Solution processed thin films 37

2.1.3.1 Spin-coating

Spin-coating is a simple method for thin film fabrication from solution including materials such as small molecular compounds, polymers, metal oxides and nanoparticle dispersions. It is well established in the OPV community for polymer blend based solar device fabrication. Spin-coating is cost-effective on lab-scale production but is mostly replaced in a scaled-up process by spray or ink jet printing techniques.

As shown in Figure 2.2 the substrate is mounted on a rotating stage and held in place by vacuum suction. A small amount of solution is applied to the centre of the substrate surface. When spun, the centripetal acceleration spreads the solution evenly across the whole substrate. Excess solution, which is unable to bind sufficiently to the substrate surface, leaves the substrate across the edges. Once the process is complete, an evenly spun film of a defined thickness remains on the substrate surface. The main

Chapter 2: Experimental parameter is the spin speed, which controls film thickness and uniformity. Solution concentration and choice of solvent are also important, because they define the viscosity and volatility of the solution. For P3HT/PCBM blends this is crucial as the drying process defines the phase separation of the blend.

Figure 2.2 Schematic of the spin-coating process: a) dropping of the solution onto the target substrate, b)

substrate spinning and c) the resulting spun thin film.

Spin-coating in air was used to produce thin films of TSCuPc, PTEBS and TiOx. P3HT, PCBM and P3HT/PCBM blends were spun under N2 atmosphere from 1,2- dichlorobenzene (Aldrich, 99%, anhydrous), employing a Laurell Technologies Corporation spin coater.

2.1.3.2 Sol-gel process

Sol-gel process is a common fabrication technique for metal oxide and ceramic thin films including TiOx.[114] Typical precursors for the solution process are metal salts or metal organic compounds such as metal alkoxides. Sol deposition is carried out by either dip-coating or spin-coating. The transition process from liquid sol to viscous or solid gel is mainly based on precursor hydrolysis and polymerisation forming metal- oxygen-metal chains and polymeric networks. In a calcination step with O2 in excess the polymeric structure is then driven towards a higher oxygen content as needed for a proper metal oxide with the correct metal-to-oxygen stoichiometric ratios.

The process was used for thin film fabrication of TiOx in Chapter 6 and 7. The precursor solution was based on a mixture of isopropanol (Fisher Scientific, HPLC

Pipette Solution

Substrate Spin direction

Spun thin film

Vacuum suction

a) _b) c)

grade), titanium (IV) isopropoxide (Ti[OCH(CH3)2]4, Sigma Aldrich, >98 %) and 2- amino ethanol (H2N(CH2)2OH, Sigma Aldrich, >99 %) as a surfactant in a ratio of 20:1:0.5 by volume. The solution was degassed and stirred for 48 hours at room temperature prior to use.

2.1.3.3 Electrodeposition

Although electrodeposition (ED) from solution is well known as a thin film deposition technique for metals, it has only recently been adapted to deposit metal oxides, and ZnO in particular. Electrodeposition of ZnO is based on the reduction of oxygen or oxygen providing precursor compounds to electrochemically generate hydroxide (OH-) in-situ at the working electrode. This leads to ZnO precipitation at the electrode and controlled thin film growth. Although zinc is already provided in the right oxidation state as Zn2+ in the bath solution from a zinc salt, the OH- precursor feed remains challenging. O2,[157] hydrogen peroxide (H2O2)[158] and nitrate (NO3-),[159]are all candidate compounds. Due to a rather low O2 solubility in aqueous solution and H2O2 instability limiting the film growth rate and control, NO3- was chosen as the precursor. NO3- ions are very soluble in aqueous solution and high film growth rates can be achieved. The process is split into two steps: NO3- is reduced to nitrite (NO2-) and OH- in the presence of H2O (Equation 2.1); OH- _{then immediately reacts to form H}

2O again and ZnO which precipitates and deposits the film (Equation 2.2). The overall reaction is summarised in Equation 2.3.[160]

NO3- + H2O + 2e-→ NO2- + 2OH- (Equ. 2.1) Zn2+ _{+ 2OH}-_{→ ZnO↓ + H}

2O (Equ. 2.2)

Zn2+ + NO3- + 2e-→ ZnO↓ + NO2- (Equ. 2.3)

Electrodeposited ZnO films were prepared in a three-electrode set up consisting of an ITO working electrode, a Ag/AgCl/KCl (3.5M) reference electrode and a platinum mesh counter electrode. Electrodeposition was carried out potentiostatically using a computer-controlled potentiostat by applying a potential vs. the Ag/AgCl electrode in a heated deposition bath containing the zinc nitrate (Zn(NO ) , Aldrich, ≥99.0 %)

Chapter 2: Experimental precursor. Precursor concentration and pH were varied for different depositions.[161] The films produced are discussed in further detail in Chapter 6.

2.1.3.4 Spray pyrolysis

Unlike other thin film preparation methods, spray pyrolysis (SP) is fairly cost effective, simple in application, scalable and ideal for metal oxide thin film deposition.[162] Similar to sol-gel and electrodeposition the technique is based on a solution process involving a metal organic precursor or metal salt, which is converted to the final metal oxide upon heat and O2 exposure. Typically, the precursor solution is sprayed by a gas jet onto a heated substrate. Primarily, the solution concentration and spray deposition time determine the film thickness, where temperature determines the film morphology and crystallinity. By influencing the O2 exposure the stoichiometric ratio between metal content and oxygen can be controlled.

The ZnO thin films presented in Chapter 6 and 7 were deposited onto heated ITO substrates from a solution of zinc acetate (Zn(ac)2, Aldrich, 99.99%) dissolved in methanol. Thin film preparation was completed by an annealing step in air for full conversion from Zn(ac)2 to ZnO.