Highly crystalline colloidal nickel oxide hole transport layer for low-temperature processable perovskite solar cell

Highly crystalline NiOX usually requires high annealing temperature (>300 °C) which is incompatible with flexible substrate and might consume high amount of energy. Herein, we demonstrate a facile emulsion process to synthesize highly crystalline, low temperature deposition (<150 °C) and solution processable NiOx nanoparticles (NPs) as a hole transport layer for the perovskite solar cells (PVSCs). A novel surfactant of tetramethylammonium hydroxide (TMAOH) was used to react with Ni(NO3)2 to form Ni(OH)2 nanoparticles (NPs). The micelles of TMAOH act as a nano-reactor containing OH− anion. The Ni+ cation enters into the nano-reactor to form Ni(OH)2 NPs inside the reactor with controlled particle size. The Ni(OH)2 NPs prepared by emulsion process are further calcined to form NiOX NPs with the particle size of 8.28 ± 2.64 nm (EP-NiOX). The smaller size of EP-NiOX NPs results in a good dispersibility and an excellent stability of NPs suspension, which can be used to fabricate uniform NiOX film without any aggregates. A power conversion efficiency (PCE) of 18.85% can be achieved using this EP-NiOX film, as compared with 16.68% using the NiOX NPs synthesized from the chemical precipitation method (CPM-NiOX). Moreover, a flexible PVSCs with a PCE of 14.28% can be fabricated using the EP-NiOX film. Except for the device performance, the quality of the EP-NiOX film shows a good batch-to-batch uniformity, resulting in an excellent reproducibility of PVSCs. This work has a potential for the development of a large-scale production of PVSCs with a high energy conservation.