A small research in google :
"Temperatures rise with increasing altitude in the stratosphere. Ozone molecules in the stratosphere absorb ultraviolet radiation coming from the Sun. The energy from the UV radiation is transformed into heat. The heating is most intense near the top of the stratosphere, so that is where the stratosphere is warmest."
In short, the stratosphere is warmest at the top because it absorbs UV radiation and also more the altitude is high more it's warm.
Hope this helps !
Photon
