Microplasma-synthesized ultra-small NiO nanocrystals, a ubiquitous hole transport material

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. We show the versatility of the synthesis process and present the superior material characteristics of the nanocrystals (NCs). The band diagram of the NiO NCs, obtained 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 test architectures, which incorporate silicon quantum dots (Si-QDs), nitrogen-doped carbon quantum dots (N-CQDs) and perovskite as absorber layers. Our results clearly show ideal band alignment which could lead to improved carrier extraction into the metal contacts for all three solar cells. In addition, in the case of perovskite solar cells, the NiO NC hole transport layer acted as a protective layer preventing the degradation of halide perovskites 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.


Synthesis of NiO nanocrystals
Atmospheric pressure direct current (DC) microplasma interacting with the liquid phase [1,2] has been employed to synthesize NiO NCs in ethanol. Supplementary Fig. S1a shows the schematic diagram of the hybrid plasma-liquid setup for synthesizing NiO NCs. 99.5% purity grade Ni foil of thickness 0.1 mm purchased from Good Fellow ltd.
was used as a source of Ni and as anode by immersing in 15 mL of ethanol (immersed area of the Ni foil was maintained to be 1.5 cm x 1.5 cm for each experiment). A Ni tube (0.7 mm internal diameter and 1 mm external diameter) was used as cathode for this synthesis. The distance between anode and cathode was kept constant at about 1.8 cm for all the synthesis in this report. Pure He gas flow at a flow rate of 50 standard cubic cm (sccm) through the Ni tube and initial voltage of 3 kV were used to create the plasma between the end of the nickel tubing and the surface of ethanol. The distance between the Ni tube and the surface of the liquid was kept at 2 mm throughout the experiment. The reaction was paused and the solution stirred every 10 min.
At the starting point a DC voltage of 3 kV was set and applied until the current reached 5 mA. As soon as the current reached the 5 mA, the current was maintained constant by gradually decreasing the voltage from 3 kV to 2 kV. A total processing time of 45 min was used for each synthesis of NiO NCs. The formation of NiO NCs started as soon as the microplasma was generated. The concentration of NiO NCs in ethanol can be varied by changing the processing time and current. Supplementary Fig. S1b shows the digital photographic image of the hybrid plasma liquid reactor with He plasma in action to synthesize NiO NCs.

TiO 2 electron transport layer
The TiO 2 compact layer was deposited onto indium-doped tin oxide (ITO) coated glass substrate using a sol-gel technique [2] . The sol of TiO 2 was prepared by mixing Titanium(IV) isopropoxide (1.56 mL), ethanol (18 mL) and triethanolamine (0.39 g) under constant stirring on hot plate at 40°C for 2 h. The solution was kept overnight and then was spin coated on the substrate at 5000 revolution per minute (rpm) for 30 s followed by annealing at 350°C for 4 h.
For making the TiO 2 mesoporous layer, TiO 2 Dyesol paste (18NR-T) and ethanol was mixed in the weight ratio of 1:4 and followed by ultra-sonication for 2 h. The ultrasonication helped to get stable well dispersed anatase TiO 2 NCs solution suitable for thin film deposition. The solution was then spin coated onto the TiO 2 compact layer coated ITO glass substrate at 2000 rpm for 60 s. After the spin coating the substrate was heated at 100°C for 10 min and then annealed at 400°C for 2 h to get a transparent sintered TiO 2 mesoporous layer.  The XPS spectrum of TiO 2 is shown in Fig. S3b which reveals two distinct peaks at 458.4 eV for the Ti 2p 1/2 and at 464.2 eV for the Ti 2p 3/2 , both in good agreement with the literature [3] . The TiO 2 layer shows good optical transmittance (Fig. S3d).

Perovskite absorber layer
Methylammonium lead iodide (CH 3 NH 3 PbI 3 ) perovskite powder with 99% purity (purchased from Xi'an p-OLED, China) was mixed in dimethylformamide (DMF) under stirring at 60 °C at 0.125 M (for CH 3 NH 3 PbI 3 1 M = 0.619.9 g/mL). The solution was then spray coated using an airbrush (Pro BD-132) with nitrogen gas flow at 1 bar in air onto the mesoporous TiO 2 layer and annealed for 40 min at 100 °C.

