a specific period of time. During this time sodium ions diffused from a thin layer of
the glass and into the clay. After the firing schedule was completed, the clay was removed from the glass, along with a
portion of the sodium ions formerly in the glass. The claim was that a dense layer of silica-rich calcium silcate was left
behind, which served as a barrier to the further migration of sodium ions during subsequent processing of the glass (if
temperatures where kept below 500°C). Although the thickness of this dealkalized layer is very thin, it was claimed to
be sufficiently resistant to chemical attack.

This whole thing sounded too good to be true, like many patents, but I gave it a try. Using a mix of EPK (Edgar Plastic
Kaolin) in water, I thinly coated one-half of a soda-lime glass slide. The other half was left bare for comparison.



After drying, the slide was fired in the kiln at 500°C for four hours. After removal of the clay by intense scrubbing under
hot water, the slide was rinsed in DI water and acetone, and blown dry. Excess glass was also removed from one end
because of size constraints. The entire slide was then coated with 30-50µm of Al in a vacuum coater, and then placed into a
scanning electron microscope (SEM) for analysis.



Due to the nature of how an SEM works, the electron beam penetrates to a depth that is determined by the accelerating
voltage used, and the density of the material being analyzed. The accelerating voltage has to be high enough to “see” the
elements that are in the sample, but in this case it also has to be low enough so that it doesn’t penetrate too deeply. After
all, I was trying to look at the composition of a thin layer under the surface of the glass, not that of the bulk material. In some
ways the deposited layer of aluminum helps alleviate this problem, as it adds thickness to the top of the sample. I tried
a few different accelerating voltages, and decided to settle on 5kV. This provided the clearest example of the differences
between the treated and untreated glass surfaces. In total, the results of four samples are presented. The first two are
spectra from the untreated side of the SLS microscope slide. The last two are from the treated side. While aluminum
dominates the results (since it is ≈50µm thick), enough counts were collected to quantify the elements in the glass surface
beneath it. The two important elements in these results are sodium and silicon. While the absolute measurements obtained
are meaningless (due to the test conditions), the spectra obtained are relevant for relative comparison.

(Continued in next post)

SLS glass, left untreated:



Looking only at the atomic percentages of sodium and silicon in this case, sodium comprises 36.5% of the total in the first
sample, and 36.9% in the second.

SLS glass, treated with EPK:




From the last two spectra, taken under the same conditions as the ones from the untreated glass, we can see that sodium
now comprises 29.3% and 28.8% of the total Na + Si.

The difference between the treated and untreated surfaces may not seem to be very great after looking at the spectra, but
remember that the electron beam penetrates into the surface of the glass much further than we need. While the
concentration of sodium ions in the treated glass will taper off towards the surface, the xray spectra depicts a loose average
over the penetration depth.

In one of my next posts, I’m going to document some practical results that I have obtained in making ITO conductive glass
films, using glass that has been treated with EPK.

ITO Coated Glass

As discussed previously, soda lime silicate (SLS ) glass needs to have a barrier layer on its surface to prevent sodium ions from
migrating into the ITO layer, which would destroy its conductivity. In this post the procedure for removing sodium from the SLS
glass is discussed, using boring pictures and bad humor.

Lacking a proper dip coater, I exploited the property of “dual use”, and borrowed the lab’s drill press. Naturally, I unplugged
it first. I would suggest that you do the same.



100g of Edgar Plastic Kaolin was mixed with 190g of DI water, and this mixture was placed under vacuum to remove the many
air bubbles that were entrapped. An SLS glass microscope slide was dipped slowly over a period of 30 seconds, and withdrawn
steadily over a period of two minutes. If there are any imperfections, streaks, whatever; just dunk it back in and try again to get
an even coating. A thin coating is key; too thick and it will crack readily upon drying. After air drying, this is what you get:



Since the process changes the composition of the glass, the slide has to be coated evenly on both sides. If only one side of
coated, you end up with a piece of *(@#*&^ like this:



Notice how the glass “cupped” upward? The top layer of the glass has less sodium than the bulk of the glass, giving it a
considerably lower expansion coefficient. But I digress. Moving along…

The slide was then placed into a kiln, and fired at 550°C for one hour. After it cooled down I took some more pictures:



The fired coating is visibly lighter in color than the “green” coating that was shown before. It’s quite tenacious, but is
removed efficaciously with the straight edge of a sharp blade. I suggest soaking the slide in water first before trying
to remove the coating, and then scraping it gently under running warm water.

