As I wrote to you earlier, please try to heat up the solution and also the substrates (at around 70 0C). Make everything quickly (transfer the hot substrates quickly from hot plate onto the spin coater and then spin the hot solution dynamically- i mean during spinning, but very fast, after 2-3 seconds the spinning starts!). A nice trick also is, after spin-coating, to anneal your films, not on a simple hotplate, but in an oven (a conventional oven, not a vacuum one). 

dissolve first MAI in DMF, very easy (just shake a little bit by hand). Then put the above solution into a vial containing the correct mg of PbCl2. Then leave it on a GOOD shaker for one day. You will end up with a crystal clear, yellow, shiny solution.

Dear Shaik Shoieb,

I will send you a letter with detail data on dissolving the lead salts at the end of this week. I have done a large number of experiments with Pb salts and I have got an experience.

Eugeniusz Małkowski

Dear Shaik Shoieb,

I am sorry for delay, but I was swamped with work even at weekend. I will definitely write you the answer to your question this week.

Best wishes

Gienek

Shaik, I think its because of the PbCl2 purity (99.999 %), you are facing solubility issues with murky solution.

Thomas: Thanks for your input, I left the solution on the stir plate for one day, still PbCl2 is not fully dissolved.

Eugeniusz: Thanks for taking out time to reply to my question, please take your time.

Sai:  Almost everyone who has reported the purity of PbCl2 is of high purity. Do you think a slightly lower purity PbCl2 will dissolve thoroughly? 

Shaik, last night, I prepared a precursor blend solution as you have suggested but we used 98% PbCl2 instead 99.999. I left the solution on stirrer without heat at moderate speed, but still noticed a foggy solution. Is that fine? Also pls suggest some spin speed for perovskite film fabrication .. 

Sai: We were successfully able to dissolve the PbCl2 and MAI in DMF, the solution was a very transparent solution as reported everywhere. I followed Thomas's suggestion:

1) Add MAI to DMF solvent, shook it for about 3-4 minutes.

2) Left the vial on the stir plate for about 10-15 minutes.

3) Added PbCl2 and placed it back on  the stir plate and left it overnight with no heat.

Hope this solves your problem.

Regarding the spin speeds for perovskite layer, there is no concrete/ strictly followed number as it might change from batch to batch and how you accelerate the spin speed plays a role too. Generally speaking, anywhere between 2000-3000 rpm is the worked out number. I highly recommend you to have the thickness optimized for your experiments.

Good luck!  

Shaik, thanks for the detailed reply. Very useful points. Could you please inform the exact quantity of MAI and PbCl2 in mg to dissolve in DMF. Also, we dont have sophisticated glove box in our lab. So, we will fabricate perovskite solar cells in open-air ambience (humidity < 40% max). 

If you follow the standard receipe, it is super easy to dissolve your perovskte solution. If I remember well (since I do not use frequently PbCl2), the purity we are using is 98%; this could help the solubility as well (better than the 99.9++% purity). As for the concentration, it is between 30-40wt% (always MAI: PbCl2= 3:1 molar ratio, very easy to calculate the adequate amount of each substance per ml of DMF). If still it doesnt work (no idea why),  try to add some PbI2 or some water in. Regarding your cells in open-air, I advise you (first) to be very careful with poisonous lead (so try to do everything in a dry-like box). Other than that, you can very easily make highly efficient solar cells at 40% humidity. I would suggest to use a porous scaffold in order to minimize the risk of poor coverage. 

Dear Sir, thanks so much for the detailed response.  Do we have to use filters such as 0.45 um/0.2 um PTFE before coating? Also, is it necessary to heat up the precursor (MAI:PBCl2 solution) before drop coating?

nothing IS NECESSARY. I never filter my perovskite solution (since  I get a bright yellow, very clear, solution). Others do, I do not see too much importance on that. People are filtering, in general, when making thin films in order to improve smoothness and avoid aggregates on the surface. We never heat up the solution before spin-caoting in the glovebox, but under air, you might find useful to have hot solution (and/or hot substrate) to improve coverage- sometimes, if you do not do that, your film is crap.  

try not to heat the solution when dissolve the components. just let it stirred for several hours.

Thank you Dr. Thomas and Yuan for your valuable inputs :)

Yesterday i tried fabricating perovskite film (1:3 PbCl2 :MAI in DMF) and annealed at 90C for 2 hrs in open air hot plate  with himidity of 20 to 25%. Even after annealing for 2 hrs, the film is not fully brown in color.Any suggestions please.

 I am attaching photograph of those films. Top : Precursor solution coated with out filter and bottom one coated with 0.45um PTFE filter.. Should i consider annealing perovksite films inside vacuum oven? Request your opinion.

