Preparation of dye-sensitized solar cells

2.4.1 Pretreatment of porous films

To optimize the adsorption of dyes, the porous ZnO- and TiO2-films have to be pre-treated first. The following procedure was established by Max Beu ¹²³ and Christoph Richter ^130,236 for ZnO.

First, the porous films are heated at 150°C on a hotplate for 1 h. After a short cooling, the films are put under a UV lamp (Eurolite, UV-tube 15 W) for 30 min to increase the number of hydroxyl groups at the surface through water adsorption ^237,238, which enhances the dye adsorption as the acidic group of the dye can form an ester binding group with the hydroxyl surface groups ⁴.

2.4.2 Adsorption procedure

The pretreated films are directly transferred into the adsorption solution. They are left inside the solution (laying on the ground of the vial with the porous film facing the bulk of the solution) for a specified time of for example 1 min, 15 min, 1 h or overnight (given in the sample name). After this time, the films are removed from the solution, rinsed thoroughly with ethanol and afterwards dried with a flow of dry N2.

Types of sensitization solutions used for adsorption of dyes (numbered for exact designation, see also Appendix 9.4, for the use of the respective sensitization solution in the cells):

1. 0.5 mM of D149 (Chemicrea) in a 1:1 mixture of acetonitrile (Roth,  99.9%,  10 ppm H2O) and tert-butanol (Sigma-Aldrich,  99.5%, anhydrous)

2. 0.5 mM of D149 + 1 mM of cholic acid (Sigma,  98%) in a 1:1 mixture of acetonitrile and tert-butanol

3. 0.5 mM of DN91 (Chemicrea) in a 1:1 mixture of acetonitrile and tert-butanol

4. 0.5 mM of DN91 + 1 mM of lithocholic acid (Sigma Aldrich,  97%) in a 1:1 mixture of acetonitrile and tert-butanol

5. 0.5 mM of DN216 (Chemicrea) in a 1:1 mixture of acetonitrile and tert-butanol 6. 0.5 mM of DN216 + 1 mM of lithocholic acid in a 1:1 mixture of acetonitrile and

tert-butanol; (b) variation: cholic acid instead of lithocholic acid

7. 0.5 mM of DN285 (Chemicrea) in a 1:1 mixture of acetonitrile and tert-butanol 8. 0.5 mM of DN285 + 1 mM of lithocholic acid in a 1:1 mixture of acetonitrile and

tert-butanol

9. 0.2 mM of J102 (see also section 1.2.3) in absolute ethanol (VWR, 99.9%) 10. 0.2 mM of J102 + 2 mM Triton® X-100 (TritonX, Sigma Aldrich)

11. 0.2 mM of J102 in ethyl acetate (Fluka, anhydrous) 12. 0.2 mM of J109 in ethyl acetate

13. 0.2 mM of J109 + 0.4 mM of cholic acid in ethyl acetate 14. 0.2 mM of WD-2 in acetonitrile

15. 0.2 mM of WD-3 in acetonitrile 16. 0.2 mM of TPA-B1 in absolute ethanol

17. 0.5 mM of the phthalocyanines (a) Vinylcarbon, (b) Phosphon, (c) Vinylphosphon or (d) AR20 in N,N-dimethylformamide (DMF, Alfa Aesar, 99.8+%)

2.4.3 UV-vis characterization of sensitized films

The sensitized films are characterized by UV-vis absorbance measurements, either in a simple transmission setup or for more scattering samples with an integrating sphere (getProbe 5393 SET) to collect all light scattered by the nanoporous films. The reflectance material is Teflon with over 95%

reflectance over the visible wavelength range ¹⁷⁴. A Tec5 diode array spectrometer (LS-C H lamp with LOE-USB MMS 1 spectrometer, Tec5) is used for both setups. For the integrating sphere setup, the light is pointed with fiber optics directly onto the back side of the FTO glass substrate, and the scattered light is collected with a collimated fiber optics at the integrating sphere output at an angle of 90° with respect to the incoming light. Maximum integration time (6500 ms) is used within the software MultiSpec Pro, and 15-20 scans are averaged for the final spectrum. Even though often a reflecting setup is used with an integrating sphere, in this work a transmission setup was chosen as the transmission or absorbance values are essential for different evaluations. In this case, the backscattered light is not captured and adds to the error of the measurement. Absorbance is then calculated from transmission assuming zero reflection.

For the long measurements in the integrating setup, an upward shift of the sample rate is observed for the whole spectrum (even after long equilibration times of lamp and sensor). Therefore the spectra (sample data) and the references are corrected by a downward shift (minimum value set to 0) and the absorbance is then calculated from these corrected data sets.

For each film, the absorbance of the non-sensitized and the sensitized film is measured, and the absorbance of the non-sensitized film is then subtracted from the absorbance of the sensitized film. So the spectra shown in the results contain only the absorbance of the dye. Additionally, the absorbance above 650 nm, where no more absorbance of the dye is expected (from solution spectra), is set to 0.

