Continuously graded-doped SnO2 for efficient n–i–p perovskite solar cells - Di Wang; Saisai Li; Zijin Ding; Jian Xu; Xingyu Chen; Qiao Zheng; Thamraa Alshahrani; Siyu Liu; Tingwei He; Xinxin Yue; Saif M. H. Qaid; Keyu Wei; et al. (Nature, 2026)