As I wrote to you earlier, please try to heat up the solution and also the substrates (at around 70 0C). Make everything quickly (transfer the hot substrates quickly from hot plate onto the spin coater and then spin the hot solution dynamically- i mean during spinning, but very fast, after 2-3 seconds the spinning starts!). A nice trick also is, after spin-coating, to anneal your films, not on a simple hotplate, but in an oven (a conventional oven, not a vacuum one). 

Thank you for the kind response. I will try preheating both the substrate and the solution before coating PbI2. We usually drop a solution on the static substrate and start spin coating. Do I have to start spinning first and coat the solution.?. Could you please suggest suitable spin speed (thickness).

yes this is what I mean by dynamically: Place the hot substrate from hotplate on the spin-coater quickly, then start spinning quickly and then spread your solution again quickly! We are using 2000 rpm but everything close to 2000-4000 rpm (depending on the type of spin-coater) for 30-45 s should work.

Thank you sir. I will try those spin speed conditions in our lab environment. 

I forgot to mention, I followed Dr. Thomas and Shaik's procedure and able to prepare a clear shiny yellow color solution :) .. 

Yesterday I tried fabricating PbCl2+MAI one step perovskite (on a non patterned glass ITO substrate), the film formation is way better than last time. However, 100 % fully covered perovskite could not be formed. I followed Dr. Thomas suggestions (Preheated both the substrate/solution and spin coated dynamically). Films were annealed at open air atmosphere with RH of < 25%. In the picture, top one is precursor solution spin coated at 3000 rpm (I could see some crystals coming on the top) and bottom one at 4000 rpm (seems to be smooth). 

VERY SHORT ANSWER: dimethylacetamide (DMAc) under heat vigorous stirring for a long while taking the relevant / appropritate safety precautions as always. I did this in ambient conditions.

http://pubs.rsc.org/en/Content/ArticleLanding/2015/RA/C4RA16253F#!divAbstract 

SHORT ANSWER:

I also had sediment problems in DMF and tried the DMF/DMSO mixture in ambient conditions while not exactly the same as some papers it was DMF 10ml and DMSO 1ml. Still got sediment.

Ambient environement... materials kept in a safe place in ambient conditions.

Ratio - 1 molar PbX2: 1 Molar MAI or FAI etc. in a vial/beaker.

DMF or DMac added and compared after heating and stirring.

I am doing this in the ambient environment taking the regular precautions. I am not making substrates or solutions in a glove box, at least not yet. Average humidity of Athens is 60% too and this varies up and down depending on the weather.

I had the same problem since before the summer and almost gave up ... BUT Thank God 3 to 4 days ago I came across this paper  'A promising alternative solvent of perovskite to induce rapid crystallization for high-efficiency photovoltaic devices'.

http://pubs.rsc.org/en/Content/ArticleLanding/2015/RA/C4RA16253F#!divAbstract

USE DMAc and heat up to the regular heating you use I think (60-80 degrees C) and stir vigorously for at least a few hours... may need at least 6 not sure. Make sure you are TAKING THE SAFETY PRECAUTIONS AND NOT NEAR SOURCES OF IGNITION and take the SAFETY MEASURES REQUIRED. 

My attempt was:

1) A few hours stirring without heat - after left to settle and saw sediment in both DMF and DMAc.

2) Most of the day at least 4-5 hours with heat and stirring- Both still had sediment although the DMAc appeared to have less although not sure if was the same as the previous day.

3) Most of the Day heat and stirring vigorously. DMAc seemed to have dissolved while stirring and cloudiness was possibly attributed to air bubbles so left to settle. DMF AGAIN SIMILAR AS BEFORE

4) After a few hours of settling and cooling DMAc solution seems to be clear DMF has Sediment.

5) Left overnight to see if it is going to precipitate any material but the DMAc seems clear so far and this has been since yesterday evening this week.

CONTINUED AND LONG ANSWER:

In the past I did not find this.... whether it was because I used a different ratio and it did not happen not sure....I can't also remember if back whether I did do a 1:1 ratio. BUT THIS PROBLEM has started to arise in the last few months from before in the SUMMER though when I wanted to start making a new set of cells. I almost gave up on it but PERSISTENCE HELPS. But it might EAT AWAY at your other tasks that you need to do in your PhD/Research targets SO TRY AND BE ORGANISED.

When the problem started occurring always got SEDIMENT IN THE VIAL OR BEAKER even though I followed the how the paper did it or maybe slightly varied.... I added DMSO to the mix to see if it helped.... I even added a bit of water... tried GBL on top of all of that which created an interesting white yellowy liquidy goupy gel but that is not what I wanted and had to safely dispose of it. For those of you who want to investigate out of curiosity what that goupy gel is be very careful as it contains lead and toxic solvents, I was very curious like a child is but I don't have the time and can't , probably someone has already studied it too... and there are SAFETY measures we all need to try and follow.

