Last december DOE released its critical materials strategy. Five rare earth metals (dysprosium, neodymium, terbium, europium and yttrium) and indium are ranked critical for clean technologies in terms of importance and supply risk.
Rare earths are not really that scarce, and in their case it is a combination of lack of concentrated ores and politics what limits their supply. The figure below shows the crustal abundance of the different naturally occurring elements, as taken from the Handbook of Chemistry and Physics. Other elements such as selenium or tellurium, key in thin film photovoltaics, are less abundant than rare earths.
