Inner morphology

With respect to structure characterization, SEM measurements can only access infor-mation about surface morphology of a small observable area [147, 148]. In fact, the

film inner morphology is of more interest for studying the photovoltaic performance of the final DSSCs, since the efficiency of charge carrier generation strongly depends on the TiO2/dye/HTMs interface, and the efficiency of charge carrier transport is greatly influenced by titania and HTM interconnected network [149–151]. Dye molecules, the birthplace of charge carriers, are adsorbed on the titania surface and then the separated electrons and holes need to travel along the corresponding n- and p-type material phases to the respective electrodes [152–154]. As the surface only has a limited fraction, the film inner structures dominate the device performance. Therefore, information about inner morphology is necessary and highly demanded.

GISAXS measurements

To obtain information about the morphology inside the films, grazing-incidence small-angle X-ray scattering (GISAXS) measurements are performed at the P03/MiNaXS beam-line of the PETRA III storage ring at DESY, as described in section 3.1.6. This advanced scattering technique is very suitable for film characterization [86, 88, 155]. It is able to parametrize the film buried and inner structures without destroying the probed samples.

Moreover, the grazing-incidence mode gives a large x-ray footprint on samples, which allows tracking the film inner domains at the nano- and meso-scales with a high statisti-cal relevance [156, 157]. 2D GISAXS data are obtained for these four titania films after calcination and are shown in figure 5.4.

The specular beam is shielded by a circular beamstop for each sample to protect the detector from oversaturation, as shown in figure 5.4. Since the directly reflected x-ray beam does not carry any structural information, shielding it enables to increase the mea-suring time for higher intensity of diffuse scattering. The intensity maximum observable in the GISAXS data is the Yoneda peak [158], which is located at the critical angle of titania (after calcination only titania remains on substrates). Conspicuous side scattering is observed in the region ranging from low qy to high qy, which indicates the existence of titania nanostructures in the probed samples. In order to obtain quantitative information regarding the titania domain size and arrangement, horizontal line cuts at the Yoneda peak position are performed along the qy direction as shown by the red arrow in the figure 5.4a. For better statistics, the line cut integrates 5 pixels which are centered at the Yoneda peak. The horizontal line cut for each film is plotted in figure 5.5a.

To extract structural information, the data obtained from horizontal line cuts are fit-ted in the framework of the effective interface approximation of the distorfit-ted wave Born approximation (DWBA) using the local monodisperse approximation (LMA) [86,88,159–

163]. Within this model, form factors with cylindrical geometry distributed over a 1D

Figure 5.4: 2D GISAXS data of spray-coated titania films with different concentration of nanoparticles after high-temperature process.a)0 wt% NPs, b)25 wt% NPs, c)50 wt% NPs, and d) 75 wt% NPs. The scattering intensity scale is shown at the right of the images. The circular black area is the beamstop. The horizontal and vertical black stripes at positions of q_z ≈1.2 nm⁻¹ and q_z ≈0.2 nm⁻¹ are the inter-module gaps of the detector. The red arrow indicates the position of the horizontal line cut.

paracrystalline lattice are used for fitting. The form factors correspond to the scattering objects with a certain size, whereas the center-to-center distance of the scattering objects is obtained from the structure factor. Both form and structure factors are assumed to be Gaussian size distributions. From data modeling, the radii of titania nanostructures (form factors) and the center-to-center distances are extracted and plotted as a func-tion of nanoparticle concentrafunc-tion in figure 5.5b and 5.5c. The data obtained from the 0 wt% NPs, 25 wt% NPs and 50 wt% NPs films are fitted with two characteristic form and structure factors, whereas the fit for the 75 wt% NPs sample requires three characteristic length scales. A fit with only 2 objects was not successful. By comparing small-sized structures, it is found that the size decreases upon the incorporation of nanoparticles, whereas the center-to-center distance increases slightly with the amounts of nanoparti-cles. As a consequence, the pore size of the titania films is related to the nanoparticle content. The pore size can be calculated using the following equation [164]:

pore size = center−to−center distance − 2 × structure radius (5.1) The titania nanostructures in the 0 wt% NPs films have a radius of (3.1 ± 0.1) nm

Figure 5.5:a)Horizontal line cuts of the 2D GISAXS data obtained from the calcined titania films. Black, red, green and blue curves represent cuts from 0 wt% NPs, 25 wt% NPs, 50 wt% NPs and 75 wt% NPs, respectively. The gray lines represent the fits. All cuts and fits are shifted along the intensity axis for clarity of the presentation. Quantitative information about b)structure radii and c)center-to-center distance are extracted and plotted as a function of nanoparticle concentration. Magenta hollow triangles represent small-sized titania structure and Violet hollow squares represent large-sized structure. Orange hollow circles indicate the nanoparticle aggregation in the 75 wt% NPs.

and a center-to-center distance of (15.0 ±3.0) nm, yielding a pore size of (8.8 ±3.0) nm.

