Representante de la Masa de Obligacionistas

5.1.1 Thermal chemical vapour deposition of bismuth oxyiodide

BiOI was grown in a two-zone tube furnace, depicted in Figure 5.1. A mass flow controller was used to independently control the flow rates of oxygen and argon, which were fed into the furnace. BiI3, which was used to supply the bismuth and iodine, was loaded into a crucible and placed into

a quartz furnace tube. Substrates were aligned within the furnace tube 2 – 5 cm from the BiI3

crucible. The furnace was heated, causing BiI3 to sublime and react with oxygen gas, forming

BiOI thin films on the substrates at temperatures between 325 – 500 ºC.

Figure 5.1 Schematic of the thermal chemical vapour deposition set up for fabrication of BiOI

films from bismuth iodide and oxygen at 325 – 500 ºC. The oxygen gas is supplied through a gas flow mixer, where it is mixed with argon carrier gas, whilst bismuth iodide powder is loaded into a crucible into a furnace. On heating the bismuth iodide sublimes, depicted by grey circles. The red-brown circles represent the gaseous intermediates from reaction between bismuth iodide and oxygen and the red lines represent bismuth oxyiodide thin films.

For BiOI films grown for chapter 6, the two-zone furnace was heated to 360 °C in the left-hand zone and 350 °C in the right-hand zone. Note that the crucible and substrates were all contained within the left-hand zone of the furnace, where < 3 ºC variation in surface temperature of substrates located up to 10 cm from the crucible was expected based on furnace temperature profiling. Thus, the temperature gradient was used only to aid gas flow. Substrates were cut to roughly 12 x 12 mm using a diamond scribe and were adhered to a microscope slide using silver paint. 500 mg BiI3 powder (Alfa Aesar, Puratronic, 99.999% metals basis) was loaded into a

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ceramic crucible. The substrates and BiI3 crucible were loaded into the left-hand side of the

furnace tube 2 cm apart. Gas flows were set to 20 mL/min argon (BOC) and 5 mL/min oxygen (Air Products) and the furnace tube was sealed (maintaining a 1:4 ratio of oxygen: argon flow rates). Films were deposited for 15, 30, 50, 60, 90 and 120 minutes, after which they were removed from the furnace and quenched in air. Between batches the furnace tube was rinsed with 37% HCl and absolute ethanol respectively, dried with an air gun and baked out at 800 °C for 4 hours to remove residual contaminants on the inner walls of the furnace tube.

For generation of a phase diagram for the CVD reaction between BiI3 and O2 in Figure

7.1, the same general procedure as above was followed. However, the ratio of argon:oxygen flowed through the furnace was varied to find the correct fraction of oxygen required to grow phase pure BiOI at each temperature. Additionally, the mass of BiI3 precursor used and the total

flow rate of gas was increased with increasing deposition temperature. This was to account for the higher vapour pressure/faster rate of depletion of BiI3 with increasing temperature, meaning

that the gas flow needed to be increased to achieve good coverage within the substrate zone. The reaction conditions are stated in Table 5.1. The deposition time was the time taken to grow a ~ 700 nm film. 325 ºC was the lowest temperature where BiOI could be grown; beyond this, the temperature was increased in 50 ºC increments between 350 – 500 ºC. Note that the temperature of the left-hand and right-hand zones was equal for this work and all work in chapters 7 and 8, as the 10 ºC temperature gradient used for work in chapter 6 was determined visually to have little effect on vapour flow and subsequent coverage of the furnace tube.

Table 5.1 Deposition parameters used to generate the phase diagram for the CVD reaction

between BiI3 and O2 (Figure 7.1). The time represents the time taken to grow a ~ 700 nm film.

