Dopant-free hole-transporting materials featuring intramolecular π-π interactions for efficient and stable perovskite solar cells - Xiaofan Gai; Huayu Bao; Cancan Gu; Zhenhu Zhang; Jianye Li; Xiaohui Cao; Shirong Wang; Xianggao Li; Guohui Yin (Chemical Engineering Journal, 2023)