This dip coating, drying, firing, and cleaning process was repeated three times, for a total of three hours in the kiln at
550°C. For the final cleaning, be sure to remove all of the clay from the glass. This can be verified by drying the slide,
and carefully looking for residue under a good light. If you need to use a mild abrasive for complete cleaning, then use
a powder, not something rigid like sandpaper. The firing process leaves a surface that is not completely smooth, and
sandpaper won’t clean it very well. Personally, I used slurry of superfine zirconium silicate to rub it clean. It doesn’t take
much, and you don’t want to overdo the cleaning. The glass surface is very hard, but you could conceivably wear it off
if you have the hyperactivity of a determined chipmunk.

Aaaaannnd...what the heck. Here are a few extra pictures showing the setup with my little DIY kiln:



The little metal hook in the first picture is used to lower everything down into the chamber. The thermocouple gets
pushed in through the top after everything’s closed up. The piece of glass above the slide keeps brick dust from
falling onto the slide below.

[Edited on 1-20-2014 by WGTR]

ITO Coated Glass

There is more than one way to coat glass with indium/tin oxide (ITO). The way that is probably the easiest and most consistent
for the average person however, would probably be the dip coating method. In this post I’m going to describe how I coated SLS
glass (surface treated in the previous post) with ITO using the drill press as an impromptu dip coater.

The ITO coating solution was prepared from 0.90g of indium metal, 0.10g of 95%/5% Sn/Sb solder, 38% HCl, 100mL of
ethyl alcohol, and some 32% ammonia solution. Some people complain about how long it takes for HCl to react with tin. I
can guarantee that if you pound the snot out of your metal like this:



…It will react very fast; within about 15 minutes. I heated up a small evaporating dish on a hot plate, and placed the small bits
of metal inside. Small amounts of 38% HCl were added gradually, about 5-10mL in all. While the HCl was reacting, the
dish was covered with a watch glass to keep the solution from splattering about. If the reaction slowed down, the cover was
removed and the solution was allowed to evaporate down. HCl was added until all of the metal was gone, and then the solution
was evaporated almost to dryness. If the evaporation is carried too far insoluble oxides can form, and then more HCl will
need to be added, and carefully evaporated yet again.

After cooling down everything, the contents of the evaporating dish are washed into a glass beaker with absolute ethyl alcohol,
about 100mL in total. I found it necessary to neutralize the solution with ammonia. This hasn’t turned up in the references
that I found, but I found this necessary in order to keep each coating from attacking the previous one. If the indium/tin
chloride salts were bought commercially, it’s probable that neutralization would be unnecessary. In this case, all I needed
was about 0.2-0.3mL of 32% ammonia solution, but this will vary depending on the amount of HCl left in the solution.
Heavy stirring is needed to ensure that any precipitates at this point will re-dissolve.

The secret sauce in this indium/tin/alcohol solution is a few tens of milligrams of cetyl trimethyl ammonium bromide. Other
surfactants could be used, but I’ve verified that this one works very well. Try to avoid anything that contains sodium, like
sodium lauryl sulfate, etc. That would defeat the point of removing sodium from the glass in the first place. Without the
surfactant, a drop of the solution on a piece of glass would first spread out, and then shrink back as the alcohol evaporates,
leaving water droplets behind on the outer fringes. The solution may even de-wet the glass slightly as it dries, leaving
behind uneven blotches of indium/tin chlorides. About 10mg of the CTAB is added at each time, and then the droplet test is
repeated. Once enough has been added, small droplets no longer form around the outer fringes as the alcohol
evaporates. If too much surfactant is added, this shows up as excess residue on the glass. Either extreme needs to be
avoided, but a little too much probably won’t hurt.