2.4.4 Preparation of a platinized counter electrode

First, a piece of cleaned FTO glass with two predrilled holes (2.5 cm x 3 cm, hole diameter 1 mm;

cleaning see above) is treated in a UV-ozone box for 15 min. To achieve a platinized counter electrode, 20 µL of a ~5 mM chloroplatinic acid solution (H2PtCl6 • 6 H2O, Sigma-Aldrich,  37.50%

Pt basis) are spread on the substrate. The solution is allowed to dry in the fume hood, and then heated at 450°C in a tube oven for 30 min (= 45 min including heating to the final temperature). The substrates are allowed to cool to RT or at least below 150°C (else cracks in the substrates are possible). This platinized FTO film can now be used as a counter electrode in the assembly of a DSC.

In some cases, the counter electrode consists of a sputtered Pt layer on a cleaned FTO glass sheet. The sputtering conditions are: DC sputtering with argon gas (Z400 setup, 50 sccm argon flow) at 400 mA for 14 s, yielding a Pt layer of 25 nm thickness. For better reproducibility of the Pt films, the heating current can be reduced to 30 mA while increasing the sputter time to 3.5 min.

2.4.5 Dye-sensitized solar cell assembly

Sealed DSCs are prepared following the description below.

The sensitized nanoparticulate film (working electrode) and a platinized counter electrode are sandwiched together with a cleaned hot-melt foil mask (Jurasol-PV-encapsulation-film Type B, Juraplast; thickness: 0.025 mm) in between the conductive sides of the electrodes facing each other.

Prior to use, the foil is sonicated first for 10 min in acetone and then for 10 min in 2-propanol. The sandwiched assembly is placed on a hotplate at 130°C, and pressed together with a hot soldering iron until the sealing foil melts and connects the two electrodes without leaving any holes (see also Figure 16). The cell is allowed to cool to room temperature.

After cooling, an electrolyte is filled into the cell by placing a drop of it onto one of the holes. The electrolyte (different electrolytes used are listed in Appendix, 9.4), consisting of 0.05 M I2 (VWR, resublimed) and 0.5 M PMII in acetonitrile (Roth,  99.9%,  10 ppm H2O) enters the cell by capillary forces and also fills up the other hole. The remnant of the electrolyte is wiped with a tissue wetted with 2-propanol. A square piece of the Jurasol foil covering both holes is added on top, and covered by a cover glass (Roth, 18 x 18 mm). A hot soldering iron is placed for a short time directly above the holes to melt the foil and seal the cell, and the complete foil is melted to ensure a more efficient sealing.

The procedure above describes the latest cell setup, which was used for most cells sensitized with indoline dyes (from cell 34 upwards). Some differences are listed below, and mentioned in the text if valid for the specified cell:

- Only one hole used for the filling of the electrolyte. In this case, a drop of the electrolyte solution is placed on top of the hole. Then the cell is placed within a vacuum lock chamber and rough vacuum is applied for some seconds. After applying atmospheric pressure again, the space within the two electrodes should be filled with electrolyte. If not, the process is repeated.

The sealing process is the same as described above.

- A different electrolyte is used for some of the cells. It contains 1 M tetrapropylammonium iodide (TPAI, Aldrich,  98%) and 0.1 M I2 (Scharlau,  99.9%) in a 4:1 solvent mixture of ethylene carbonate (Aldrich, 98%) and acetonitrile (Roth,  99.5%)

- To this electrolyte, for some cells also a lithium salt is added, with 0.1 M concentration of LiClO4 (Sigma Aldrich,  98.0%)

Figure 16 – Schematic representation of the assembly of a sealed DSC, viewed from the top and in cross-section.

2.4.6 Preparation of wire solar cells

For electrochemical measurements of sensitized ZnO layers on metal wires, cells have to be prepared out of the deposited wires. The ZnO-coated wires are treated similarly to the ZnO films, with slight variations to make the procedure compatible to textile substrates. That leads (in short) to the following procedure:

1. Heating of laminated samples to 100°C in an oven, afterwards ensuring that the lamination is still intact.

2. Inserting of the laminated ZnO-coated wire into the sensitization solution (see 2.4.2) for 1 h or 15 min (specified in the text or in the sample name), and rinsing with ethanol or acetonitrile.

The sensitized substrates can now be used for the solar cell assembly. In the case of the wire substrates, the solar cell consists of a compartment made of a platinized FTO glass and a clean glass object slide (Roth) connected with glue (hot melt glue, Pattex). The compartment leaves enough space for the sensitized sample including the lamination and a spacer to avoid a contact of the sample and the Pt counter electrode. For long-term electrochemical analysis like EIS, the compartment is sealed with glue at the top to avoid electrolyte evaporation. The cell setup is depicted in Figure 17.

Figure 17 – Schematic cell setup for wire cells, shown from the top and in cross-section. The cell is illuminated from the back, also including absorption of light by the electrolyte.