Search for radiating heat into space. There were some reports about 8-10 years back. Occasionally you will see people working on materials that have narrow emissions spectra, but I think they may have other things in mind, like combining them with photovoltaics to make solid state heat engines. Which has the potential to be much more efficient than thermocouples. Thermocouples are so far from their theoretical limits that boosting the conversion rate by single digit percentage points is more than doubling efficiency, which is how we ended up with a new thromocouple material on the front page of HN a month ago.