Microplasma-synthesized ultra-small NiO nanocrystals, an ubiquitous hole transport material for next generation solar cells
We report on a one-step hybrid atmospheric pressure plasma-liquid synthesis of ultra-small NiO nanocrystals (2 nm mean diameter), which exhibit strong quantum confinement and demonstrate their application as a hole transport material for solar cell devices. We show the versatility of the synthesis process and present the nanocrystals (NCs) superior material characteristics. The band diagram of the NiO NCs, measured experimentally, highlights ideal features for their implementation as a hole transport layer in a wide range of photovoltaic (PV) device architectures. As a proof of concept, we demonstrate the NiO NCs as a hole transport layer for three different PV device architectures, which incorporate silicon quantum dots (Si-QDs), nitrogen-doped carbon quantum dots (N-CQDs) and perovskite as absorber layers. Our results clearly show an improved stability and facilitation of the carrier extraction into metal contacts for all three solar cells. In addition, in the case of perovskite solar cells, the NiO NCs hole transport layer acted as a protective layer preventing the degradation of halide perovskite from ambient moisture with a stable performance for > 70 days. Our results also show unique characteristics that are highly suitable for future developments in all-inorganic 3rd generation solar cells (e.g. based on quantum dots) where quantum confinement can be used effectively to tune the band diagram to fit the energy level alignment requirements of different solar cell architectures.