Deposition methods

The titania/PS-b-PEO composite films are deposited from the corresponding solutions usually onto solid substrates for film characterization. Common substrates are silicon wafers (Si-Mat, (100)-orientation, 100 mm in diameter) and soda-lime glass sheets (Carl Roth GmbH, 26 × 76 mm²). In order to meet various experimental requirements, the silicon or glass substrates are cut into smaller pieces using a diamond cutter. The cut substrates are then cleaned in a sulfuric acid bath as described below. Several deposition techniques are applied to prepare films onto the substrates, such as spin coating, spray coating and solution casting. In the present thesis, the titania/PS-b-PEO composite films are deposited via either spin coating or spray coating. The HTM layer is mainly produced with spin coating, in some special occasions, it is prepared by solution casting.

Substrate cleaning

To ensure that the deposited films are not contaminated by surface impurities on the substrate, silicon and glass sheets are cleaned in an acid bath before using them as sub-strates. Moreover, the cleaning process can keep a defined state of the substrate surface, which is essential to ensure reproducibility. The acid bath is prepared according to the procedure established in reference [115]. DI water (H2O), hydrogen peroxide (H2O2) and sulfuric acid (H2SO4, 98 %) are mixed in sequence in a glass beaker residing in a water bath at room temperature. The amounts of each component are give in table 4.2. After mixing, the water bath is heated to 80 ^◦C gradually. Separately, the substrates are loaded into a teflon holder and rinsed with DI water to remove dust particles from the substrate surface. Once the temperature reaches 80 ^◦C, the teflon holder is immersed into the acid bath, and kept for 15 min. Thereafter, it is taken out and transferred to a beaker with pure DI water. Each substrate is rinsed with DI water to remove acidic traces before being dried under nitrogen flow. After drying, the substrates are stored in a clean sample box for further use.

component amount

H2O 54 mL

H2O2 84 mL

H2SO4 198 mL

Table 4.2: Volumetric composition details of the acid bath used for substrate cleaning.

Spin coating

Spin coating is a procedure to obtain uniform thin films on flat substrates. The process of spin coating can be classified into four steps. (1) The coating material, containing the molecules dissolved in a solvent, is applied onto a substrate. (2) The substrate is rotated at high angular speed and consequently the majority of the coating material is flung off the substrate. The high-speed spinning is necessary for the formation of homogeneous films. The centrifugal force caused by spinning spreads the liquid evenly on top of the substrate. Larger centrifugal force usually lead to better film homogeneity. (3) Airflow caused by the rotation makes the majority of the solvent evaporate rapidly. (4) After solvent evaporation, a homogeneous plasticized film is achieved on the substrate. For spin coating, higher angular speed leads to thinner final films. Furthermore, film thickness also depends on the solvent, the viscosity and concentration of the solution. In case of films derived from pure polymer solutions, the film thickness dcan be estimated using an empirical formula.

d =Cω⁻¹²c₀M

1 4

W (4.1)

where C is an empirical constant, ω is the angular speed,c₀ is the solution concentration and M_W is the molecular weight of the polymer. The solution concentrations need to be in a range, where the thickness is linearly correlated to the solution concentration [116].

For samples deposited by spin coating a Delta 6 RC TT (S¨uss MicroTec Lithography GmbH) spin coater is applied. The parameters of spin coating including acceleration speed, angular speed of rotation and rotation time are first set. The pre-cleaned substrate is placed on the rotation table of the spin-coater and is then held via an applied vacuum.

It is important to note that the center of the substrate has to be aligned to the rotation axis. A certain amount of solution is dripped on the substrate by a pipette. After the rotation stops, the sample is dismounted from the spin-coater and its back side is wiped with a clean tissue to remove residual solution.

