Tailoring p-Type Charge-Transfer-Doped Hole Transport Layer for All-Inorganic CsPbBr3 Perovskite Solar Cells

Organic–inorganic hybrid perovskite solar cells (PSCs) have shown impressive photoelectric conversion efficiency (PCE) but suffer from inevitable degradation when exposed to air and light. Replacing the organic cations with inorganic Cs+ to yield all-inorganic CsPbX3 perovskites can improve the stability under environmental pressure. Among them, cesium lead bromide (CsPbBr3) perovskite shows great environmental tolerance under illumination, humidity, heat, and oxygen attack. However, the energy gap between the carbon electrode and CsPbBr3 limits the carrier separation–transfer rate. Herein, p-type charge-transfer-doped NiO nanocrystals are employed as hole transport materials in all-inorganic CsPbBr3 PSCs with a fluorine-doped tin oxide (FTO)/SnO2/CsPbBr3/NiO-l-Cys/carbon structure. This combination reduces the charge-carrier recombination by decreasing the hole transport potential energy-level barrier between the perovskite and the hole transport layer. The coordination of the energy structure associated with the interfacial charge extraction–transfer action leads to a remarkable enhancement in PCE (9.61%) and a much higher open-circuit voltage (1.614 V), surpassing those achieved by hole-free devices (7.35% and 1.504 V, respectively). The PSC device assembled with NiO nanocrystals displays good stability under high humidity and temperature for 30 days. The greatly improved PCE, coupled with good stability, demonstrates the great potential of l-cysteine-doped NiO for future use as a hole-transporting material in all-inorganic PSCs.