I don’t have any recommendations for solar fans, but I bought two of the Geek Aire 12" rechargeable battery fans this year:
https://www.amazon.com/Geek-Aire-Rec…/dp/B078PBRD68
The price has gone up on the 12" model by about $15 since I bought mine last month.
So far, I really like them a lot. They are sealed motor/weather-resistant, like barn fans are supposed to be. The 16" model actually shows horse barns in the advertising materials… at the time, I couldn’t justify paying that much more for a larger fan. Instead of traditional low/med/high settings, they have a dial. On the highest setting, they move a lot of air-- as much as an equivalent size shop fan. The battery only lasts a few hours on the highest settings, though. On the lower settings, the battery will last all day and then some. I hang them in the morning, then charge them in the evening. It takes maybe a couple hours at most to completely charge them from 0. You can also just used them plugged in like a normal fan.
I have electricity in my barn, but I only have one outlet and it’s poorly placed for stall fans. Last year, I had extension cord city going on, which made me super nervous. These have been a welcome alternative! I am curious to see how long it takes before the batteries start to weaken.