4 Influence of the coadsorbate
4.1 Influence of the coadsorbate on dye adsorption
Figure 37 shows the absorbance of ZnO films sensitized with different indoline dyes either with or without a coadsorbate. For more than one film sensitized by a given sensitization procedure the absorbance is shown, while not all films gave functioning solar cells and thus will no longer appear in the following sections. Similar to the films adsorbed with a coadsorbate (section 3.1), also films without a coadsorbate show a high absorption already by an observation by eye, and usually they appear darker than films adsorbed with a coadsorbate. The measurements with an integrating sphere show the same unexpected flattening of the maximum as discussed in section 9.1.1, p. 187, even for absorbance values near 1. This was attributed to a changed scattering background for the pure ZnO film and the film with adsorbed dye, and thus mainly the absorbance between 600 and 700 nm can be discussed. An absorbance spectrum of D149 in solution is shown in Figure 37(a) for comparison with the spectra of the dyes adsorbed to the ZnO film.
Sensitization for 15 min
For a sensitization time of 15 min, the absorbance of the films without coadsorbate in Figure 37 is higher than the absorbance of films with coadsorbate at λ between 600 and 700 nm. This increase could be caused either by the adsorption of more dye molecules, or by a stronger aggregation of the adsorbed molecules, or both. If a full maximum would have been measured, these cases could have been discerned by a comparison of the absorbance at the maximum and the full width of the peak at
xix Additionally, one film with a slightly different deposition procedure is compared (film/cell [70], sensitized for
half maximum for the different films. On the other hand, the fact that a maximum cannot be resolved in the spectrometer is an indication of a very high light harvesting efficiency for the cells, which is beneficial for current generation in the cells.
A normalization of the spectra to values at 635 nm (see Figure 38, p. 108) shows that the absorbance for higher wavelengths is higher for films without coadsorbate than with coadsorbate, which indicates a higher aggregation of dye molecules when a coadsorbate is not employed. This is observed for all four sensitizers, similar to previous results for D149 121,126. Similar findings were also described for another indoline dye D131 and a squaraine dye 17,127, whereas for D149 with or without coadsorbate in TiO2-based DSCs also an increase in the amount of dye is observed, and additionally a redshift of the spectrum for lower concentration of the coadsorbate was seen 108. However for ZnO films deposited with different parameters, the sensitization without coadsorbate unexpectedly led to a lower amount of dye molecules on the films 16.
400 500 600 700
0.0 0.5 1.0 1.5 2.0 2.5
D14915minCA [61]
DN9115minLCA [60]
D14915min [47]
DN9115min [48]
DN9115min [66]
D149 in DMF
Absorbance (integrating sphere)
Wavelength / nm
400 500 600 700
0.0 0.5 1.0 1.5 2.0 2.5
DN21615minLCA [52]
DN28515minLCA [63]
DN21615min [49]
DN21615min [50]
DN28515min [51]
D149 in DMF
Absorbance (integrating sphere)
Wavelength / nm
400 500 600 700
0.0 0.5 1.0 1.5 2.0 2.5
DN2161h [67]
DN2161hCA [68]
DN2161hLCA [70]
Absorbance (integrating sphere)
Wavelength / nm
Figure 37 – Absorbance (measured with an integrating sphere) of ZnO films sensitized with different indoline dyes with and without a coadsorbate. (a,b) Films sensitized for 15 min and (c) cells sensitized for 1 h. Lighter colors indicate films sensitized without a coadsorbate, while darker colors indicate sensitization with a coadsorbate. Dashed lines of the same color in (a) indicate different films sensitized by the same sensitization procedure. A spectrum of D149 dissolved in dimethylformamide is added for comparison in (a,b).
(a) (b)
(c)
For the films in the present work, the lower amount and lower aggregation of the sensitizer dye on the ZnO surface when adsorbed with a coadsorbate is explained by the co-adsorption of the coadsorbate and thus a steric hindering of dye aggregation, and the blocking of adsorption sites at the ZnO surface by coadsorbate molecules, thus decreasing the maximum amount of dye that can be adsorbed. For D149 a coadsorbate even leads to the suppression of an adsorption of a second monolayer, which was observed via adsorption isotherms on ZnO 126.
In contrast to the sensitization with coadsorbate, where D149 showed the narrowest “peak” and thus the least amount of dye on the ZnO surface (but similar aggregation, see Figure 20, p. 69 and Figure 21, p. 71), without coadsorbate D149 leads to a broad spectrum with higher aggregation than DN216 and DN285, whereas DN91 also shows high aggregation in one film, see Figure 38, p. 108.
The higher aggregation of D149 is probably observed because of the formation of a second monolayer of dye molecules 126.
650 675 700
0.0 0.5
DN21615minLCA [52]
DN28515minLCA [63]
DN21615min [49]
DN21615min [50]
DN28515min [51]
Normalized absorbance (integrating sphere)
Wavelength / nm
650 675 700
0.0 0.5
D14915minCA [61]
DN9115minLCA [60]
D14915min [47]
DN9115min [48]
DN9115min [66]
Normalized absorbance (integrating sphere)
Wavelength / nm
650 675 700
0.0
0.5 DN2161h [67]
DN2161hCA [68]
DN2161hLCA [70]
Normalized absorbance (integrating sphere)
Wavelength / nm
Figure 38 – Absorbance normalized to the minimum at about 450 nm for ZnO films sensitized with different indoline dyes with or without a coadsorbate. (a,b) Films sensitized for 15 min and (c) cells sensitized for 1 h. Lighter colors indicate films sensitized without a coadsorbate, while darker colors indicate sensitization with a coadsorbate. Dashed lines of the same color in (a,b) indicate different films sensitized by the same sensitization procedure.
(a) (b)
(c)
Two causes are possible, why this behavior is not (as strongly) observed for the double-anchor dyes with longer alkyl chain: First, the steric hindrance of a longer alkyl chain and a second anchor group strong enough to either prevent such a second monolayer or at least lead to a distance of the molecules that does not lead to a much larger aggregation, and second, the weaker binding of D149 compared to for example DN216 15 results in a higher affinity of dye molecules to each other, compared to the double-anchor dyes. For DN91 the smaller steric hindrance seems to be small enough to lead to higher aggregation, as for some films the aggregation is also enhanced more.
Sensitization for 1 h
For cells adsorbed for 1 h, the depression of the curve maximum becomes even more pronounced than for the cells adsorbed for 15 min, see in Figure 37(c). Only film [70], which showed a decreased scattering due to a change in the deposition xx, shows a considerably higher absorbance in the maximum and also a broader spectrum, which suggests that the film contains more dye than the otherwise similarly sensitized film [68]. Comparing the dye amount of the standardly sensitized films with or without a coadsorbate, the sensitization without coadsorbate leads to a broader spectrum and thus more dye adsorbed, simply by the absence of coadsorbate molecules and thus more available binding sites. The dye molecules however do not become much stronger aggregated, compare the normalized spectra in Figure 38, p. 108. This further supports the assumption made for the sensitization for 15 min, that the steric hindrance of the longer alkyl chain and second anchor group at least partly prevents aggregation.
xx For film [70], a higher voltage was applied during the first few seconds of EosinY/ZnO resulting in a more transparent ZnO film. As the aggregation is similar to other films sensitized for 1 h, the different transparency is probably the cause for the somewhat different color of the film as seen by eye (more pink compared to the more