At the n where the PFs are maximised, the κ el are < 1 W m -1 K -1 , whereas at the largest n = 10 22 cm -3 they reach relatively large values of 1-5 W m -1 K -1 . Given that the κ el appears in the denominator of Eq. 1, this reinforces the finding that large n are detrimental to TE performance.