There are a lot of factors you're not considering. Reducing heat output is good, yes, but putting a layer between your chips and their heat sink is bad! Even though the total heat output is lower, the chip temperature will be higher, because it will be more insulated. There is no way around this; any heat-to-power device acts as insulation compared to a plain heat conductor. You will also have added weight and cost. In a phone, where the heat differences are small, the electricity gained will be almost nothing. So actually we're not likely to see this in phones.

This adds weight and volume that are bad for a mobile phone. Also, the size must be enough to process all the heat from the phone. In systems with a big heatsink you will need a bigger device between the heat source and the heatsink. This will not be a problem for phones, but it may be a problem for notebooks.

Also, with a difference of 30C (60F) they increased the maximum efficiency from 1% to 4%, but that temperature difference is probably too hot for a phone in your pocket.

It's probably better to have a smaller phone, or to use the additional space/weight in a bigger battery.