NiO nanocrystals hole transport layer
A continuous layer of NiO NCs was deposited onto the perovskite layer using an airbrush (Pro BD-132) spray coater. 1 mL of NiO-ethanol sol via nitrogen gas flow at 1 bar was spray coated onto the perovskite layer followed by annealing for 10 min at 120°C. During the spray coating of NiO-ethanol sol no degradation of the perovskite layer was observed as the contact time of ethanol with perovskite layer was negligible, before ethanol can damage the perovskite layer it evaporated at 120°C.

Gold (Au) metal contacts
Au metal contact was deposited onto the NiO NCs hole transport layer using a sputtering technique (Moorfield minilab DC/RF magnetron box sputter system). Argon plasma at constant current of 0.15 A for 60 min with a working pressure of 1.5 x 10 -2 mbar was used to sputter Au from a 99.99% pure Au target.

Silicon quantum dots layer
Thin films of Si-QDs have been deposited using a flat parallel electrode atmosphericpressure plasma system where silane (SiH 4 ) was used as precursor, being delivered directly into the reactor together with the other gases. The details of the synthesis process and material characterization have been reported elsewhere [4][5][6] .

Nitrogen doped Carbon Quantum dots layer
N-CQDs were synthesized using atmospheric pressure microplasma-liquid interactions.
The atmospheric pressure microplasma setup consists of a direct-current (DC) source, a carbon rod as the cathode and a hollow nickel tube as the anode. At first citric acid (1.051 g, 7.8 mmol of COOH groups) and ethylenediamine (556 μL, 15.6 mmol NH 2 groups) were mixed in 10 mL de-ionized (DI) water (1:2 molar ratio of COOH:NH 2 ) to make a solution suitable for the synthesis of N-CQDs. The carbon electrode was immersed in this solution while the nickel tube was brought close to the surface of the liquid and the gap was maintained within 2 mm throughout the experiment. The distance between anode and cathode was approximately 2 cm. Pure He gas flows at 60 sccm through the Ni tube and a current of 6 mA between the electrodes were used to create the microplasma. The distance between the nickel tubing and the surface of the water was adjusted to maintain a constant discharge voltage of 1.3 kV. The reactions were carried out for 30 minutes in total to give a yield of 1 mg/mL of N-CQDs in water. The detail synthesis process and material characterization have been reported in a separate communication [7] .

Materials characterization
Transmission electron microscopy (TEM) was carried out using a JEOL JEM-2100F equipped with a field emission electron gun operated at 200 kV. Atmospheric plasma synthesized NiO nanocrystals-ethanol colloids with no further purification was directly dropped on a holy carbon coated Cu grid of 300 meshes and evaporated overnight to prepare the samples for TEM study.
The chemical compositional analysis was performed by X-ray photoelectron spectroscopy (XPS) using an X-ray source (Al = 1486 eV) and the Kratos Axis Ultra DLD spectrometer. X-ray spot size was 400 µm 2 . The sample analysis chamber pressure was maintained at 10 -9 bar for all measurements. Current and voltage were 10 mA and 15 kV respectively during the measurements. The NiO nanocrystals in ethanol without further purification was spray coated onto Si (N-type, 100) substrate for XPS analysis. Specific region scans were performed at a resolution of 0.05 eV and pass energy of 20 eV. Calibration was performed using the C 1s peak located at 284.5 eV. Directly synthesized colloidal solutions of nanocrystals were transferred into quartz substrates for UV-VIS spectroscopy.
X-ray diffraction (XRD) measurements were performed with a Bruker diffractometer with a Cu K α line of 0.1541 nm.

Figure S10
Current density vs. voltage plot of the metal oxide-perovskite device under one sun condition (AM 1.5G) for different number of days exposed in ambient atmosphere (a) device with NiO hole transport layer and (b) device without NiO hole transport layer.