Electroluminescent refrigerators

Solid-state optical refrigeration, which is featured by carrying the internal thermal energy away through light emission, has progressed rapidly after the first demonstration of photoluminescent (PL) refrigeration in 1995 (Epstein et al., 1995). To date, it has been shown that an Yb-doped LiYF₄ cooler can reach a temperature of 114 K and give a cooling power of 750 mW at room temperature (Melgaard et al., 2014). Further improving this kind of rare-earth-doped coolers to an even lower cooling temperature will be a difficult task. This is because the cooling process involves the thermalization of dopants and becomes inefficient when the thermal energy is comparable to the energy difference between discrete dopant levels in the ground-state manifold (Sheik-Bahae and Epstein, 2007). Different to the doped coolers, a semiconductor luminescent refrigerator is not restricted by such limitation and is expected to have lower operation temperature and higher cooling power. Moreover, it can be directly integrated with other semiconductor devices. These attractive features have stimulated extensive research on semiconductor luminescent refrigeration (Sheik-Bahae and Epstein, 2007, 2009). However, the required external efficiency for cooling is as high as nearly unity such that the realization of semiconductor PL refrigeration was not achieved until recently by Zhang et al. (2013) on CdS mircorod.