Preparation of Electrolyte Solution

The solid polymer electrolyte solution containing PEO, 20% KI. and I, (10% of KI) in acetonitrile solvent was prepared. KI and I, were mixed in acetonitrile and stirred at 50°C for 3^1 hours using a magnetic stirrer. The KI:I, composition was fixed at 10:1 (w/w). This polymer electrolyte solution was poured into a petri dish to form a film. Figure 5.11(a) shows the PEO film in the petri dish after drying in vacuum at room temperature for ~7 days to eliminate traces of solvent. On characterization, the ionic conductivity was found to be 2.02 x 10^-5 S/cm.

Fabrication of Dye-Sensitized Solar Cells

As mentioned elsewhere, a conventional DSSC/PSSC has the following components:

• 1. Glass substrate coated in fluorine-doped tin oxide (FTO)
• 2. Nano-/micro-porous, nanocrystalline, semiconducting TiO, for the working electrode
• 3. A dye or perovskite sensitizer
• 4. Electrolyte composed of solid polymer electrolyte film having maximum ionic conductivity + I₂ (for V/1," redox couple)
• 5. Platinum-coated counter electrode

In the subsequent discussions, the techniques adopted for fabrication of DSSC/ PSSC and its components are described. Selection of material for a component plays an important role in maximizing cell efficiency [22-25]. In our laboratory, we have successfully developed DSSC/PSSC assemblies using FTO glass as the substrate.

For laboratory-scale DSSC/PSSC fabrication, we cut FTO (Sigma Aldrich, USA) glass plate (lxl cm²) and cleaned it thoroughly in an ultrasonic bath with acetone, chloroform, and isopropyl alcohol. A blocking layer of Ti0₂ solution (Sigma Aldrich, USA) was spin-coated on the FTO glass plate. The Ti0₂-coated FTO substrate was annealed at 500°C for 30 min in a programmable muffle furnace [26-28]. To prepare the porous Ti0₂ working electrode, Ti0₂ paste was coated on as-annealed

titania-coated FTO substrate using the doctor’s blade method, followed by heating at 500°C for 30 min. This provided a nice porous (10-15 nm pore diameter) TiO, film of approximately 10 mm thickness. The counter electrode was prepared by coating a layer of platinum (spin-coating chloroplatinic acid) on another piece of FTO-coated glass followed by calcination at 400°C for 30 min [29]. The working electrode (FTO/ Ti0₂) was dipped into the (N3) dye solution overnight. The solid polymer electrolyte with redox couple was sandwiched between the working and the counter electrode. The assembly of a typical DSSC/PSSC is shown in Figure 5.12.

Characterizations

X-Ray Diffraction

The phenomenon of diffraction of X-rays from a crystal was first discovered in 1913 by W. L. Bragg and W. H. Bragg using ZnS crystal. There are four components in a typical XRD instrument (Figure 5.13):

• 1. X-ray source
• 2. Sample stage
• 3. Receiving optics
• 4. X-ray detector.

The optics and X-ray detector are situated on the focusing circle while the sample stage lies at the center of the circle. The angle between the sample and X-ray source is 0 (Bragg’s angle) and the angle between the X-ray detector and the projection of X-rays 20. In a typical XRD investigation, the sample is mounted on the sample holder and illuminated by the X-ray beam. The X-rays are scattered by each atom in the sample [30-35]. These scattered beams interfere constructively or destructively depending whether the beams superimpose in phase or out of phase with each other.

The planes of the sample at which the X-rays scatter are called reflecting planes. The processing system records the position of the interference maxima, corresponding to constructive interference. Each peak position corresponds to different planes present in the sample.

X-Ray Diffraction Data Analysis

After collecting the XRD patterns, the first task is to assign its peaks to the different planes present in the sample. Each plane is represented by a unique set of Miller indices. Therefore, every peak is assigned a Miller index corresponding to the plane it represents. There are three methods of assigning Miller indices to the diffraction peaks:

• 1. Comparing the measured XRD pattern with the standard database (JCPDS cards)
• 2. Analytical methods
• 3. Graphical methods.

XRD is an important characterization technique that not only provides information regarding the crystallinity of the sample but it is also used to measure the particle size present in the sample indirectly. All the parameters regarding the crystalline arrangement of the system can be evaluated by XRD analysis in Figures 5.14 and 5.15. Analysis of XRD data gives the unit cell parameters of the system. In our study, we used XRD technique to analyze the course of the reaction as well as to study the effect of heat treatment over time.

In our study, XRD played an important role in analyzing the crystallite size. Actually, at the grain boundaries, recombination sites exist. Large crystallites enhances the efficiency of photovoltaic devices. Therefore, an investigation of crystallite size is critical in order to optimize the performance of the devices [36].

FIGURE 5.15 Typical XRD instrument