Why can't a hot-air balloon go any higher then the troposphere

In principal, yes.

Explanation: If you heat up the air so it is warmer than the surrounding air, then you will have a buoyancy force. But we almost always use helium or hydrogen when going up to the stratosphere. Using light gases, you get a much larger buoyancy force for the same volume than you can get with hot air. Hope this helps.

Because of the heat in the air balloon so the more you go up the colder it gets and then the air balloon will just die.

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