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DOI: 10.1002/aenm.201702722 Inorganic HTLs are low-cost materials with versatile optoelectronic properties, superior physical and chemical stabilities.[2e,7] Generally, they can be classified into binary, ternary compounds, etc., according to their element composition, and ternary oxides are substantially attractive and promising in PVSCs application due to their good conductivity and tunable optical and electrical properties. Recently, sol-gel synthesized CuyCrzO2 particles with size >100 nm as HTL in PVSCs have been reported, yielding a PCE of 17.19%.[8] CuGaO2 nanoplates with 50–75 nm in diameter and 10–15 nm in thickness were synthesized by the microwave-assisted method at 230 °C and the PVSCs based on 345 nm thickness CuGaO2 HTL have been achieved with a PCE of 18.51%.[9] Co1–yCuyOx thin film was demonstrated as HTL into PVSCs via high-cost magnetron cosputtering technique, displaying a relatively low PCE of 10%.[10] Considering that small nanoparticles (NPs) favor the formation of a high-quality film including well-packed (i.e., elimination of the voids and gaps) and functional morphology (i.e., smooth and compact film), inorganic NPs with diameters in a few nanometers fabricated by facile methods to ensure low-cost and goodquality film for highly efficient HTLs in PVSCs still demand. Spinel NiCo2O4, with Ni occupying octahedral sites and Co occupying both octahedral and tetrahedral sites, is a p-type semiconductor with a conductivity of 500 S cm−1, at least two orders of magnitudes higher than that of NiO and Co2O3. It has been applied in photodetector, oxygen reduction, Li-ion batteries, and electrocatalysis due to its outstanding merits like electrochemical activity, environmental friendliness, low-cost, and abundance.[12] Several synthesis methods of NiCo2O4, such as thermal decomposition, combustion, electrospinning, spray pyrolysis, sol-gel, hydrothermal, and pulsed laser deposition, have been exploited.[11a,12b–d,13] However, the synthesized NiCo2O4 featuring different microstructures or large particle size is unsuitable for forming films, let alone the complicated fabrication processes, high temperature, and even toxic chemical agents (such as NH4F). Furthermore, surfactants such as ethylene glycol (EG) are typically added to decrease the particle size (≈10–20 nm) and maintain monodispersion,[14] making the application of NiCo2O4 in PVSCs more difficult considering the degraded electrical properties resulting from surfactant ligands. The above issues This study proposes a novel strategy of controllable deamination of Co–NH3 complexes in a system containing Ni(OH)2 to synthesize ultrasmall ternary oxide nanoparticles (NPs), NiCo2O4. Through this approach, ultrasmall (5 nm on average) and well-dispersed NiCo2O4 NPs without exotic ligands are obtained, which enables the formation of uniform and pin-hole free films. The tightly covered NiCo2O4 films also facilitate the formation of large perovskite grains and thus reduce film defects. The results show that with the NiCo2O4 NPs as the hole transport layer (HTL), the perovskite solar cells reach a high power conversion efficiency (PCE) of 18.23% and a promising stability (maintained ≈90% PCE after 500 h light soaking). To the best of the author’s knowledge, it is the first time that spinel NiCo2O4 NPs have been applied as hole transport layer in perovskite solar cells successfully. This work not only demonstrates the potential applications of ternary oxide NiCo2O4 as HTLs in hybrid perovskite solar cells but also provides an insight into the design and synthesis of ultrasmall and ligand-free NPs HTLs to enable cost-effective photovoltaic devices.