Solar cell assembly

Two types of titania-based solar cells are produced within the scope of the present work, ssDSSCs and hybrid solar cells. The difference is that no dye is involved in the architecture of hybrid solar cells whereas ssDSSCs need a dye. The production steps of both solar cells are demonstrated in figure 4.10. The individual steps are discussed in more detail below.

Figure 4.10: Schematic illustration of the fabrication of ssDSSCs and hybrid solar cells.

Both cells start with a FTO-coated glass substrate. a) FTO sheets are etched to obtain a desirable pattern. b) The patterned FTO sheets are cleaned with organic solvent. c) The compact titania layer is deposited as electron blocking layer. d) The mesoporous titania films are produced. e) Dye molecules anchor to the nanostructured titania. For hybrid solar cells, this step is not present. f) The HTMs are coated on top of the (dyed) titania films. g) The gold back contact is deposited on top to finalize the solar cells.

Substrate preparation

The production of solar cells starts with the preparation of the bottom electrode. A trans-parent conductive oxide, namely fluorine-doped tin oxide (FTO), is used as the bottom electrode. The FTO sheets are purchased from Solaronix with a size of 10 × 10 cm², a thickness of 1.0 mm and a resistance of about 15 Ω ⁻¹. First, the FTO substrate are cut into pieces of 2.1 × 2.1 cm². To avoid short circuits in the device, a small stripe of FTO is etched away as shown in figure 4.10b. Therefore, one part is covered with a scotch tape. Then, zinc powder is placed on the the tape-free area. Afterwards, HCl (12 M) is dripped on the zinc powder, the uncovered FTO is etched away. After etching, the substrates are rinsed with DI water to remove leftovers of zinc powder and HCl before tearing off the protective tape. Afterwards, the patterned FTO sheets are cleaned with four organic solvents in sequence in a ultrasonic bath for 10 min. The used solvents are:

Alconox® detergent solution (16 mg mL⁻¹), ethanol (99.8 %), acetone (99.9 %) and 2-propanol (99.8 %). The cleaned substrates are subsequently treated with oxygen plasma for 10 min to remove organic residues.

Hole blocking layer

A compact titania film is used as hole blocking layer. The detailed sol-gel synthesis is described in section 4.2.1. The deposited method, spin coating, is explained in detail in section 4.2.3. It is important to note that a narrow stripe of FTO is protected by a piece of tape, making sure this area is not coated the compact titania layer. After spin coating, the tape is removed and the compact film is calcined for 1 h at 450 ^◦C with a heating ramp of 5 ^◦C/min. The final architecture is shown in figure 4.10c.

TiCl₄ treatment

The compact titania layer is treated with a TiCl4 bath to improve the adhesion to meso-porous titania layer above [120]. First, the compact titania coated FTO sheets are rinsed with DI water before placed into a beaker with 50 mL DI water inside. Here, make sure all the substrates are immersed in DI water. Afterwards, 1 mL of the TiCl4 solution (2 M) is added drop-wise. The beaker is then moved into a water bath, which is heated up to 70 ^◦C gradually. After the water bath reaches 70 ^◦C, the samples are kept at this temperature for 30 min. The substrates are subsequently taken out from the TiCl4 bath and rinsed with DI water and ethanol in sequence before drying with nitrogen. Finally, the samples are calcined at 500 ^◦C for 30 min with a heating ramp of 600 ^◦C/h.

Mesoporous titania films

Mesoporous titania films obtained from TTIP and from EMGT are used as photoanodes for ssDSSCs and hybrid solar cells, respectively. The sol-gel synthesis is described in details in section 4.2.2. The titania sol-gel solution is deposited onto the compact titania coated FTO substrates by spin coating or spray coating. Coating methods are explained in section 4.2.3. After film deposition, the titania/PS-b-PEO composite films derived from TTIP undergo high temperature calcination to burn away the polymer template, while for EGMT the polymer template is removed by UV irradiation to obtain porous titania films (figure 4.10d). For ssDSSCs, the porous titania structures are again treated with a TiCl4 bath.

Dye loading

Dye loading is only used for ssDSSCs. After the second TiCl4 treatment, the mesoporous titania films are taken out from the furnace at 80 ^◦C before again being treated with oxygen plasma for 30 min. Afterwards, the samples are directly soaked into a D149 or D205 dye solution and kept inside for 20 h. The dyed samples are then taken out from the dye solution and rinsed with acetonitrile to remove superfluous dyes which do not anchor to the titania surface. In the end, the samples are dried with nitrogen (figure 4.10e).

HTM backfilling

For ssDSSCs, the dyed titania films are backfilled with P3HT or spiro-OMeTAD to achieve the active layer (figure 4.10f). The P3HT layer is produced via spin coating, while the spiro-OMeTAD layer is prepared via solution casting.

For hybrid solar cells no dyes are employed for the device fabrication. The achieved mesoporous titania films are pre-soaked in the host solvent of the P3HT solution for 45 min before backfilled with P3HT by spin coating. To obtain more efficient titania photoanodes, artificial superstructures are introduced onto mesoporous titania films. The detailed process of master production is described in section 4.2.5. The features of the superstructured mesoporous titania films and the corresponding hybrid solar cells are shown in chapter 8.

Gold electrode

A gold electrode is used as the counter electrode for both ssDSSCs and hybrid solar cells. Producing the gold contact is the last step in the solar cell fabrication process, as shown in figure 4.10g. In this work, the gold contact is deposited via thermal evaporation.

The samples are placed upside down on into a shadow mask. During evaporation, the gold contact, in form of a pixel, can only be deposited onto the samples through these openings. The evaporation chamber is evacuated to a pressure of less than 3×10⁻⁵ mbar before about 180 mg of gold is thermally evaporated for 3 min. After the gold electrode is deposited successfully, the performance of solar cells is ready to be tested. The active area of one pixel is defined as the region where the gold contact overlaps with the FTO.

During solar cell test, no shadow masks are applied. The accurate area is evaluated from its optical image via the software ImageJ v1.42q.