With the indium/tin chloride solution prepared, the clean glass slide is dip coated in the same manner as it was previously with
the clay. The slide is slowly lowered into the solution with the aid of the drill press. At this point it is important to allow the
solution to settle. Keep vibrations to an absolute minimum. After holding the slide quietly in the solution for 60 seconds,
withdraw it at a rate of about 1” per minute. Your hands are steady, right? Keep the upward motion of the slide smooth,
without jerking the crank handle unsteadily. Any unsteadiness will show up as uneven deposition on the glass, although this
really isn’t the end of the world if it happens. The ITO film will still “work”.

Once the slide has been coated (it will be dry almost as soon as you pull it out), it is fired in a kiln at 500°C for two minutes. In
my particular situation, there was a 1 minute ramp up to 500°C, 2 minutes hold time at 500°C, and then quick removal from
the kiln after this. Avoid higher temperatures, because the sodium ions in the bulk of the glass can begin to migrate back
to the surface, causing havoc with the ITO film.

Now for the fun part… This dip coating and firing process needs to be repeated at least 10 times before the bulk resistivity of the
ITO film reaches its minimum. Also, the thinner the coating is, the lower its bulk resistivity. While thinner coats will measure
at higher resistance with an ohmmeter than thicker ones will, for a given resistance a series of thinner coats will be more
transparent than the thicker ones. This is due to the lower bulk resistivity inherent with thinner coats.

When the slide is coated with the first few coats of ITO, not much color change is noted other than a slight darkening of the
glass. With about 20 coats, the glass begins to take on a yellowish reflection in the light. With about 30 coats, a violet
coloration; and with about 40 coats, a violet to bluish reflection.

Once the glass has been coated with as many coats as you want, the hot glass is taken immediately from the kiln at 500°C,
and allowed to cool down in a reducing atmosphere. This may sound intimidating, but it’s really not. Here’s the special
ITO-glass-cooler-downer-doohickey:



It consists of a plastic garbage bag that has been carefully taped around the end of a short cardboard tube. The tube, of
course, has been clamped into a vise. The bag is filled with a few mL of ethyl alcohol, and then inflated with nitrogen gas (or
some other inert gas). The nitrogen not only vaporizes the alcohol, but serves to exclude oxygen. In operation, the hot
slide is placed directly into the tube (as can be seen), a glass dish held over the end of the tube, and then the bag is quickly
untwisted and squeezed, to purge the tube of oxygen. This process also carries alcohol vapors across the hot glass, which
reduces the ITO film slightly. If the glass is merely allowed to cool in ambient air, the ITO film will have a resistance that is
5-10 times higher than it will if it is cooled in this reducing atmosphere. A reference in a previous post directed the use of
a tube furnace at 600°C, with 0.1% hydrogen in argon flow for one hour, but my method works just as well, and is much
simpler.

If you don’t see this dramatic reduction in film resistance, the glass was probably allowed to cool off too much before it was
put into the tube, or there was too much oxygen in the bag. The ITO film has to cool down below about 300-400°C before it
can be safely removed to the room air, but I just leave it in there for a few minutes. This ensures that it doesn’t re-oxidize
as soon as I take it out.

This picture was taken after 40 coating/firing cycles, and a reduction:



It was tricky to capture the reflections just right. I had to drop the exposure way down to get a good image. The reflection of
the coating ranges from violet to blue when viewed at an angle. When observed straight through it is practically
transparent, with a slight yellow coloration.

After reduction, and with 40 coatings, I managed to get the sheet resistance down to about 60-70 ohms per square. Right
before the reduction, the sheet resistance was about 300-400 ohms per square.

Thanks bfesser. This has been quite a fun project.

When I get the chance I'll post a picture of the latest solar cell that I put together. In direct sunlight it's delivering about 0.375V
open circuit, and about 0.6mA short circuit current. Ideally I'd like to see 0.5V and 5mA/cm² readings respectively. If I can get
a couple of mA out of it, though, I'll probably just provide the documentation and stop there.

And yes, it is easier just to buy it! When I looked through this thread again, I figured that's the conclusion most people will
come to. It's certainly my conclusion.

[Edited on 1-21-2014 by WGTR]

Die Sensitized Solar Cell (DSSC) help

Quote: Originally posted by extacandpeacht1970

Wow, this topic is such inforvative! Good job and thanks for the useful info. It's just like reading some decent Englishessays review - lots of useful info from experienced people.