Spray deposition

Spray coating is a process in which the substrate collects dynamic droplets dispersed in a gas. The process of generating these small droplets is known as atomization, which is achieved by pushing out the liquid from a spray nozzle with the aid of a compressed carrier gas [117]. The shape of the spray pattern on the substrates, like solid stream, hollow cone and full cone, strongly depends on the spray nozzles. By selecting the right type of nozzle, a large variety of patterns can be achieved. In this work, a full cone nozzle is chosen for spray deposition, which gives a round shaped area on the substrates with complete spray coverage. Before being sprayed out of the nozzle, a part of the liquid is usually swirled within nozzle and the other part is non-spinning liquid that bypasses an internal vane.

When the liquid runs out through the exit point of the nozzle (usually denoted as orifice), the full conical pattern is achieved. By changing the vane design and/or the distance between the vane location and the orifice, the spray angle and liquid distribution of the spray cone can be tuned appropriately. Full spray cone finds itself most widely used in industry.

The process of spray coating with a full cone is schematically illustrated in figure 4.9.

The whole liquid breakup is divided into three flow regimes based on the volume frac-tion during liquid atomizafrac-tion as described below, which has been reported by Jenny et al. [118].

Figure 4.9: Schematic illustration of liquid atomization with three flow regimes.

• regime I: This regime is named as dense regime. In this region the collision and coalescence of droplets occur frequently, which is the main characteristic of the dispersed

phase dynamics. Due to the abundant existence of droplets, they easily collide with their neighbors and merge together. The dispersed phase volume ration is above 10⁻³.

• regime II: This regime is denoted as dilute regime. In this regime the continuous phase turbulence is dispersed significantly, which dominates the dispersed phase dynamics.

The collision and coalescence of droplets can be neglected since the single droplet is far away from its neighbors. The dispersed phase volume ration is in the range of 10⁻³ to 10⁻⁶.

• regime III: This regime is referred as very dilute regime. Here, the continuous phase turbulence scarcely exists. Small and isolated droplets are presented instead. The dispersed phase volume ration is less than 10⁻⁶.

During spray coating the solvent evaporation from the droplets has to be considered as an important factor for the determination of the spray parameters. For example, if solvent evaporates completely during the droplet transport period, the substrate collects a layer of powders rather than a film layer. A very small amount of solvent makes the droplets stick to the substrate and enable tthem to fuse with pre-arrived droplets. The pressure of the carrier gas and nozzle-to-substrate distance are the key parameters to control the solvent evaporation of the droplets. In the present thesis, the spray device is mounted on a spray set up which allows to regulate the pressure of the carrier gas and nozzle-to-substrate distance. The spray nozzle is orientated perpendicular to the substrate and the nozzle-to-substrate distance is set to 16 cm. The pressure of the carrier gas is kept constant at 2 bar during the whole spray coating process. Under this gas pressure, the flow rate of the coating solution is adjusted to be 25-30 µL s⁻¹. The substrate is kept at 80 ^◦C during the whole process. Two spray protocols are applied in this work: one is using 10 s spray shots and 10 s pause between subsequent shots instead of continuous spray deposition. The other protocol uses that 1 s spray shots and 1 s pauses. A thicker film can be achieved via spraying more solution.

Solution casting

Usually thick films can be obtained by solution casting. The film thickness can be tuned by the solution concentration and the amount of solution dripped on the substrate. Com-pared to spin coating and spray coating, solution casting is very cost-effective as no material is lost during the deposition process. However, it lacks homogeneity. Due to Marangoni flow inside a droplet during the solvent evaporation [119], coffee-rings tend to be produced at the solution edge, which destroys the film homogeneity. For the present experiments, solution casting is used to deposit spiro-OMeTAD layers. The substrate is placed onto a flat metal plate before a defined amount of the coating solution is dripped

on the substrates. It is important to note, the substrate needs to be fully covered. It takes much more time for total solvent evaporation as compared to spin coating and spray coating. After solvent evaporation, a spiro-OMeTAD layer is formed, which can be used directly as a HTM layer in ssDSSCs.