Temperature LHS / °C Temperature RHS / °C Oxygen flow rate / mL/min Argon flow rate / mL/min [Flow oxygen] [Total gas flow]

/ % Mass BiI3 / g Time / min 325 325 0.2 23.1 0.9 0.5 120 325 325 5 18 21.7 0.5 120 350 350 0.3 25 1.2 0.5 60 350 350 5 20 20.0 0.5 60 400 400 0.3 30 1.0 1.0 20 400 400 5 25 16.7 1.0 20 400 400 6 24 20.0 1.0 20 450 450 0.4 40 1.0 1.0 7.33 450 450 5 35 12.5 1.0 7.33 450 450 8 32 20.0 1.0 7.33 500 500 0.5 50 1.0 1.0 2 500 500 10 40 30.0 1.0 2

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For all other work in all other sections of chapter 7 BiOI was deposited between 325 – 500 ºC where the ratio of oxygen:argon flow rates through the furnace was roughly 1 %. The deposition conditions used are given in Table 5.2. The conditions at 350 ºC were used to grow films for all work in chapter 8.

Table 5.2 Deposition parameters for BiOI films grown for work in section 7.1.1 onwards. The

conditions at 350 ºC were used to grow films for all work in chapter 8.

Temperature LHS / °C Temperature RHS / °C Oxygen flow rate / mL/min Argon flow rate / mL/min [Flow oxygen] [Total gas flow]

/ % Mass BiI3 / g Time / min:s 325 325 0.2 23.1 0.9 0.5 120 350 350 0.3 25 1.2 0.5 60 400 400 0.3 30 1.0 1.0 20 450 450 0.4 40 1.0 1.0 7:20 500 500 0.5 50 1.0 1.0 2

5.1.2 Atmospheric pressure spatial atomic layer deposition of nickel oxide

A detailed description and schematic of the Vertical Cambridge University Close Proximity atmospheric pressure spatial atomic layer deposition reactor was given in section 4.3.2.2. Nickel oxide was deposited using bis(methylcyclopentadienyl)nickel(II), Ni(CpMe)2 (97 %, Sigma-

Aldrich), and oxygen gas (BIP®, Air Products) as precursors. Ni(CpMe)2 was heated to 60 °C to

increase its vapour pressure (from 1.6 Pa at room temperature to 22.7 Pa at 60 ºC) and thus its gas fraction in the carrier gas stream at the gas manifold. Note that at temperatures above 60 ºC the precursor degraded. A spacing of 90 μm between the gas manifold and substrate was used. Argon gas was bubbled through the Ni(CpMe)2 bubbler at 250 mL/min, which was diluted by argon gas

flowing at 50 mL/min through the metal dilution line. Oxygen gas flowed directly from a cylinder at a rate of 100 mL/min and diluted by a flow of argon through the oxygen dilution line at 500 mL/min. The argon gas flow rate through the inert gas lines was 1000 mL/min. The substrate was held on the heated platform at a temperature of 350 ºC and was moved back and forth below the gas manifold at a speed of 10 mm/s, unless otherwise stated. The number of movements below the gas manifold was varied to achieve the desired thickness, where the growth per cycle was 0.14 nm/cycle (i.e. per backwards or forwards movement) for a substrate speed of 10 mm/s.

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5.1.3 Solution processed nickel oxide

Solution processed NiOx was used as the hole transport material for BiOI solar cells. 1454 mg

nickel nitrate hexahydrate (Sigma-Aldrich) was dissolved in 5 mL anhydrous ethylene glycol (Sigma-Aldrich, 99.8 %) and 334 μL ethylenediamine (Sigma-Aldrich) inside a glovebox. The solution was stirred for 1 hour and filtered through a 0.2 μm syringe filter. Substrates were cleaned sequentially in soapy deionised water, deionised water, acetone and iso-propyl alcohol for 10 minutes each in an ultrasonic bath. They were dried with an air gun and further cleaned by oxygen plasma for 10 minutes. The nickel nitrate solution was cast onto the substrates at 5000 rpm for 45 s using a spin coater. The substrates were dried on a hot plate at 125 ºC for 30 minutes then annealed at 300 ºC for 1 hour. They were quenched by removing from the hot plate directly onto aluminium foil.