From what I can remember (I'm basing this of my days of liquid cooling & overclocking on gaming PC systems... so I'm not sure how much of this will actually carry over) using the peltier to cool something is only one half of the equation. While you can use the "cool" side on your phone, you may also need something on the hot side to pull excess heat away. The basic setups for CPU's was to have the peltier on the chip, and the waterblock attached to the hot side of the peltier. With that setup you could theoretically go below room temp.

My best guess in your application, since you said that blasting the AC at it cools it down; is that you MAY be able to find a peltier that has a high enough power rating (in a small enough package) that would fit the bill....but I can't say for sure. Also, since you need a thermal paste between the peltier and back of the phone to effeciently transfer heat, it could make for messy handling of the assembly.

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