For the sample with nanoparticles, the smaller structure radii and larger center-to-center distances give rise to larger pores. The mesopore sizes are of (15.0 ± 6.0) nm, (18.8 ± 4.0) nm and (15.6± 3.0) nm in the 25 wt% NPs, 50 wt% NPs and 75 wt% NPs samples, respectively. The large-sized structures in the 0 wt% NPs, 25 wt% NPs and 50 wt% NPs samples and middle-sized structure in the 75 wt% NPs film have the same radii of (4.7

± 0.1) nm, irrespective of nanoparticle addition, whereas the center-to-center distances have similar trend as the distances between small-sized structures. Via equation 5.1, the pore size between large-sized structures can be calculated as well. The average pore

size is (16.0 ± 2.0) nm in the 0 wt% NPs film, and it increases to approximately 20 nm upon nanopartilces being added in the preparation, but it decreases to (16.0 ± 3.0) nm with further increasing nanoparticle content to the weight ratio of 75 %. The third form factor in the 75 wt% NPs sample originates from nanoparticle aggregates, which are observed in SEM measurements as well. When the nanoparticle concentration in the film exceeds a certain threshold, a part of them tend to agglomerate rather than homogeneously distributing in the sol-gel-synthesized titania network. The large cluster has a structure size of (100±16) nm and an average pore size of (130±40) nm. Through the comparison of scattering intensities from different form factors, the volume ratio of each form factor an be computed as described in section 2.4.3. Through calculation, the large clusters are rare in the 75 wt% NPs film with a volume fraction of only 4.3 %.

In general, the inner morphologies in all films (from GISAXS measurements) are quite in agreement with the surface morphologies obtained from SEM measuremnets. During high-temperature calcination, the removal of template polymer leads to a certain collapse of titania nanostructures [165–169], whereas the presynthesized crystalline nanoparticles hinder a titania nanostructure collapse [170]. As a consequence, we find larger pore sizes in the 25 wt% NPs and 50 wt% NPs samples compared to the 0 wt% NPs film.

However, the ligands outside of titania nanoparticles are removed by calcination step as well, which will also cause the collapse of the titania nanostructures, and this effect presumably exceeds the stabilization effect of nanoparticles in the the 25 wt% NPs sample.

This is attributed to be the reason that the pore sizes decrease with further increasing nanoparticle concentration up to 75 wt%.

N2 adsorption–desorption isotherms

The presence of mesopores in these four samples is confirmed by nitrogen desorption isotherms as shown in figure 5.6. The measurements of nitrogen adsorption-desorption isotherms are performed on a Quantachrome Autosorb-1 instrument as de-scribed in section 3.1.4.

The N2 adsorption-desorptionn isotherms for four samples can be identified as the adsorption and desorption branch of a type IV curve, according to IUPAC classifica-tion [171]. Type IV is a sign of the existence of mesopores in the probed samples [171].

In this type, a monolayer of adsorbed nitrogen is pervading upon surface of titania nanos-tructures at low relative pressure, while at high relative pressure formation of multilayer starts. When the pressure continues increasing to a point that matches Kelvin-type rules in mesopores, nitrogen condensation takes place, which gives a sharp increase of adsorp-tion volume as shown in figure 5.6. Moreover, hysteresis between the adsorpadsorp-tion and

Figure 5.6: Nitrogen adsorption-desorption isotherms measured at 77 K of mesoporous ti-tania films after calcination. a) 0 wt% NPs, b) 25 wt% NPs, c) 50 wt% NPs, and d) 75 wt% NPs. Green solid circles indicate nitrogen absorption and red hollow circle represent nitrogen desorption.

desorption curves is observed in all samples, which also reveals the presence of mesopores.

After N2 reaches saturation, desorption, which the opposite of the adsorption, usually takes place. As nitrogen evaporation from mesopores typically occurs at a lower pressure than for capillary condensation, a hysteresis is seen. The hysteresis in figure 5.6 can be categorized as type H1 loop, indicating the pores have uniform size and shape [171–173].

All samples exhibit pore size distribution with two peaks as shown in figure 5.7. The extracted pore sizes are in good agreement with the value obtained from GISAXS mea-surements. The trend that pore size increases with addition of nanoparticle and then decreases with further increasing nanoparticle concentration has been confirmed again.

The detailed textural parameters derived from nitrogen adsorption–desorption isotherms are summarized in table 5.3.

Amongst all films measured, the 0 wt% NPs film has the lowest Brunauer–Emmett–Teller

sample name BET surface area pore volume pore size (m² g⁻¹) (cm³ g⁻¹) (nm)

0 wt% NPs 80.1 0.28 8.8, 16.1

25 wt% NPs 193.1 0.72 17.1, 20.0

50 wt% NPs 225.2 0.80 17.9, 20.0

75 wt% NPss 145.3 0.46 15.0, 16.7

Table 5.3: Textural parameters of these four mesoporous samples extracted from nitrogen adsorption–desorption isotherms. Figure 5.7 shows that each film has two dominant pore sizes.

(BET) surface area of 80.1 m² g⁻¹ and and pore volume of 0.28 cm³ g⁻¹. Both param-eters increase significantly with introduction of nanoparticles into titania films. The 50 wt% NPs film possesses highest BET surface area (225.2 m² g⁻¹) and pore volume (0.80 cm³ g⁻¹), almost four times larger than for the 0 wt% NPs sample. Larger surface are predicted to anchor more dye molecules on titania surface, which is very helpful for charge carrier generation. More pore volume enables higher degree of HTM percolation, thereby more interfaces between n- and p-type semiconductors. As discussed before, the interface plays a crucial role in the final device performance. Furthermore, larger pore sizes are easier for efficient infiltration of a hole-transporting conjugated polymer, which can lead to a higher pore filling fraction. Therefore, ssDSSCs based on the 50 wt% NPs photoanodes are expected to have a excellent performance in terms of specific surface area, pore volume and pore size.