Correction to: Nature Energy https://doi.org/10.1038/s41560-017-0016-9 (2017); published online 23 October 2017

In the version of this Review originally published, the Fig. 7a y-axis unit was incorrectly given as ‘mW cm2’; it should have read ‘mW cm–2’. This has now been corrected in all versions of the Review.