Why are spin-coated polymer solar cells more efficient?
I've done spin coating for some other experiments - but in my opinion it's the gold standard probably because of the programmability and the variable thickness of coating that can be achieved. In spin coating the spin surface can be made to rotate at stepped speeds. So intially it can spin at slow speeds (400–600rpm, this is when you pipet out the solution you want to coat) and then the speed can be stepped up in stages upto around 2000–3000rpm. This kind of stepped spinning (and the spinning speed) gives a very uniform coating after the solvent dries off. But yeah, the uniformity also depends on the particle size, the concentration of the solution etc. Apart from all that the thickness can go into micrometer scales (verified this with SPR once). To get higher thickness you just have to change the spin speed - so the range of thickness you can achieve with the same concentration of solution cannot be matched. So yeah - I guess that's why it's preferred.Horizontal dipping is like an ancient method in my opinion. In spin coating the pipetting part can be automated to let out exact volume of the solution and the batch to batch variation would be less. With dip coating though, we can't be sure that the coating thickness will be uniform across batches - and there's a high chance to get surface defects. So QC/QA people hate this method possibly. But I would say horizontal dipping is nice and simple when you do it in the laboratory scale. But going into the industry with that is - meh.