Lead containing solar cells... how are these greener?

Perhaps you should reread the article as it specifically states:

Additionally, the cells contained 60 percent less lead than the lead-based, single-junction (non-tandem) perovskite solar cell holding the current record efficiency of 22.1 percent. The reduction in lead content and improved efficiency for a low-bandgap cell represent a significant step toward practical, more environmentally friendly perovskite tandem solar cells, the researchers say.