1. Solar cell efficiency tables (version 50)
2. Burger, B.; Kiefer, K.; Kost, C.; Nold, S.; Philipps, S.; Preu, R.; Rentsch, J.; Schlegl, T.; Stryi-Hipp, G.; Willeke, G.; Wirth, H.; Warmuth, W. Photovoltaics Report; Frauhofer ISE, Feb 26, 2018; https://www.ise.fraunhofer.de/content/dam/ise/de/documents/publications/studies/Photovoltaics-Report.pdf.
3. Ad-Hoc Working Group on Defining Critical Raw Materials. Report on Critical Raw Materials for the EU; European Commission, May 2014; http://www.catalysiscluster.eu/wp/wp-content/uploads/2015/05/2014_Critical-raw-materials-for-the-EU-2014.pdf.
4. Moss, R.
L.; Tzimas, E.; Willis, P.; Arendorf, J.; Thompson, P.; Chapman, A.; Morley, N.; Sims, E.; Bryson, R.; Pearson, J.; Tercero-Espinoza, L. Critical Metals in the Path Towards the Decarbonisation of the EU Energy Sector. Assessing Rare Metals As Supply-Chain Bottlenecks in Low-Carbon Energy Technologies. JRC Scientific and Policy Reports, EUR 25994; Joint Research Center, European Commission, 2013; https://setis.ec.europa.eu/sites/default/files/reports/JRC-report-Critical-Metals-Energy-Sector.pdf
5. The composition of the continental crust