Planar perovskite solar cells with long-term stability using ionic liquid additives by Bai, S, Da, P, Li, C, Wang, Z, Zhongcheng, Y, Fan, F, Maciej, K, Xianjie, L, Sakai, N, Wang, J, Huetter, S, Bucheler, S, Fahlman, M, Gao, F, Snaith, H