Filtering any sediments in the yellowy clear solution of my DMF mixture would alter the concentration that would be on the substrate so that would not be useful for me ensuring my repeatability.

So seeing the paper above.... or below or here :

http://pubs.rsc.org/en/Content/ArticleLanding/2015/RA/C4RA16253F#!divAbstract

I added the crystals of PbI2 and FAI in the same beaker at a 1:1 molar ratio and then added 1ml of DMAc. Did this again but the other mixture had 1ml DMF instead.

The DMF did not dissolve the perovskite fully and has a sediment in the bottom whereas the DMAc seems to do this. See the paper and my images too in copying them with a different material since I used FAI instead of MAI.

Below is a description for those who have time to read that you although you STILL need to STIR VIGOROUSLY AND UNDER HEAT FOR A LONG TIME.

DMAc films gave 15% efficiencies... in the method they used in a nitrogen glove box and measuring in an ambient atmosphere.

Who has been finding it difficult to mix a 1:1 ratio molar concentration in 1ml of solution of Pbx2 and FAI..... I did until I noticed the above paper. I put the powders in the same vial and add the solvent... may not be the best way but that is how it worked before and it was fine until I saw this sediment issue.

PbI2 powder container has had a lot of contact with ambient humidity and is several months old maybe over a year. 

FAI Kept in stored in an ambient environment was made a few months ago by people in the team... I believe this was very moisture saturated even though there was para-film around the container I used.. As I put it in the micro-scale to weigh them... appeared like the crystals you see in salt for fries/chips or the food which is old and saturated with moisture such as salt which does not come out of the bottle when you are trying to add it to your chips or food etc. (SALT NaCl food additive table salt or sea salt) which has had moisture stick them together ... in the same way as I removed the FAI crystals with a small spatula I saw the same effect.

The PbI2 I used is stored in an ambient environment and is in a still in powder form and seems visually unaffected by such contact with moisture.

I have been trying to do this 1 molar ratio of the two in 1ml of solution too and had the same problem with 0.001 Moles of PbI2 and 0.001 Moles of FAI or MAI. Every time I used DMF I got this white sediment. 

Others whom I know had no issue when they did this in the argon glove box so I had a feeling it either was the age of the DMF I was using which may be too fresh although I think they had the same aged DMF as me... or the ambient environment since an inorganic expert informed me old DMF is better due to it decomposing to have more amines in it. The bottle I had been using was 98% concentration about 3 litres and not older than a few months.

I used an old bottle of DMAc though so this may also be a factor which increased its solubility if this is also true for DMAc. 

I would suggest to everyone to check out the paper which uses DMAc instead of DMF. 

check out the difference between DMF and DMAc in the attached images.

The clear solution has approximately DMAc 1mL with a 1:1 molar ratio of FAI and PbI2 whereas the DMF has the same but you see sediment

I put in PbI2 and FAI in the same vial first and then added the magnet and solvent and then closed the vials and started stirring. Make sure safety is measures are taken and that this can be done in a fume hood. The DMF is from a relatively new bottle a few months old while the DMAc bottle I used seemed to be older.

But Note:

DMAc required heating and vigorous stirring for a long time

PbI2 stored in ambient conditions and is not fresh so has acquired humidity externally seems unaffected by this humidity.

FAI looked like sticky salt when I took it out.

This might help people you talk to about ithttp://pubs.rsc.org/en/Content/ArticleLanding/2015/RA/C4RA16253F#!divAbstract

I have heard that the older the DMF the better because over time it has more amines in it whereas fresh DMF is not as good. Moisture in the powders apparently help dissolve the powders we are working with.

Regarding the method though ... you need to stir it with heat for a long time quite vigorously. I don't think leaving it for an hour under heat will work... needs time but for sure is better than DMF alone. Let me know how you you all get on... I will see if I can repeat the same thing with a larger quantity of material so I can add more solvent for XRD analysis as you need a large amount of material to get a good XRD. It should be fine provided the concentration is the same. I don't think the quantity should make any difference whatsoever but I am there are experts who may know better.

I hope this helps everyone and thanks to the group who discovered this for us. Will post this on the other pages with similar questions ... hopefully will help them.

Regards to everyone

Harry

I just want to say... I checked the solutions and while the DMAc is better... it is a bit murky and so has not fully dissolved the solution as I thought.

during heating liquid (GBL/DMF) in a hot plate,what speed did you select?and during stirring the solution (like mixture of CSI,SnI2,GBL) what speed did you select?